Snap gauge is GO and NOGO type gauge. That means snap gauge consist 2 fixed measured distance or gap, one is known as GO and another one is known as NOGO.
GO means your job or workpiece is OK and NOGO means your workpiece is rejected.
In snap gauge, value of GO is always greater than NOGO.
Let set an example,
we have a snap gauge of size
12.500 +/_ 0.050 mm
GO size = 12.550 mm
NOGO size = 12.450 mm
and it is measuring criteria for a work piece.
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According to snap gauge, 25.55 mm size is acceptable for a workpiece. When we push snap gauge towards the workpiece the only area of GO cover i.e. workpiece is accurate.
If Snap gauge able to move till NOGO area that means workpiece is undersized.
Snap gauge is usually checked for outer surface, which can easily be observed favorable or not.