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148 Part III: Inspection and Test
Part III.A.3
EXAMPLE 14.1
Imagine we’re trying to assemble a 5" long plug into a 4" deep bore. If the diameter of
the bore was 2.0000" and the diameter of the plug was 1.9999", the plug would have to
be straight within (or probably below) 0.0001" to ensure that it would fit and function.
On the other hand, if your plug diameter was 1.9995" it could have a straightness deviation
of almost 0.0005" and still fit. This is a form example as opposed to location but the
concept is very similar.
In our example, we’ll use an inside feature. Our drawing states that we have a diameter
tolerance of 1.5000" + 0.0005" so our MMC would be 1.5000". In addition, we have
a positional location tolerance of 0.001" applied at MMC.
The intent of positional location when applied at LMC or MMC is to ensure an
acceptable amount of clearance between the part we’re making and its mating part. If
the size of our inside feature is at MMC (smallest diameter) we need to be within 0.001"
location for the mating part to fit our part properly. The larger we make our inside diameter,
the more “room” we have for the location to be outside of the 0.001" location
tolerance zone with the clearance remaining acceptable. Whatever amount above MMC
(above our minimum) our diameter becomes is applied as a bonus to the tolerance of
the positional location. If the diameter of our feature is 1.5005", the five ten thousands
of an inch we are above MMC would be applied to the location tolerance, which then
becomes 0.0015" (Clark 2008).
If we look at the diameter tolerance and the location tolerance as two sides of a balanced
equation, whatever we add to one side we also add to the other side to keep the
two sides in balance:
1.5000" (diameter at MMC) = 0.001" (location).
If we were to add tolerance, then
1.5000" + 0.0005" (diameter) = 0.001" + 0.0005" (location)
1.5005" (diameter) = 0.0015" (location)
1.5000" + 0.0005" .001 M A B C
C
2.000"
B
4.000"
A
Figure 14.12 Form tolerance xample.