The industry standard is referred to as the ten percent rule. This practice requires that 10% of the product tolerance is divided between the GO AND NOGO gauges. For plug gauges we use a plus tolerance on the GO member; and a minus tolerance is applied to the NOGO member. Ring gauges receive a reverse tolerance; GO member is minus and NOGO is plus tolerance.
This 10% rule results in gauge tolerance always being included in the part tolerance by up to 10%. This rule results in the possibility that up to 10% of good product is rejected; but no bad product would pass inspection.
A part with a hole diameter requirement of 1.000” +/- 0.001”. Therefore hole diameter range is 0.999” to 1.001”. The product has a combined tolerance of 0.002”; 10% of this combined product tolerance is 0.0002”. Divide 0.0002” by 2 = 0.0001”.
ANSI/ASME B89.1.5 provides a tolerance chart to define the recommended class tolerance. In the above case 0.0001” is classified as a Z tolerance. The GO member would be specified as 0.999” plus 0.0001” tolerance and the NOGO would be specified as 1.001” minus 0.0001”. The gauge would be marked 0.9990” ” 1.0010” class Z GO/NOGO plug gauge.
Selecting a higher gagemaker tolerance of X or XX will consume less product tolerance and allow the acceptance of slightly more product but with less gauge wear and shorter life expectance on the gauge.