Why Temperature Compensated gaging is a good investment
Temperature compensated gaging offer’s greater accuracy overall in
the production of precision components. If manufacturers want to
produce consistent and repeatable precision measurements then
temperature needs to be considered. Otherwise measurements that
one day appear to be correct are not correct another day, when
temperatures are different. Temperature compensated gaging
corrects all measurements to the international standard reference
temperature of 20 degrees C. If manufacturers are serious about
quality then temperature must be taken into account. Temperature
compensated gages may be more expensive than conventional gages
because they are more complex, but the benefit is that they
improve repeatability, reproducibility and consistent overall
accuracy in production processes.
As a result, users of temperature compensated gaging save costs
of rework and scrap, and they achieve greater process control by
improving Cps and Cpks. Users of temperature compensated gaging
experience cost savings that pay for their investment in just a
few weeks, and they demonstrate equipment process improvements in
the order of 100% or better. A typical user experienced the
following results:
Note that the results on the left were obtained without using
temperature compensation, while the results on the right used
temperature compensation. The temperature compensated results show
Cpk improving dramatically in both tests. Tighter statistical
grouping of results means that fewer bad parts are produced,
process becomes more efficient, fewer parts are scrapped or
reworked, costs are reduced and higher quality parts are made, so
customer satisfaction is improved.
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Determining the need for Temperature Compensation
Precision manufacturing increasingly calls for tight tolerances.
Through call-out or implied reference to ISO 1, dimensions are
usually specified at 68°F
/ 20°C
(ISO 1 - 1975: “The standard reference temperature for industrial
length measurements is fixed at 20°C”
). A few degrees departure from 68°F
(20°C),
by workpiece, master or gage, can cause certain critical
dimensions to change size by an undesirable percentage of
tolerance. It can be important to identify in advance those
critical dimensions that, when measured on the shop floor, may be
subject to significant thermal variation, so that steps to avoid
such errors may be taken.
The following guidelines suggest a method for determining when to
use temperature compensation so as to avoid thermal errors.
1) Review “Tolerance Ratios” – the ratio of total tolerance to
nominal dimension. The amount of thermal expansion that a
workpiece will experience for a given change in temperature is
directly related to the overall dimension of the feature to be
measured and the coefficient of expansion of the object’s
material. The greater the thermal expansion in relation to the
allowable tolerance, the more likely it is that temperature
compensation may be required. For example, if total thermal
expansion over the given temperature range represented only 1% of
total tolerance, it is unlikely that temperature compensation
could be justified. However, if thermal expansion amounted to 30%
or more of total tolerance a good case could be made for
eliminating thermally induced measurement errors through the use
of temperature compensation.
By way of illustration consider the following: A 4 inch (100mm)
diameter aluminum piston or a 4 inch (100mm) bore in an aluminum
housing, each with a total tolerance of .0006 inch (0.016mm) might
be produced in a plant that experienced year round temperature
fluctuations of 20°F
(11°C).
This temperature range could cause measurement of the part to vary
by .0010 inch (0.026mm), or 162% of tolerance, which would suggest
a strong case for using temperature compensation.
2) An approach taken by a major automotive company is as follows:
a) Estimate
the year-round range of temperatures that will be experienced
while measuring the workpiece at its shop floor measuring station.
b) Apply
that thermal range and an approximate coefficient of expansion to
the dimension to be measured.
c) Express
the result as a percentage of the allowable tolerance and
determine if it is acceptable to ignore this variation.
d) If
that variation exceeds a certain limit (say, 10% for the purposes
of this example) specify temperature compensation for the
respective gage.
Although there are several temperature compensation methods in use
today, it is well to be warned that some have severe limitations.
Systems that tend to work the best independently monitor the
temperature of the
master, workpiece, and gage
fixture in real time. They typically use light-touch, durable
contact sensors, and are more accurate and repeatable than
non-contacting alternatives.
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