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Chapter 9
C. Gage Selection, Handling,
and Use
Part II.C.1
1. FACTORS IN GAGE SELECTION
Select gages according to the feature
or characteristic to be measured, the
applicable tolerance, the 10:1 rule, etc.; the
accuracy, resolution, and capability of the
test instrument, and determine whether
the type of measurement should be direct,
differential, or transfer. (Application)
Body of Knowledge II.C.1
There are many factors to consider in the selection of a measuring or gauging
instrument or system for a particular manufacturing inspection operation.
The Rule of Ten
The rule of ten—sometimes referred to as the 10:1 rule or the gage maker’s rule—
serves as a baseline or beginning of the selection process. It states that inspection
measurements should be better than the tolerance of a dimension by a factor of
10, and calibration standards should be better than inspection measurements by
a factor of 10.
For example, assuming that the tolerance on a shaft diameter is ±0.025 mm
(±.0010 in.), the increment of measurement on the inspection instrument should
be as small as 0.025/10 = 0.0025 mm (.00010 in.).
Similarly, the increment of measurement of the calibration standard for the
inspection instrument should be as small as 0.0025/10 = 0.00025 mm (.00010 in.).
Once the smallest increment of measurement for an instrument has been determined,
then candidate instruments need to be evaluated in terms of the degree of
satisfaction they offer relative to the following performance criteria:
1. Accuracy. The ability to measure the true magnitude of a dimension.
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