Proceedings of
th
th
6 International & 27 All India Manufacturing Technology, Design and Research Conference
(AIMTDR-2016)
College of Engineering, Pune, Maharashtra, INDIA
December 16-18, 2016
Tool Path Planning Strategies for CNC Machining of Free
Form Surfaces using Surface Properties
Mandeep Dhanda1 and S.S.Pande2
Computer Aided Manufacturing Laboratory, Department of Mechanical Engineering,
Indian Institute of Technology, Bombay, Mumbai–400076, India
E-mail: [email protected], [email protected]
ARTICLE INFO
ABSTRACT
Keywords:
Freeform Surface CNC Machining
Tool path planning
Isoplanar Isophote and
Curvature Based Strategies
Today designers create variety of complex freeform surfaces on parts using CAD tools. It has been a
challenge for CAM systems to efficiently generate part programs for machining these complex
surfaces on multi axis CNC machines. This paper reports the development and comparison of tool
path planning strategies based on intrinsic properties of free form surfaces to machine them
efficiently on 3-axis CNC machines. Two path planning strategies have been compared viz. Isophote
based surface subdivision with isoplanar path and Curvature based isoscallop path. The software
developed takes CAD part model in STL format as the input and generates tool paths which are
finally post-processed to FANUC format. A critical comparison was made between the surface based
strategies (Isophote and Curvature) and Isoplanar strategy (commercial software), for parts with
varying complexities. Results indicate that the Curvature based tool path planning strategy gives
much promising results compared to the Isophote based and the commercially available Isoplanar
strategies in terms of enhanced product quality and productivity.
1.
Introduction
Today freeform complex surfaces are widely used in
various automotive, aerospace, domestic product as well as
medical implant manufacturing industries. CAD technologies
enable the designers to model freeform surfaces with varying
complexities to meet various functional and aesthetic
requirements of the products [1]. It is a challenge for the
CAM/CNC systems to generate efficient part programs to
manufacture these surfaces on multi-axis CNC machines to get
better product quality and higher productivity. Tool path
planning is thus, a fundamental task in CNC process planning
[2].
Literature reports extensive research efforts directed
towards various approaches for efficient tool path planning.
Prominent among them are Isoparametric, Isoplanar and
Isoscallop sculptured surface machining strategies [2]. Each one
is, however, beset with its own advantages and limitations.
Isoparametric and Isoplanar [2],[3]strategies result into varying
scallop height throughout the surface, while the Isoscallop
strategy is computationally complex and time consuming
[4].The main drawback of all these existing strategies is that
they are more biased towards the machine coordinate axes (X,
Y, Z) and are essentially feature blind. They do not take into
consideration variations in part geometry and their intrinsic
surface properties such as surface normal and curvature during
tool path planning [3]. If the properties of the surface are
considered during tool path generation, the quality of CNC
programs can be definitely improved. However, no rigorous
research work has been reported in this direction. [8], [10].
The objective of the present work is to develop and
compare tool path planning strategies based on surface
properties (Isophote and Curvature) for machining of freeform
surfaces on 3-axis CNC machines. Our aim is to find out which
strategy is more efficient by comparing the results for surface
based strategies with those for Isoplanar strategy available in
the commercial software.
2.
Tool path planning strategies
The tool path planning strategies under comparison are
presented one by one.
2.1 Isoplanar strategy
This strategy is most popular and is implemented in all
commercial CAM software. The cutter paths herein, are
obtained by the intersection curves between the freeform
surface and a number of parallel vertical planes (eg. X-Z in a
VMC) in the CNC machine Cartesian space. Despite the
equispaced successive parallel cutting planes, the roughness
(scallop height) varies continuously throughout the part. This is
because scallop depends on the cutter geometry, side step as
well as the radius of curvature of the surface in a plane normal
to the direction of the cutter velocity vector at the cutting point
[2].The main drawback of the isoplanar machining strategy is
that it is feature blind i.e. variations in part geometry (surface
curvature) are not taken into consideration during tool path
planning. Commercial software using the isoplanar strategy do
not give any guidelines to the user to choose the step over (side
step). It is thus, not possible for the user to predict the scallop
height variation aprior, for a chosen side step but resort to trial
and error methods to keep the scallop height in check [3].
Adaptive surface based strategies like Isophote and Curvature
based overcome this drawback of the Isoplanar strategy.
785
ISBN: 978-93-86256-27-0
th
th
6 International & 27 All India Manufacturing Technology, Design and Research Conference (AIMTDR–2016)
College of Engineering, Pune, Maharashtra, INDIA
2.2 Isophote based strategy
Isophotes are curves joining points of equal light intensity
from a given source. Isophote divides the surface into regions
based on the inclination angle of the local surface normal with
the tool axis (Z) [5]. Regions lying between isophote curves are
considered almost flat so that an efficient localized tool path can
be generated by considering each region separately. Fig.1 shows
typical isophotes for a surface, associated regions (A-D) and
tool paths therein [7]. In this work, Isophote software developed
by Aniket et.al [7] is used. Various steps in Isophote based path
planning algorithm are presented.
1. Isophote Generation: This module has two functional
subparts viz. Mesh Offset and Isophotes development [8].The
Mesh Offset module identifies and separates out the surface to
be machined from the entire part (STL CAD model) by
calculating the normal of each facet and its visibility with
respect to tool axis (Z) using vector dot product. The STL part
file is then offsetted by a distance of one tool radius using the
area average unit normal at each vertex of the mesh to generate
the CL surface.[8]
o
o
B (13.28 -26.55 )
o
o
D (39.28 - 53 )
A (0.01 -13.28 )
C (26.55 -39.82 )
o
o
o
o
Fig. 1. Isophote regions and tool paths [7]
The Isophote development module divides the offsetted
surface into different regions which essentially contain facets
inclined to Z-axis within a certain range of angles. The range of
inclination angle is computed for the entire model and divided
equally into sub ranges [8]. Fig.1 shows some typical ranges.
2. Step over Computation: In each isophote region, zigzag
tool path strategy is employed. The Step-over (side step) value
is adaptively computed for each isophote region with a view to
keep the maximum scallop height less than the user specified
limit. The localized step over is a function of tool radius, r;
maximum scallop height, h and average inclination angle for a
given isophote region φavg. [8] The step over, w is given by
(Fig. 2).
w = 2 x cosФavg x ( r2 + (r – h )2)1/2
(1)
3. Tool Path Generation: The CL (offset) surface (STL
mesh model) is sliced using the Y-step over corresponding to
the maximum step over (side step) calculated in each region.
Isoplanar strategy is followed and zigzag tool paths are
generated in each isophote region in X-Y plane (Fig.1). Due to
the adaptive nature, the tool paths would have varying side steps
in each region (Fig.1). A strategy to stitch the tool paths in each
isophote region was developed to generate the complete tool
path. The post processed NC program obtained from our
developed software was simulated on commercial simulator,
Vericut7.0.1. It was validated with some benchmark case
studies as well as by actual machining trials [8].
2.3 Curvature based strategy
Curvature is another property other than surface normal
which defines the nature of the surface [9]. In this work,
Curvature based adaptive path planning strategy developed by
Parth et al. [10] is used. Important steps in this algorithm are
presented.
1. Curvature Estimation: CAD part model in STL format is
pre-processed to extract Facet and Normal data. Curvatures
were estimated on the STL mesh using fitting of Paraboloid
[11]. Based on the values of the curvature obtained, side step
lengths are computed [12]. As before, the STL part mesh is
offset along the direction of average normal at each vertex by
the radius of the cutter to generate the cutter location (CL)
surface. Tool path planning is done on the CL surface.
2. Tool Path Generation: The tool path generation strategy
is based on isoscallop strategy [4] combined with a refining
strategy to minimise the number of cutter contact (CC) data
points. The forward steps for each path satisfy the criteria of
chordal error (lengths) using circular arc approximation [1]. The
tool paths are generated on the CL surface.
Tool paths generation is done starting from the initial
master path which was chosen along one of the lines of
curvature on the surface, viz., along major principal curvature
(K1) direction. The point having the least value of curvature
(K2) on the surface is selected as the starting point (Fig. 3).
Fig. 3. New master path derived from initial master path [10]
Fig. 2. Step over computation [5]
These values are used for generating the tool path in each
region.
ISBN: 978-93-86256-27-0
786
With the change of tool radius, the offset surface would
changes. But the starting point is so chosen that it remains
independent of the tool radius, as it is chosen on the original
surface. This starting path is used for offsetting based on
isoscallop strategy [4].
3. Tool Path Refinement with Master Path Generation:
After computing the offset path (from the previous Master
path), it needs to be refined to remove redundant paths for each
segment for meeting the tolerance constraint. Once the new
Tool Path Planning Strategies for CNC Machining of Free Form Surfaces using Surface Properties
master path is identified satisfying the smoothness criteria [10],
data of all the previous paths including the initial master path
are discarded and the whole surface is covered with tool path
that are derived from this new master path. Fig.4 shows the
refined tool paths generated from the new Master path, where
every point for the next path is computed from the
corresponding point on the current path. The interval between
these two “parallel” points is governed by the scallop height
requirement, tool radius and the radius of curvature at the point
on the current path. As a result, any redundant machining is
eliminated while staying within the tolerance limits, thus
improving the path efficiency.
50.8 mmpm; Spindle Speed: 900 rpm; Allowable Scallop
Height: 0.05 mm. Tool path generation was done for
Isophote[7] and Curvature[10] based strategies and postprocessed to FANUC format. It was decided to compare the
generated NC programs with the one generated by commercial
CAM software (MasterCAM) for the same parameters.
MasterCAM needs user to input side step for isoplanar strategy
and does not give any guidance to choose side step for
controlling scallop heights. To solve this problem, Isophote
based NC program generated by our system was analysed to
extract minimum and maximum values of side steps. Using
these limit values, NC programs were generated for isoplanar
strategy in MasterCAM and compared with Isophote and
Curvature based strategies.
The NC programs for all strategies were simulated and
verified in Vericut to check the tool paths (Fig.6). Scallop
height was measured on the simulated machined surface at 30
different locations across the surface using Vericut. Minimum,
maximum and average scallop heights were recorded. The
results are shown in Table 1.
Table 1
Scallop heights
Min. (µm)
Max. (µm)
Avg. (µm)
Spread(µm)
Fig. 4. Surface covered with tool paths derived from the new master path
The generated CL data was post processed to FANUC
format and the CNC program was tested on the commercial
CNC simulator Vericut as well on the actual machining trials
for various benchmark case studies.
The results obtained for all the three strategies are
discussed in the next section.
3.
Curvature
Strategy
Isophote
Strategy
45
58
50
13
24
53
39
29
Isoplanar Strategy
Max. Side step
Min. Side step
(1.11mm)
(0.77mm)
48
23
69
38
54
27
21
15
Table 1 shows that both Isophote and Curvature based
strategies give average scallop height below the desired value
(50microns) while the isoplanar scallop value is highly
dependent upon the side step chosen. For all the cases, a range
(min, max.) is seen to exist about the average scallop height
indicating the spread.
Results and discussion
The developed system with Isophote and Curvature based
strategies was tested on various freeform parts to study its
ability for adaptively generating tool paths for controlling
scallop height during finishing. A typical case study is
presented here.
3.1 Case study-sculptured surface machining
Fig. 5 shows the CAD model of the part created in
Solidworks and saved in STL file format which forms input to
the developed system. The STL file contains 23530 facets.
Fig. 6. Simulation of NC program (isoplanar) in vericut
A critical comparison of the three strategies is presented in
the sections to follow.
3.3 Isophote and curvature based strategies
Table 1 shows that the variation in scallop height (spread)
across the surface is much less for Curvature based tool paths
(13 microns) than the Isophotes based (29 microns). Scallop
frequency plot for Curvature strategy (Fig.7) shows that the
highest frequency is very close to the target value (50 microns)
and the spread is very narrow (13 microns).
Fig. 5. CAD model
3.2 CNC code generation and simulation
To generate NC part program the following parameters
were chosen: Tool diameter: 6.35mm (Ball end Mill); Feed:
787
ISBN: 978-93-86256-27-0
th
th
6 International & 27 All India Manufacturing Technology, Design and Research Conference (AIMTDR–2016)
College of Engineering, Pune, Maharashtra, INDIA
Frequency
that although the step over is kept constant, the scallop height
goes on varying for both the cases. Scallop frequency plot of
isoplanar (Fig. 9) shows that spread is large (49-71) microns in
isoplanar as compared to Curvature (46-59) microns and
Isophote (25-53) microns. To reduce this scallop spread,
sidestep will have to be reduced which would ultimately affect
the productivity. Max. Scallop in Isoplanar is at value of
71microns, whereas for Curvature and Isophote it is at 59
microns and 53 microns respectively. It indicates that more
values in isoplanar scallop deviates from specified limit.
Though the values look concentrated in range (49-53) microns
for isoplanar (Fig.9), larger spread beyond specified limit makes
it unacceptable.
8
7
6
5
4
3
2
1
0
Scallop(mm)
Fig. 7. Scallop frequency plot–curvature based
However, about 30- 40% of scallop values are exceeding
the target, though the maximum frequency is around the target.
Narrow spread shows the near isoscallop nature of surface
which is good from the point of view product quality.
In comparison scallop frequency plot for isophote strategy
(Fig. 8), though shows highest frequency at the target value, has
a much wider spread (29microns). Minimum scallop is as low
as 24microns. This shows that the side steps have been chosen
too conservatively even in regions of less complexity, giving
smaller scallops. Coupled with inefficient path stitching
strategy, the Isophote was seen to take more machining time
than the Curvature based tool paths [10].
8
7
6
5
Frequency
4
3
2
1
0
Scallop (mm)
9
8
7
Fig. 9. Scallop frequency plot-isoplanar (max. side step)
Reducing side step substantially to 0.77mm, (Fig.10), all
the scallop values are pushed below the specified limit
(50microns).The range varies from 24-38 microns with
maximum frequency around 24-26 microns. This shows that the
side step chosen is too conservative which would significantly
reduce the productivity.
6
Frequency
5
4
3
2
1
0.020
0.022
0.025
0.027
0.030
0.032
0.034
0.037
0.039
0.042
0.044
0.046
0.049
0.051
0.053
0
Scallop (mm)
Fig. 8. Scallop frequency plot–isophote based
This is primarily because of the difference in approaches
in the two adaptive planning strategies. Isophotes based tool
paths are generated on the surface by diving it into regions
(Isophotes) based on the normal inclination angle. This is
followed by the isoplanar strategy in each region and a path
stitching strategy. As a result, redundant machining takes
place in some locations. Due to the region wise tool paths,
there are increased number of tool retractions which reflects
in the higher (simulated) machining time. None of these
problems are observed in curvature based tool paths as it
follows adaptive tool path planning strategy and once tool is
engaged it machines the surface in a continuous manner. The
Curvature based adaptive strategy is thus, much superior
compared to the Isophote.
3.4 Isoplanar strategy
Fig. 9 and 10 shows the scallop height distribution for the
two isoplanar based NC codes generated by MasterCAM with
side steps of 1.11mm and 0.77mm respectively. It is observed
ISBN: 978-93-86256-27-0
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0.020
0.021
0.022
0.024
0.025
0.026
0.027
0.028
0.030
0.031
0.032
0.033
0.034
0.036
0.037
0.038
Frequency
8
7
6
5
4
3
2
1
0
Scallop (mm)
Fig. 10. Scallop frequency plot–isoplanar (min. side step)
While generating isoplanar tool path using MasterCAM, it
is not possible to predict the scallop height variation for a
chosen step over. The user has to perform trial and error
method to choose the proper step over to keep the scallop
height in check. No guidance is provided by the software. On
other hand the developed system (Isophote, Curvature)
adaptively computes different step overs for entire surface
region to maintain the scallop height below the user specified
value. In the steeper regions of the part where the scallop
height varies is expected to be high, additional tool paths are
automatically introduced to control the scallop height. In this
Tool Path Planning Strategies for CNC Machining of Free Form Surfaces using Surface Properties
[3]
S. Ding, M.A. Mannan, A.N. Poo, D.C.H. Yang, Z. Han, “Adaptive isoplanar tool path generation for machining of free-form surfaces”;
Computer-Aided Design; Volume 35; 2003; Pages 141-153.
[4] Hsi-Yung Feng, Huiwen Li, “Constant scallop height tool path generation
for three-axis sculptured surface machining”, Computer-Aided Design,
Volume 34, 2002, Pages 647-654
[5] Zhonglin Han Daniel C. H. Yang, “Iso-phote Based Tool-path Generation
for Machining Free-form Surfaces”, Journal Of Manufacturing Science
And Engineering,656-64/Vol.121, November199
[6] Zhonglin Han, Daniel C.H. Yang, “Iso-phote Based Tool-Path Generation
for Machining Free-Form Surfaces”, Journal of Manufacturing Science
and Engineering, Volume 121, 1999, Pages 656-664.
[7] Aniket Chaudhary, S.S. Pande.” Isophote Based Tool Path Planning
Strategy for Sculptured Surface CNC”, Proceding of 5th International &
26th (AIMTDR-2014) Conference, December 12th–14th, IIT Guwahati,
India.
[8] Chaudhary A. A., “Isophote based tool path planning strategy for
sculptured surface machining”, M. Tech. Thesis, Indian Institute of
Technology Bombay, 2014.
[9] Giri V., Bezbaruah D., Bubna P., Choudhry A. R., “Selection of master
cutter paths in sculptured surface machining by employing curvature
principle”, International Journal of Machine Tools and Manufacture, Vol.
45, 2005, pages 1202-1209
[10] Thakar Parth Bipin, “Adaptive Strategy for Planning Efficient Tool Paths
for CNC Machining of Freeform Surfaces”, M. Tech. Thesis, Indian
Institute of Technology Bombay, 2014.
[11] Surazhsky T., Magid E., Soldea O., Elber G., Rivlin E., “A comparison of
Gaussian and Mean curvature estimation methods on triangular meshes”,
In: Proceedings of IEEE, 2003, pages 1021-1026.
[12] Rong-Shine L., Koren Y., “Efficient tool path planning for machining
freeform surfaces”, ASME Transactions, Vol. 118, 1996, pages 20-28.
respect, the isoplanar strategy used in the commercial
softwares is feature blind.
Comparing all the three strategies, it is observed that
curvature based strategy is the best one which provides near
isoscallop surface machining with least spread of values due to
its adaptive nature.
4.
Conclusions
This paper reported a critical comparison of the capabilities
of three tool path planning strategies for sculptured surface
CNC machining. Case study shows that the Curvature based
strategy is much superior and efficient than Isophote and
Isoplanar based strategies in terms of near isoscallop machining
with least spread of values. It provides a nice trade-off for
improving productivity and product quality.
Acknowledgment
The financial support received by the first author from the
NCAIR (National Centre for Aerospace Innovation and
Research) of IIT Bombay-India is gratefully acknowledged.
References
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[2]
Byoung K. Choi and Robert B. Jerard, “Scupltured Surface Machining
Theory and applications” Kluwer Academic Publishers, 1998.
Ali lasemi, DeyiXue, PeihuaGu, “Recent development in CNC machining
of freeform surfaces”, A state-of-the-art review, Computer-Aided Design,
Volume42, 2010,Page 641-654
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ISBN: 978-93-86256-27-0