How to determine the proper gagemaker’s tolerance for a GO/NOT GO cylindrical gaging application

One of our most frequently asked question is "Which gagemaker’s class of tolerance should be used for selecting plain plug and ring gages to check internal and external diameters of parts?"

The industry standard is referred to as the Ten Percent Rule. This common rule of practice requires that 10% of the product tolerance is divided between the GO and NOT GO gauges. For plug gages a plus tolerance is applied to the GO member and a minus tolerance is applied to the NOT GO member. Ring Gauges receive the reverse tolerance direction so that the GO member is minus and the NOT GO member is plus.

The 10 Percent Rule results in gauge tolerance always being included in the part tolerance by up to 10%. This rule results in the possibility that 10% of good product may fail inspection but that no bad product would ever pass inspection.

Example 1

A part has a hole diameter requirement of 0.500" +/- .001". Thus the hole diameter is 0.4990" - 0.5010". The product has a tolerance of 0.002". 10% of the product tolerance is 0.0002", which is divided between the GO and NOT GO gages. Divide 0.0002" by 2 = 0.0001". This is your gage tolerance. On the industry standard gagemaker’s tolerance chart, 0.0001" is classified as a class Z tolerance. The GO member would be specified as 0.4990" plus 0.0001" tolerance (or 0.4990-Z) and the NOT GO would be specified as 0.5010"minus 0.0001" (or 0.5010-Z). The gage would be assembled and marked as 0.4990"- 0.5010" Class Z GO/NOT GO Plug Gage w/handle.

Example 2



A part has a hole diameter requirement of 0.500" +/- 0.0009". Thus the hole diameter is 0.4991" - 0.5009". The product has a tolerance of 0.0018". 10% of the product tolerance is 0.00018", which is divided between the GO and NOT GO gages. Divide 0.00018" by 2 = 0.00009". This is your gage tolerance. On the industry standard gagemaker’s tolerance chart, 0.00009" is less than class Z tolerance (0.0001), but greater than class Y tolerance (0.00007). In order to stay within the Ten Percent Rule, class Y would be chosen. The Go member would be specified as 0.4991" plus 0.00007" tolerance (or 0.4991-Y) and the Not Go would be specified as 0.5009" minus 0.00007 (or 0.5009-Z). The gage would be assembled and marked as 0.4991"- .5009" Class Y GO/NOT GO Plug Gage w/handle.

Selecting higher precision gagemaker’s tolerances for cylindrical plug and ring gages such as class X or XX will consume less product tolerance and will allow the acceptance of slightly more product but with less gage wear life and at greater expense.