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Six step Gages
The NPTF & ANPT system begins with the use of the L-1 gage having a 1 turn in or out limit (2 turns total) the same as the NPT system. The purpose of this gage is to measure the pitch diameter size of the part. It is necessary to refine the standoff (distance from the step to the part) to a more accurate measurement because the 2-turn total tolerance limit must be divided into 3 equal zones. These three zones are known as the minimum, basic, and maximum zones, and the part is referred to as a minimum part, basic part, or maximum part.
The basic zone is the linear area within 1/3 turn from the face of the part, in or out. If the plug gage is standing out from the face (or datum point) of the internal part more than 1/3 of a turn you have a minimum part because the step on the gage stops within the minimum zone. Likewise, if the plug stands in more than 1/3 of a turn you have a maximum part. The inverse applies to external threads. Knowing the type of part, you can pick the two appropriate steps of the six step gage. B/Bt for Basic, Mn/Mnt for Minimum, Mx/Mxt for Maximum. The face of the part should be between the two steps.
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