The use of the go/no-go fixed limit gage is considered one of the simplest forms of pass/fail attribute inspection. If the go fits and the no-go does not, the product is considered in tolerance. When inspecting a cylindrical hole, if the go does not enter, the hole is too small (undersize). If the no-go enters, the hole is considered too large (oversize). The questions and problems do not begin to arise until the size of the hole drifts closer to either the small side of the tolerance or the large side of the tolerance. This is when the discussion of the selected gage diameters, tolerances and gaging force come into play.
Go and no-go gage dimensions and tolerances are derived based on the product dimensions and tolerances. The general rule of thumb is that the go plug gage diameter (plus tolerance) would represent the smallest diameter of the product tolerance for the hole. The no-go plug gage (minus tolerance) would represent the maximum diameter of the product tolerance for the hole. The sum of the go and no-go gage tolerances should not equal more than 10% of the total product tolerance (the 10% Rule). The go gage is fairly straightforward. The go must pass freely through the entire length of the hole. The fun starts when the no-go is used. The amount of force required for the gage to enter the hole becomes a point of discussion. This is due to a certain level of subjectivity. The questions asked are: How far is the no-go allowed to enter? How much force should be applied? Should the tolerance on the no-go gage be plus?