Thread Gaging Notes

  • Thread gaging is one method of testing the acceptability of machine screws, SEMS, nuts and threaded holes in general. It is possible that parts which are near a limit could be accepted by one gaging method and rejected by another method. A screw thread is considered acceptable when it passes any of the allowed gages in ANSI B1.3 provided that the gages used are within the tolerances specified by this standard.
  • Many commercial applications will accept a screw that may not pass a gage test but will accept a mating nut. It is important to know a users requirements concerning thread quality when testing machine screw threads,
  • "Go" gages check either the maximum material limit, or size, to assure interchangeability in the assembly of maximum material mating parts.
  • "No Go" gages inspect the functional diameter limit of item internal threads. In applying the "No Go" gage, the functional diameter is acceptable when the gage does not pass the inspected threads. I some cases the "No Go" gage can indicate out-of-roundness of the pitch cyclinder.
  • Thread gages are normally set to master gages with a range of calibration precision such as Class X (looser tolerance) or Class W (tighter tolerance). They are also set by technitions with slight variations in hand pressure. For those reasons, gage setting setting variables should be considered during thread inspection. The "No Go" gage can also be confused by undersized, erron laden threads which actually undersize on pitch diameter but with helix, flank or related out of tolerance conditions large enough to not allow the gage to enter the required number of turns. One example
  • For the above reasons, thread inspection should take into consideration gage and operator variables when passing quality judgements on machine screw thread quality.

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