Lean Six Sigma Quality Transformation Toolkit (LSSQTT)*
LSSQTT Tool #11 Courseware Content
“Basic Measurement, Geometric Relationships, Broader Data-based Issues”
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Foundational metrology and measurement issues
Accurate data, total quality systems, kaizen, lean, six sigma
Metrology and inspection system services in quality
Historical background on metrology
Form, fit, finish and function, geometric underpinnings
Foundational metrological and measurement issues
Basic measurable features in geometric dimensioning
Basic principles and devices for measurement and data collection
Shifting toward the metric system
Surface quality: focused foundational metrological issue
*Updated fall, 2007 by John W. Sinn.
Foundational
Metrology
Measurement Issues
And
Basic metrological principles and tools are
important in the context of quality systems and data
gathering. Important principles include basic
measures and standards, precision and accuracy, and
calibration. Several metrological tools are necessary
to help implement these principles. These include
gage blocks, surface plates, and other gages and
direct measurement devices. Each of these will be
briefly presented and discussed in this section. As
well, selected other specific measuring information
and devices will be pursued elsewhere in this tool,
focused directly on metrology issues and
circumstances. The foundational metrological and
measurement issues are shown graphically nearby.
Additionally, several basic measures are useful in
industry to help give meaning, through standard
values, for inspection. These include length, time,
mass, temperature, electric current, light, and others.
The levels of standards and values which are
commonly used revolve around working levels,
calibration,
functional,
reference
and
national/international standards.
Precision and accuracy are functional
interpretations of the standards in production.
Precision is the closeness with which a measurement
can be read directly from a measuring instrument. It
is also considered as the smallest marked increment
on the instrument. By contrast, accuracy is the
measure of how close the reading is to the true size of
the part. Calibration relates as the comparison of a
measurement,
standard
or
instrument
of
known/dependable accuracy with another standard or
instrument to detect, correlate, report or eliminate by
adjustment, any variation in the accuracy of the
device being compared. Calibration systems
generally involve procedures to enable a comparison
of the instrument being calibrated with a standard
having a higher degree of accuracy.
Hand held measurement devices include rules,
calipers, micrometers, dividers, small hole gages, and
telescoping gages. Most of these are also direct
measurement devices since they are read directly on
the instrument. Indirect gages require comparison to
a standard to get an actual final measure. Related to
this, bench type inspection devices include the
vernier height gage and dial indicators. Likewise, the
use of light to view a product or component shape
relative to desired shape as noted on a template, is
what occurs with an optical comparator. This is
staging a shape on the comparator and projecting it
for comparison purposes onto the screen, relative to
known values.
More recently, surface plates and other
traditional metrological tools are being used less and
less in favor of coordinate measuring machines
(CMM). The CMM is a device, based largely on
traditional surface plate logic, having built in
overhead measurement capability with the traditional
values of surface plate technology as the basis. By
using overhead measurement systems, tremendous
precision, often lost in traditional hand setups, is
gained. Through the use of high quality machine tool
precision being built in, CMM's also take full
advantage of computer numerical control (CNC)
logic, further explained in later tools.
It is important to recognize that high quality,
precision, direct computer controlled measurements
for the automated workplace will often be achieved
through the use of the CMM.
Even where
automation is not the question, CMM technology
may be the answer. The CMM can be programmed
for various parts, and assuming an organization must
inspect a part on a repeat basis, the program can be
called up and recycled for significant gains in
productivity--realizing there was a cost involved in
writing the program. The direct computer control
(DCC) feature of the CMM also enables reverse
engineering, taking dimensions directly from the
component being inspected. The reverse engineering
system will also become increasingly essential in the
emerging concurrent engineering environment of the
future.
This also relates to the need for evaluating our
measurement systems to determine measurement
error and other inconsistencies. The following
procedures are intended to assist in the analysis of
gage repeatability and reproducibility (R & R),
sometimes called measurement analysis. This is
particularly appropriate and helpful for analyzing
shop floor (and other) gauges and instruments, and
their operators, for usefulness in the broader
inspection system. It is also true, however, that these
R & R values will increasingly be required for
documentation as suppliers and vendors--to be
shipped with product and/or provided in other ways
to demonstrate general capability in the inspection
and quality system.
Yet the fundamental reason for the R & R
analysis is to assure as best we can that our gages are
in fact accurate, and that our operators are using them
correctly. Gage analysis would be a logical area of
pursuit if we are seeing unexplainable variation in
our X bar and R charts, or if we have eliminated
virtually all other possibilities--the gages simply
should be evaluated on a regular basis. This includes
relational connections to maintenance, indicating that
when the gages do not check out as repeatable it is
likely they are needing repair or calibration, or both.
Although eventually data collection may occur
semi-automatically via computer terminals at
workstations, currently most data will be manually
measured at the gage and manually recorded on
forms. This establishes the basis for the system, in
conjunction with the gage and procedure for use-manually collected data. It is important that all
operators and others understand the importance of
taking good measures and recording this data
accurately. Much analysis time will be spent--and
decisions made on the basis of this data, and thus, it
is vitally important that it be done carefully.
Similarly, the forms are only as good as the
people make them. Through use, and based on team
inputs, the forms should continue to evolve toward a
satisfactory collection, organization and analysis tool.
Part of the point is, as we use the forms, as a key part
of the system, we should be jotting down notes,
talking to our supervisor about them, observing how
others use them, and so on. Perhaps most important,
we should be listening to what our customers are
saying about the forms as well as the rest of the
system. This includes people up and down the line,
in quality functions, engineering personnel, other
team members, and so on--all internal customers.
This also, obviously, includes our external customers.
Accurate Data, Total Quality Systems,
Kaizen, Lean, Six Sigma
Before proceeding any further, it would be
well to begin/continue emphasizing the need to
gather and handle data carefully and accurately.
During collection and manipulation, such as
organizing into columns or in forms, it is vitally
important that we be mindful of the need to use
extreme care and precision in our work. As we will
see, increasingly, the data will formulate the basis of
virtually all decisions in the workplace and
throughout the organization. If we do not exercise
extreme care and caution when measuring and
recording, and perhaps as we calculate, we may be
helping to put ourselves, and others, out of work.
The sample we use as the basis upon which to
draw all of our conclusions must be part of that
carefully accomplished thinking for deriving accurate
data about characteristics.
How frequently to
measure, when and where, how to measure, size of
subgroup, who should do it, and what to do with the
data, are all the beginnings of the right questions to
ask about the quality system. This all must be done
through careful interaction and team work with
persons in quality, engineering, and throughout
manufacturing--getting accurate sampled data is not
easy--but it is vitally important since so much relies
on this foundational data. All of this must be
carefully thought through and designed with strong
and careful communication from and with customers
and suppliers, internal and external.
Virtually all else that we will do, and certainly
all other tools to be studied, are based on and around
the basic statistical information given here at this
time--with some expansion and elaboration. But it is
very important that we understand what is happening
here now--and beyond. It is also vitally important
that we begin to take steps to integrate this into all
that we do and all that we are about--from the point
of the raw material being ordered to the finished
product being shipped. Just as the basic SPC forms
the basis for our technical future, the attitudes that we
must build and engender for all of our people are
vitally important--all of the tools in the world cannot
make any difference if I do not want to learn them or
use them.
With statistical feedback provided, standards
can either be altered or created. Clearly, however,
based on quality data generated in production,
standards can and should be derived or altered.
Equally as useful, based on statistical feedback,
control in processes can be pursued. Statistical
analysis of the process can be pursued by gathering
information which is documented/stored. If designed
properly this can provide a data base which will
prove invaluable for comparative and analytical
purposes related to the processing functions--again
enabling better decisions and ongoing improvement
if we use the data properly. The basic relationships
inherent in data, SPC, teams, documentation, and
ongoing improvement are shown in a nearby graphic.
Assuming the data base is built-up over time,
much useful productivity information should be
afforded. For example, if the rate of the task is
speeded up, how many conditional units are produced
and/or rejected? Thus, has productivity or quality
actually been improved? Similarly, for training
purposes, by observing results based on statistical
feedback, employees can gain insights and
understanding about when they have mastered a task,
providing a sufficient quality level. But this all takes
a disciplined, well trained, and "comfortable with
numbers" work force--not quickly or easily achieved.
We would be well advised to remember that it
is desireable to have statistics which clearly provide
documentation of quality levels as well as other
general communication assistance. Statistical
information can provide an excellent communication
and documentation system for internal as well as
external purposes. Increasingly, technological
organizations are demanding clear records of the
statistical process control report for a given lot or
shipment. But as well, and an extremely important
reason for using statistics, is the need to be able to
communicate effectively and quickly with internal
and external customers, upstream and downstream.
If I do not use statistics properly, I increase the
likelihood that customers will go somewhere else
with their business.
One of the key reasons for using statistics is to
actually "know" what we are talking about. It is one
thing to say "oh about half" versus saying 50%. Or if
we say "well, quite a few of the products were
defective", rather than saying "20% of the products
were defective", it is different. Yet again, if we say
"10 of the products had 3 defects each, yet none of
them were actually defective", it can make a
significant difference. The difference is that we are
being more precise when we place a proper
numerical indicator into the discussion. This is part
of what will increasingly be demanded by our
customers--and what we must demand of our
suppliers. A broad-based model, forming a key part
of the quality system, includes virtually all
components being discussed in the toolkit, as shown
in the graphic earlier. This is all part of the basis for
the more robust systems based on data and
documentation for improvement. This all relates to
wanting to be better informed as we make improved
decisions, rather than "seat of the pants" decisionmaking.
Based on precision of communication, and of
knowing better when we say something, what this
actually means, is at the root of the need to use
statistics and data. We must understand and use
statistics to facilitate enhanced competitiveness,
pivotal to our ability to improve the workplace and
quality ongoing. The ability to stay healthy and
competitive is directly related to our knowledge of
statistics, and how we use the statistical tools to solve
technical problems and work with one another.
The actual inspection systems can be analyzed
for kaizen, as with any other function. Development
of lean systems where the actual process of
inspection is imbedded in the other work as a value
adding process is not a trivial matter and definitely
can be improved in most cases. Knowing WIP in
inspection, takt time, thruput and so on is worthy of
pursuit and can likely lead to kaizen. Six sigma ties
directly to gauging and inspection since we must
have reliable data collected at the point of production
in order to develop robust data for the basic
improvement and problem solving system.
Metrology And Inspection
Services In Quality
System
Metrology is the science of measurement
based on some known standard. Inspection is the
comparison of existing materials or components to
known standards or values. If we are going to build
effective quality systems it is important to first
determine quality characteristics upon which
standards can be based, understand the relationship to
customer and supplier needs and issues through
vendor certification, know the metrological tools
available and how to apply them, and finally build it
all around reliability principles known to be effective.
The following section provides further information
and helps to put it all into a context appropriate for
building the necessary quality system--focused on
measurement and metrology.
Measurement scales are of two types, nominal
and interval. Nominal measurement scales have
numbers assigned for the sole purpose of
differentiating one object from another. This could
include identification of lots and locations of product
in various stages of production. Interval measurement
scales are measurement systems for classification
which includes an equality of units. This means that
there are equal distances between observation points
on the scale. Not only can we specify the direction of
the difference but we can indicate the amount of the
difference as well. This could refer to temperature
scales, and any application with all the characteristics
of the interval scale plus absolute zero capabilities
enabling statements involving ratios of two
observations, such as "twice as long" or "half as fast".
The relationship of nominal and interval
measures to the attribute and variable discussion
presented in the previous tools is important. As
related to the operator on the floor and the broader
system, the nominal scale is a relative attribute
measure such as a go no-go gage used for
comparative purposes. The interval scale would be a
variable measure which may or may not be a standard
scale graduated in known units of measure. For
purposes of moving the discussion forward it is
suggested that the increments of measure in the
interval devices and systems should be standard
units, enabling articulation with other devices and
systems for suppliers and customers.
Several metrological principles and tools are
important in the context of quality systems. Important
principles include basic measures and standards,
precision and accuracy, and calibration. Basic
measures are useful in industry to help give meaning,
through standard values, for inspection. These
include length, time, mass, temperature, electric
current, light, and others. The levels of standards and
values which are commonly used revolve around
working levels, calibration, functional, reference and
national and international standards.
Precision and accuracy are functional
interpretations of the standards in production.
Precision is the closeness with which a measurement
can be read directly from a measuring instrument. It
is also considered as the smallest marked increment
on the instrument while accuracy is the measure of
how close the reading is to the true size of the part.
To facilitate quality systems, several general
metrology and inspection considerations must be kept
in mind. For example, people must be adequately
trained to use measuring equipment. It is simply
fruitless to proceed without adequate training.
Concerning
environmental
issues,
ambient
temperatures in labs and plants must be controlled.
Also an environmental issue, lighting in facilities
must be adequate and appropriate for inspection. Of
course, cleanliness is of significance in general but
also dust/humidity through filtering/ventilation must
be addressed.
Effective inspection occurs in part through
several systems' components, primarily emphasizing
the operator. Establishment of quality standards must
occur and be evaluated periodically. A system for
inspection planning must be put together. How does
this occur? What is the organizational structure that
encourages good planning and interaction, both for
inspection as well as other elements related to
quality?
Development and use of inspection
instructions must be accounted for, as a part of
production, certainly to include training for the same.
The system, regardless of who does it, must
include internal audits for quality and methods to
handle reporting media, with documentation and
feedback for improving the system? Once reported,
how does analysis of data occur, and once analyzed,
how does this information get used? Does the system
account for good communication and feedback about
what was found in inspection? The degree to which
each element in the system is applied varies with the
product, customer requirements, production phases,
cost, ability of workers, time available, and perhaps
other factors. However, the design and use of the
elements and their system is a key factor contributing
to top rate inspection functions and quality overall.
If the quality system and overall production
system, within a cultural environment of change, is
dynamic and improving on-going, the quality of work
life should be improving in very real ways. The
workplace should be getting cleaner, certainly
including air quality, climate, lighting, general
housekeeping and so on. And along with all else,
clear criteria for acceptance or rejection must be
identified and communicated, as well as how
decisions are made for shutting down production,
who files what reports and to whom, and so on, all
designed to help improve the system. These all
suggest numerous key roles for technologists as
fundamental services to be performed within the
context of supplier and customer relationahips.
Historical Background On Metrology
Metrology, the science of measurement,
became necessary around 6000 B.C. when prehistoric
people discovered agriculture. To find the fertile soil
needed to grow crops, early humans had to move
away from caves and make their own shelter, thus
beginning architecture. As the need for more
sophisticated materials for shelter increased so did
the need for metrology. Nearly three thousand years
ago the cubit came into when hand fitting became
necessary to put on roofs and to distinguish property
boundary.
The cubit, like many other units of measure,
was based on the human body. Although not broken
down into smaller increments until the Romans, the
Egyptians were able to build a pyramid with 756 feet
on each side to a height of 481 feet. As long as tools
for building structures and other items were relatively
crude, their exact size was of little consequence.
When metal came into use it was discovered that size
could be controlled to duplicate any tool that had
proved effective.
This continued the marriage
between tools and metrology during biblical times
and well beyond.
Since various people (besides the Egyptians)
were discovering the need for metrology, different
units were developed. This created conflicts in
property boundaries, weights of grains, etc., when
nations were taken over by new governments. One
of such occurrences was after the Romans left
England and political authority was divided among
the Saxon kings. Although primary interest of the
kings was conquest, the "Law of Edward" evolved.
This law was an accumulation of Anglo-Saxon creeds
dating from after the birth of Christ. This was one of
the earliest examples of government standardization
and consistency.
During the Renaissance, the mechanical clock,
and early, crude, machines were built. In addition,
the slide rule came into use, the micrometer was
invented, and the British yard became the legal
British standard until the 1800's. In 1718 another
improvement in standardization was the so-called
"yard stick", started for a permanent standard of
length. The reason was that the British yard in metal
form was relatively inaccessible to the individual
craftsman where standardization is of the utmost
importance. At this point, manufacturing was still at a
stage in which each shop had its own individual
standards.
As improvements in metrology and machining
slowly continued, James Watt decided to improve the
steam engine. When he did so, Watt ran into the
serious limitations of available cutting and machining
tools. Watt, and others, up to this period would get
around the lack of accuracy in machining through the
use of crude hammers and forming tools. Watt could
not make the steam engine sufficiently precise until
accurate tools were evolved. It was not until the late
1700's that boring bars and other tools of sufficient
ridgidity were available that a somewhat true cylinder
could be bored. The steam engine represented one of
the key turning points in metrology since precision
and accuracy were combined with metallurgy and
design to meet with successful conversion of energy
resources into power.
As demand for manufactured products
increased consistent with the standard of living, Ely
Whitney and others recognized the wastefulness of
handicraft methods. When replacement parts were
needed they had to be hand fitted. This required a
considerable amount of time. If the hand fitting
could be eliminated more time could be spent making
products rather than fixing them. Whitney also
recognized that if more than one worker were to
contribute to a final product, the product of each must
be controlled dimensionally. This required the use of
tolerances, capable machines, and accurate, precise,
repeatable measuring devices. Whitneys' idea(s) was
proven in the late 1700's and are now know as the
principle of interchangeability of parts, all pivotal for
mass production.
During the 1800's devices such as the surface
plate, a micrometer that could measure to .001 inch,
and gage blocks were developed. In addition, by
1850 most of the basic machine tools had been
developed. These included the engine lathe, the
index milling machine and the first screw machine
made completely of metal. In the early 1900's mass
production, as presently known, began and the car
industry was started. In order to succeed in the car
industry many leaders of industry needed to increase
the capabilities of metrology and the machine tool.
As the auto industry and WW II drove the
need for accurate measuring devices, tools such as
the dial indicator, the air gage, and the electric gage
were developed.
Along with these inventions
developments in automatic tools and new ways of
measuring quality were being tested. This included
developments such as numerical control (NC) lathes
and statistical quality control (SQC). In the 1960's
and 1970's final inspection was found to be expensive
in terms of one defect part making a whole assembly
unacceptable. This began the idea of inspecting parts
during and after their previous operation, sampling
and process gauging. In- process gauging now plays
an important role in reducing waste and improving
quality.
With increasing automation in manufacturing,
the need for automated measurement is becoming
much more apparent. Presently, developments in
self-diagnostic machines and systems, machine
vision, and lasers are playing an important role in
automation.
In addition coordinate measuring
machines and microscopes are being used as speed in
measuring becomes more critical and electronic
components become smaller. In the future, more
accurate, reliable and faster instruments will be used
more
frequently
in
computer
integrated
manufacturing systems. These instruments will be
developed due to the increasing need for tighter
tolerances in industries such as aerospace and
automobile manufacturing. This is also driven by the
need to reduce costs and enhance delivery schedules,
consistent with increasing customer demands for
improved quality.
Form, Fit, Finish and
Geometric Underpinnings
Function,
Limiting simply says the range of dimension, as with
.003, but is not directional relating to the
specification. Tolerances also relate to specification
(or "spec") limits in various sections during the
discussion of control charting.
Allowances indicate contact/space between
mating components. Clearance is free space allowed.
Interference is when specific amounts of negative
clearance is required, as in the case of press fits.
Specific fits include the following:
1.
2.
3.
4.
5.
Quality systems, and in particular metrology
and inspection, must assure form, fit, finish and
function. These will be further addressed in terms of
surface quality considerations, tolerances and
allowances, and dimensions and shape. It must be
understood that this relates considerably to design
and engineering functions, as well as production
functions in the technological organization, not to
mention function after a product is in service, or
reliability.
Surface quality considerations include
roughness, and finer irregularities in surface texture,
usually resulting from processing but not necessarily
limited to processing. Surface quality could also be a
function of corrosion or other physical impact beyond
processing. Roughness height/width/waviness and
other errors of form are all typical concerns when
considering surface quality. Measuring surface
quality is generally accomplished by moving a fine
stylus or probe across the surface of the component
being examined. However, a more precise method is
interferometry. This is a quality/metrology method
involving putting light on the objects' surface and
measuring the interference in light waves. Surface
quality is discussed in detail in a later part of this
tool.
When parts are designed to go together, some
tolerance is typically allowed. Tolerance says how
much deviation from standard can be allowed, above
or below, positive or negative. These are generally
given in bilateral, unilateral or limiting tolerances.
Bilateral is where the part can vary + .003, but is + in
either direction from the specification. Unilateral
allows only deviation in one direction +.003 or -.002.
Running/sliding--intended to rotate, slide.
Locational clearance fits--stationary parts but
freely assembled.
Transition locational--accuracy of location is
apparent, but some clearance or interference
can be achieved.
Locational interference--light pressure for
assembly.
Force or shrink--heavy pressing to assemble.
These allowances also relate to specifying quality in
the design function, and function in mechanical
applications, all discussed in other parts of the
toolkit.
Base line dimensioning is useful in
specification of quality characteristics. It is where all
dimensions and measures are given from a common
reference point. This eliminates tolerance build up
and provides all measures to a common reference
point. This is a spin off of computer numerical
control, absolute programming, where all dimensions
come from X, Y and Z coordinate intersection in
machining and measuring operations. As will be
noted later, this also ties into CAD-Math data, and
new product development. Geometric dimensioning
or tolerancing also relates here since all forms/shapes
are treated similar to base line and geometric
dimensioning.
Several forms or types of
measurement are of concern for metrological issues.
These
include
perpendicularity,
cylindricity,
angularity, runout, straightness, flatness, roundness,
squareness,
parallelism,
concentricity,
and
eccentricity. Each of these will be briefly explored
and addressed in a later section.
Foundational
Metrological
Measurement Issues
And
Several basic metrological principles and tools
are important in the context of quality systems and
data gathering. Important principles include basic
measures and standards, precision and accuracy, and
calibration. Several metrological tools are necessary
to help implement these principles. These include
gage blocks, surface plates, and other gages and
direct measurement devices. Each of these will be
briefly presented and discussed in this section. As
well, selected other specific measuring information
and devices will be pursued elsewhere in this tool,
focused directly on metrology issues and
circumstances.
Several basic measures are useful in industry
to help give meaning, through standard values, for
inspection. These include length, time, mass,
temperature, electric current, light, and others.
Levels of standards and values which are commonly
used revolve around working levels, calibration,
functional, reference and national/international
standards. These are further identified as follows:
1.
2.
3.
4.
5.
Working level--used at the work center and on
the shop floor, throughout the plant.
Calibration standards--working level gages are
calibrated here, often done out in the shop.
Functional standards--used in metrology labs
to calibrate company standards.
Reference standards--certified to the US
Bureau of Standards, used in lieu of national
standards, done in the metrology lab.
National and international standards--final
authority to which all standards are traced.
Related to this, and defined earlier, precision and
accuracy are functional interpretations of the
standards in production. Precision was indicated to be
the closeness with which a measurement can be read
directly from a measuring instrument, also considered
as the smallest marked increment on the instrument.
By contrast, accuracy was defined as the measure of
how close the reading is to the true size of the part.
Calibration is defined as the comparison of a
measurement,
standard
or
instrument
of
known/dependable accuracy with another standard or
instrument to detect, correlate, report or eliminate by
adjustment, any variation in the accuracy of the
device being compared. Calibration systems
generally involve procedures to enable a comparison
of the instrument being calibrated with a standard
having a higher degree of accuracy. This will be
defined further in a later section.
Gauge blocks are metal blocks, typically
accurate to a millionth of an inch. They are useful in
laboratories to serve as one of the most accurate
references available. Other tools/gauges are
calibrated from gage blocks. Gage blocks come in
various grades and should be selected for the type of
work to be accomplished. Gage blocks have two
working surfaces which are flat, smooth and parallel,
and they are made of special steel which combats
changes in dimension and is hardened to avoid wear.
A surface plate is a reference surface which is
relatively flat, for basing measures upon. Typically
made of cast iron or granite, they must possess (l)
sufficient strength to support a test piece, or setup,
with dimensional stability; and (2) sufficient
accuracy for measurements required. Cast iron plates
have the characteristics of greater strength to weight
ratio, relative to granite. The cast iron plate is less
likely to chip/fracture and it can be wrung without
damaging the surface, whereas granite may gouge.
Also the cast iron plates are magnetically useful in
terms of special setups. By contrast, granite plates
have the characteristics of being non-corrosive, and
they do not burr or get craters. The granite plate has
better flatness tolerances, better thermal stability, and
they are non magnetic.
Surface plate/gage block accessories for
metrological inspection include a toolmakers flat,
which provides a smaller measure and is similar to
the surface plate, but more precise. Angle plates are
used in perpendicular set-up and measurements, and
also as an elevating device.
Parallels are a
support/elevating mechanism consisting of matched
pairs with parallel working surfaces. Likewise, Vblocks are useful for locating/holding rounds while
Sine bars and plates are used to measure angular
setups.
Several traditional types of gages are useful
for inspection work. Among these are general fixed
size, pneumatic and hand held (direct/indirect)
gauges. General gages are useful for gauging large
numbers of similar components in an efficient
manner, particularly linear and angular dimensions,
taper,
roundness,
concentricity,
eccentricity,
parallelness and contours. Fixed size gages are used
for accept or reject situations, as in snap gages for
outside diameters, and rings for rounds and shapes.
Plugs are used to check internals (all are go-no-go
types).
Pneumatic gauges simply measure air
leakage surrounding or within the gauge as it is in
contact or close proximity with the component being
inspected. These are easy to read and have no
moving parts, yet they are sensitive to the surface of
the product being studied.
Hand held measurement devices include rules,
calipers, micrometers, dividers, small hole gages, and
telescoping gauges. Most of these are also direct
measurement devices since they are read directly on
the instrument. Indirect gages require comparison to
a standard to get an actual final measure. Related to
this, bench type inspection devices include the
vernier height gage and dial indicators. Likewise, the
use of light to view a product or component shape
relative to desired shape as noted on a template, is
what occurs with an optical comparator. This is
staging a shape on the comparator and projecting it
for comparison purposes onto the screen, relative to
known values. Often, a vellum or clear plastic print
will be overlaid on the screen for direct comparison
of part to dimensions.
More recently, surface plates and other
traditional metrological tools are being used less and
less in favor of coordinate measuring machines
(CMM). The CMM is a device, based largely on
traditional surface plate logic, having built in
overhead measurement capability with the traditional
values of surface plate technology as the basis. By
using overhead measurement systems, tremendous
precision, often lost in traditional hand setups, is
gained. Through the use of high quality machine tool
precision being built in, CMM's also take full
advantage of computer numerical control (CNC)
logic, further explained in a later tool, relating to
automation.
It is important to recognize that high quality,
precision, direct computer controlled measurements
for the automated workplace will often be achieved
through the use of the CMM.
Even where
automation is not the question, CMM technology
may be the answer. The CMM can be programmed
for various parts, and assuming an organization must
inspect a part on a repeat basis, the program can be
called up and recycled for significant gains in
productivity--realizing there was a cost involved in
writing the program. The direct computer control
(DCC) feature of the CMM also enables reverse
engineering, taking dimensions directly from the
component being inspected.
Basic Measurable Features In Geometric
Dimensioning
Part of what the previous section leads into is
the area of geometric dimensioning and tolerancing
(GDT). GDT is a system of communicating in
design and technical work, related to drafting and
prints, now most often done as computer aided
drafting (CAD). This also opens the door for CADmath data as one of the key numerical systems for
communicating from design to production and
beyond. More important within the current section
and tool, GDT is a bridge between the design side
and measurement, since by specifying various
features and conditions in design, information for
quality and measurement is also being defined for
important characteristics in production. This section
provides specific details, at an introductory level,
relating measurement and design through GDT, with
a focus on straightness, flatness, roundness,
cylindricity, parallelism, perpendicularity, angularity,
circular runout, and total runout. In all cases,
examples are given similar to the way they will
actually be used in design and quality "print takeoff"
applications.
Straightness. The simplistic definition of
straightness relates to deviation from straightness as
specified in a component, typically a flat or a
cylinder. The straightness measure is typically
referenced to another surface or an axis in the
component. Three types of straightness are of
concern, surface elements, axis and centerplane.
Surface elements relate two surfaces one to the other,
axis straightness indicates axial straightness relative
to surfaces, and centerplane relates the centerplane
and corresponding surfaces as a single unit. Each of
the straightness measures are graphically described
nearby:
∅ = .020
(a)
∅
(b)
.510
.490
∅ = .020
(c)
.020
.510
.490
Straightness Feature.
Example (a) is showing a surface straightness
specification. (b) is straightness relative to an axis.
Both are relative to a cylinder as provided by the
diameter symbol and centerline. Example (c) is a
centerplane on a flat.
Measurement of the straightness characteristic
would occur by rotating a cylinder on its axis and
checking for variation along the length. In a flat, the
deviation would be checked across the surface, first
one side and then turned, and the other checked. In
both cases the MMC allowable deviation in any
single plane would be as specified in the box.
Flatness.
Another key measurable in
metrology, either in the end product or in equipment
required to do production, is flatness. Flatness is a
geometric form which applies to a continuous
surface, as related to waviness. It is important for
various reasons, such as mating components'
integrity, surface seal capacity, and overall fit and
function. When it is specified, it is not within the
context of a datum, but what is termed an optimum
plane. The optimum plane is referenced within two
imaginary lines as a tolerance zone, where the
specified flatness surface must remain. Flatness
cannot be gauged in the common go, no-go, sense of
the term, but rather it must be measured.
Flatness, while critical, is typically separated
from accumulated size tolerances or associated
measurement issues. The feature or characteristic is
shown as a side view of a circular disc in the
following specification:
a
c
b
Looking Down On A Disc.
Assuming the part under discussion is a disc, looking
downward, as in a top view, the three points where
the part is suspended are represented by the points a,
b and c. After leveling the suspended component
relative to the measuring surface, multiple measures
can be made across the entire component surface.
Roundness. Roundness measurement is a
form tolerance which applies to cylindrical parts.
The tolerance zone for roundness is two concentric
circles where the stated value provides the actual
tolerance. Graphically the roundness measure would
appear as follows:
.001
.003
∅ .500
Flatness Feature.
What is being communicated is that the surface on
the top plane must be maintained within a tolerance
zone of .001, across the entire face.
The
parallelogram is the symbol for flatness, shown in the
traditional GDT specification box.
For measurement purposes, after the optimum
plane is established, a full indicator measure must be
made of the plane. This would typically be measured
on a surface plate or coordinate measurement set up
where three independent yet equivalent points are
established.
Adjustable jackstands would be
adjusted, measured independently, and used as a
three point support for the object being measured.
With the part suspended atop the jackstands, and after
the part has been leveled at the three jackstand points
relative to the surface plate, the specified tolerance
can be checked.
.490
Roundness Feature.
The graphic indicates that the diameter ( ∅ ) of the
part must conform to a size dimension of maximum
size .500 and minimum .490. The roundness (O) of
the part must be held to a tolerance of .003.
Significantly, it should be noted that the roundness is
not contingent upon the diameter. While there is no
required relationship, it is also true that the indirect
relationship will frequently relate the two. The more
stringent the diameter measure, the more likely the
roundness measure is to be held.
The actual measurement of roundness is
compounded by the reality that roundness appears to
be measurable by standard two point direct
measurement equipment such as calipers and
micrometers.
But two point measures can be
misleading due to any two point across from each
other (180 degrees apart) may or may not be
concentric or eccentric. This is also compounded by
the eccentric/concentric issue being different at all
points along the length of the cylinder. Effectively,
the actual functional outside diameter of the
component will be the actual next size up cylinder
which could slip over the component. Moreover, the
"lobe error" as it is called, will have more than only
two lobe points at each "slice" of the diameter being
measured.
Typically, in non-critical parts or applications,
the lobe error issue can be dealt with by taking the
average of multiple diameter measures with two point
measurement equipment. But for more critical part
measures the method will require a surface plate set
up, “V” blocks, and a dial indicator. The dial
indicator used in the full indicator mode (FIM) can
give a fairly robust measure of overall lobe behavior
as defined in concentricity or eccentricity terms. This
is depicted graphically nearby:
a
.003
∅ .500
.490
Cylindricity Feature.
The feature control box is specifying that the cylinder
must have all surface points along the length within a
stated tolerance zone of .003. The tolerance zone is
specifying that all roundness measures, straightness
of surface, and taper from end to end will lie within.
The compounding features of cylindricity are
graphically illustrated by the following geometric
forms presented earlier with reference to roundness,
now further expanded.
c
b
Concentricity Or Eccentricity.
Simplistically, circles ab and cb as separate entities,
may be round at the two points indicated on each.
Whether they are similar at the infinite other two
point possibilities around the circumference
potentials, or length if a shaft, will remain a question.
Gathering additional points and averaging will help
in gaining a more accurate measure. Placing a
shafting on centers and rotating on axis, or rotating in
a rotary table, can provide alternative methods for
measurement relative to V blocks.
Similarly,
gathering multiple measures at each slice of the same
length/diameter, and averaging can assist. Use of the
CMM for the same functions, as with many
applications, can reduce numbers of moving parts in
the measure, and human interference, potentially
reducing the overall error.
Cylindricity. When the individual features of
roundness, straightness and taper are combined into
one measure, the more complex issue of cylindricity
is addressed. Roundness represents, theoretically,
only one slice along the diameter of the cylinder. By
contrast cylindricity seeks to connect all of the slices
along the surface of the cylinder, and compare both
sides of the cylinder simultaneously. Graphically, the
cylindricity measure is shown nearby:
Taper As Part Of Cylindricity.
Not only is taper notably an issue, but multiple tapers
and other irregularities must be ascertained. As
referenced in the previous graphic showing the
tolerance zone of .003, the part showing multiple
tapers would be measured to determine compliance
within the specified zone.
Measurement sophistication must match or
exceed the complexity of the feature specification in
precision and accuracy. The key difference between
roundness measures previously discussed and the
cylindricity features is that straightness and taper
must now be determined and maintained. As the
simple roundness measures are accomplished, now
the question of other complexities must be checked
by determining total cylinder geometry, sometimes
referred to as total runout. In this case, as well as
representing and averaging multiple roundness
measures on the surface and constructing these into
the geometrical form, straightness, taper and other
elements must be represented. This could include
elements such as concentricity and eccentricity.
These could be established by running a dial
indicator or CMM probe along the axis of the
cylinder in multiple iterations.
nearby:
Probe
This is illustrated
Or Dial
Cylinder Rotated On Centers.
As the part is rotated, systematically, the indicator or
probe is used to establish points on the surface.
Points are consistently and incrementally established,
recorded and averaged. As the more complex
aggregate values are identified and plotted, the
geometric form of the cylinder will be established for
analysis.
Parallelism. When surfaces or axes must be
controlled relative one to the other for parallel
conditions it is referred to as parallelism. At least
one datum is called out as a plane or axis relative to
another plane or surface. This may result in a
tolerance zone between two planes with a datum as a
plane. The tolerance zone could be the axis of two
planes as a tolerance zone when a datum is a separate
plane for an axis to surface parallel condition. Or the
tolerance zone could be expressed as a cylinder
where the parallel condition is the axis of the cylinder
relative to another axis as the datum. In all cases the
parallel specification must express a tolerance zone
within which the parallel condition must fall, shown
as two parallel slanted lines in the feature box. This
is shown graphically nearby:
.003 A
The graphic indicates that feature A must be parallel
to datum A. Thus, all surface points on A must fall
within a tolerance zone of .003 relative to datum
plane A. This type feature would generally be
measured at a surface plate with a dial indicator or at
a CMM. Various locations on the surface could be
checked to determine that none fall outside the
tolerance zone.
Perpendicularity. Perpendicularity applies to
conditions or features where orientation of parts nust
be 90 degrees one to the other. The tolerance zone is
represented by two parallel planes apart by the
specified distance, and 90 degrees to the datum. This
is communicated as providing a .003 tolerance zone
for feature A relative to datum plane A in the
drawing above.
Once again, the basic measurement system
would probably require a surface plate and dial
indicator set up as a minimum. The preferred method
would generally be a CMM to characterize the
perpendicular feature on the outside dimension being
specified. It should be noted that the feature, as
identified, is only specifying the outside surface, and
that if perpendicularity of the inside surface were
desired this would need to be a separate feature.
.003 A
-A-APerpendicularity Feature.
Parallel Feature.
Angularity. Where angles must be specified
in geometric form, the term is generally referred to as
angularity. The angularity feature, while seemingly
less complex than some other features, specifically
cylindricity, does bring forward some interesting
complexities in the GDT communication system.
Since angularity requires a reference of the angle to
some 90 degree datum, this introduces what is termed
primary and secondary datums in the GDT system.
This is illustrated nearby graphically:
.003 A
B
30
-A-B-
geometry, sometimes referred to as total runout. The
key differences in cylindricity and roundness, relative
to runout, is that the cylindrical features are specified
to a datum such as an axis or diameter. This is
presented as circular runout and total runout. The
less complex of the two is circular runout, and this
will be presented first as part of the basis for total
runout.
In the case of circular runout, multiple
roundness measures of the surface must be
represented. These were discussed in measurement
terms as running a dial indicator probe along the axis
of the cylinder in multiple iterations, taking
individual "slice" measures at various locations. This
is illustrated nearby:
FIM
Dial Indicator
Angularity Feature.
The angle of the specified surface is 30 degrees
relative to primary datum A. However the tolerance
zone of .003 is aligned by secondary datum plane B,
shown as a 90 degree reference.
As a measurement task, sine plates or bars
would be used in the traditional setup at a surface
plate to achieve the specified angle. The sine plate is
an adjustable angle device, providing the desired end
result, while the sine bars would be set with gage
blocks used to geometrically configure the
appropriate angle in set up. The preferred method
would be to use a CMM and characterize the angle
with direct hard probe hits which are converted into
an angularity measure with the algorythyms of the
software of the machine. Use of the CMM would
permit placing the part to be measured directly into
the work envelope of the machine, and taking the hits
for rather immediate conversion of data into angle
measure. By contrast, the traditional set up would
require configuring the 30 degree angle of the part to
become parallel to the surface plate through the use
of the sine plate or bar technology. Use of the CMM
would not only be quicker, but the CMM would
likely enhance the overall precision of the
measurement exercise, reducing measurement error.
Circular runout. Runout was introduced
earlier within the context of cylindricity.
As
discussed earlier, as the simple roundness measures
are accomplished, the question of other complexities
must be checked by determining total cylinder
Cylinder Rotated In V Blocks.
As the part is rotated in v blocks, systematically, the
indicator or probe is used to establish points on the
surface. Circular measure points are consistently and
incrementally established as high elements or slices
of the cylinder. As the more complex aggregate
values are identified and plotted, the geometric form
of the cylinder will be established for analysis.
Graphically, the circular runout measure is
shown nearby:
.003
A
-A-
Circular Runout.
The feature control box is specifying that the cylinder
must have all circular measured points along the
surface within the stated tolerance zone of .003. The
tolerance zone is specifying that all circular measures
on the shoulder feature must fall within .003 relative
to the datum surface A. After taking a minimum of
two separate slice measures, preferably three or more,
the tolerance compliance can be determined as runout
from the datum. Significantly, the specified datum
A requires use of v blocks to reference the actual
total diametral characteristic, relative to use of axis
on centers. By using the v block to rotate the
component within, the actual circular runout will be
read at the specified point on the shoulder. However,
it must be remembered that the v block is measuring
from only two surface points, those being the highest
points on the surface from which they are referenced.
Concern over use of v blocks applies to both circular
as well as total runout, and the concern also helps
explain why more sophisticated technologies such as
CMM have been pursued.
Total runout. Total runout relates to circular
runout in much the same way as roundness relates to
cylindricity. While roundness is one measured
"slice", similar to circular runout, the more complex
total runout relates all geometric considerations
relative to the cylinder within the context of runout.
Thus, the total runout feature is a composite control
for rotating parts taking into consideration roundness,
concentricity, straightness, taper, and part surface
profile. As related to end surfaces in rotating parts
total runout would be concerned with wobble,
perpendicularity and flatness of surface. As a
composite, the total runout is a critical feature for
control of rotating components in reference to
balance, vibration and the overall dynamic in
operation. Similar to cylindricity, the total runout
feature requires a FIM for depicting part control,
giving the composite surface geometry and
dimension.
Converting the following fractions
thousandths of inch decimal readings nets:
1/3 = .333 inch
2/3 = .666 inch
3/8 = .375 inch
1/8 = .125 inch
1/16 = .062 inch
Foundational instruments and devices used in basic
measurement to derive the measurements shown
above are defined as rulers, vervier calipers, and
digital gages. This is not exhaustive but should
suffice as an introduction for most persons getting
started in measurement instruments and devices. The
ruler is a foundational device since the vernier scale
which is essentially the ruler, is used as the basis for
many other important measuring devices. This
includes the vernier caliper and the micrometer,
although these devices are changing rapidly to
become digital or in the case of calipers, often dial
instrument.
A vernier scale is attached to an instrument so
that it may slide in a path parallel to the line of
measurement. The main scale is also parallel to the
line of measurement. Both scales are mounted so
that readings can be made with minimum parallax
error, meaning that incorrect readings result because
of misalignment in graduations on scales.
Advantages of vernier scales include:
1.
Basic Principles And Devices
Measurement And Data Collection
For
Most of the data which will be increasingly
important in the quality systems in the future will be
determined, measured and recorded in thousandths as
a decimal value. Technically this is written as .001
for one thousandth, and if in inches, we would say
.001 inch. If this were in hundredths it would be
written as .01 inch, and in tenths as .1. To convert
from a fraction into decimal we divide the lower half
of the fraction into the upper half. Thus, if the
fraction 1/2 inch were provided, 2 divided into 1
equals .5, and if this is done in the thousandths level
of accuracy we would say .500 inch. If the fraction
1/4 inch were provided, 4 divided into 1 equals .25,
and again if done in the thousandths level of accuracy
we would say .250 inch.
into
2.
3.
Amplification is achieved by design and is not
dependent on moving parts.
No interpolation is required.
There is no theoretical limit to the scale range.
Disadvantages of vernier scales include:
1.
2.
3.
The vernier scale is located on the instrument
used and is dependent on the instrument for
accuracy.
The reliability of readings depends sufficiently
upon the observer relative to most instruments.
The discrimination is limited to the person
using the gage to a great extent.
The advantages and disadvantages of vernier scales
are shown graphically nearby. Related
to
the
caliper, digital gages are the fastest developing type
of high precision measurement and gauging. Their
ability to provide two or more scales plus their
relatively low cost explains their popularity. The
concept to keep in mind, however, is that the digital
instruments are only as accurate as their mechanical
parts, regardless of how far the zeros in the readout
extend. Most digital instruments are easy to use but
some have features that may assist in different
situations. The best way to learn about the general
and added features of a digital instrument is to read
the operators manual. Some general advantages and
disadvantages follow.
Advantages of digital
instruments are identified as:
1.
2.
3.
4.
Can be zeroed out at any point within their
range.
Reduce the visual error.
Instruments can be tied into real time SPC.
Remote operation.
Disadvantages of digital instruments are identified as:
1.
2.
Digital readout can be misguiding due to the
digital accuracy not corresponding to the
mechanical accuracy of the device.
Less rugged relative to standard vernier
instruments.
The advantages and disadvantages of digital
instruments are shown in summary form in a nearby
graphic.
It is important to note, relative to instruments
and devices for measuring, using vernier and digital
systems, that the applications are virtually unlimited.
This can include rulers, dial indicators, calipers,
micrometers, and others in terms of hand held
instruments. But it can also include placement on
machines and equipment such as hard gages, or
dedicated systems for extracting variable measures
for data gathering. This is important in the context of
the previous tools on attribute data transitioning to
variable for enhanced ability to know if components
and parts are in compliance with specifications as
characteristics.
A distinction should also be made between
destructive and nondestructive testing in inspection.
Nondestructive testing (NDT) implies that some
inspection of the item will be accomplished with
inspection techniques that can find surface or internal
defects without destroying the item. By contrast, the
item will be rendered non-useful through destructive
testing. For example, arc welds in a nuclear reactor
receive 100% NDT inspection for defects because the
cost of a failure here can be great. The primary used
NDT techniques are the following: Liquid or dye
penetrate testing, magnetic particle testing,
radiographic inspection, neutron radiographic,
ultrasonic testing, Eddy current testing, acoustic
emission, thermal inspection, optical holography,
among others.
Shifting Toward The Metric System
One of the key areas of development in basic
measurement is the shift which is occurring toward
metric as the standard. This has been shifting
gradually worldwide, to the extent that most of the
rest of the world is metric while the US remains an
English system. Over the next several years there
will be a continued effort to move the US into the
metric measurement system, lead primarily by
organizations doing global business. The bottom line
is, we need to have everyone on the same sheet of
music, certainly including the units of measure, as
well as instruments, data collection and
documentation.
Advantages of the system.
By way of
introduction, it should be pointed out that there are
several advantages in the metric system. This
includes the fundamental reality that there is one
basic unit for each quantity which directly and
logically relates to all other units. Decimals are used
exclusively, with no fractions, providing a more
precise and accurate basis for measurement in all
applications. The inherent gains in use of the metric
system apply directly to our need to be able to
communicate with the least ambiguity and confusion
in logical mathematical ways, how production is
occurring. This is aided by elimination of long rows
of zeros, simple and absolute symbols, and the
system being comprehensive in the sense that all
measures are covered under one umbrella. Other
advantages relate to the need for shifts in ISO and QS
standards as all parties move toward solid global
supplier and customer systems for producing product.
Getting a handle on the meter. By way of
relative comparison, one millimeter (mm) is about
the thickness of a dime. Ten mm are roughly equal
to the size of the small fingernail, or one centimeter
(cm). Ten cm are equal to one meter, a meter being
slightly longer than one yard. A 100 mm cigarette is
just under four inches, 35 mm film is about an inch
and a half, and a standard door is roughly two meters
in height. One thousand meters is called a kilometer
(km). The basis for all measures in the system is the
meter, with units of 10 being the fundamental
building blocks in the system. This provides a
nomenclature system as follows:
meter = m
millimeter = mm
centimeter = cm
decimeter = dm
kilometer = km
10 mm = 1 cm
1000 mm = 100 cm = 1 m
1000 m = 1 km
10 dm = 1 m
If we were to translate this into decimals, as is the
case for precision purposes, as converted from typical
fractions in the English system, they might appear to
be similar, at a quick glance, all based on the
millimeter and inch. The example in this case
provides increments of 1/8 inch, or .1250 decimally,
converted to millimeters:
Fraction
Decimal
1/8
.1250
1/4
.2500
3/8
.3750
1/2
.5000
5/8
.6250
3/4
.7500
7/8
.8750
1 inch = 1.000 =
mm
3.175
6.350
9.525
12.700
15.875
19.050
22.225
25.400
While there are similarities, and the English system
appears
quite
satisfactory,
problems
(or
opportunities) can be noted in longer distances where
we must resort to feet and inch conversions in
illogical increments, yards and miles even less
logical, and so on. The point is that the metric
system, with its 10 placed logic system, is much more
logically designed, particularly advantageous for
precision and accuracy in measurement.
Surface Quality: Focused Foundational
Metrological Issue
In 1962, representatives of the United States of
America Standards Institutions, the Canadian
Standards Association, and the British Standards
Institution signed an American, British, and Canadian
declaration of accord in standards for surface texture.
The standards set up criteria for the standardization
of tracer-point measuring instruments and roughness
samples.
The term "Machine Finish" refers to the
geometric irregularities produced on the surface of a
solid material by the cutting action of a tool, by
abrasives, or by other finishing devices. In fact,
designers put some explanatory notes such as "Rough
Grind," "Lap," "Smooth Grind," etc., on their
drawings in order to secure the finish they want.
Actually, they are specifying a method of
manufacture rather than a surface finish.
Each type of tool or machining operation
leaves its own individual markings. For example, a
shaper with around-nose cutting tool produces long,
regularly spaced U-shaped furrows having sharp
ridges: an end mill makes a curved pattern with half
the curves extending in one direction and half in the
other. Transverse markings (those across the
direction of tool motion) are the result of the profile
shape of the tool and the rate of feed. Longitudinal
markings (those in the direction of tool motion) result
from the irregular cutting action of the tool during the
process of chip removal, changes in speed or in the
condition of the tool, or small variations in the
uniformity of the material being worked.
Fundamental theory of measurement. A
variety of instruments are available for measuring
surface roughness and surface profiles. The majority
of these devices employ a diamond stylus which is
moved at a constant rate across the surface,
perpendicular to the lay pattern. The rise and fall of
the stylus is detected electronically (often using a
LVDT device), amplified and recorded on a strip
chart, or processed electronically to produce AA or
rms readings. The unit containing the stylus and
driving motor may be hand-held or supported in the
work piece or other supporting surface. The
resolution of these devices is determined by the
radius of diameter of the tip of the stylus. When the
magnitude of the geometric features begins to
approach the magnitude of the tip of the stylus, great
caution should be used in interpreting the output from
these devices.
Profilometer. The Profilometer is recognized
as one of the best, if not the first, practical tracer-type
electronic instrument for measuring surface
roughness. It is a mechanical-electronic shop
instrument for measuring directly the average height
of surface roughness, in millionths of an inch, of all
types of surfaces produced by machining, grinding
and finishing operations. It is composed of three
principal parts.
This includes the tracer, the
amplimeter, and suitable piloting means. The tracer
consists of an electromagnetic reproducer made up of
a moving coil, to which is attached a diamond tracing
point, and a stationary permanent magnet.
The diamond point has a 90 degree included
angle and a .0005” radius hemispherical tip. It moves
up and down as it is guided over the roughness
irregularities of the surface being measured. The
motion of the coil in the magnetic field generates a
voltage that is proportional to the roughness
irregularities since the tracer is constrained to move
in a direction perpendicular, or vertical, to the
nominal surface being measured.
The amplimeter, an electronic amplifier with
micro inch instrumentation and controls, amplifies
the voltage produced by the tracer, and activates the
meter. Since the meter may be of the averaging type,
(approximately two-second period) the height of the
single scratches is not obtained, but rather the reading
indicating the average height of the irregularities of
the surface being traced. Suitable piloting must be
provided to move the tracer over the work surfaces.
This is often done manually on both internal and
external surfaces, using tracers with suitable skids.
However, it is often automated or mechanized as is
the case in the Surfcom product by Brown and
Sharpe.
Limitations of tracer point measurement.
It is not practical to form the point so that it would be
perfectly sharp and slender enough to reach into
every scratch, no matter how small. Such an
extremely fine point would tend to catch on the
irregularities of the surface and either deform them or
break off (the probe point). Obviously, it is also
possible that the actual data being generated will be
affected by the size and shape of the stylus. Another
factor which limits the accuracy of tracer point
analyzers is the skid, which supports the tracer
diamond. Both the size and the position of the skid
directly affect the reading attained, because the
motion of the diamond in relation to the skid rather
than the actual movement of the tracer point with
respect to the surface is measured.
Surface quality terminology. As the surface
of all materials finished by milling, turning, grinding,
and other operations is composed of myriad tiny
irregularities, certain terms descriptive of those
irregularities should be thoroughly understood. This
includes roughness, flaws, roughness width cutoff,
waviness and lay. Each is further defined below:
1.
2.
3.
Roughness, the factor controlled by the use of
the micro inch surface finish designation, is a
recurrent irregularity typical of the surface. It
refers to all those deviations from a nominal
surface which are characteristic of the surface,
regardless of the crest-to-crest distance of the
irregularities.
Flaws, such as scratches, dents, and other local
damage, are irregularities or imperfections
which occur only at a few places in a piece of
material; they are not typical of the surface as
they are usually caused by conditions other
than the normal machining process.
Roughness width cutoff refers to the ability
of an instrument to differentiate between fine
pitch and coarse pitch roughness irregularities
4.
5.
hence permits a better description of a surface
to be made when necessary. It is a matter of
the adjustment of the sensitivity of the
instrument in gauging the irregularities of the
surface rather than a characteristic of the
surface being assessed.
Waviness is surface irregularity of greater
spacing than occurs in roughness. It may be
the result of warping, vibration, or the work
deflecting during machining.
Lay is the term used to refer to the direction of
the predominant tool marks, grain, or pattern
of the surface roughness.
Related to several of the terms, it should be noted that
all surface roughness measurements or comparisons
are normally taken across the lay. It is this direction
which gives the best comparative value, and the
highest reading on tracer point analyzers.
An example setup for surface quality. This
provides one approach to deriving surface quality
data in a typical manufacturing environment. The
principles and information can be applied to many
applications, and it is intended to be start-up in
nature. The basis of this setup is the SURFCOM
profilometer by Brown and Sharpe, although other
equipment could be used. The SURFCOM includes
the following:
1.
2.
3.
4.
5.
5.
Tracing driver and cord, hookups.
Measuring stands and pickup.
Amplifier/recorder and power supply setup.
Appropriate
software
for
calculating
descriptive statistics, printer and other support
setup.
User's guide to the SURFCOM.
Specimens might be 3µs, 6µs, and 1µs.
Typical procedures would include a SOP, such as is
shown here. It should be recognized that this is
summarized, and should be studied further in actual
user manuals for specific equipment. Procedures
should be considered adequate to get started. The
first set of steps (1-14) are oriented primarily toward
calibration, while the remainder of the steps (15-21)
deal with actual data acquisition.
1.
2.
3.
Read appropriate chapter’s of the user’s guide
to become further acquainted with surface
texture terminology and the SURFCOM
equipment being used.
Calibrate the SURFCOM machine following
proper instructions. Also refer to appropriate
sections of the user’s guide.
Push the power switch located on the far left
hand side of the amplifier/recorder vertically
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
upwards to turn the equipment "on". Wait at
least 15 minutes for equipment warm up
before proceeding.
Set the vertical magnification to 2K. Press the
buttons marked (<) and (>) located in the
upper
right
hand
corner
of
the
amplifier/recorder.
Press the yellow preset button so that the
yellow LED (light) for the row containing the
label "Length" lights up. Also. the green LED
labeled "in" on the right side of the display
should light up. The yellow preset or data
button should display a value of 0.16. This is
the traversing length of the pickup stylus on
the specimen.
Set the "sens" switch to 1 .
Press the yellow button labeled "Average" so
that its LED light is off.
Press the green output button so that the green
LED at the top row of the outputs is on. Press
the parameter select button below the ra label.
The green LED on this button should be on.
The display is now ready to output Ra values.
Place the calibration specimen under the
pickup on the worktable.
Ensure the pickup axis is parallel to the
specimen surface. If not parallel, reset it by
re-tightening the skid-skidless knob.
Ensure that the stylus tip is vertical to the work
piece surface. Caution: Care should be taken
with lowering the pickup. If lowered in
excess of the CAL adjuster, the red light of
the CAL adjuster will glow and may cause
damage to the pickup and stylus tip.
Lower the pickup carefully using the tracing
driver’s elevating block until the yellow light
on the zero pointer is set to the middlemost
position.
Note: The CAl adjuster should not be
changed while measuring is in progress.
Press the "meas" button on the amplifierrecorder to start the measuring process. This
should be done once the pickup has made
contact with the specimen and the zero pointer
is set at the middlemost position. The display
should show a Ra (average surface roughness)
value of 118 µ in (micro inches).
While the measuring is taking place, check to
see that the zero indicator does not exceed the
calculation range for the entire traversal length
(neither of the two red lights at the extreme
ends should glow). Remove the calibration
specimen from under the pickup.
Actual measurement would occur as apart of the
next several steps, all based on proper calibration.
The example being discussed is designed around Xbar and R charting, and gage R and R, as well as
capability analysis.
15.
16.
17.
18.
19.
(a) Place a face of the specimen under the
pickup.
(b) Adjust the pickup height and retighten the
pickup clamp knob. Ensure that the stylus tip
is vertical to the work piece surface and
properly positioned in general.
(c) Ensure that the pickup axis is parallel to the
work piece surface. If it is not parallel, set it
properly by re-tightening the skid-skidless
selection knob and making other adjustments.
(d) Carefully lower the pickup using the
tracing driver’s elevating block until one of the
yellow lights in the zero indicator which is
labeled CAL. This is also called the CAL
adjust, to be illuminated (care should be taken
while lowering, if lowered in excess the red
light at the extreme end of the CAL adjuster
glows) by gently turning the CAL adjuster (a
rotatable knob) to illuminate the middle most
yellow light of the zero adjuster.
(e) Check the status of the "auto return"
button. If it is required that the stylus should
automatically return to its original position
press the button so that the display reads "on".
(f) Press the "meas" button on the tracing
Driver or the measure button on the front panel
of the amplifier-recorder, to start the
measuring process. At the end of the process
the display will show the Ra (average surface
roughness) value over the traversing length.
(g) While measuring is progressing, check to
see that the zero indicator does not exceed
calculation range for the entire traversal length
(Neither of the two red lights at the extreme
ends should glow).
(h) The CAL adjuster should not be changed
while measuring is in progress.
(i) Record the Ra value on the printed forms.
This form contains the results. Repeat the
above procedure at four more different points
on the same side of the specimen.
Using another specimen, repeat the above.
Create a printout of all data collected.
Perform and print all basic SPC calculations
including X-bar and R charts, Cpk, R and R
and print all as well as histograms and charts,
by following the procedures listed in the
software package. To determine R and R
follow instructions in the toolkit.
Provide an analysis and interpretation of
information identified in previous items.
20.
21.
Depending on level of analysis desired,
perform a comparison between the two (or
more) sets of data using ANOVA, looking for
significant differences in variation. This
should be done using appropriate software.
Similarly, the data could also be used to
perform Design of Experiments (DOE) using
appropriate software to further analyze the
data.
This example is further explored in several
subsequent tools within the toolkit.
Questions, analysis, other issues. Several
questions should be raised to assist the reader further
focus on the issue related to surface quality, data
collection, inspection and analysis in general. These
relate to most of the topics previously presented and
discussed, but in most cases, are only the beginning.
While the profilometer is the basis for traditional
surface quality measures, uses of lasers and other
light interference systems will be the more likely
approaches for the future. This is true since the
profilometer, while reasonably accurate, is slow and
time consuming. It is also vulnerable to deviations
and mis-interpretations due to operator error, types of
surface generation equipment, and other factors and
elements.
Yet, the above presentation should help the
reader begin to understand elements of the
relationships inherent in surface quality, reliability
and functioning of product and other general
relationships. It should also serve to assist the reader
in beginning to relate the necessity of properly
calibrated equipment, as well as care and knowledge
in collection and acquisition of the actual data so vital
to the improvement process.