Centralizing Acme Screw Thread Ring Gage
The thread form for Centralizing Acme screw threads is the same as General Purpose Acme Threads with the exception that the Centralizing Acme Screw Threads have a limited clearance at the major diameter of both the internal and external screw threads, and are used extensively for feed screws. The restricted clearance enables a bearing at the major diameter of the thread to preserve alignment of the thread axis preventing wedging on the flanks and part sagging.
Five classes-of-fit are used with the Centralizing Acme threads: 2C, 3C, 4C, 5C, and 6C. For any combination of the five classes of threads covered within the standard (ANSI B1.5) some end-play or backlash will result. Although 5C and 6C are intended to allow for restricted backlash they are not recommended for new designs.