Pitch diameter and thread gauges

The pitch diameter is the notional diameter at which the thread flanks of bolt and nut bear on each other; it lies between the major and minor diameters. It is the most important dimension of a thread: it determines whether bolt and nut fit together, how much clearance they have and how the load is distributed across the flanks. The tolerance class, for example 6g, relates mainly to it. Major diameter and pitch can be roughly checked with a calliper, but the pitch diameter requires gauges or measuring instruments.

Thread gauges

• GO ring gauge: Must screw on easily over the full thread length; checks that the thread is not too large.
• NOT GO ring gauge: Must not screw on by more than two turns; checks that the thread is not too small.
• Thread plug gauge: The same principle for internal threads, with a GO end and a NOT GO end.
• Thread micrometer and three-wire method: Measure the pitch diameter as an actual value, required for inspection reports and for tight tolerances.
• Profile projector: Shows the magnified thread profile, for small threads below 10 mm.

A thread is within gauge when the GO gauge and the NOT GO gauge both give the correct result. All gauges at RS Randack are calibrated and traceable; inch and special threads have their own gauge sets.

How RS Randack tests

When the thread rolling machine is set up, the first parts are checked with a ring gauge or the matching nut and with the thread micrometer, then at fixed sampling intervals; more frequently for fine threads and close tolerance classes. For large threads, the pitch diameter is documented and can be included in the inspection report on request. Parts that are to be coated are checked by RS Randack before and after coating so that the gauge still fits once the coating has been applied.

What you should specify

Tolerance class, whether a dimensional report on the pitch diameter is required, and whether the thread must be within gauge after coating.