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"All
Dimensions are Applicable at 20° C (68° F)"
Tight Tolerances Feel the Heat
Close tolerances are usually
specified for a good reason! For example, we use accurate specifications
to maximize performance and quality of tight fitting, mating parts. Once
the drawings are finished we send them off to manufacturing, and get on
with the next job. It is assumed that machine tools and inspection
equipment will insure that the right dimensions are obtained. And those
tools are remarkably capable. Except that they can be susceptible to
variation if exposed to changes in temperature. Production shop floor
temperatures are rarely precisely controlled.
The result is that machine tools, positioning systems and gages can
produce varying readings depending on prevailing temperatures. In many
cases the range of dimensional variation can exceed the total tolerance
specified for a feature dimension! For example, a steel diameter such as
a crankshaft bearing journal might measure 3.0000 inches at 90° F
but at 70° F it will measure 2.9996 inches. That .0004 inch can
represent a large part of the total tolerance. If the journal was
undersize by that amount, and the mating bearing was oversize by the
same amount, or the reverse was true, what effects would this have on
overall performance of the assembly?

According to ANSI (Y14.5M-1982), "Unless
otherwise specified, all dimensions are applicable at 20° C (68° F)".
The very first standard published by ISO dealt with the same issue. ISO
1 (1975) ratified an international agreement reached originally in 1931.
Consisting of just a single sentence, it states: "The standard reference
temperature for industrial length measurements is fixed at 20° C."
On the shop floor, however, these
standards are usually overlooked, ignored or unknown. Consequently, an
opportunity to maintain high quality and process capability is often
missed. Although parts may appear to operators to be within tolerance at
the time of manufacture, they could be out of tolerance if measured
under controlled conditions at 68° F. Over the days and months there
will be unnecessary variation in dimensions if they are compared at the
true reference temperature.

There is a Solution
But things don’t have to be this way.
Modern technologies such as automatic electronic temperature
compensation can correct for these errors, if designers bring the need
for consideration of thermal effects to the attention of the production
floor. All you have to do is to state clearly on drawings: "All
Dimensions are Applicable at 20° C (68° F)" (or other reference
temperature, if appropriate). This will act as a reminder that
tolerances are critical and that thermal variation should be minimized.
Automatic Temperature
Compensation Systems provide users with an economically priced method of
minimizing thermal errors in precision dimensional measurements. They
sense temperatures of the key elements of a measurement system (that is,
the workpiece, the gage and the setting master), calculate the amount of
thermal error in that system and output a correction signal. That
correction, or compensation, can be applied automatically to a precision
gage so that measurements reflect true dimension as if all the elements
were constantly at the International Reference temperature of 68° F/20°
C.
Environmental control such as air
conditioning, heating and liquid coolant are expensive and inaccurate.
These methods cost hundreds of thousands of dollars to install and
operate, and can achieve repeatable thermal control only to within ± 2
or 3 degrees F at best. Temperature compensation systems cost less than
a tenth of such solutions, and achieve temperature measurement accuracy
in the order of ± 0.5 degrees F.
Temperature compensation
systems are designed to work with all electronic gages, including
automated, column and other bench top gaging systems. Hundreds of such
systems are currently in service in plants making automotive powertrain
components, bearings, railroad equipment, printing machinery and similar
products. However, production teams who were given adequate warning that
temperature fluctuations would generate measurement inaccuracies
installed these systems.
The Designer’s Role
It is not enough to expect those on the
production floor to remember to minimize thermal variations. Production
personnel deal with many dimensional specifications, and the majority
does not require such accurate finishes that temperature needs to be
considered. As a result, they may well not be aware of the need to
consider temperature on a close tolerance feature unless they are
alerted. The designer of the part, however, will know that a close
tolerance is being specified, and should attract attention to it by
providing an alert on the drawing.
It may be useful to remember that as a
rule of thumb, when total tolerance represents 1,000th or less of the
nominal dimension of a part feature, temperature is probably going to
significantly affect measurement accuracy.
It is important to realize that all gages
and positioning systems are affected by temperature. Although there may
be offsetting affects caused by the gage or master expanding or
contracting along with the part, they invariable change at different
rates, due to their different effective expansion coefficients. And it
is extremely difficult to control temperatures in a production
environment.
Gages are now being used to measure to
discriminations and accuracies measured in Microns and Tens of
Millionths of an Inch. Influenced by the competitive drive for improved
quality, efficiency and manufacturing techniques, tolerances have been
steadily getting tighter. Tolerances have become so tight, in many
instances, that the subtle effects of temperature have quietly become
significant and often unnoticed.
Over the last one hundred
and fifty years tolerances have steadily and regularly decreased. During
the Industrial Revolution a tolerance of 0.1mm was considered tight. By
1950s and 60s 0.01mm tolerances were appearing on drawings. Now it is
not uncommon to find tolerances measured in .001mm units (microns).
Thermal expansion of part features during
the production and post-production processes can now exceed total
tolerance. However, since the effects of temperature are so subtle, and
the eye cannot discern changes in dimension that are measured in microns
or tenths of thousandth of an inch, it is easy to overlook this
phenomenon. The time to think about this issue is during the design
phase and the preparation of technical drawings.
The same technological trend that has
driven tolerances down into the microscopic realm has provided the
know-how for dealing with thermal problems. Electronic temperature
compensation systems can correct for temperature-induced errors in real
time, but most gages do not have such systems built into them unless the
end-user requests them. Designers of components can assist everyone by
giving early notification that temperature needs to be considered for
the tight tolerances they specify. Just clearly mark drawings:
"All Dimensions are Applicable at 20° C (68°
F)".
Top
So,
you don’t think you have a temperature problem?
The Myth
It’s easy to believe
that your precision inspection systems and gages are giving you accurate
dimensional control. Your gage readouts or SPC charts tell you that you
are in control, right? And those gages have been shown to have a Gage
R&R below 10%, so they must be correct. No problem!
Would it shake your
confidence if you learned that despite a 10% Gage R&R, gage accuracy can
drift over time by as much as 50% to 100% of tolerance without you being
aware of it? If a gage appeared to give you accurate readings but was in
fact off by a large factor, what would it be doing to your process? Your
process could be out of control and you wouldn’t know it. Wouldn’t it be
like being on a diet and thinking you were shedding several pounds a
month, only to find that the scales were wrong and you were, in fact,
gaining weight? Well, that is the kind of invisible effect that
temperature changes can have on shop floor gages. And those gages are
performing a vital function! Don’t be fooled by the myth of short term
Gage R&R. Gages that operate on the shop floor, even in supposedly
controlled environments, more often than not are subject to temperature
induced drift. It is just too subtle an effect to be obvious, so it is
often overlooked or ignored.
However, you can easily
test this for yourself. Measure the same feature on a workpiece
repeatedly, in the same gage, over a period of a few weeks. Write each
measurement down. If you have a thermometer you might also want to write
down the ambient temperature at the time of each measurement. After you
have taken several measurements, compare them. You will probably see
variation, and this variation will probably correlate with recorded
temperature variations. (Of course, there may also be other thermal
influences than ambient variations that would account for these
differences. Machining operations, coolants, washers, etc., may be
causing temperature changes as well.) Fig 1 shows an example of such a
study. The same crank pin on the same crank shaft was measured about 60
times over a period of two weeks at a major automobile engine plant, on
the same shop floor gage. As can be seen from the graph, measurements
varied by 13 microns on a part with control limits 14 microns apart!
That is almost 100% variation. However, the gage used for this test had
been certified to have a Gage R&R of better than 10%. Temperature
accounted for 90% of the variation seen in this chart.
Well, what about those
Gage R&R and Process Capability studies?
Gage
studies are generally performed in controlled situations. They are
performed over a short period of time or in temperature controlled
environments, so that temperature does not have time to have an
influence. The effects of temperature are deliberately eliminated.
However, gages are then used in shop floor environments where ambient
temperatures fluctuate from hour to hour and month to month, and where
workpieces vary in temperature as the result of operations or seasonal
ambient variations. Of course, if no further tests are performed, this
fact will never become apparent. You put your trust in the gage.
However, you will be producing parts that vary significantly in relative
size over time, none the less. Fig. 2 illustrates the variations that
can affect a one inch (25.4mm) steel diameter. Other diameters of the
same material would be affected proportionately, and aluminum parts show
twice the thermal growth.
Over the long term,
temperature will cause uncorrected gages to produce varying dimensions.
The argument that changes in ambient temperature will cause offsetting
errors in workpiece and gage is generally untrue. Usually the gage and
workpiece expand and contract at differing rates as temperatures change.
They have different effective coefficients of expansion. Gages are
particularly prone to exhibiting unexpectedly high effective thermal
coefficients, due to their complex geometries, mixtures of components,
electronic drift, etc. They have been seen to exhibit coefficients 10 to
20 times greater than that of a workpiece. Moreover, it is worth
repeating that workpieces are frequently exposed to operations that can
cause them to be at varying temperatures – washers, coolants, machining
all effect temperature, so they are frequently not at the same
temperature as the gage.
So-called "tempering" – the use of air
conditioning to reduce the extremes of temperature fluctuations, while
not completely controlling the temperature at the ISO standard 20°C /
68°F – can also lead to misplaced confidence in gages. Tempering tends
to keep shop floor temperatures within +/- 10°F (5.5°C) of some
arbitrary value other than the ISO standard 20°C / 68°F. Precision
measuring instruments can still be rendered inaccurate by this much
variation (see Fig 3. and the accompanying paragraphs describing the
effects of 5°C variation), and tempering ignores process induced
temperature variations such as have already been mentioned, i.e. from
machining, coolants, washers, etc.
A long term test can reveal the problem,
and suggest a solution.
By testing a measurement system over a
long term, while operating temperatures vary naturally in the regular
shop floor environment, you can obtain realistic data about true shop
floor gage performance. In the example shown in Fig 3 data was taken
from a gage in three runs, each of which was conducted over a period of
several hours.
In the first run of 100 parts, conducted
over a short period of time, temperatures were held reasonably stable.
Ambient temperature rose by just 0.9°C (1.6°F) during the test. In the
second run, the same 100 parts were measured while ambient temperature
gradually increased by 5°C (9°F). Look at the negative effect that this
had on true process capability.
The dramatic improvement obtained by
using automatic temperature compensation can be seen by looking at the
results of the third run. In the final long term run temperatures were
again allowed to rise by 5°C (9°F) while the same 100 parts were being
measured, but in this case "temp comp" was applied. These results were
obtained from a regular shop floor post process precision gage,
operating in a typical shop floor environment. Many such gages have
successfully applied temperature compensation systems so as to obtain
such improved results.
Temperature Compensation
Temperature compensation
systems consist of a microprocessor based controller equipped with
temperature sensors and a selection of I/O ports. They communicate with
electronic gaging systems over analog or digital interfaces. Their
temperature sensors are designed to be attached to the gage fixture so
as to monitor temperatures of the part being measured, the gage fixture
and the calibrating master or masters during gaging operations.
These temperature compensation systems
can be programmed with the effective coefficients of expansion of the
gage, part and master. During measuring operations they continuously
sense temperature variations and compute and send a correcting signal to
the gage in real time. The gage then displays the correct measurement
after eliminating thermal errors.
Temperature compensation systems have
been installed on automatic, bench top and hand held gages. Users are
extremely pleased with the robust performance of these systems, often
volunteering to be the subjects of technical articles and papers for
publication. More recently, Albion Devices, Inc., Solana Beach, CA
(www.AlbionDevices.com) has begun to install systems on in-process gages
for applications such as grinding and honing. These systems actually
sense the temperature of parts and in-process gages during the machining
operation and provide a correcting offset to the gage. The result is
that dimensions are measured as if temperatures were being controlled at
reference temperature (20° C / 68° F) while operating temperatures are
considerably different than this. However, since the system is
displaying the size that parts would be at if they were at reference
temperature, they can be machined directly to final size without having
to allow time for cooling before making final measurements and
performing a finishing grind.
Don’t miss an opportunity to improve
quality and process capability.
By turning a blind eye
to the issue of temperature you can miss a valuable opportunity to
improve quality and process capability. As tolerances become
increasingly tight even a small temperature variation from the ISO
standard 20°C / 68°F will cause significant gaging error. It is easy to
be fooled into thinking that you don’t have a problem. But just because
your data is grouped nicely, it does not necessarily mean that it is
accurate (Fig 4). As a repeatable gage drifts with temperature it will
appear to continue to give good results. We tend to believe what the
indicator or the gage display shows. However, that data may be grouped
tightly, while still being considerably off target. Don’t be misled.
Top
Gage R & R and Temperature on the Shop Floor
The specified accuracies of gages as
determined by Gage R & R studies can only be achieved if they operate at
20° C (68° F)
"Gage R & R" is a
standard for determining the ability of a gage and its operator to
obtain the same result while taking successive measurements. It is
important to understand, however, that a gage with a good R & R is not
necessarily accurate. It may just keep giving the same wrong answer. For
example, a gage that read 3.0008 inches 30 times in succession while
measuring a 3.0000 inch diameter would produce an excellent Gage R & R
result. It would have remarkable repeatability, or "precision". However,
it would not be giving accurate readings. This is what happens to gages
as temperatures change. A heated 3.0000 inch diameter measures 3.0008 –
until it cools down later. Since the true dimension should be that which
is obtained at 20° C (68° F), if temperatures are not at 20° C (68° F)
then measurement inaccuracies will occur, while the gage will appear to
be giving good readings. The specified accuracies of gages can only be
achieved if they operate at 20° C (68° F) (reference temperature) and
their masters and workpieces are at the same temperature, or if
compensation is made for differences in temperature from the reference
temperature.
Gages that operate on
the shop floor are subject to temperature induced drift. Gage studies
are generally performed in controlled situations. They are performed
over a short period of time or in temperature controlled environments,
so that temperature does not have time to have an influence. The effects
of temperature are deliberately eliminated from these studies. However,
gages are then used in shop floor environments where ambient
temperatures fluctuate from hour to hour and month to month, and where
workpieces vary in temperature as the result of operations or seasonal
ambient variations.
A long term test can reveal the problem,
and suggest a solution.
By testing a measurement system over a
long term, while operating temperatures vary naturally in the regular
shop floor environment, you can obtain realistic data about true shop
floor gage performance. Gages that produce a Gage R & R of 10% or less
in a controlled test may obtain results that are closer to 100% or more
if temperatures are allowed to drift during the test as they would in
normal operating conditions on the shop floor. Similarly, process
capability studies can be heavily influenced by temperature.
In the example shown in Fig 1 data was
taken from a gage in three runs, each of which was conducted over a
period of several hours. In the first run of 100 parts, conducted over a
short period of time, temperatures were held reasonably stable. Ambient
temperature rose by just 0.9°C (1.6°F) during the test. In the second
run, the same 100 parts were measured while ambient temperature
gradually increased by 5°C (9°F). Look at the negative effect that this
had on true process capability.
The dramatic improvement obtained by
using automatic temperature compensation can be seen by looking at the
results of the third run. In the final long term run temperatures were
again allowed to rise by 5°C (9°F) while the same 100 parts were being
measured, but in this case "temp comp" was applied. These results were
obtained from a regular shop floor post process precision gage,
operating in a typical shop floor environment. Many such gages have
successfully applied temperature compensation systems so as to obtain
such improved results.
Similar studies conducted by Ford Motor
Company personnel at their Livonia Transmission Plant revealed process
improvements of over 100%.
Gage R & R and Temperature Compensation
The addition of temperature compensation
to a gage that obtains a 10% R & R in an environment that is completely
controlled at 20° C (68° F) cannot improve the system performance.
Indeed, since the addition of any components to any measuring system
will inevitably add some error. So, under perfect environmental
conditions, Albion Devices, Inc., cannot claim to improve Gage R & R.
Indeed, under such circumstances our systems may contribute to a
measurement system degradation of a percentage point or so.
However, shop floor measurement systems
do not perform under perfect conditions from day to day. Rather, they
are exposed to thermal variations which, as shown above, can cause their
actual R & R to vary by over 100%. Albion’s temperature compensation
systems are intended to eliminate most of this thermally induced drift.
Thus, Gage R & R might drift by 100% on the shop floor without
temperature compensation while temperatures were uncontrolled. If
temperature compensation were applied to the same gage, R & R would be
held closer to the value established under controlled conditions (say,
10%), give or take some minor system error. In other words, under shop
floor conditions, true Gage R & R would be in the region of 100%. Adding
temperature compensation to the same gage would bring R & R back down to
10% +/- some minor system error.
Top
Eliminate Temperature Electronically and
Improve Capability
Precision dimensional
control is an important part of manufacturing, yet it is still common
practice to ignore the single largest cause of precision measurement
error: temperature. Typical tests of precision measuring systems fail to
address this problem. A short-term capability study on a process or
gaging system may lead to the belief that a very acceptable G R & R, Cp
or Cpk can be obtained. However, it may hide the fact that over the long
term, thermal effects can cause significant deterioration in
performance.
A recent study
demonstrated not only the detrimental effect of temperature over the
long term on an apparently good process control gage, but also the
benefit of using electronic temperature compensation to overcome this
problem. A short term study of a gage used to measure mass produced 18
mm (0.71 inch) diameter spool valves indicated that the process could be
controlled with a Cp value of 2.77. That should be pretty good. However,
a long-term study, during which ambient temperature increased by just 5°
C (9° F), showed a deterioration of the Cp value to 1.56. Moreover, Cpk
fell from 2.12 to just 0.35! Fortunately, the Quality Manager
responsible for system implementation provided for the gage to be
equipped with an electronic temperature compensation system from Albion
Devices, Inc., that uses Albion’s proven and patented thermal correction
method. When this was connected, the same long-term test (with
temperatures rising by 5° C) produced a Cp of 3.11 and a Cpk of 2.81.
Table A and Fig. 1 summarize these results.
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Table A |
Run I: At Stable (almost) Temperature
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Run II: While Temperatures increased by 5° C, without
Compensation
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Run III: While Temperatures increased by 5° C, with Temperature
Compensation
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Cp |
2.77
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1.56
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3.11
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Cpk |
2.12
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0.35
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2.81
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Most shop floor gages are subject to
significant thermal variation. They may easily see 5 to 10° C (9 to 18°
F) temperature changes between early morning and noon, particularly in
summer. Very few mass production plants find it practical or affordable
to control temperatures accurately. If a gage run-off is performed in
such a factory over a short term, say, half-an-hour to an hour only
small thermal variations will be experienced during the test. However,
the tool or gage will be used over much longer periods of time, and then
it will be exposed to greater thermal changes. It is rare to test a gage
over several hours, or while temperatures change a few degrees, but if
the gage is going to be used under such conditions, it should be tested
under such conditions.
The gage referred to above
was evaluated in this way and its performance was improved as a result
of understanding its thermal behavior. In the test one hundred parts
were run through the gage three times. The conditions were changed for
each run, while data were recorded.

First, the short-term
capability of the gage was assessed. The one hundred parts were sent
through a conveyor that fed the automatically controlled gage. The gage
controller is equipped with computerized data collection and SPC
software. It calculates statistics and displays them in real time. It is
intended for use on a production line immediately following a grinding
operation to produce outside diameters to micron tolerances.
While
attempts were made to control ambient temperatures during this run,
there was nevertheless a typically unavoidable morning ambient increase
of 0.6° C during the approximately 50 minutes that it took to measure
the parts. Fig. 2 plots each measurement taken during the run
independently and shows a trend line that indicates a shallow
deterioration of approximately 2 microns in mean value since the
temperature rose slightly during the course of the data gathering. Since
the thermal change was not large, Fig. 3 shows a histogram with an
apparently very acceptable distribution.
Now take a close look at
the results of the second run, during Now take a close look at the
results of the second run, during which ambient temperature rose by just
5° C, such as might easily be experienced in a morning of production.
Fig. 4 shows a trend line in which mean diameter varies by 8 microns.
Note that the mean value decreases. Of course, the reverse might occur
during an evening or night shift as temperatures dropped. Since in this
case the dimensions are apparently decreasing, this indicates that the
gage or gage tooling is expanding at a higher rate than the workpiece.
As further studies determined, the gage was in fact expanding at a very
high rate, which is not uncommon. Gages and gage fixtures are typically
complex mechanical and electrical assemblies that for several reasons
are heavily affected by thermal changes. For example, steel expands at
an approximate rate of 6.8 parts per million per degree F, and aluminum
at a rate of 13 parts per million per degree F. By comparison, this gage
expands and contracts at the rate of 33.3 parts per million per degree
F. Since the gage fixture expands as temperature increases, the meauring
probes must extend further so as to contact the workpiece, thus giving
the impression that the part is smaller.
In a practical
application this could lead to acceptance of oversized parts. In a
system in which machine compensation (automatic feedback to a machining
operation) was being used, the gage would actually instruct the machine
tool to make oversized parts. On most shop floors, temperatures vary
continuously. As temperatures change, so will measured values of
critical dimensions. Fig. 5 shows the effect that the 5° C drift had on
the distribution of measured diameters during the second run. Clearly,
the wider distribution of measured data points led to a considerable
reduction in calculated capability (Cp) and the trend in the direction
of decreasing mean size caused a major deterioration of Cpk value
(capability centered around nominal size).
For the third and final
run, a GageComp temperature compensation system from Albion Devices,
Inc., of Solana Beach, CA (www.albiondevices.com) was added to the
gaging system. GageComp’s custom designed industrial sensors monitored
temperatures of workpieces, the setting master and the gage as parts
were measured. GageComp then calculated a correction based on
predetermined correction coefficients, and electronically sent this
correction in real time to the gage. (See Fig. 8). The computerized gage
mixed the correction from GageComp with the measured dimension to
produce a temperature corrected measurement of each part. Effectively,
the gage now displayed the dimension that would have been obtained if
each measurement had been taken while parts, master and gage were held
at the International Reference Temperature of 20° C (68° F).
Compare Figs. 3, 5 and 7 (runs 1, 2 and 3
respectively), and you will see that original gage capability (run
number 1, Fig 3) is severely compromised when temperature variations
influence measurements (Fig 5). By applying temperature compensation
while temperatures changed in run number 3 (Fig .7), original gage
capability (run number 1) is restored. In fact, it is even improved.
As Fig. 6 shows, the
mean dimensional measurement remains flat when temperature compensation
is applied. After smoothing out inherent gage R & R variation, average
measurements were virtually unaffected by the change in ambient
temperature. Further, as was seen in Table A and Fig. 1, Fig. 7 shows an
improvement in both Cp and Cpk over run number 2 (in which ambient
temperature increased by 5° C) and even run number 1, in which
temperatures increased by just 0.6° C. This last observation
demonstrates that process improvement can be obtained from using
temperature compensation even in environments in which temperatures are
held stable to within a degree or two, which is the best that can be
held in large plant areas.

Electronic temperature
compensation systems are in wide use in a variety of applications. They
can interface to just about any electronic gage, but it is best to think
of them as a separate, distinct system from that of the mechanical
gaging assembly. When discussing this subject with gage suppliers it is
helpful to keep this in mind and to differentiate responsibilities
between vendors of the different system components. It can be useful to
establish criteria for performance of the gage first at stable
temperature, then secondly under typical shop floor thermal variations.
By separating these requirements, specific responsibilities can be
established for performance under varying conditions.
Temperature compensation
requires unique experience and expertise and is probably best left to
those who specialize in the subject. They can then be held accountable
for achievement of reasonable thermal correction specifications.
A
relatively small investment of time and money in temperature
compensation can pay huge dividends. Increased process capability (see
Fig. 1) is well known to yield cost savings, and more importantly, to
provide a competitive edge. Mass producers of discrete parts are
invariably under pressure to produce. Suppliers of production and
inspection equipment feel the same urgencies. It is understandable that
they may want to avoid the seemingly time consuming task of a long-term
study. However, as the above results clearly reveal, a short-term study
of capability is not necessarily a true test. Customers who rely on such
studies to certify the quality of a supplier’s process should beware.
Similarly, Quality Managers and Manufacturing/Process Engineers
everywhere should take note of this opportunity to improve process
capability and thereby gain significant advantage in the marketplace and
on the bottom line.
Top
Albion's Temperature Compensation Methodology
Albion Devices, Inc., develops and manufactures
temperature compensation systems which correct gages and positioning
systems for thermally induced variations such as these.
All of Albion’s compensation
systems use the same basic methodology in their applications. They take
into account temperature variations and customized, individual
coefficients of expansion for each "element" of a measurement system,
namely part (workpiece), master and gage.
Tests conducted by Ford Motor Company early in the
1990s, at their Lima Engine Plant in Ohio, showed that critical
dimensional measurements on crank shaft pins were significantly affected
by variations in temperature in the natural factory environment.
A part was repeatedly gaged some
sixty times, at the same station, over a period of approximately two
weeks (Fig. 1). The gage was known to be highly reliable, and repeatable
to tenths of a micron. The only significant variable was environmental
temperature, which turned out to be responsible for a range of 13
microns of dimensional variation in the group of measurements.

Albion conducted tests
to prove that it could correct for errors such as these. For the
purposes of the tests conducted in order to generate the data displayed
below, Albion’s GageComp-S Single Channel Temperature Compensation
System was attached to an automotive piston gage.
The gage was mastered while ambient temperature
was at 19° C (66° F). Measurements of the same 99mm piston were then
taken repeatedly while the piston and gage slowly changed temperature
over a range of approximately 8° C (14° F), from 18° C to 26° C (64° F
to 79° F) as the environmental temperature changed (Fig. 2). As can be
seen, while the piston and gage each changed temperature at roughly the
same rate, and by about the same amount, significant uncompensated
measurement variation of some 14mm occurred due to the differences in
coefficient of expansion of gage (36.7 parts per million per ° C) and
part (18 parts per million per ° C), while the electronically generated
compensated dimension showed only a small range (about .5 micron) of
variation.
It is not unusual for gages to have a surprisingly
high coefficient of expansion. A number of factors combine to cause this
effect, including mechanical design considerations, mixtures of
materials used in the construction of the fixture and the effects of
temperature on some electronic transducers.
The object of Albion’s Temperature Compensation
systems is to correct gage dimensions so that they read as if
measurements were made with part, master and gage at the International
Reference temperature of 68° F/20° C, as stipulated by ISO 1. This test
demonstrates the ability of Albion's systems to correct for well over
90% of thermal error. The remaining uncertainty is due to influences
such as gage R & R, accuracy of temperature readings and thermal
gradients.
Albion’s temperature
compensation instruments and approach have been used successfully in a
variety of applications. They include in-process dimensional control,
post process measurements and hand held, bench top and automatic gaging
in a wide variety of industries.
Top
The Thermal Error Index (TEI)
A
conference, held in Seattle, WA a few years ago now and sponsored by
ASPE, focused on, among other things, issues relating to the effects of
temperature on manufacturing processes. There is clearly an increasing
awareness of this phenomenon, which is becoming more important as
machined part accuracies become more critical. Mr. Kenneth Blaedel, of
the Lawrence Livermore National Laboratory, delivered a particularly
relevant half day tutorial on thermal effects in precision engineering.
He referred to a useful formula for calculating the Thermal Error Index
contained in ANSI B89.6.2 and referenced again in ANSI B89.1.12M-1990.
The
Thermal Error Index (TEI) is intended as an estimation of the maximum
possible measurement error due to all thermal effects. It was originally
conceived as a tool to assure that temperature control is adequate for
the calibration of measuring equipment, as well as the manufacture and
acceptance of workpieces. It recognizes that if measurements are not
made with all elements of the gaging system being at 68°F (20°C),
temperature-induced measurement errors will occur. It also acknowledges
that there are uncertainties in estimating coefficients of expansion,
and in accurately measuring temperatures.
The
formula addresses Nominal Differential Expansion (NDE) and the
Uncertainty of Nominal Differential Expansion (UNDE). NDE represents the
net variation in dimension which can be anticipated as a result of
differences in the assumed values of the coefficients of expansion in
part, master and gage. UNDE represents the difference between the
handbook values and the true values of the coefficients of expansion.
The difference is often approximated at 10% of the handbook value.
Temperature Variation Error (TVE) is also considered. TVE is defined by
ANSI as an estimate of the maximum possible measurement error induced
solely by deviation of the environment from average conditions. TVE is
determined from the results of two drift tests, one of the master and
the comparator and the other of the part and the comparator. The TEI
formula, which results in an answer expressed as a percentage of the
total tolerance, or Working Tolerance (WT), is as follows:
TEI = [(NDE+UNDE+TVE)/WT]
x 100
A
calibration, part manufacture, or acceptance procedure complies with
this standard if it is carried out with all components of the
measurement system at 68°F/20°C, or if it can be shown that the TEI is a
reasonable and acceptable percentage of the working tolerance. In
applying TEI, for example, to the acceptance testing of a coordinate
measuring machine, ANSI declares that the measuring environment is
unacceptable if the TEI is greater than 50%. ISO is currently
considering the concept of TEI for adoption into their own standards.
Example:
Consider
measuring 3 inch diameters of aluminum parts to a tolerance of ± .0002
inches using a steel master and a gage comprising both steel and
aluminum. The nominal coefficient of expansion (COE) is about 6.5 ppm
(parts per million) per degree F for steel, about 13.1 ppm/°F for
aluminum and the effective COE of the comparator (gage) is 10 ppm/°F.
Temperatures in the environment can vary by as much as 20°F, but are
measured to 1°F, (so they can actually vary by as much as 21°F). Assume
that the comparator is mastered immediately before each measurement and
therefore the temperature of mastering and measuring will be close.
Case #1: If no
compensation is applied to correct for NDE, then
TEI = [(.000416 + .000116 +
.000015)/.0004] x 100 = 137% where,
NDE = (13.1 - 6.5) ppm/°F
x 21°F x 3 inch = .000416 inch representing the contribution from the
difference between the nominal COEs
of the steel master and the aluminum part
UNDE = 1.85 ppm/°F x
21°F x 3 inch = .000116 resulting from the sum of the uncertainty of the
COE of the aluminum part, estimated here at about 1.3 ppm/°F (i.e. 10%
of the nominal COE) and the uncertainty of the COE for gage block steel,
estimated from a number of studies to be about .55 ppm/°F,
TVE = .000015 inch as obtained from a
drift check over the course of perhaps 1 minute,
WT = .0004 inch
Without compensation for
NDE, this environment is clearly unacceptable.
Case #2: If compensation
is applied to correct for NDE, then
TEI = [(.00002 + .000116 +
.000015)/.0004] x 100
= 38% where,
NDE = (13.1 - 6.5) ppm/°F
x 1°F x 3 inch = .00002 inch because correction for NDE can only be made
in this case to the nearest 1°F,
UNDE = 1.85 ppm/°F x
21°F x 3 inch = .000116 inch, the same as above
TVE = .000015 inch as obtained above, and
WT = .0004 inch
With compensation for
NDE, and using the guideline that a TEI of less than 50% defines an
acceptable environment, this environment is acceptable to make the
measurement.
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Temperature compensation of in-process gages
during grinding operations.
Measurements: Automatic gages are used to
control dimensions in grinding processes. The gage is positioned so that
its contacts can measure dimensions of ground work pieces while they are
being machined. Feedback is provided to the grinding machine controller
so as to regulate the depth of cut. Errors may be caused by temperature
changes induced by machining energy, ambient fluctuations and changes in
coolant temperature that can cause dimensions and settings of work
pieces, gages and setting masters to vary.
Temperature Compensation
Approach: Albion’s GageComp Temperature Compensation System uses a
proprietary "Diamond Series" sensor to monitor temperatures of workpiece
and master and another sensor to track the temperature of the gage
fixture.

GageComp applies
separate thermal coefficients for each of these three "elements" and
computes and transmits a correction via analog signal to the process
control gage, where correction amounts are mixed with gaged dimensions
to give resulting net compensated measurements for the measured
diameter.
Gage
Modification: Gage heads are engineered to incorporate Albion’s DS-1
workpiece/master temperature sensor. The sensors are positioned so that
they come in contact with the machined surface of the work piece, or as
close thereto as possible. These sensors are coolant resistant, durable
and designed with the tough environmental considerations of this
application specifically in mind. A LG-1 gage temperature sensor is also
mounted on the gage head.
Characterization: To
determine effective coefficients of expansion (COEs), empirical testing
is performed on masters, sample workpieces and the gage head (the
"elements" of the measurement system). The uncompensated gage is used to
make measurements for these tests with compensation turned off, so that
true changes in dimension can be noted. Coefficients are then calculated
and compensation is then turned on, to verify the results of the thermal
correction applied.
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