^ . 1 ^
L
AN INVESTIGATION OF INTERMITTENT VERSUS CONTINUOUS
CUTTING METHODS OF TOOL LIFE TESTING
by
JAMES LEO T H O A ^ S , B . S . in I . E .
A THESIS
IN
INDUSTRIAL ENGINEERING
Submitted to the Graduate Faculty
of Texas Technological College
in P a r t i a l Fulfillment of
the R e q u i r e m e n t s for
the Degree of
MASTER OF SCIENCE
IN
INDUSTRIAL ENGINEERING
Approved
Accepted
August, 1969
r^
ACKNOWLEDGMENTS
I a m deeply indebted to Dr. B. K, L a m b e r t for his d i r e c t i o n
of this t h e s i s and to the other m e m b e r s of my c o m m i t t e e , D r . C. L.
Burford, Dr. P . Kc Koh, and P r o f e s s o r W. D„ SandeL
I wish to
thank the d e p a r t m e n t technicians C. D. Mittan, J . L. Gibbs, and
Co E, Hipp for t h e i r a s s i s t a n c e in setting up the e x p e r i m e n t a l equipment.
I a m also indebted to the Texas Tech Industrial Engineering
D e p a r t m e n t for t h e i r s p o n s o r s h i p of this t h e s i s .
I v^ish to e x p r e s s my appreciation to my wife and to my p a r e n t s
without w^hose constant patience and understanding this r e s e a r c h
would not have been completed.
11
TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS
ii
LIST OF TABLES
v
LIST OF ILLUSTRATIONS
vii
Chapter
I.
II.
III.
PURPOSE AND SCOPE
1
Introduction
1
Purpose
4
L i t e r a t u r e Survey
4
EXPERIMENTAL DESIGN, EQUIPMENT,
AND PROCEDURE
30
G e n e r a l Considerations
30
Equipment
32
Design of the E x p e r i m e n t
37
Experimental Procedure
42
ANALYSIS OF FLANK WEAR AND
FORCE DATA
Flank Wear Data for I n t e r m i t t e n t and
Continuous Methods
F o r c e Data for I n t e r m i t t e n t and
Continuous Methods
iii
46
47
63
IV
Chapter
Page
Tool Life Data and Economic
Analysis
IV.
79
CONCLUSIONS AND RECOMMENDATIONS
FOR FURTHER RESEARCH
90
Flank Wear Data
90
F o r c e Data
92
Tool Life Values and Economic
Considerations
A r e a s for F u r t h e r R e s e a r c h
94
95
LIST OF REFERENCES
98
APPENDIXES
A.
D y n a m o m e t e r Calibration Curves
102
B.
E x p e r i m e n t a l Data
104
LIST OF TABLES
Table
Page
1.
Cutting Tool M a t e r i a l s and C h a r a c t e r i s t i c s . . .
20
2.
List of Equipment and Use
32
3.
I n t e r m i t t e n t Method t - T e s t s for Flank Wear
Between Opposite Cutting Edges
49
Continuous Method t - T e s t s for Flank Wear
Between Opposite Cutting Edges
51
t ' T e s t s for Flank Wear Between Intermittent
and Continuous Testing Methods
59
I n t e r m i t t e n t Method t - T e s t s for Cutting F o r c e s
Between Opposite Cutting Edges
64
I n t e r m i t t e n t Method t - T e s t s for Longitudinal
F o r c e s Between Opposite Cutting Edges . . .
66
Continuous Method t - T e s t s for Cutting F o r c e s
Between Opposite Cutting Edges
68
Continuous Method t - T e s t s for Longitudinal
F o r c e s Between Opposite Cutting Edges . . .
69
t ' - T e s t s for Cutting F o r c e s Between I n t e r mittent and Continuous Testing
Methods
75
t ' - T e s t s for Longitudinal F o r c e s Between
I n t e r m i t t e n t and Continuous Testing
Methods
76
S u m m a r y of Tool Life v e r s u s Cutting Speed
Curves
83
4.
5.
6.
7.
8.
9.
10.
11.
12.
v
VI
Table
13.
14.
Page
Minimum Cost Economic Analysis of
I n t e r m i t t e n t and Continuous
Testing Methods
87
Maximum Production Rate Economic
Analysis of I n t e r m i t t e n t and Continuous Testing Methods
87
15.
I n t e r m i t t e n t Testing Method
105
16.
Continuous Testing Method
108
LIST O F I L L U S T R A T I O N S
Figure
1.
Page
F o r c e s A c t i n g on a Single P o i n t Tool
During a Turning Operation
7
2.
T y p e s of W e a r R e s u l t i n g in T o o l F a i l u r e
3.
T y p i c a l T o o l Life v s . Cutting Speed
Curve.
R e l a t i o n s h i p of H a r d n e s s and T e n s i l e
S t r e n g t h to W o r k p i e c e and T o o l
Materials
15
5.
S t a n d a r d Tool G e o m e t r y N o m e n c l a t u r e
18
6.
F l a n k W e a r v s . Cut L e n g t h (or T i m e ) for
4.
V a r i o u s Cutting S p e e d s - C a r b i d e T o o l s . . . .
7.
8.
9.
10.
11.
12.
13.
, T h r e e Diraensional Lathe Dynamometer
M e a n F l a n k W e a r - T i m e C u r v e s for
I n t e r m i t t e n t Method
M e a n F l a n k W e a r - T i m e C u r v e for
C o n t i n u o u s Method
10
12
24
35
53
54
M e a n F l a n k W e a r v s . Cutting Speed for
I n t e r 171 it t e n t Method
56
M e a n F l a n k W e a r v s . Cutting Speed for
I n t e r m i t t e n t Method
57
D i f f e r e n c e B e t w e e n InteriTiittent and
C o n t i n u o u s Method F l a n k W e a r
62
Mean Cutting F o r c e v s . T i m e for
I n t e r m i t t e n t Method
71
vii
Vlll
Figure
14.
15.
16.
17.
18.
Page
Mean Cutting F o r c e v s . Time for
Continuous Method
72
Mean Longitudinal F o r c e v s . Time for
I n t e r m i t t e n t Method
73
Mean Longitudinal F o r c e v s . Time for
Continuous Method
74
Tool Life C u r v e s for I n t e r m i t t e n t
Testing Method
81
Tool Life Curves for Continuous Testing
Method
19.
Calibration Curve for Cutting F o r c e , F
20.
Calibration Curve for Longitudinal
F o r c e , F.
82
. . . .
103
103
CHAPTER I
PURPOSE AND SCOPE
Introduction
Tool life studies play a vital role in determining the optimuin
cutting conditions for economical metal removal operations.
As
more is known about the metal cutting process, conservative handbook values for the machining variables traditionally used by industry
may gradually be replaced with values based on economical considerations.
Only continuous r e s e a r c h in this area and the application of
the results, whether it be directly or by handbook revisions, will enable manufacturing organizations to operate economically under the
ever increasing production requirements of today.
It must be recognized, however, that each r e s e a r c h study
determines tool life values for a particular set or sets of workpiece
m a t e r i a l s , tools, machining variables, etc. , and even under the
same carefully controlled experimental conditions spreads in tool
life values of as much as 10:1 have been found (1). Many explanations
have been offered for this wide dispersion such as tool differences,
m a t e r i a l differences, method of holding the work, length to diameter
ratio, etc. , but as of yet no one has completely answered this dilema (1)
1
The one factor which does not vary in most experimental procedures is the manner in which tool life measurements are taken.
Regardless of the tool life criterion chosen, the conventional experimental procedure is:
1, Determine the tool life criterion to be used,
2,
Begin cutting with the test tool for a short period
of time, usually one-half to one minute,
3,
Remove the tool from the work and take the necessary
measurements,
4,
Continue cutting with the same tool for the next
specified time period,,
5,
Remove the tool again from the work and take the
required measurements,
6,
Repeat this procedure with the same tool until
the end of the test (2).
Lambert (3) and Sowinski (4) have cast doubt on this method
of tool life testing.
They show that different tool life values are ob-
tained when using the conventional, or intermittent, raethod and
when using an alternate, or continuous, method of tool life testing.
The continuous method allows the tool to cut without interruption
for a longer period of time with only one measurement taken at the
end of the period.
These two papers, however, are limited in scope.
Lambert
compares only one continuous cutting time length, 4 minutes, with
the same time length using the intermittent method and the cutting
stopped at 0. 5, 1. 0, 1. 5, 2. 0, 3. 0, and 4. 0 minutes.
Sowinski
tests to find a carbide insert which will r e s i s t chipping during an
intermittent cut for a particular metal type, V-57 iron-base hightemperature alloy.
Both recognized and established variability be-
tween the two methods, but neither investigated the nature of this
variability; i. e. , if there is a critical cutting speed - cutting time
combination whereby it makes no difference v/hich testing method
is used.
Another problem is encountered when applying the results of
0
tool life tests to disposable carbide inserts.
It is normally assumed
that tool life values obtained for one cutting edge will remain constant for all cutting edges on the same insert.
Leon (5) has shown
this assuraption to be incorrect for successive edges of an insert
when using flank wear as the tool life criterion.
More specifically,
flank wear values of the first and fourth edges differed significantly,
at the five per cent level, at every one of the three speed levels
tested.
At the highest and lowest speed levels, Leon also showed that
the first and third edges had significantly different flank wear values.
These results indicated a "cumulative effect" on flank wear as successive edges were tested.
Leon did not, however, c o n s i d e r the possibility of using only
the two opposite edges of each i n s e r t in o r d e r to offset this "cumulative effect. "
Purpose
It was the purpose of this r e s e a r c h to investigate:
1.
The effect of cutting speed on tool life values
obtained when using an i n t e r m i t t e n t and a continuous method of tool life testing,
2.
The effect of cutting speed on tool life values
obtained using opposite cutting edges of a carbide
insert,
3.
The relationship existing between speeds for
opposite cutting edges and testing methods to
d e t e r m i n e if a c r i t i c a l s p e e d - t i m e condition
exists,
4.
The effect of any variability in tool life values
between edges and methods on economic models
of the cutting operation.
L i t e r a t u r e Survey
The m e t a l cutting p r o c e s s has been under careful investigation
p r i m a r i l y in the l a s t sixty y e a r s b e c a u s e of the work of F . W. Taylor
in the e a r l y 1900's (6).
Many r e l a t i o n s h i p s , f o r m u l a s , e t c . , have
been developed which a t t e m p t to verify by e m p i r i c a l m e a n s what
physically happens in the p r o c e s s of rennioving of m e t a l from a w o r k piece by a cutting tool.
P r o b a b l y one of the b e s t ways to evaluate the efficiency of a
m e t a l cutting operation is to r e l a t e the "ease of machining" to the
different conditions which effect the p r o c e s s .
The cominonly used
t e r m to e x p r e s s this relationship is "machinability r a t i n g . " This
rating is n o r m a l l y e x p r e s s e d as a percentage and i s d e t e r m i n e d by
comparing how the workpiece m a t e r i a l under consideration m a c h i n e s
with r e s p e c t to a standard material^__Sj
Le.el^ given a rating
of 100 p e r c^nt,
In evaluating the machinability rating of a m e t a l , the followin
/
c r i t e r i a m a y be considered:
1.
Magnitude of the cutting forces on the tool,
2.
Quality of the workpiece surface finish,
3.
Life of the cutting tool between r e s h a r p e n i n g s ,
under standardized conditions,
4.
F o r m and size of chips,
5.
P o w e r consumption of the m a c h i n e ,
6.
Cutting t e m p e r a t u r e s ,
7.
Rate of cutting under a standard force,
8.
Rate of nietal r e m o v a l (7,8),
8
curve is obtained which may be utilized to convert in-process recorder readings to force values.
Surface finish is also an important consideration but is not often taken into account in machining calculations.
The machining
operation is set up using previously determined values of the metal
cutting variables and the resulting surface finish is noted.
If the
finish is not acceptable, the process variables are changed until the
finish meets specifications.
Normally, surface finish will innprove
with increased cutting speed, decreased feed, decreased depth of
cut, increased workpiece temperature, and improved friction conditions between the work and tool (7, 9).
Tool life is the primary machinability factor controlling the
cost of a cutting operation, and for this reason most machinability
ratings of workpiece materials are based on tool life values only.
Tool life is a term in the metal removal industry which has many
definitions.
A coiTiiTion definition is the time for a tool to go from a
"sharp" condition to a condition considered to be "dull" (10).
Often
tool life is specified in terms of equivalent cutting speed, i . e . , the
cutting speed at which a standard value of cutting time, such as 60
minutes, is obtained under a given set of cutting conditions (7, 11).
A tool ceases to cut efficiently and reaches the end of its useful life because of:
).
Flank wear - - Caused by abrasion or wear on the
flank below the cutting edge;
2.
C r a t e r w e a r - - Caused by the moving chip w e a r i n g
a cup in the tool face in back of the cutting edge; which
gradually grows l a r g e r and finally c a u s e s the cutting
edge to c r u m b l e ;
3.
Chipping - - Caused by the breaking out of s m a l l chips
from the face or flank at the cutting edge; usually due
to m e c h a n i c a l or t h e r m a l shock on b r i t t l e tool m a t e r i a l s ;
4.
Various combinations of the above (11, 12).
These types of failures a r e shown graphically in F i g u r e 2.
Often
included as a type of tool failure is the complete breakdown of the cutting edge.
This type of failure, however, is not a common o c c u r -
r e n c e with p r o p e r l y designed and p r o p e r l y selected t o o l s .
Tool life is dependent on many f a c t o r s , some kno"wn and some
still unknown.
Basically, t h e r e a r e four types of v a r i a b l e s known
to affect tool life.
They a r e :
1.
Machining v a r i a b l e s - - Feed, speed, and depth of cut;
2.
Workpiece v a r i a b l e s -- P h y s i c a l and chemical p r o p e r t i e s ;
3.
Tool v a r i a b l e s - - Tool m a t e r i a l and geometry;
4.
Cutting condition v a r i a b l e s - - Cutting fluids, ambient
t e m p e r a t u r e of workpiece, and the frequency or length
of cut (3, 9).
10
Crater
Wear
Wear Land
Width
Flank
Wear
Chipping
F i g . 2.
Types of Wear Resulting in Tool F a i l u r e ,
10
11
The m a n n e r in which t h e s e v a r i a b l e s effect tool life, the common
methods to e x p e r i m e n t a l l y d e t e r m i n e tool life, and the economical
c o n s i d e r a t i o n s of tool life will be discussed in the r e m a i n d e r of this
section.
Relationship of Tool Life to Machining Variables
The t h r e e p r i m a r y machining v a r i a b l e s a r e :
1.
Cutting speed - - The velocity at which the workpiece
moves p a s t the tool or at which the tool moves p a s t
the workpiece; e x p r e s s e d in feet per minute (fpm);
2.
Feed - - The distance the tool advances longitudinally
or into the workpiece for each revolution of the workpiece; e x p r e s s e d in inches p e r revolution (ipr);
3.
Depth of cut - - The n o r m a l distance fronni the original
cutting surface to the freshly cut surface; e x p r e s s e d in
inches.
Of the t h r e e v a r i a b l e s listed above, cutting speed influences
tool life by the g r e a t e s t amount (9, 10, 11). So g r e a t is this effect
that Taylor developed the following equation:
y
where:
V = Cutting Speed,
T = Tool life,
T^ = C
12
n = D i m e n s i o n l e s s constant depending on tool and v/orkpiece v a r i a b l e s ,
C = D i m e n s i o n l e s s constant depending also on tool and
workpiece v a r i a b l e s ; n u m e r i c a l l y equal to the cutting
speed giving one minute tool life (6).
Upon examining this equation, it is obvious that as speed inc r e a s e s tool life d e c r e a s e s in an exponential m a n n e r .
In fact, when
plotted on log-log graph p a p e r , tool life v e r s u s cutting speed values
obtained under n o r m a l operating conditions will plot approxiinately
as a s t r a i g h t line.
A typical curve is shown in F i g u r e 3.
Of the two remaining machining v a r i a b l e s . Cook states that feed
has m o r e effect on tool life than depth of cut (10).
M a t e r i a l : Steel
Tool: Tungsten carbide
Depth of cut: 1/16 inch
F e e d : 0. 062 ipr
50
100
Tool Life (min)
F i g . 3,
Typical Tool Life v s . Cutti]\r, Speed Curve,
log-log scale (15).
13
If these two variables are incorporated into a tool life equation, the
resulting expression is:
V T^ d^ f^ = C
where:
V, T, n, C = Defined as before,
d = Depth of cut,
f = Feed,
X, y = Dimensionless constants again depending on workpiece and tool variables (7, 11).
Relationship of Tool Life to Workpiece Variables
It would be very convenient to be able to relate tool life directly
with known properties of the Vv'^orkpiece material.
Many researchers
have spent countless hours in performing tool life tests for a particular
workpiece materia], and consequently there are several sets of tabularized data available on this subject (7, 11, 13, 14, 15).
The Tool Engineers Handbook lists the following six material
variables as having an effect on tool life:
1. Hardness,
2.
Tensile strength,
3.
Chemical composition,
4.
Microstructure,
14
5.
D e g r e e of cold work,
6.
S t r a i n hardenability (7).
H a r d n e s s and tensile strength have been shown to have a rough
c o r r e l a t i o n to cutting speed for a given tool life and may be used
to e s t i m a t e (roughly) the cutting speed to use for a specific r a a t e r i a l .
This r e l a t i o n s h i p is shown in F i g u r e 4 for different workpiece and
tool m a t e r i a l s .
Exact r e l a t i o n s h i p s between the chemical composition of the
workpiece and tool life have not as yet been d e t e r m i n e d .
However,
the g e n e r a l effect of alloying elements on tool life is known.
For
e x a m p l e , the addition of silicon (0.0 - 2.0%) or nickel (0.0 - 5.0%)
to c e r t a i n types of s t e e l s d e c r e a s e s tool life values, w h e r e a s the addition of sulphur (0. 0 - 0 . 3%) or phosphorus (0. 00 - 0. 15%) may inc r e a s e tool life (7).
Of c o u r s e , these effects could vary c o n s i d e r -
ably if the l i m i t s shown a r e exceeded.
Workpiece m i c r o s t r u c t u r e has also been shown to have a c o r r e l a t i o n with the life of a cutting tool (7, 8, 11, 16, 17).
In g e n e r a l ,
any constituents in the workpiece m i c r o s t r u c t u r e which a r e h a r d e r
than the tool m a t e r i a l tend to d e c r e a s e tool life.
F o r exainple, hard
insoluble p a r t i c l e s of aluminum oxide in the workpiece s t r u c t u r e
d e c r e a s e tool life b e c a u s e of their a b r a s i v e action on the tool face.
C o n v e r s e l y , some types of softer p a r t i c l e s such as m a n g a n e s e sulfide in s t e e l and lead in steel and b r a s s have a v e r y beneficial effect
15
Workpiece Key
(1) - Magnesium and
3000
(2) (3) -
2000
(4) (5) (6) (7) -
0)
alloys.
Brass andbronze.
Aluminum and a l l o y s ,
plastics,
Cast i r o n .
Carbon steel,
cast steel.
Alloy s t e e l .
Stainless steel,
Monel m e t a l s .
Cast Tool
Material
High Speed
Steel
1—1
o
CO
o
0)
m
•{—"
50
•H
-I-
^
+
4-
100 150 200 300 400 600
B r i n e l l H a r d n e s s (Log scale)
U
+
4-
20,000
50,000
1
1
100,000 200,000
Tensile Strength (Log Scale)
F i g . 4.
Relationship of H a r d n e s s and Tensile Strength
to Workpiece and Tool Alriterials (7, 11).
16
on tool life.
It is believed these particles decrease the tendency for
localized pressure welding to take place at the tool-chip or tool-workpiece interface, thereby creating better machined surfaces and longer
tool life (8).
Cold working the workpiece raaterial is often beneficial to tool
life depending on the amount of carbon found in the metal.
In low
carbon steels, up to 0. 3%, cold working improves tool life by lowering the friction encountered in machining; however, cold working of
steels with a carbon content of over 0.4% will reduce tool life in some
cases (11).
Metals usually increase in strength and hardness when strained
or deforined.
This property of "strain hardenability" is nornially
evaluated by the exponent n in Meyer's hardness relationship:
Load = ka
where:
k = Constant of the material corresponding to a fictitious,
underformed hardness, or a special relative hardness
value;
a = Diameter of the impression made by a Brinell ball at
varying values of load;
n = Slope of the resulting curve plotted on log-log graph
paper (7, 16).
17
A low value of n is reported to have beneficial effects on tool life and
surface quality, provided that comparisons are made on materials
of comparable hardness (7).
Relationship of Tool Life to Tool Variables
The two types of tool variables having an influence on tool life
are tool geometry and tool material.
The standard single point tool
geometry is given in Figure 5. Of the seven factors describing tool
geometry, the three having the most pronounced effect on tool life
are the side rake angle, the side cutting edge angle, and the nose
radius (7, 11, 18).
The side rake angle is important because it directs chip flow
to the side of the tool holder and permits easier feeding of the tool
into the work.
When the tool feeds more easily into the work, forces
on the tool tip decrease thereby increasing tool life.
The side cutting edge angle determines the true length of cut
which in turn determines the distribution of the cutting forces on
the tool.
Increasing this angle also increases tool life, but only
up to a point.
Beyond this critical point, chatter develops result-
ing in deterioration of surface quality and tool life.
The nose radius determines the quality of the machined surface as v/ell as tool life.
Increasing the nose radius generally
18
/r=
J
K
LJC
Nose
Side Cutting
Edge Angle
ladius
Side
Rake Angle
Side
Relief A
F i g . 5.
h
End Cutting
Edge Angle
Back
Rake Angle
^/
^ .
End
Relief Angle
Standard Tool G e o m e t r y Nomenclature,
19
improves surface finish, but if the increase is excessive tool chatter
may be induced which results in a tool life decrease.
As can be seen, compromises raust be made among these factors
depending on the application.
Several sets of tabulated data are avail-
able which give satisfactory tool geometries for particular tool-v/orkpiece combinations (7, 11, 13).
The remaining tool variable, tool material, has a more pronounced effect on tool life than the geoiTietry (9). Obviously, the tool
material must be harder than the metal to be cut at the elevated
temperatures occuring during the cutting process. A very hard
material, however, is brittle and might chip or break easily.
There-
fore, the basic properties of a good tool material should be:
1. Hardness greater than the workpiece,
2.
High strength retention at elevated temperatures,
3.
Microstructure which is wear resistant (9).
In summary, the general types of cutting tool materials in use
today, when they appeared, their maximum cutting speeds, maximum cutting temperature, and major constituents are listed in Table
1.
Relationship of Tool Life to^ Cutting Condition Variables
Based on the previous discussion, it should be obvious tb.ot when
metal cutting can be carried out under conditions of lower temperatures,
20
TABLE 1
C U T T I N G T O O L M A T E R I A L S AND
C H A R A C T E R I S T I C S (18).
Material
Approximate
Y e a r of
Appearance
Maximum
Cutting
Speeds
(fpra)
MaximuiTi
Cutting
Temperatures
(°F)
Carbon steel
1800
25
400
0 . 7 - 1 . 2 % Carbon
High-speed
steel
1850
35
500
T u n g s t e n and
Manganese
High-speed
steel
1890
75
1000
T u n g s t e n and
Chromium
Cast alloys
1915
100
1500
Stellites
Super-highspeed steel
1928
150
1600
Cobalt
Carbides
1930
300
2000
Tungsten,
T a n t a l u m , and
Titanium
Ceramics
1955
1600
2200
Oxides
l o n g e r t o o l life can b e e x p e c t e d .
Major
Constituents
Cutting t e m p e r a t u r e r e d u c t i o n is
one of t h e functions of a cutting fluid.
The o t h e r functions a r e to
r e d u c e f r i c t i o n , p r o t e c t the m a c h i n e d s u r f a c e froi-n c o r r o s s i o n ,
and to c a r r y a w a y c h i p s f r o m the cutting zone (8, 18).
21
An incidental i m p r o v e m e n t which r e s u l t s from using a cutting
fluid is that the possibility of a built-up-edge is reduced.
Coincid-
ing with this reduction is a d e c r e a s e , or at least not an i n c r e a s e , in
t o o l - w o r k p i e c e friction (8).
When friction is reduced, tool t e m p e r a -
t u r e will d e c r e a s e , t h e r e b y giving b e t t e r tool life.
Of the t h r e e types of cutting tool failures given in Figure 2 the
m o s t commonly used c r i t e r i o n to judge tool life endpoint is flank
w e a r , or the w e a r land width.
It is believed that going beyond a
c r i t i c a l w e a r land value causes an excessive t e i n p e r a t u r e i n c r e a s e
in the tool, t h e r e b y causing a d e c r e a s e in the h a r d n e s s of the cutting
edge (16).
This softening of the tool m a t e r i a l quite naturally r e s u l t s
in s h o r t e r tool life and a decline in the cutting p r o c e s s efficiency.
Values of flank w e a r often used to determine tool life range from
0. 005 to 0. 060 inch, and a r e governed a l m o s t exclusively by the
ma-chining environment (1, 16, 19),
F o r exaraple, if surface quality
is the limiting f a c t o r , v e r y little wear can be t o l e r a t e d .
In addition,
w e a r land widths of only a few thousandths on a form tool give r i s e
to l a r g e t h r u s t and longitudinal forces which may r e s u l t in large
tool deflections and a loss of dimensional stability (1).
If m e t a l
r e m o v a l is the only c r i t e r i o n , hov/ever, the tool might possibly be
run until complete breakdown of the cutting edge.
The a v e r a g e wear,
land for carbide tool endpoint is usually chosen as 0.030 inch (10,
22
14, 18, 19). High speed s t e e l tools a r e often run until a w e a r land
of 0.060 inch develops (14, 18).
C r a t e r w e a r is found p r e d o m i n a t e l y v/hen machining tough, strong
s t e e l s in conjunction with a continuous chip (5). Since this is a r a t h e r
limited c a s e , c r a t e r w e a r is l e s s frequently chosen to d e t e r m i n e
tool f a i l u r e .
The two types of f a i l u r e s just d i s c u s s e d a r e by no m e a n s all inclusive.
A m o r e complete listing is given by the A m e r i c a n Society
of Tool and Manufacturing E n g i n e e r s (ASTME) a s :
1.
Coraplete failure - - Tool completely unable to cut;
2.
Flank failure - - O c c u r r e n c e of a c e r t a i n size of worn
a r e a on the tool flank (usually based on a c e r t a i n width
of w e a r land or a c e r t a i n volume of m e t a l worn av/ay);
3.
F i n i s h failure - - O c c u r r e n c e of a sudden, pronounced
change in finish on the work surface in the direction
of e i t h e r i m p r o v e m e n t or d e t e r i o r a t i o n ;
4.
Size failure - - O c c u r r e n c e of a change in dimension(s)
of the finished p a r t by a c e r t a i n amount;
5.
Cutting force (or power) failure - - I n c r e a s e of the
cutting force (tangential force), or the power consuiTLption, by a c e r t a i n amount;
6.
T h r u s t force failure - - I n c r e a s e in the t h r u s t on the
tool by a c e r t a i n amount, indicative of end w e a r ;
23
7.
Feeding force failure - - Increase in the force
needed to feed the tool by a certain amount, indicative
of flank wear (7, 11).
If flank wear is chosen as the tool life criterion and this value is
plotted against the length of cut (or time) for various speeds, the
family of curves shown in Figure 6 results.
It can be seen that
there are three distinct phases to each curve.
Phase
These phases are:
I - Characterized by a small amount of flank wear
occurring in a very short period of time.
This
phase is sometimes called the "break-in" period and usually lasts from one to two minutes
depending upon the type of tool, workpiece,
etc.
Phase II - Characterized by a gradual increase in flank
wear.
This phase, although considered by
many researchers to increase linearly, is
usually parabolic in shape and may be approximated by a straight line only in a small region
(5).
The duration of this phase depends on the
machining conditions.
Phase III - Characterized by a rapid increase in flank v/ear
until tool failure.
This transition zone has been
called the "tempcratare sensitive region" by
24
V1<V2<V3<V4
(D
f—1
Length of Cut (F^) or Time (min)
F i g . 6. Flank Wear v s . Cut Length (or Time) for
Various Cutting Speeds (8, 18).
25
Chao and T r i g g e r since it is at this point that
rapid i n c r e a s e s in tool t e m p e r a t u r e s a r e noted
which, as stated before, reduce the p r o c e s s efficiency (1, 20).
Although c r a t e r w e a r is a m o r e difficult m e a s u r e m e n t method
to utilize and usually o c c u r s in limited c a s e s , t h e r e is one i m p o r tant point to c o n s i d e r .
Both flank w e a r and c r a t e r wear m a y occur
s i m u l t a n e o u s l y under c e r t a i n cutting conditions, especially when
using carbide t o o l s .
The i n t e r r e l a t i o n s h i p between these two types
of vi/ear is v e r y vague.
One study v/as p e r f o r m e d using radioactive
m e a s u r e m e n t techniques to evaluate the flank w e a r , c r a t e r w e a r ,
and total w e a r for v a r i o u s cutting s p e e d s .
The study concluded that:
1. Approximately sixty p e r cent of the total tool w e a r
took place at the c r a t e r .
2.
Flank w e a r measurenaents gave a reasonably good
indication of the total tool w e a r (21).
Machining E c o n o m i c s
As noted previously, tool life has a m.ajor effect on the economics
of a cutting p r o c e s s .
The total cost of a cutting operation, C , can
be b r o k e n down into many individual components, depending on the
situation.
The major components a r e :
26
C- = Idle c o s t / p i e c e ,
C^ - Cutting c o s t / p i e c e ,
C^^ - Tool changing c o s t / p i e c e ,
C^g = Tool regrinding c o s t / p i e c e ,
C^(j = Tool d e p r e c i a t i o n c o s t / p i e c e ,
Cpf = P r e m a t u r e failure c o s t / p i e c e (9).
The total cost equation would then be:
Cp = Ci + Cc + C^c + C^g + C^d + Cpf .
[l]
If it is a s s u m e d that disposable carbide i n s e r t s a r e used, tool
r e g r i n d i n g costs can be eliminated and p r e i n a t u r e failures can be
a s s u r a e d to be relatively infrequent and can also be eliminated froni
the cost equation.
T h e r e f o r e , a simplified total cost equation, a s -
suming a fixed depth of cut, can be given a s :
Cp = xTc + y T c / T + xT^ (T^/T) + xT^
where:
X = A v e r a g e operating cost p e r minute for labor and
overhead,
y = Cost of an individual cutting edge,
T = Tool life,
Tc = Cutting time p e r p a r t ,
T^ - Tool changing t i m e ,
Te = Non-cutting tiine (10).
[2]
27
T h i s equation m a y be simplified and r e w r i t t e n a s :
Cp = x [ T e
+ T^
(1 + R / T ) ]
[3]
where:
R = T^j + y / x .
Using this r e l a t i o n s h i p , the m i n i m u m cost tool life, T=s may be
obtained a s :
T* - R (1 _ 1)
[4]
where:
R = Defined previously,
n = Exponent in T a y l o r ' s tool life equation (10).
A s s u m i n g a fixed depth of cut, T a y l o r ' s tool life equation may be
w r i t t e n to i n c o r p o r a t e feed a s :
V
T""
C,
T,
f""
= C ,
or
V = C / T^ f"^
[5]
where:
V,
n = Defined as before,
f = Feed,
rn = Dimensionless constant depending on workpiece and tool v a r i a b l e s (7, 11).
Substituting Equation [4] into this e x p r e s s i o n r e s u l t s in:
V = C / (T*)" f'".
[6]
The value for feed used in this equation will n o r m a l l y be adjusted
to some rnpximum value, f''',
consistt^nt with the p r o c e s s r e q u i r e i n e n t s
28
(excluding tool w e a r ) .
The resulting m i n i m u m cost cutting speed would
thus b e :
Vmin = C/
(T=:=)" (£*)"".
[v]
It should also be realized that some situations will be encounte r e d w h e r e it is n e c e s s a r y to produce at the m a x i m u m production
r a t e consistent v/ith the p r o p e r levels of the machining v a r i a b l e s .
In this situation, the v a r i a b l e to rainimize v/ould be the production
t i m e p e r p i e c e , Tp.
Given the same a s s u m p t i o n s as before, the total production
t i m e p e r piece can be given a s :
Tp = T^ + Td ( T ^ / T ) + T^
[8]
where:
T = Tool life,
Tc = Cutting time p e r p a r t ,
T 1 = Tool changing t i m e ,
Te = Idle or non-cutting time (14).
S i m i l a r l y , the tool life for m a x i m u m production r a t e , T^^^^'
m a y be found to be:
T m a x = Td < ^ _ 1 »•
Again, substituting this e x p r e s s i o n into Equation
r e s u l t s in:
t^l
[sj
29
JLt should be noted that the minimum cost and maximum production rate cutting speed expressions both contain the exponents "n"
and "C" of Taylor's tool life equation.
If varying values of these
two constants are obtained when different cutting edges and methods
of tool life testing are used, then expressions
reflect this difference.
[?] and [lO] will
Therefore, the type of cutting conditions and
the production requirements actually encountered should determine
what values of these constants to use in arriving at the profjer cutting speed.
Investigation of these variances and their effects on
the economic models discussed here were the primary objectives of
this research.
Chapter II describes the equipment used in the experiment, the
experimental design, and the experimental procedure.
Chapter III
presents the results of the experiment and the analysis of these results.
Chapter IV gives the conclusions obtained from the experi-
rnent and recommendations for further research.
CHAPTER
II
E X P E R I M E N T A L DESIGN, EQUIPMENT,
AND PROCEDURE
G e n e r a l Considerations
This r e s e a r c h was conducted using a turning operation under
single point cutting conditions.
A single point turning operation
was s e l e c t e d for s e v e r a l r e a s o n s .
F i r s t , it is a common industrial
operation utilized by manufacturing concerns which m u s t generate
cylindrical shapes.
Second, well defined economic models for
single point turning have been developed and could be used in the
analysis.
Third, an engine lathe and the a,ssociated e x p e r i m e n t a l
equipment w e r e available for u s e .
Finally, experiraental tooling
c o s t s would have been e x c e s s i v e if a drilling or milling operation,
for e x a m p l e , was chosen.
A single type of workpiece m a t e r i a l , SAE 1018 cold rolled steel,
having the following c h e m i c a l composition, was chosen for use in
/
the e x p e r i m e n t :
1.
Carbon
0. 15 to 0. 20%,
2.
Manganese
0.60 to 0.90%,
30
31
3.
Phosphorus
^ . 0 4 % maximum,
4.
Sulfur
0. 05% m a x i m u m ,
5.
Silicon
0. 10 to 0. 30% (7).
SAE 1018 s t e e l was chosen p r i m a r i l y b e c a u s e of its wide i n d u s t r i a l
u s e , r e l a t i v e l y low cost, and availability from local s o u r c e s .
This
type of v/orkpiece m a t e r i a l was a l s o chosen because of its relatively
low machinability rating of 65% (7). Metals in this raachinability
range u s u a l l y produce a m e a s u r a b l e araount of flank w e a r in the cutting tirae i n t e r v a l s used in the e x p e r i m e n t .
The tools selected w e r e DoAll, type SPG-422, grade DO-16,
disposable carbide i n s e r t s having a chemical composition as follows:
1.
Tungsten carbide
79%,
2.
Cobalt
9%,
3.
Titanium carbide
8%,
4.
Tantalum carbide
4% (22),
The i n s e r t s w e r e 1/2 by 1/2 by 1/8 inch with a 1/32 inch nose radius
and an 11 d e g r e e end relief angle.
The cutting tools w e r e mounted
in a 5/8 inch s q u a r e shank tool holder having a 15 degree side cutting edge angle.
The DO-16 grade was found to be the type s p e c i -
fied when inaking light roughing and finishing cuts, t h e r e f o r e , it was
ideally suited for the niachining conditions used in the e x p e r i m e n t .
B e s i d e s the fact that disposable i n s e r t s w e r e r e l a t i v e l y incx]:>ensive
as c o m p a r e d to conventional high-speed steel tools, they also reduced
32
the tool changing time in the experiment.
Intermittent cuts utilized
in this research required removing and replacing the tool eight times
per insert under a particular cutting condition.
Since the tool hold-
er was permanently mounted in the dynamometer, the inserts were
removed simply by loosening a screw clamp on the tool holder.
Thus, no change in the relative position of the tool and the workpiece was necessary and consistency of the experimental procedure
throughout the research was assured.
A mechanical chip breaker was used to prevent the formation
of continuous chips which would foul the tool and workpiece.
In ad-
dition, a safer working environment for the machine tool operator
was obtained.
It was decided not to use cutting fluids in the experiment as
dry cutting conditions facilitate faster tool wear and eliminate any
uncontrollable effect a cutting fluid may have on tool life.
Equipment
The equipment of prime importance in this research included
the engine lathe, tool dynamometer, Dynagraph recorder, and measuring raicroscope. A complete listing of equipment is given in Table
2,
33
TABLE 2
LIST OF EQUIPMENT AND USE
Equipment
Use
1.
Engine lathe
Perform turning operation
2.
Tool dynamometer
Measurement of tool forces
3,
Beckman Dynagraph
recorder
Record tool forces
4.
Gaertner micrometer
microscope and meas'
uring fixture
Measurement of flank wear
5,
Tool holder
Hold carbide inserts
6.
Chip breaker
Break up continuous chips
7.
Three-jaw universal
chuck
Hold work at headstock
8.
Live center
Support work at tailstock
9.
Stop watch
Measure cutting time
10,
Aluminum shims
Adjust height of dynamometer
11.
High intensity desk larap
Illuminate insert edge under
microscope
34
Engine Lathe
The lathe available for use was a 1952 miodel, type DLNE,
W e r k z e u g m a s c h i n e n F a b r i k , 7.5 h o r s e p o w e r engine lathe equip-ped with a t h r e e - j a w u n i v e r s a l chuck and live center at the t a i l stock.
P r e v i o u s r e s e a r c h e r s have deterinined spindle speed a c -
c u r a c y to be approxiraately - 5 R P M (3, 5), t h e r e f o r e , no further
v e r i f i c a t i o n was felt to be needed for this r e s e a r c h .
All m e c h a n i s m s
w e r e carefully checked for defects in o r d e r to prevent u n n e c e s s a r y
e x p e r i m e n t a l e r r o r from being introduced into the r e s u l t s ; none
w e r e found.
After inspecting the machine and periodically through-
out the e x p e r i m e n t , it was lubricated to i n s u r e smooth operation.
Tool D y n a m o m e t e r
The d y n a m o m e t e r used was a one-piece, three component tool
d y n a m o m e t e r consisting of two extended octagonal r i n g s , one above
the o t h e r .
The b a s i c operation of this type of force m e a s u r i n g in-
s t r u m e n t was d i s c u s s e d in Chapter I.
F i g u r e 7 i l l u s t r a t e s the
d y n a m o m e t e r used in this r e s e a r c h and the t h r e e forces it is capable of m e a s u r i n g .
Recorder
A six channel Beckman Dynagraph r e c o r d e r was utilized to r e cord the output resulting fronn the dynamoineter s t r a i n gages during
the cutting operation.
The output, in t e r m s of m i l l i m e t e r s of pen
35
0)
o
^1
o
^ 53 y
o
f1 L,
»^ ;S o
W) ^
Tl
bJO.2
;^ o rt
U ^ K
II
O
II
II
4-> ^1
P^ h h
^1
0)
0)
s
>>
Q
(I)
I—I
(V5
O
•1-1
CO
S
P
0)
<u
iV
o
o
tx^
36
d e f l e c t i o n , w a s r e c o r d e d on s t r i p c h a r t s .
P r o p e r c a l i b r a t i o n of p e n
d e f l e c t i o n to t o o l f o r c e s f a c i l i t a t e d r a p i d and a c c u r a t e f o r c e d e t e r mination f r o m the s t r i p c h a r t r e a d i n g s .
Two of the s i x a v a i l a b l e
c h a n n e l s w e r e u s e d ; only t h e c h a n n e l for cutting f o r c e , F , and the
c h a n n e l for l o n g i t u d i n a l f o r c e , F^, w e r e c o n n e c t e d to the d y n a m o meter.
T h e t h i r d a v a i l a b l e c h a n n e l for the r a d i a l f o r c e , F , w a s
r
not u t i l i z e d b e c a u s e of the i n s i g n i f i c a n t effect of t h i s f o r c e on t o o l
life (7).
Microscope
Flank w e a r m e a s u r e m e n t s w e r e taken using a G a e r t n e r twenty
power m i c r o m e t e r m i c r o s c o p e equipped with t h r e e lens mounted
h a i r l i n e s ; one s t a t i o n a r y h a i r l i n e and two m o v a b l e p a r a l l e l h a i r lines.
T h e p a r a l l e l h a i r l i n e s and the i n s e r t edge w e r e a l i g n e d
w i t h t h e s t a t i o n a r y h a i r l i n e , w h i c h c o r r e s p o n d e d to a z e r o w e a r
land w i d t h .
The p a r a l l e l h a i r l i n e s w e r e t h e n m o v e d u n t i l t h e y w e r e
c e n t e r e d o v e r t h e l o w e r edge of the w e a r l a n d .
The d i s t a n c e the
p a r a l l e l h a i r l i n e s w e r e m o v e d to c e n t e r t h e r a o v e r the l o w e r edge
of t h e w e a r l a n d , i . e . , the w e a r land w i d t h in u n i t s of 0, 0001 inch,
w a s t h e n d i r e c t l y r e a d f r o i n the m i c r o m e t e r d r u m .
A high i n t e n s i t y
d e s k l a m p w a s u s e d to i l l u m i n a t e the i n s e r t edge u n d e r the m i c r o s c o p e to f a c i l i t a t e p o s i t i o n i n g of t h e i n s e r t and to e n a b l e a c c u r a t e
m e a s u r e m e n t of the w e a r land w i d t h .
37
D e s i g n of the E x p e r i m e n t
T h i s e x p e r i m e n t w a s p e r f o r m e d u s i n g four i n d e p e n d e n t v a r i ables:
t e s t i n g m e t h o d , c u t t i n g e d g e , cutting s p e e d , and cutting t i m e ;
and t h r e e d e p e n d e n t v a r i a b l e s :
itudinal force.
flank w e a r , cutting f o r c e , and l o n g -
A s i n d i c a t e d e a r l i e r in t h i s c h a p t e r ,
workpiece
\
^
m a t e r i a l , t o o l t y p e , and t o o l h o l d e r g e o m e t r y w e r e helii )fconstant
V
t h r o u g h o u t the e x p e r i m e n t .
ables:
A l s o fixed w e r e two m a c h i n i n g v a r i -
feed and d e p t h of c u t .
It w a s d e c i d e d to hold the m a c h i n i n g v a r i a b l e s of feed and depth
of cut c o n s t a n t for s e v e r a l r e a s o n s .
F i r s t , feed and depth of cut
do not h a v e a s g r e a t a n influence on tool life a s d o e s cutting speed
(9, 10, 11).
S e c o n d , m o s t e c o n o m i c m o d e l s d e v e l o p e d for single
point t u r n i n g specify a n o p t i m u r a cutting s p e e d for m i n i m u m c o s t
o r m a x i m u m p r o d u c t i o n r a t e a t a fixed depth of cut.
Since a depth
of cut is g e n e r a l l y d e t e r m i n e d by the p a r t c o n f i g u r a t i o n and a s u i t a b l e feed is d e t e r m i n e d by c e r t a i n p r o c e s s r e q u i r e m e n t s , s u c h a s
s u r f a c e f i n i s h , it s e e m e d l o g i c a l to fix t h e s e v a r i a b l e s .
Finally,
e c o n o m i c r e s t r i c t i o n s f o r c e d t h e s e v a r i a b l e s to be held c o n s t a n t .
T h e e x p e r i m e n t would h a v e g r o w n e n o r m o u s l y in s i z e and e x p e n s e
if m o r e t h a n one l e v e l of e a c h f a c t o r had b e e n u s e d .
T e s t c u t s w e r e m a d e to d e t e r m i n e p r o p e r l e v e l s of t h e fixed
m a c h i n i n g v a r i a b l e s , feed and depth of cut, w h i c h would yield
38
c h a t t e r free cutting and produce forces capable of being m e a s u r e d
without damaging the d y n a m o m e t e r .
The levels s e l e c t e d w e r e :
1.
Feed -
0.00315 ipr,
2.
Depth of cut -
0. 075 i n c h e s .
Independent V a r i a b l e s
^'
Testing method. - -
Two testing methods w e r e utilized, an
i n t e r m i t t e n t method and a continuous method.
The b a s i c e x p e r i -
m e n t a l p r o c e d u r e s for each w e r e d i s c u s s e d in Chapter I.
The i n t e r m i t t e n t method utilized eight one-minute cutting tirae
i n t e r v a l s p e r cutting edge.
T h r e e i n s e r t s , with two cutting edges
p e r i n s e r t , w e r e utilized for each speed level t e s t e d .
This combi-
nation r e s u l t e d in six cutting edges per speed level and a total of
240 m i n u t e s of cutting under the i n t e r m i t t e n t method.
In o r d e r to
eliminate any effect on w e a r r a t e due to heat build up in the i n s e r t ,
a m i n i m u m of four minutes of cooling time was allowed between each
one minute cut.
The t h r e e time i n t e r v a l s selected for use under the continuous
method w e r e 4, 6, and 8 m i n u t e s .
Continuous cuts w e r e made at each
of t h e s e tirae i n t e r v a l s and one flank w e a r m e a s u r e i n e n t was taken at
the end of the p e r i o d .
out the e x p e r i m e n t .
F o r c e readings w e r e taken continuously throughAgain, t h r e e i n s e r t s and tv/o edges p e r i n s e r t
w e r e utilized at each of the t h r e e time i n t e r v a l s for each speed level
39
so t e s t e d .
This combination r e s u l t e d in a total of 540 minutes of
cutting u n d e r the continuous method.
2.
Cutting edge. - - Opposite edges of the carbide i n s e r t s w e r e
used in this r e s e a r c h in o r d e r to extend the study of Leon (5) on
tool life differences between consecutive cutting edges of the same
carbide insert.
In this way, the knowledge gained from Leon's
study was expanded and used to b e t t e r understand the w e a r of c a r bide i n s e r t t o o l s .
3.
Cutting speed. - -
Cutting speeds n o r m a l l y encountered in
production situations for the tools and workpiece m a t e r i a l specified
m a y range from 250 - 550 surface feet p e r minute depending on the
values of the machining v a r i a b l e s used (7, 11, 16).
Therefore,
five equally spaced speed levels in this range w e r e chosen for use
in the e x p e r i m e n t .
Since the engine lathe did not have an infinitely
v a r i a b l e spindle speed feature, equal speed spacing was a c c o m plished by fixing the spindle speed and varying the workpiece diam e t e r in equal i n c r e m e n t s .
To obtain the ranges of speeds d e s i r e d ,
the spindle speed was fixed at 710 revolutions p e r minute.
The five
workpiece d i a m e t e r s and the resulting cutting speeds a r e given
in F i g u r e 8.
4.
Cutting t i m e . - -
d e c i m a l stop watch.
Cutting time was m e a s u r e d with a Galco
M e a s u r e m e n t of cutting time with a stop watch
was d e t e r m i n e d to be of sufficient a c c u r a c y for the p u r p o s e s of
40
this r e s e a r c h .
Cutting t i m e was m e a s u r e d from, the instant of con-
t a c t between the tool and workpiece to the end of the p a r t i c u l a r tiine
i n t e r v a l u n d e r investigation.
M e a s u r e m e n t s w e r e taken at the end of each tirae period given
below:
1.
I n t e r m i t t e n t method - 1, 2, 3, 4, 5, 6, 7, 8 m i n u t e s ,
2.
Continuous method - 4, 6, 8 m i n u t e s .
WORKPIECE
DIAMETER
(inches)
CUTTING
SPEED
(fpm)
1.75
325.3
2.00
371.8
2.25
418.3
2.50
464.8
2,75
510.3
F i g . 8.
Cutting Speeds Resulting F r o m the Specified
Workpiece D i a m e t e r s .
Dependent V a r i a b l e s
1.
Flank w e a r . - -
Flank w e a r is one of the m o s t common meth-
ods of d e t e r m i n i n g tool life.
E x c e s s i v e v/ear on the flank of the tool
r e s u l t s in rapid i n c r e a s e s in tool t e m p e r a t u r e with a resulting
41
decrease in cutting edge hardness (7). Coinciding with the softening of the tool raaterial is a rapid tool wear rate and a resulting
drop in cutting process efficiency.
Therefore, flank wear was a
logical criterion for this research.
Measurements of wear land width were taken at the end of
each of the eight one-minute cuts under the intermittent method
and at the end of each specified time period under the continuous
method.
The Gaertner microscope described previously was uti-
lized to make the flank wear raeasurements.
2.
Cutting force and longitudinal force. - - The cutting and
longitudinal forces acting on the cutting tool were measured using the
three component dynaraometer coupled to a Beckman Dynagraph recorder.
Previously developed calibration curves were then used to
convert in-process recorder readings of pen deflection to force
values.
At each speed level, three inserts were used for the interraittent method and three inserts were used for each of the three
continuous time lengths, resulting in a total of 60 carbide inserts and
120 cutting edges being utilized.
Total cutting time for the experi-
ment was 780 minutes. A total of 94. 5 feet of steel bar stock
was used.
42
Experiraental Procedure
The s t e e l b a r s utilized in this e x p e r i m e n t w e r e previously p u r chased for two s e p a r a t e r e s e a r c h studies and consequently they had
b e e n raachined to some extent.
They w e r e , hov/ever, determined to
be usable for this r e s e a r c h and w e r e machined to the p r o p e r d i a m e t e r
p r i o r to the beginning of the e x p e r i m e n t .
The l a r g e s t d i a m e t e r 21
b a r s , each 24 inches long, w e r e machined to the l a r g e s t d i a m e t e r
specified, 2 . 7 5 i n c h e s .
The reraaining 21 b a r s , each 30 inches
long, w e r e machined to a 2. 0 inch d i a m e t e r .
In this way, the first
t h r e e l e v e l s of cutting speed w e r e run on the first set of 2 1 b a r s by
machining each b a r to the next lower d i a m e t e r after the experiraental
cuts w e r e m a d e .
s e t of 2 1 b a r s .
The l a s t two speed levels w e r e run on the second
This eliminated the need for machining the b a r s to
the next lower d i a m e t e r after the third cutting speed level, thereby,
conserving e x p e r i m e n t a l t i m e .
Since these b a r s w e r e used p r e v i o u s -
ly, they contained the n e c e s s a r y countersunk hole for mounting in
the t a i l s t o c k l i v e - c e n t e r .
In o r d e r to mount the d y n a m o m e t e r on the lathe, the tool post
w a s r e m o v e d and the d y n a m o m e t e r was s e c u r e l y clamped in the
T - s l o t s on the lathe c a r r i a g e .
The tool holder was mounted in the
d y n a m o r a e t e r and an i n s e r t clamped in p l a c e .
The p r o p e r tool tip
height was next adjusted by placing aluminum shims under the
43
dynamometer.
This adjustment was necessary to insure that the
tool tip height coincided with the rotational axis of the workpiece.
One end of a workpiece was mounted in the three-jaw chuck
and the live center in the tailstock was inserted into the previously
drilled countersunk hole in the opposite end.
The dynamometer was
connected to the recorder and the recorder pens zeroed before the
start of the cutting operation.
To insure that the proper cutting edge of the carbide insert
was being used, the opposite edges of each insert were numbered
" 1 " and "2" v/ith a felt tipped pen.
The first three repetitions
under each testing method-cutting speed combination were run with
three inserts using the number " 1 " edge.
The remaining three
repetitions for that combination were run using the sanae three
inserts but now using the number "2" edge.
This method blocked
off'any variability existing betv/een cutting edges and faciliated the
analysis to determine if there were significant differences in tool
life values between opposite edges of the same insert.
When the tool was mounted in the tool holder and the dynamometer and recorder were connected and checked, the desired depth
of cut was then obtained.
This was accomplished by turning the
cross-feed handwheel mounted on the compound rest until the tool
tip engaged the workpiece surface,
A micrometer slip rin^ mounted
on the handwheel was rotated to show zero depth of cut on the scale.
44
The tool was then moved longitudinally past the end of the workpiece
and the handwheel rotated to move the tool inward until the d e s i r e d
depth of cut was obtained.
Next the p r o p e r feed and spindle speed w e r e selected by l e v e r s
on the h e a d s t o c k .
The lathe power supply was turned on at this
point to facilitate engaging of the g e a r s in the headstock.
When t h e s e steps w e r e accomplished, the cut was ready to
be m a d e .
The stop watch was checked for operation, spindle lever
engaged, and the feed m e c h a n i s m engaged.
At the instant of con-
t a c t between the tool and workpiece, the stop watch was s t a r t e d .
When the p r e s c r i b e d t i m e period was over, the feed lever v/as d i s engaged, the tool backed away from the work, and the spindle lever
disengaged ending the cycle.
At this time the speed level, edge
n u m b e r , repetition n u m b e r , and the time interval w e r e noted on
the r e c o r d e r s t r i p c h a r t adjacent to the pen deflection r e a d i n g s .
The tool was removed from the holder, mounted under the
m i c r o s c o p e , and the flank w e a r m e a s u r e d and r e c o r d e d .
The tool
was then r e i n s e r t e d in the tool holder after the 4 minute cooling
period or replaced with a new tool depending on the cutting method
being u s e d .
Each group of t h r e e tools, when through cutting,
w e r e placed in envelopes with the speed and testing method noted
on the o u t s i d e .
Also placed in the envelopes was a sample of the
45
chips produced by the particular cut.
The chip color, texture, and curl
characteristics aided in the analysis of the results of the experiment.
When the cuts on a workpiece were complete, a non-test carbide insert was placed in the tool holder and the workpiece was
machined to the next lower diameter.
The bar was then set aside for
use at the next speed level and another workpiece was mounted in
the lathe.
This cycle was repeated until all five speed levels had
been run.
Periodically during the experiment, the dynamometer calibration was checked to insure that no changes had taken place which
would affect the force readings obtained; no significant changes in
calibration were found.
After completion of the experiment, the recorder readings were
converted from inillimeters of pen deflection to force values by means
of the appropriate calibration curve equation given in Appendix A.
All data obtained in the experiment are given in Appendix B.
The following chapter presents an analysis of the flank wear
and force data.
CHAPTER III
ANALYSIS OF FLANK WEAR AND FORCE DATA
This chapter presents the analysis of the experimental flank wear
and force data obtained in the research.
two assumptions.
Inherent in the analysis were
First, it was assumed that flank wear values ob-
tained for a specific speed-tirae combination were normally distributed about a mean value, W. This assumption was utilized by
Lambert (3), and was shown to hold true for high-speed steel tools
in a study at Purdue University (23). Second, the flank wear values
used to indicate tool life end point for each cutting speed were smaller in this research than those discussed in Chapter I, since the
cutting time intervals v/ere necessarily shorter than normally used
in studies of this type.
Therefore, it was assumed that the relation-
ship between flank wear and time was linear with tiine and that the
smaller flank wear values v/ould yield tPie same tool life-speed relationship as larger values.
The analysis to be presented is divided into three sections as
follows:
1.
An analysis of the flank wear data for opposite cutting edges, and intermittent and continuous methods
.o
47
of tool life t e s t i n g ;
2.
An a n a l y s i s of the force data for opposite cutting
e d g e s , and i n t e r m i t t e n t and continuous methods of
tool life testing;
3,
An a n a l y s i s of the effect of differences between
flank w e a r values for both testing methods on a
m i n i m u m cost and m a x i m u m production r a t e economic models of t h e cutting p r o c e s s .
Flank Wear Datai for Intermittent and Continuous Methods
The f i r s t step in the data analysis involved determining if significant differences between flank w e a r values obtained frora opposite
cutting edges at each s p e e d - t i m e corabination existed.
To p e r f o r m
the a n a l y s i s , a t - t e s t was utilized to d e t e r m i n e if the differences
between the sample m e a n s , -x and y, of two n o r m a l l y distributed
v a r i a b l e s , X and Y, were significant.
The t statistic and its de-
g r e e s of freedom, d. f. , a r e given by:
t =
X
V
- y
2
2
nxSx + nySy
and
d. f. = n^x + n y -
where
2
n n (n + n
X y^ X
y
^x + ^y
_ 2)
;_
48
X = Sample m e a n from distribution X,
y = Sample m e a n from distribution Y,
S^ = E s t i m a t e of population v a r i a n c e of distribution X,
2
S
= E s t i m a t e of population v a r i a n c e of distribution Y,
n^ = Sample size taken frora population X,
ny = Sample size taken from population Y (24, 25, 26).
The r e s u l t s of the t - t e s t s a r e p r e s e n t e d in Tables 3 and 4.
F r o m t h e s e t a b l e s , it can be seen that t h e r e a r e no significant
differences between flank w e a r of opposite cutting edges for either
testing method.
In s e v e r a l c a s e s , however, the mean flank wear
values for opposite edges were widely s e p a r a t e d , but the large
v a r i a n c e s a s s o c i a t e d with the wear values kept the related t-values
s m a l l and insignificant.
T h e s e r e s u l t s do indeed substantiate the work of Leon (5).
The r e s u l t s of L e o n ' s study indicated that t h e r e was a "cumulative effect" on flank w e a r when using the four cutting edges of an
i n s e r t consecutively.
The p r e s e n t study indicates that this effect
may be eliminated by using only the two opposite cutting edges.
Since t h e r e w e r e no significant differences in flank wear b e tween opposite edges at any s p e e d - t i m e combination for either
t e s t i n g method, the t h r e e flank w e a r values for each edge w e r e
grouped t o g e t h e r , the resulting six values a v e r a g e d , and this
r
49
TABLE
3
I N T E R M I T T E N T M E T H O D t - T E S T S F O R F L A N K WEAR
B E T W E E N O P P O S I T E CUTTING EDGES
Speed
Time
t
d.f.
510
(VI)
1
2
3
4
5
6
7
8
0.229
0.534
2.213
0. 356
0.196
0. 171
0. 379
0.845
4
4
2.78
2.78
II
II
tl
II
II
II
II
II
II
II
II
II
II
II
II
II
II
II
tl
II
II
II
II
II
1
2
3
4
5
6
7
8
0.277
1.606
0. 169
0.196
1. 512
0.533
0.098
0, 383
11
II
II
II
II
II
II
II
II
II
II
II
II
11
II
II
II
II
II
II
II
II
It
II
II
11
II
II
II
II
M
II
1
2
3
4
5
6
7
8
0.612
0.478
0.831
0.891
O.96I
0.739
0.892
1.452
II
II
II
II
II
II
tl
II
II
II
II
II
II
II
II
II
II
II
II
II
It
II
II
II
II
II
II
II
1
2
3
4
5
6
0.802
0.919
0. 364
0. 571
1. 000
0.400
II
II
II
II
II
II
II
II
II
II
II
II
II
II
11
II
II
It
II
II
II
II
II
II
465
(V2)
418
(V3)
372
(V4)
t Q25
4
Result
Not S:Lgnificant
Not S:Lgnificant
50
TABLE
Speed
-
325
(V5)
Time
3 - Continued
d.f.
7
8
0.242
0.267
1
2
3
4
5
6
7
8
1.706
1.260
1. 109
1. 183
1.400
1. 114
0.853
0.775
2.78
II
Result
'.025, 4
II
Not Significant
tl
II
I II
^x = ^y = 3
51
TABLE 4
CONTINUOUS METHOD t - T E S T S FOR FLANK W^EAR
BETWEEN OPPOSITE CUTTING EDGES
Speed
Time
510
(VI)
4
6
8
0.653
0. 195
0. 520
465
(V2)
4
6
8
0.870
1. 332
0. 095
418
(V3)
4
6
8
2. 516
0.252
1.911
372
(V4)
4
6
8
0.280
0.204
0.516
325
(V5)
4
6
8
0.843
2. 000
1.999
^x = ^y - 3
^•^-
*. 025
4
Result
Not Significant
52
value used a s the m e a n flank w e a r value for each combination.
The
r e s u l t i n g flank w e a r - t i m e c u r v e s for both testing methods a r e p r e s e n t e d in F i g u r e s 8 and 9.
Some i m p o r t a n t o b s e r v a t i o n s may now be made from these two
curves.
In F i g u r e 8, the shapes of the curves generally follow those
of other r e s e a r c h e r s as d i s c u s s e d in Chapter I.
The slopes of the
c u r v e s in this figure a l s o indicate that at the t h r e e highest speed
l e v e l s , the w e a r r a t e i n c r e a s e d m o r e rapidly than for the two lower
I
i
speed l e v e l s .
This o b s e r v a t i o n shows the well established effect of
cutting speed on flank v/ear holds t r u e in this r e s e a r c h .
In Figure 9,
I
I
«
the "wear r a t e for the t h r e e highest speeds is also higher than for
,
I
the lowest two s p e e d s , but only for the time interval from four to
six m i n u t e s of continuous cutting.
F r o m six to eight minutes of
continuous cutting, the second and third highest speed levels actually have an equal or lower slope than the lowest two speed l e v e l s .
This indicates that the continuous method yields lower wear r a t e s
than the i n t e r m i t t e n t method as cutting time i n c r e a s e s .
In addition, t h e r e was a definite grouping of the data under
both testing m e t h o d s , with the m e a n flank w e a r values at the
highest t h r e e speed levels being s e p a r a t e d from those of the lowe s t two speed l e v e l s .
This seeras to indicate that a " c r i t i c a l " cut-
ting speed range for the given machining conditions o c c u r r e d b e tween 418 and 372 feet p e r minute, whereby flank wear values
f
53
510 fpm
• ^ 4 6 5 fpm
600--
,. 418 fpm
500
400
in
I
o
CO
o
300
^
O
372 fpm
> 0 325 fpm
0
200 1—!
100 -0
"1-
6
Time (Minutes)
F i g . 8.
Mean Flank W e a r - T i m e Curves for
I n t e r m i t t e n t Method.
8
54
600 - 5 10 fpm
500 - /
.-J© "^^^ fpna
400 - -
418 fpm
LO
I
o
CQ
(D
300
O
f1
5) 372 fpm
d
d)
200
..,--® 325 fpm
0)
100
ot
H
y
8
T i m e (Minutes)
F i g . 9.
M e a n F l a n k W e a r - T i m e C u r v e for C o n t i n u o u s
Method.
55
a h o v e t h i s r a n g e t e n d to b e "higher and i n c r e a s e f a s t e r t h a n w e a r
values below this range.
T h i s c r i t i c a l r a n g e of s p e e d s m a y be s e e n
m o r e c l e a r l y in F i g u r e s 10 and 1 1 .
U n d e r the i n t e r m i t t e n t m e t h o d ,
c h a n g e s in flank w e a r r a n g i n g f r o m 218 x 10"
to 140 X 10"
inch at eight m i n u t e s
i n c h a t four m i n u t e s w e r e found b e t w e e n the t h i r d and
forth speed levels.
U n d e r t h e c o n t i n u o u s m e t h o d , the c h a n g e s in
flank w e a r b e t w e e n t h e s a m e two s p e e d l e v e l s r a n g e f r o i n 190 x 10"
a t s i x m i n u t e s to 133 x 10"-' inch a t four m i n u t e s .
T h e s e c o n d p o r t i o n of t h e a n a l y s i s involved d e t e r m i n i n g if diff e r e n c e s e x i s t e d b e t w e e n flank w e a r v a l u e s of the two t e s t i n g m e t h o d s
F l a n k w e a r v a l u e s u n d e r t h e i n t e r i n i t t e n t m e t h o d for four, s i x , and
e i g h t m i n u t e s of c u t t i n g v / e r e c o m p a r e d v/ith t h o s e o b t a i n e d u n d e r the
continuous method at the s a m e t i m e i n t e r v a l s .
The d i f f e r e n c e s w e r e
c o m p a r e d u s i n g the t - p r i m e s t a t i s t i c , t ' , d e s c r i b e d by H o e l (10).
T h i s s t a t i s t i c t e s t s t h e h y p o t h e s i s t h a t t h e s a m p l e m e a n s f r o m two
n o r m a l d i s t r i b u t i o n s a r e e q u a l w h e n t h e v a r i a n c e s of e a c h d i s t r i b u t i o n a r e u n k n o w n and c a n n o t be a s s u m e d to be e q u a l .
population m e a n s a r e equal
approximate t distribution.
t'
=
X-
iU^ ~MY^'
When t h e
*^^ s t a t i s t i c p o s s e s s e s a n
The t ' s t a t i s t i c is given by:
y
w i t h a s s o c i a t e d d e g r e e s of f r e e d o m ,
d.f. , of:
56
8 rain.
600
/
^ 6 min.
500
4 min.
400 - in
I
o
r-l
w
300
0)
200
f—I
100 -
0 T-^.v
325
372
418
465
510
Cutting Speed (fpm)
F i g , 10,
Mean Flank Wear v s . Cutting Speed for
I n t e r m i t t e n t Method.
57
600 - -
i> 8 m i n .
500 '
© 6 min.
400 - in
I
4 min.
o
I—(
CO
300
o
u
0)
200 f—t
^.
'^
100
0
t.
V-F
325
372
418
465
510
Cutting Speed (fpm)
Fig. 11.
F l a n k W e a r v s . Cutting Speed for Continuous
Method.
58
2
2
S
n
d.f. =
S
n
^
1
2
S
2
S
^
^
_ 2
^
,_iL)2+(_y_)2
^x+1
ny+1
W h e r e : All symbols a r e as d e s c r i b e d previously.
The r e s u l t s of the t' t e s t s for the flank w e a r values obtained
u n d e r each testing method a r e given in Table 5. After four minutes
of cutting, the i n t e r m i t t e n t flank w e a r was significantly g r e a t e r
than the continuous flank w e a r except at the 418 feet p e r minute
speed l e v e l .
After six m i n u t e s , flank w e a r was significantly dif-
f e r e n t between methods except at the 465 and 418 feet p e r minute
speed l e v e l s .
After eight m i n u t e s , flank w e a r at the 510 feet p e r
minute speed level was the only non-significant value, with the r e maining four speed levels having significantly different flank w e a r
between testing m e t h o d s .
These r e s u l t s indicate that t h e r e was
slightly m o r e v a r i a b i l i t y between testing methods after four and
eight m i n u t e s of cutting than after six minutes of cutting.
T h e s e r e s u l t s w e r e compared with those of L a m b e r t (3), at
four m i n u t e s of cutting and w e r e found to p a r t i a l l y a g r e e with his
results.
L a r a b e r t found no significant difference between testing
m e t h o d s at his highest speed level of 465 feet p e r minute and lov/e s t feed level of 0. 00393 inches p e r revolution.
In this r e s e a r c h .
59
TABLE
5
t ' - T E S T S F O R F L A N K WEAR B E T W E E N I N T E R M I T T E N T
AND CONTINUOUS T E S T I N G M E T H O D S
Speed
Time
t'
510
(VI)
4
6
5.608
2.452
1.606
11
6
-6
2.201
2.447
2.447
Significant, I > C
Significant, I ^ C
Not Significant
3.937
0.618
4.270
12
7
11
2. 179
2. 365
2.201
Significant, I ^ C
Not Significant
Significant, I ^ C
1.223
0.592
3.081
9
11
10
2.262
2.201
2.228
Not Significant
Not Significant
Significant, I ^ C
2.860
3.612
6.452
11
11
12
2.201
2.201
2. 179
Significant,
Significant,
Significant,
2. 181
3.090
6.496
12
14
12
2. 179
2. 145
2. 179
Significant, I ^ C
Significant, I ^ C
Significant, I ^ C
8
465
(V2)
4
6
8
418
(V3)
4
6
8
372
(V4)
4
6
8
325
(V5)
4
6
8
d.f.
nx = ny = 6
I = Mean, inte r m i t t e n t method
C = Mean, continuous method
^,025,
d.f.
Results
I^C
I^C
I^C
60
t h e r e was a significant difference at the s a m e speed level (level 2)
and cutting t i m e .
The feed, however, was slightly lov/er in this
study at a level of 0. 00315 inches p e r revolution.
The r e m a i n d e r
of the data at four m i n u t e s of cutting a p p e a r e d to a g r e e v e r y well
with L a m b e r t ; i . e . , as the speed d e c r e a s e d , flank w e a r values of
the'two methods r e m a i n e d significant for the lowest two s p e e d s .
The insignificance of the flank w e a r values after four rainutes of
cutting at the t h i r d speed level s e e m e d to further justify the previous
conclusion that this was a c r i t i c a l cutting speed region and that furt h e r r e s e a r c h in the a r e a is needed.
The cutting t i m e s of six and eight minutes produced flank w e a r
c o m p a r i s o n s which w e r e not e n t i r e l y unexpected.
The insignificant
differences in flank w e a r between the two testing methods after six
m i n u t e s of cutting at the second and third highest speed levels p r o b ably r e s u l t e d b e c a u s e at these speeds a buipt-up edge was unlikely
to f o r m .
If a b u i l t - u p edge does not form, it has been shown the
tool f o r c e s tend to be higher than when a built-up edge is p r e s e n t (16).
T h e r e f o r e , higher forces indicate the mean flank w e a r values during
the continuous cutting p r o c e s s would be higher and t h e r e f o r e c l o s e r
to the flank w e a r during the i n t e r m i t t e n t cut.
This fact raay be seen
m o r e c l e a r l y upon r e - e x a m i n i n g F i g u r e s 8 and 9. At the second
speed level, the w e a r r a t e can be seen to be r a t h e r rapid and a l m o s t
l i n e a r in n a t u r e under the i n t e r m i t t e n t method.
Under the continuous
61
method, the wear after six minutes at the second speed level was
slightly higher than for the first and highest speed level.
There-
fore, insignificant flank wear differences between methods were
to be expected at this time interval.
At the longest cutting time of eight minutes, the only speed
at which the flank wear difference between raethods was insignificant was at the highest speed level.
This result may be explained
by examining a plot of the difference in flank wear values between
testing methods as given in Figure 12. The highest cutting speed
is the only speed level where this difference decreases from sLx
to eight minutes of cutting time.
In addition, it should be noted
that four of the five speed levels incidate a decrease in flank wear
difference between methods from four to six minutes of cutting.
Thus, Figure 12 indicates that the transition from six to eight
minutes of cutting time was the "critical" time period being sought
as a portion of this research.
It is important to note that in all research studies in this area
(3, 4), including the present study, flank wear values for the continuous testing method were lower than those for the intermittent
method.
It is believed that this result stems from the fact that
a portion of the workpiece material remains on the cutting edge,
and during an intermittent cut this bit of material is reraoved each
time the tool contacts the work.
Since this small portion of
62
14 0-© 4 6 5 fpm
120--
418 fpm
100
in
I
o
80
en
0)
V
A
60--
O
'I y
O
P
^
40
u
c
It!
E
20
0
8
T i m e (Minutes)
F i g . 12.
D i f f e r e n c e B e t w e e n I n t e r m i t t e n t and
C o n t i n u o u s Method F l a n k W e a r .
63
m a t e r i a l is p r e s s u r e welded to the cutting edge, when removed it
t e n d s to chip away some of the edge.
This leads to a m o r e rapid
w e a r r a t e during the i n t e r m i t t e n t tool life testing method.
F o r c e Data for Inter rait tent and Continuous Methods
T a b l e s 6 and 7 give the r e s u l t s of a c o m p a r i s o n of cutting forces
and longitudinal forces between opposite cutting edges for the i n t e r m i t t e n t t e s t i n g method.
It can be seen from Table 6 that the only
significant difference in cutting force between edges o c c u r r e d at the
highest speed level during the sixth minute of cutting.
Since the
second edge produced the lower cutting force, the significant difference can probably be attributed to variability in the data r e sulting from the tools, workpiece s t r u c t u r e , and the machine tool
system.
Tables 8 and 9 give the r e s u l t s when the forces for opposite
cutting edges under the continuous method w e r e c o m p a r e d .
The
information p r e s e n t e d in the tables shows one t-value which is
significant.
This value o c c u r s in Table 9 when comparing longi-
tudinal force differences at the highest speed level after six minutes of continuous cutting.
Although the t-value found in Table 8
for the cutting force at the s a m e speed level after six minutes of
cutting is insignificant at the five p e r cent level, it is significant
at the ten p e r cent level.
These r e s u l t s s e e m to verify the
64
TABLE
6
I N T E R M I T T E N T M E T H O D t - T E S T S F O R CUTTING F O R C E S
B E T W E E N OPPOSITE CUTTING EDGES
Speed
Time
t
d.f.
510
(VI)
1
2
3
4
5
6
7
8
0.692
0.460
0. 140
0. 525
0.542
4 . 128
1.993
2.621
4
4
1
2
3
4
5
6
7
8
465
(V2)
.418
(V3)
372
(V4)
Result
•^- 025, 4
2.78
2.78
N o t S]Lgnificant
Not S:Lgnificant
II
It
It
II
It
It
II
II
It
tl
Significant
Not S:Lgnificant
II
It
It
tl
0. 166
0,449
0. 314
0. 007
0. 353
0.221
0. 143
0. 000
It
It
It
tl
II
tl
tl
It
tl
It
II
It
II
It
It
II
11
It
It
It
It
It
It
It
1
2
3
4
5
6
7
8
0.579
0. 325
0. 335
0.405
0.796
0,619
0.654
0.617
It
It
It
It
II
It
II
It
II
It
It
II
II
It
It
tl
II
tl
II
II
II
II
tt
II
1
2
3
4
5
6
7
8
2 . 160
1.674
0. 512
0.835
0. 005
0. 178
0. 210
0.712
It
It
It
II
It
tl
It
It
It
It
It
It
It
It
II
II
II
It
II
II
II
II
It
tt
F
65
TABLE
Speed
Time
325
(V5)
1
2
3
4
5
6
7
8
^x = ny = 3
6 - Continued
d.f.
1.193
1.738
1.273
1.570
1.293
0.909
1.221
1.415
-.025, 4
Result
2.78
Not Significant
66
TABLE 7
INTERMITTENT METHOD t - T E S T S FOR LONGITUDINAL
FORCES BETWEEN OPPOSITE CUTTING EDGES
Speed
510
(VI)
465
(V2)
418
(V3)
372
(V4)
Time
1
2
3
4
5
6
7
8
0.577
0.226
0. 320
0.256
0. 004
0. I l l
0. 122
0. 002
1
2
3
4
5
6
7
8
^•^-
"^.025, 4
Result
4
4
2.78
2.78
Not S Lgnificant
Not S Lgnificant
II
It
It
It
It
It
It
It
II
tl
tl
II
It
It
It
It
tl
tl
1. 155
1. 155
0. 000
0.000
0.816
1. 155
0.817
0. 368
II
It
It
It
It
It
It
It
It
It
It
It
tl
II
It
It
II
It
It
It
It
It
tl
II
1
2
3
4
5
6
7
8
0.817
0.551
1.092
1.092
1.804
2.065
1.731
0.710
It
It
It
II
It
It
II
It
It
It
It
It
tt
It
It
It
II
It
It
It
II
It
It
It
1
2
3
4
5
6
7
8
0. 365
0. 311
0.518
0.412
0. 164
0.004
0.741
0.816
It
II
It
tl
tt
II
II
II
It
It
It
II
II
It
It
It
It
II
It
II
tl
It
II
It
. r
67
TABLE
Speed
Time
325
(V5)
1
2
3
4
5
6
7
8
....
nX
-
n.
0.814
1.442
0.864
1.585
1.586
0.598
1. 135
1. 191
7 - Continued
^•^-
*. 025, 4
Re s alt
4
4
2.78
2.78
Not Significant
Not Significant
II
It
It
It
It
It
It
It
It
tl
II
11
68
TABLE 8
CONTINUOUS METHOD t - T E S T S FOR CUTTING FORCES
BETWEEN OPPOSITE CUTTING EDGES
Speed
Time
510
(VI)
4
6
8
1.008
2.242
0.834
465
(V2)
4
6
8
0.497
0.488
0.714
418
(V3)
4
6
8
1.026
1.039
0.732
372
(V4)
4
6
8
0.691
1.000
0. 160
325
(V5)
4
6
8
0.623
0.219
0.700
nx = Hy =: 3
d.f.
4
4
II
Result
-.025, 4
2.78
2.78
It
Not Significant
Not Significant
It
It
69
TABLE
9
CONTINUOUS M E T H O D t - T E S T S F O R LONGITUDINAL
F O R C E S B E T W E E N O P P O S I T E CUTTING EDGES
Speed
510
(VI)
Time
d.f.
4
6
8
0.000
3..169
0. 309
465
(V2)
4
6
8
0. 000
0.283
1.295
418
(V3)
4
6
8
0.866
1,914
0. 327
372
(V4)
4
6
8
4
6
8
0.614
1. 155
0.517
2. 160
1.411
0.489
325
(V5)
^ x = ^y
4
4
It
'.025, 4
2.78
2.78
tl
Result
Not Significant
Significant
Not Significant
Not Significant
70
conclusion that the six minute cutting speed range is an area where
more detailed research is needed.
There were no significant dif-
ferences in either of the two forces measured after eight minutes
of cutting for both of the testing methods.
Force data for the intermittent method is plotted in Figures 13
and 14, and plotted for the continuous method in Figures 15 and 16.
The figures indicate that as speed increases, the cutting force for
both testing methods decreases; the primary exceptions being the
highest speed level in Figures 13 and 15 under the intermittent
method.
This deviation from the general trend probably resulted
from a rapid increase in flank wear at the high speed level which
was observed in the discussion on flank wear in the previous
section.
It should also be noted that in all cases cutting forces exceeded
longitudinal forces.
This result agrees with those reported by
Tourret (15).
To test if significant differences in forces existed between testing
methods, a t' test was perforraed on the data.
The results of these
tests are presented in Tables 10 and 11.
The information in Table 10 shows that at four minutes, the
cutting force was significantly different between testing methods
at the first and second highest speed levels.
After six minutes,
the cutting force was significantly different at the second, third.
71
130
510 fpra
,^ O 372 fpm
120 -
—O 325 fpm
110
en
'V
Pi
o
JO—-—-O 418 fpiri
• j^^-^
465 fpm
100 -J
^
o
o
o
2r
^.^'
vO^
o
hi
WD
-.0-"
-
©-•
90
!3
U
o
80
0
t
8
T i m e (Minutes)
F i g . 13.
Mean Cutting F o r c e v s . T i r a e for Interm i t t e n t Method.
72
130
120
»j
••„__*•
fg)
^
9
• —•••—© 325 fpm
—
—
110 - -
0 465 fpm
— - ^ 372 fpm
5 10 fpm
418 fpm
CO
O
100 - -
&.
o
o
u
o
^
90
--
Pi
•iH
+J
-P
u
Pi
o
80
0
t
-f
[-
"i"—-I—~H
4
5
6
--!
f8
T i m e (Minutes)
F i g . 14.
M e a n Cutting F o r c e v s . T i m e for Continuous
Method.
73
90--
•^-0 372 fpm
510 fpm
80
...-O 325 fpm
70
© 418 fpm
CO
13
Pi
pi
O
o
o
;H
o
60--
r-l
PI
13
+->
•H
50
. . ^ 465 fpm
W)
Pi
o
©—
Pi
©——.
—0^
cv5
•0)
40
ot
7
8
Time (Minutes)
F i g . 15.
Mean Longitudinal F o r c e v s . Time for
I n t e r m i t t e n t Method.
74
904-
80--
o^"
*">0 325 fpm
J /^ f p m
^ 4 6 5 fpm
70CO
'V
PI
O
510 fpm
g
60'
;^
o
©^
P!
13
50
W)
-
©-
o
Pi
^
40>-
ot
8
Time (Minutes)
F i g . 16. Mean Longitudinal F o r c e v s . Time for
Continuous Method.
75
TABLE
10
t ' - T E S T S FOR CUTTING FORCES BETWEEN
INTERMITTENT AND CONTINUOUS
TESTING METHODS
.Speed
Time
t
510
4
12
8
5. 358
2. 179
2.647
464
4
6
8
418
d.f.
*. 025., d.Jf
Result
11
2. 179
2.262
2.201
Significant, I ^ C
Not Significant
Significant, I ^ C
2. 377
2.805
2. 100
8
6
10
2. 306
2.447
2.228
Significant, C ^ I
Significant, C ^ I
Not Significant
4
6
8
1.733
2.670
1.208
12
10
12
2. 179
2.228
2. 179
Not Significant
Significant, C ^ I
Not Significant
371
4
6
8
1.036
3. 153
2.080
11
10
11
2.201
2.228
2.201
Not Significant
Significant, I ^ C
Not Significant
325
4
6
8
1.737
1.456
0.867
9
6
12
2.262
2.447
2. 179
Not Significant
Not Significant
Not Significant
6
9
^x = ny = 6
I = M e a n , inte;r m i t t e n t m e t h o d
C = Mean, continuous raethod
F
76
TABLE
11
t ' - T E S T S FOR LONGITUDINAL FORCES BETWEEN
INTERMITTENT AND CONTINUOUS
TESTING METHODS
Speed
Result
Time
t'
d.f.
510
4
6
8
2.551
3.000
2.231
8
11
12
2. 306
2.201
2. 179
Significant,
Significant,
Significant,
464
4
6
8
2.536
5.005
4.811
6
10
9
2.447
2.228
2.262
Significant, C}> I
Significant, C ^ I
Significant, C ^ I
418
4
6
8
0.406
1. 140
1.239
11
11
11
2.201
2.201
2.201
Not Significant
Not Significant
Not Significant
371
4
6
8
0. 557
2. 388
1.206
7
9
11
2. 365
2.262
2.201
Not Significant
Significant, I ^ C
Not Significant
325
4
6
8
2.012
2.259
0.724
12
9
10
2. 179
2.262
2.228
Not Significant
Not Significant
Not Significant
•
^ x = ^ v. = 6
C = Mean, continuous method
^025,
d.f.
I^C
I^C
I^C
77
and fourth highest speed levels.
Finally, at eight minutes, differences
between cutting forces v/ere significant only at the first and highest
speed level.
Table 11 shows similar results.
After four rainutes of cutting,
longitudinal forces between methods were again significantly different at the first two highest speed levels. At six minutes, differences were significant at the first, second, and fourth highest
speed levels.
Eight minutes of cutting yielded differences which
were significant at the first and second highest speed levels.
In general, it can be concluded that as speed decreases, the
differences in force values between methods become insignificant.
This conclusion probably stems from the fact that a built-up-edge
forms more rapidly at the lower speed levels.
The built-up-edge
changes the effective rake angle of the tool thus causing the cutting and longitudinal forces to decrease.
During an interraittent
cut, the built-up-edge is broken off at each contact with the workpiece.
This increases the flank wear and causes an increase in
the cutting and longitudinal forces.
During the continuous cut,
the built-up-edge forms, then breaks off, and reforms again;
repeating this cycle until the end of the cut. This cyclic action
causes a larger fluctuation in cutting and longitudinal forces for
the continuous cut, thereby reducing differences which may exist
between methods to an insignificant level.
78
Also to be noted is the fact that in both tables the forces for one
method are not constantly greater or lesser in magnitude than those
for the other method.
The intermittent forces are greater than the
continuous forces at the first and fourth speed levels, with the converse being true at the second, third, and fifth speed levels.
These
inconsistencies are believed to be caused by differences in the workpiece structure, tools, or by unexplainable effects due to a built-ip-edge
at certain speed-time-method combinations.
This is an area definitely
warranting further study.
Several observations may also be made when the t'-tests between
testing methods for flank wear and forces (Tables 5, 10, and 11) are
compared.
Significantly different flank wear values when compared
with significantly different force values coincide only at the 510 and
465 feet per minute speed levels after four minutes of cutting and at
the 372 feet per minute speed level after six minutes of cutting.
The coincidence at these particular combinations probably resulted
from the higher speeds causing a higher flank wear rate and a corresponding increase in force values.
The shorter time intervals
at which this coincidence occurred further verified the time effect
on flank wear rate. Nonsignificant differences in flank wear and
force values coincide only at the 418 feet per minute speed level after
four minutes of cutting. At the lowest speed level of 325 feet per
' .
79
minute, the flank wear values for all time intervals are significantly
different, while all of the force values are not significant.
These observations lead to one basic conclusion; in this research,
force values tend to be very poor indicators of the flank wear on an
insert.
Of the two forces raeasured and analyzed, however, the
longitudinal forces seemed to be better indicators of flank wear than
cutting forces; since the results of the t'-tests coincided seven times
w^hen comparing wear with longitudinal forces and only four times
when comparing wear with cutting forces.
Tool Life Data and Economic Analysis
In order to corapare the results of a minimura production cost
and maximum production rate economic models applied to each testing method, tool life curves must first be drawn.
Tool life values,
when plotted on log-log graph paper, generally result in a straight
line according to the Taylor tool life equation VT^ = C discussed in
Chapter I.
Tool life values in this experiraent were obtained utilizing the
assumption stated at the beginning of this chapter.
In this assump-
tion it was stated that smaller values for flank wear were chosen
as the tool life end point since the relationship between flank wear
and cutting time was assumed to remain linear throughout time.
Using this assuaiption, a flank wear value was chosen which was
80
common to the flank wear-time curves for both methods at the speed
levels under consideration.
The cutting tirae required to reach the
specified flank wear was then read directly from the abscissa of
each graph.
It will be recalled that the flank wear-tirae curves given in Figures 8 and 9, pages 53 and 54, presented the first three speeds and
the last two speeds grouped separately.
Therefore, it was necessary
to select two different flank wear values depending on the speed level
as follows:
SPEED (fpm)
FLANK WEAR (inches x 10"^)
510, 465, 418
375
372, 325
175 .
Using these flank wear values, the resulting log-log plots of
tool life versus cutting speed are presented in Figures 17 and 18.
These results are summarized in Table 12.
Figures 17 and 18, as well as the data sumraarized in Table 12,
indicate considerable differences in expected tool life values between
intermittent and continuous cutting.
For the three highest speed
levels, the intermittent method yielded lower n and C values than the
continuous method.
For the lowest two speed levels, the intermit-
tent method yielded a higher n value but a lower C value than the
continuous method.
81
a
13
<D
<U
to
W)
7i
6
7 8 9
Time (Minutes)
F i g . 17.
A.
510, 465, 418 feet p e r minute speed l e v e l s ;
0. 00375 inch flank w e a r c r i t e r i o n .
B.
372, 325 feet p e r minute speed levels;
0, 00175 inch flank w e a r c r i t e r i o n .
Tool Life Curves for Intermittent Testing
Method.
82
2000 ..
P.
0)
w
to
Pi
•H
O
8 9
Time (Minutes)
F i g . 18.
A.
510, 465, 418 feet p e r minute speed l e v e l s ,
0. 00375 inch flank w e a r c r i t e r i o n .
B.
372, 325 feet p e r minute speed l e v e l s ,
0. 00175 inch flank w e a r c r i t e r i o n .
Tool Life C u r v e s for Continuous Testing
Method.
83
TABLE
12
SUMMARY OF TOOL LIFE VERSUS
CUTTING S P E E D CURVES
Testing
Method
Speed Level
(fpm)
n
C
Intermittent
510, 465, 418
0. 348
740
Intermittent
372, 325
0. 554
700
Continuous
510, 465, 418
0. 636
1250
Continuous
372, 325
0. 477
795
84
F r o m observing t h e values of C, the cutting speed for one minute
tool life, it may be initially concluded that the continuous testing
method gave b e t t e r tool life than the i n t e r m i t t e n t testing method
u n d e r the s a m e machining conditions.
The values for n, the slope
of the log-log tool life c u r v e , n o r m a l l y indicate how fast the w e a r
r a t e changes with changes in cutting speed.
In this c a s e , however,
no conclusion may be made as to which testing method is b e t t e r since
one method did not consistently yield a higher or lower n value than
the other method.
In o r d e r to d e t e r m i n e economic differences between testing
m e t h o d s , an analysis was p e r f o r m e d using the two economic models
p r e s e n t e d in Chapter I and s u m m a r i z e d h e r e .
The t o t a l cost p e r p a r t , ^, resulting from a single point t u r n ing operation was given in Chapter I as [2] :
^ = x [ T c + T^ + Td ( T c / T ) ]
+ Y(Tc/T)
w h e r e : All symbols a r e as given p r e v i o u s l y .
B a s e d on the total cost equation above, the m i n i m u m cost cuttine speed, V . , a s s u m i n g a fixed depth of cut and feed can be
& ^
» mm'
°
^
given by:
Vr a m
• = C /' (^T r a .m)' ^
where:
n, C = Constants in T a y l o r ' s tool life equation,
Tmin ~ Tool life for minimun"i production cost, and
85
1
equals R.(n - 1) where R = T^j + Y/X.
The m i n i m u m cost p e r p a r t , ^ j ^ j ^ ' ^^^Y then be given a s :
<min = X [ T e + ( T e ) „ i „ / d - n ) ]
where:
(Tc)j^ij^ = Cutting t i m e at V^^^^^ and equalsTTdL/12 V^-^^
d = D i a m e t e r of w o r k p i e c e ,
L = Length of cut,
f = Feed.
When the n e c e s s i t y to produce at the m a x i m u m production rate
is c o n s i d e r e d , the production time p e r piece is the v a r i a b l e which
is m i n i m i z e d .
The resulting tool life for m a x i m u m production r a t e
"^max' ^^ given in Equation [9] a s :
T
= T
max
U -'] •
d
If this equation is substituted into the Taylor tool life equation,
VT^ = C, the r e s u l t i n g equation for the cutting speed for m a x i m u m
production r a t e Vj^3^_j^, i s : '
V
_ p
^ max
/
^ '
/-T^
\n
^ max^
The cost of operating at t h e s e conditions may be found by the following cost equation:
^max = X [ T e + (T^)max /
1"^]
where:
(Tc)max = Cutting time at V^^^^^ .
86
F o r the p u r p o s e s of this a n a l y s i s , the follov/ing values w e r e a s s u m e d to hold for the cutting operation:
X = 15 cents p e r minute,
R = 4,
L = 10 i n c h e s ,
Tg = 0, 5 m i n u t e s ,
f = 0,00315 inches p e r revolution,
di =^2.75 inches (used with 510, 465, and 418 fpm grouping),
d2 = 2 , 0 0 inches (used with 372 and 325 fpm grouping),
T^ = 1,0 m i n u t e .
Using the values of n and C obtained from the tool life curves
and the above a s s u m e d v a l u e s , the r e s u l t s of the economic analysis
for m i n i m u m cost and maximura production r a t e a r e given in
Tables 13 and 14, r e s p e c t i v e l y .
F r o m t h e s e two t a b l e s , it can be seen that the continuous t e s t ing method yields lower costs in e v e r y case under both economic
models.
T h e r e f o r e , it should be obvious that tool life values ob-
tained u n d e r i n t e r m i t t e n t cutting conditions should not be applied
to a situation w h e r e the tool will actually be cutting under continuous
cutting conditions.
Using the m i n i m u m cost model, the t h r e e highest
speed levels yielded a 16.4% d e c r e a s e in costs when using the continuous method instead of the i n t e r m i t t e n t method of tool life t e s t i n g .
At the lowest two speed levels using the s a m e economic raodel,
87
TABLE
13
MINIMUM COST ECONOMIC ANALYSIS O F
I N T E R M I T T E N T AND CONTINUOUS
TESTHNIG METHODS
Testing
Method
Speed Level
(fpm)
1.
Intermittent
510, 465, 418
3 7 2 , 325
7 . 50
3 . 22
365
366
151. 5
159. 8
2.
Continuous
510, 4 6 5 , 418
3 7 2 , 325
2 . 30
4 . 38
739
411
126. 7
123. 6
TABLE
T-•^min
V .
mm
( M i n u t e s •') (fpm)
Vmin
( C e n t s)
14
MAXIMUM PRODUCTION RATE ECONOMIC ANALYSIS
O F INTERMITTENT AND CONTINUOUS
TESTING METHODS
Testing
Method
Speed Level
(fpm)
^max
'^max
(Minutes) (fpixi)
1.
Intermittent
510, 465, 418
372, 325
1.88
0.81
2.
Continuous
510, 465, 418
372, 325
0.57 1780
1. 10
761
594
789
'^max
(Cents)
96.0
78. 5
60. 3
70. 1
88
there was a 24. 9% decrease in costs from the continuous to the intermittent method.
To illustrate these results, suppose a tool life
experiment to determine economical cutting conditions was run in
the 510 to 418 feet per minute speed range using an intermittent
testing method.
In actuality, however, the production operation
called for a continuous cut of seven minutes.
If an intermittent test-
ing method was used and the results applied to the continuous cutting
situation, a penalty cost of 24.8 cents per piece would result from
using the wrong testing method.
When using the maxiinum production rate economic model, the
three highest speed levels yielded a 37. 2% decrease in costs when
using the continuous method instead of the intermittent method.
There
v/as a 10, 7% decrease in costs from using a continuous instead of
an interinittent method at the two lowest speed levels. Using the
same assumptions as in the previous illustration, the penalty cost
for using the wrong testing method under the maximura production
rate criterion would be 35. 7 cents per part.
It should also be noted that no coaiparisons of cost data were
made between the two grouped sets of speeds within a testing raethod.
The primary reason for not comparing these results was that the
flank wear criterion for each grouping was considerably different.
Lambert (3) showed that as the flank wear criterion increased,
values of the exponent n in the Taylor tool life equation decreased
Y
89
and values for C in that same equation increased.
Therefore, it was
felt that a valid comparison between the two sets of grouped data for
each testing method could not be obtained.
A summary of the results of this research and conclusions which
can be drawn from these results are presented in Chapter IV. Areas
for further research are also included in this chapter.
CHAPTER
IV
CONCLUSIONS AND RECOMMENDATIONS
FOR FURTHER RESEARCH
The p r i m a r y p u r p o s e s of this r e s e a r c h study w e r e to investigate:
1,
Flank w e a r and force differences between opposite
cutting edges of a carbide i n s e r t cutting tool, and
2,
Differences in flank w e a r and tool life values b e tween the inte rinittent and continuous methods of
tool life t e s t i n g .
The r e s u l t s p r e s e n t e d in Chapter III indicate that these p u r p o s e s
were accomplished.
In the following s e c t i o n s , the r e s u l t s a r e
s u m r a a r i z e d and conclusions a r e then drawn from these r e s u l t s .
Flank Wear Data
Mean flank w e a r values for opposite cutting edges w e r e first
analyzed for both testing methods utilizing the t - s t a t i s t i c to t e s t
for differences between sample raeans from tv/o n o r r a a l populations.
In all c a s e s , it was found that flank w e a r values for opposite
90
91
cutting edges did not differ significantly.
This result was important
since it extended the research of Leon (5) on flank wear differences
between consecutive edges of a carbide insert when using an intermittent tool life testing method.
Leon concluded that there seemed
to be a "curaulative effect" on flank wear as consecutive cutting
edges were utilized.
The present study substantiated Leon's results
and led to the conclusion that for both testing methods the "cumulative effect" may be eliminated by using the opposite edges of an
insert.
It should be pointed out, however, that one edge per insert
could also achieve the same result, and quite possibly other combinations of cutting edges as well.
Therefore, more research is felt
to be needed in which the econoraics of different edge combinations
are investigated.
Equal spacing of the cutting speed levels brought out the obvious
grouping of the mean flank wear data for both testing methods. The
resulting wear from the three highest speed levels were definitely
seen to be separated from the lowest two speed levels.
Frora this
result, it is concluded that somewhere in the 372 to 418 feet per
minute speed range is a "transition zone" in which the rate of flank
wear tends to increase as speed increases.
In order to test for flank wear differences between testing methods,
the t' -statistic was used to test differences between sample means
from two normal distributions when the distribution variances are
92
unknown and cannot be assumed to be equal.
several observations were made.
From the analysis,
It was found that intermittent
cutting yielded a higher mean flank wear than continuous cutting.
In addition, it was seen that flank wear differences between testing
methods v/ere slightly greater at the four and eight minute cutting
time periods than at the six minute period.
Upon re-exaraining
Figure 12, page 62, the large differences in flank wear between
methods at cutting times of six and eight minutes at all but the
highest speed level leads to the conclusion that this time period may
be "critical" to flank wear and is believed to warrant further investigation.
Force Data
The force data recorded by the dynamonaeter was analyzed
using the same statistical tests ysed for flank wear.
The analysis
for differences in forces between cutting edges resulted in two
t-values being significant:
1.
One cutting force difference under the intermittent method at the highest speed level after six
minutes of cutting, and
2.
One longitudinal force difference under the continuous method again at the highest speed level
after six minutes of cutting.
93
Although the first significant t-value above was attributed to differences in the workpiece structure, tool material, etc. , the important point was the time at which both occurred.
This result leads
to the conclusion that data in the cutting time range of six minutes
exhibits unusual variability and suggests that further research in
the area is warranted.
The t' test between testing methods leads to the general conclusion that as speed decreases, differences in force values between
methods tend to become insignificant.
This result was believed to
stem from the presence of a built-up-edge and its effect on force
reduction discussed previously.
Since a built-up-edge forms quickly
at low speeds, forces for both methods were expected to be low. The
continuous method had a built-up-edge throughout the cutting cycle,
which resulted in low forces and correspondingly low flank wear
values.
Even though the inte rinittent method produced the cyclic
action of formation and removal of the pressure and heat welded
built-up-edge, the speeds were low enough so that a small amount
of flank wear took place on the cutting edge.
Therefore, at low speeds
the differences in wear were not separated by a significant amount.
Since this result was in almost direct opposition to the result
obtained for flank wear, force values are concluded to be poor
predictors of flank wear especially at the lower speed levels.
I^ongi-
tudinal forces did, however, seem to be slightly better indicators
94
of flank w e a r than cutting f o r c e s ,
L a m b e r t (3) alsc concluded that
longitudinal f o r c e s w e r e b e t t e r p r e d i c t o r s of flank w e a r than w e r e
cutting f o r c e s .
Tool Life Values and Economic Considerations
When tool life c u r v e s w e r e plotted for both testing methods, the
f i r s t conclusion drawn is that t h e r e a r e considerable differences
in the c u r v e s .
Two c u r v e s w e r e r e q u i r e d for each method because
of the grouping of the data at the t h r e e highest and at the two lowest
speed l e v e l s .
It was shown that in e v e r y c a s e , the values of C, the cutting
speed for one minute tool life, w e r e higher for the continuous method
than for the i n t e r m i t t e n t method.
This leads to the conclusion that
the continuous method gives longer tool life than the i n t e r m i t t e n t
method.
Differences between testing methods w e r e not immediately
evident upon examining the values for n, the slope of the log-log
plots of tool life, since one method did not produce consistently
s m a l l e r or l a r g e r n values than the other method.
The r e s u l t s of the economic a n a l y s i s showed that the continuous
method yielded lower production costs than the i n t e r m i t t e n t method
for both economic m o d e l s c o n s i d e r e d .
Using the m i n i m u m cost
m o d e l , i n c r e a s e s in production costs of 16. 4% and 24, 9% at the
95
three highest and two lowest speed levels, respectively, resulted
from using the intermittent method rather than the continuous raethod.
Using the maximum production rate model, increases in production
costs of 37. 2% and 10. 7% at the three highest and two lowest speed
levels, respectively, resulted from using the intermittent method
rather than the continuous method.
The economic analysis justifies the conclusion that tool life
values obtained using an intermittent testing method should not be
used to determine economical cutting conditions for a continuous
cutting operation.
The research has verified the feelings of this
investigator in that experimental tool life testing methods should
coincide as closely as possible with the actual production cutting
method to be used in order to properly apply the resulting tool life
data.
Areas for Further Research
During the discussion of the results of this research, several
areas warranting further investigation were raentioned.
These areas
and several additional areas are now presented:
1.
Additional studies in which speed levels are concentrated in the critical speed range of 372 to 418 feet
per minute.
96
2.
Further studies similar to this research but with
longer cutting times for both testing methods and
with the use of a cutting fluid.
3.
Additional studies similar to this research using
different workpiece materials, more feed levels,
and more cutting edge combinations.
4.
Investigations of the same nature as this study in
which tool-workpiece temperatures are monitored
in order to determine if the causes of the "cumulative effect" found in flank wear values among cutting edges of carbide inserts are temperature
related.
5.
Studies to determine the effects of using other
grades of carbide inserts for a specific cutting
situation and an economic analysis of the results.
6.
Additional studies to determine more quantitatively
the differences existing between carbide inserts of
supposedly the same grade and chemical composition.
7.
Studies in which the factors responsible for significant differences in tool life values are quantitatively
deterinined and used to formulate a model to predict
97
their effect on this variability.
Some factors which
should be investigated are:
a.
Workpiece microstructure,
b.
Tool composition and grade,
c.
Machining variables,
d.
Workpiece holding method,
e.
Longitudinal distance of the tool from the
end of the workpiece.
LIST OF REFERENCES
(1) M a z u r , J o s e p h C. "Tool Life T e s t i n g , " A m e r i c a n Society
of Tool and Manufacturing E n g i n e e r s , P a p e r No. MR68610, 1968.
(2) B r i e r l e y , Robert G. , and Siekmann, H. J. Machining
P r i n c i p l e s and Cost Control. New York: McGrawHill Book Company, Inc, , 1964,
(3) L a m b e r t , B. K. "An Analysis of the Reliability of Tool Life
P r e d i c t i o n . " Unpublished Ph. D. d i s s e r t a t i o n , Texas
Technological College, August, 1967.
(4) Sowinski, L, A, "New Turning Tool Test, " A m e r i c a n
Machinist, Vol. 109, No. 6, March 15, 1965.
(5^ Leon, J o s e R. ' A n Analysis of Tool Life Variation Among
Cutting Edges of Carbide I n s e r t s . " Unpublished M a s t e r ' s
t h e s i s , Texas Technological College, May, 1969.
(6) T a y l o r , F . W. "On the A r t of Cutting Metals. " T r a n s actions of ASME, Vol. 28, 1907.
(7) A m e r i c a n Society of Tool and Manufacturing E n g i n e e r s . Tool
E n g i n e e r s Handbook. P r e p a r e d by ASTE Technical Publications C o m m i t t e e . New York: McGraw-Hill Book
Company, Inc, , 1959.
(8) Black, P a u l H. Theory of Metal Cutting.
Hill Book Company, Inc. , 1961.
New York:
McGraw-
(9) L a m b e r t , B. K. I, E, 5351 - Advanced Production Design
C l a s s Notes. Texas Technological College, F a l l , 1968.
(10) Cook, Nathan H, Manufacturing A n a l y s i s . Reading, M a s s . :
Addison Wesley Publishing Company, Inc. , 1966.
98
99
(11) ASTME T e c h n i c a l Publications C o m m i t t e e . Machining With
C a r b i d e s and Oxides. New York: McGraw-Hill Book
Company, Inc. , 1962.
(12) H a m , Inyong. " F u n d a m e n t a l s of Tool Wear, " A m e r i c a n
Society of Tool and Manufacturing E n g i n e e r s , P a p e r
No. MR68-617, I968.
(13) ASME Re s e a r c h , C o m r a i t t e e on Metal Cutting Data and Bibliography. Manual on Cutting of M e t a l s . Ann A r b o r ,
Michigan: A m e r i c a n Society of Mechanical E n g i n e e r s ,
1952.
(14) C u r t i s s - W r i g h t Corporation. United States A i r F o r c e
Machinability Report. Woodridge, New J e r s e y , Vol. 1,
1950; Vol. 2, 1951; and Vol. 3, 1954.
(15) T o u r r e t , R. P e r f o r m a n c e of Metal Cutting Tools.
B u t t e r w o r t h ' s Scientific Publications, 1958.
London:
(16) Battelle M e m o r i a l Institute. . An Evaluation of the P r e s e n t Understanding of Metal Cutting. Columbus, Ohio: 1959.
(17) Shaw, M. C. , and S m i t h , P . A. "How Workpiece S t r u c t u r e
Affects Tool Life, " A m e r i c a n Machinist, Nov. 12, 1951.
(18) Vidosic, J. P . Metal Machining and F o r m i n g Technology.
New York: The Ronald P r e s s Company, 1964.
(19) Boulger, F r a n c i s W. "What is Known Today About Metal
C u t t i n g , " ASTE Collected P a p e r s , Vol. 58, Book 1,
P a p e r No. 44, 1958.
(20) Chao, B. T. , and T r i g g e r , K. J. " T e m p e r a t u r e Distribution
of Tool-Chip and Tool-Work Interface in Metal Cutting, "
T r a n s a c t i o n s of ASME, Vol. 80, 1958.
(21) Opitz, H. , and Hake, O. "Radioactive Wear M e a s u r e m e n t s ,
An Approach to Rapid Routine Testing for Tool P e r f o r m a n c e , " M i c r o t e c n i c , Vol. 10, No. 1, 1956.
(22) M e t a l l u r g i c a l P r o d u c t s Division, G e n e r a l E l e c t r i c Company.
Application of Cemented Carbide Data, 1964.
V
100
'(23) W a g e r , J. G. , and B a r a s h , M. M. "The Nature of the D i s t r i bution of the Life of HSS Tools, and its Significance in
Manufacturing. " P u r d u e University, L a F a y e t t e , Indiana,
1967.
(24) Hoel, P a u l G. Introduction to M a t h e m a t i c a l S t a t i s t i c s .
York: John Wiley and Sons, Inc. , 1966.
New
(25) Duncan, A. J. Quality Control and Industrial S t a t i s t i c s .
Homewood, Illinois: Richard D. Irwin, Inc. , 1959.
(26) H i c k s , C h a r l e s R.
Experiments.
1964.
Fundamental Concepts in the Design of
New York: Holt, Rinehart, and Winston,
APPENDIXES
A - D y n a m o m e t e r Calibration Curves
B - E x p e r i m e n t a l Data
101
.
I.
102
APPENDIX
A
D Y N A M O M E T E R C A L I B R A T I O N CURVES
103
CO
40
F c = 9.82 D - 7.00
u
0)
-»->
0)
s
D = M i l l i m e t e r s of pen
deflection
30
•H
20 PI
o
•H
•<->
V
(U
10
r-i
0)
P
0
40
480
+
120
4
160
200
4240
4280
4320
Applied F o r c e (Pounds)
F i g , 19.
^
Calibration Curve for Cutting F o r c e , F ^ .
20
F t = 2 7 . 7 7 D - 10.25
15
D = M i l l i m e t e r s of pen
deflection
CO
U
•H
• iH
10
Pi
o
•iH
5 -
o
r-l
m
0)
0
D
40
80
120
160
200
240
280
320
Applied F o r c e (Pounds)
F i g . 20.
Calibration Curve for Longitudinal F o r c e , F . ,
104
APPENDIX B
E X P E R I M E N T A L DATA
105
TABLE
15
I N T E R M I T T E N T T E S T I N G IVIETHOD
T h e v a l u e s g i v e n b e l o w a r e the r a e a n s and v a r i a n c e s of the
t h r e e s a m p l e s p e r c u t t i n g edge t a k e n a t e a c h cutting s p e e d - t i m e
combination.
V
E
T
FW
S2
^c
Ft
C u t t i n g s p e e d , feet p e r m i n u t e ;
E d g e of i n s e r t b e i n g u s e d ;
Cutting t i m e , m i n u t e s ;
M e a n flank w e a r , i n c h e s x 1 0 ~ ^ ;
Variance, inches x 1 0 ~ ^ ;
Mean cutting force, pounds;
Mean longitudinal force, pounds.
V
510
(VI)
465
(V2)
FW
1
2
3
4
5
6
7
8
213
283
390
473
523
533
593
627
233
233
100
1433
433
234
33
234
101
102
110
111
117
123
129
135
168
237
92
90
38
6
38
62
55
59
62
66
71
78
82
82
258
580
500
339
401
401
258
258
1
2
3
4
5
6
7
8
210
277
367
463
526
550
600
643
400
233
233
933
434
900
900
933
108
107
111
103
114
115
120
119
80
80
18
311
2
2
2
8
64
64
69
71
71
75
80
82
258
258
451
305
305
451
433
305
1
2
3
4
5
6
200
240
307
377
440
497
700
400
933
433
700
223
92
91
95
95
98
99
38
0
14
14
2
8
43
43
45
43
43
43
16
16
0
16
16
16
106
TABLE 15 - Continued
V
S2
7
8
553
617
.203
3033
100
102
26
14
50
48
64
16
1
2
3
4
5
6
7
8
193
263
303
373
413
480
557
633
1033
233
233
433
233
700
1433
263
93
93
94
95
96
99
101
102
56
27
42
26
42
18
42
62
48
48
45
43
45
48
45
50
16
16
0
16
0
16
0
64
1
1
2
3
4
5
6
7
8
180
243
273
310
340
387
417
467
100
1033
933
700
300
933
433
233
96
96
96
96
97
98
100
101
24
24
12
18
26
14
14
0
59
61
57
57
57
55
62
66
192
108
112
112
112
112
112
144
2
1
2
3
4
5
6
7
8
190
230
317
367
397
440
480
543
700
1300
7233
11433
10133
14700
14700
8134
93
95
97
99
102
101
105
104
38
27
26
54
50
42
97
56
68
68
68
68
73
73
75
73
63
112
112
112
48
48
16
48
1
1
2
3
4
5
6
7
8
133
163
177
203
227
237
257
287
233
1633
2133
1233
433
133
133
33
106
112
113
122
117
122
120
125
24
38
24
99
26
38
75
26
57
66
71
73
79
82
89
89
16
48
16
41
48
113
112
112
., .
,
372
(V4)
S^
FW
2
418
(V3)
\
/
S2
T
E
Fc
Ft
107
—TABLE
V
15 - Continued
E
T
FW
S2
Fc
S2
Ft
S^
372
(V4)
2
1
2
3
4
5
6
7
8
123
140
167
190
210
230
253
283
233
300
133
400
400
700
433
433
95
101
108
114
117
120
122
119
32
42
161
57
62
123
202
105
55
64
66
71
71
82
82
80
63
64
144
16
16
64
64
146
325
(V5)
1
1
2
3
4
5
6
7
8
147
170
177
187
200
217
230
250
633
1600
1633
933
700
833
700
400
93
96
97
95
101
106
107
107
27
24
38
26
24
54
50
50
55
55
55
57
59
68
68
68
64
64
64
16
0
63
63
63
2
1
2
3
4
5
6
7
8
120
140
150
163
177
197
217
240
100
100
100
233
133
133
33
100
104
113
114
116
117
117
119
120
158
153
327
314
267
220
153
123
.66
78
68
73
75
75
80
82
337
449
449
192
208
208
144
209
108
TABLE
16
CONTINUOUS T E S T I N G M E T H O D
T h e v a l u e s g i v e n b e l o w a r e the r a e a n s and v a r i a n c e s of t h e t h r e e
s a m p l e s p e r c u t t i n g edge t a k e n a t e a c h cutting s p e e d - t i r a e c o r a b i nation,
V
E
T
FW
S
F^
F-t;
V
= C u t t i n g s p e e d , feet p e r r a i n u t e ; •
= E d g e of i n s e r t b e i n g u s e d ;
= Cutting tirae, m i n u t e s ;
= M e a n flank w e a r , i n c h e s x 1 0 " ^ :
= V a r i a n c e , i n c h e s x 10"'^^;
= Mean cutting force, pounds;
= Mean longitudinal force, pounds.
E
T
FW
S2
Fc
S^
s2
Ft
1
510
(VI)
465
(V2)
418
(V3)
1
4
6
8
367
447
597
2633
18533
9033
95
103
114
2
42
226
50
50
66
64
64
144
2
4
6
8
340
467
543
700
2533
12033
90
117
103
63
27
152
50
75
62
64
64
305
1
4
6
8
323
410
490
433
3100
6100
108
115
108
98
123
153
57
62
66
208
16
144
2
4
6
8
340
517
483
300
9733
3733
103
109
114
99
153
14
57
64
78
208
112
16
1
4
6
8
263
397
363
1033
2633
1733
100
106
108
14
18
2
57
62
76
112
16
64
2
4
6
8
327
380
430
233
6100
700
105
114
104
32
105
38
64
87
80
16
337
337
109
TABLE
V
E
16 - Continued
T
FW
S2
Fc
s2
Ft
S2
1
4
6
8
160
200
233
100
100
233
108
106
117
255
24
146
64
71
80
449
16
48
2
4
6
8
153
197
240
1033
433
100
116
111
103
25
24
60
1
4
6
8
130
160
180
300
100
100
116
118
115
170
18
147
71
80
78
112
48
112
2
4
6
8
153
180
200
1233
100
100
122
119
122
2
8
74
87
87
82
0
0
64
*--
372
(V4)
325
(V5)
73 (D
75
16
112
75