Working thread plug gages are the fixed limit functional gages used to inspect product threads on a pass/fail basis. If the go fits and the no-go does not, your parts are in tolerance. The function of the gages is straightforward. However, how do we know our thread plug gages are in tolerance? We inspect, measure or calibrate them. These three terms can mean different things to different people. For our purposes we will use “inspect” to mean visual inspection for damage and foreign material. “Measure” will be an actual measurement of a specific feature. “Calibration” incorporates the other two and includes the recorded measurements taken in a controlled environment with stated uncertainties in an accredited lab.
Measuring Pitch Diameter by the Three Wire Method.