Screw Thread Insert (STI) Gage Tolerances

I was recently asked: The ThreadTech Software program says that the pitch diameter tolerance for a #10-32 UNF-3B STI Screw Thread Plug Gage is 0.0001" but the STI Screw Thread Plug Gages we own were certified at 0.0003". Why is this?

This is an interesting question because it brings to light a disparity between the STANDARDS and the REAL (screw thread gage) WORLD.

The STI screw thread plug gage tolerance was established quite some time back by Emhart Teknologies for their Helicoil screw thread insert product line. They decided to use Class W for the STI screw thread plug gages, but keep the gage information proprietary so that they could be the sole source for STI screw thread plug gages. The thing about the STI screw thread plug gages is that they are not all that unique, and any screw thread gage maker could make them if he knew the screw thread pitch diameters.

So what happened was that as screw thread gage makers discovered the screw thread pitch diameters for the screw thread insert requirements they began making the STI screw thread plug gages using the Class X tolerance. They did this because that is what ALL screw thread working plug gages use as a tolerance. Eventually the proprietary Helicoil screw thread insert requirements became known and published. The official requirement for the STI screw thread plug gages was set at Class W. In conflict to the requirements of the standard the screw thread gage makers continued to make the STI screw thread plug gages using Class X tolerance. I can only guess at the thought process. Maybe the the screw thread gage makers realized that they had been making the STI screw thread plug gages as Class X for years and no one complained. They saw that the STI screw thread plug gages seemed to work just fine in application. Alternately the screw thread gage makers may have considered the cost of making the STI screw thread plug gages with the extra tight tolerance and rejected the concept immediately. I know of only one screw thread gage maker who made the gages to the standard, but to remain competitive in the STI screw thread plug gage marketplace they recently discontinued the practice. Today there is no STI screw thread plug gage maker that I know of that is making STI screw thread plug gages to the Class W tolerance as standard product. If you are willing to pay the extra to get the Class W tolerance, the STI screw thread plug gages are available as a special make item.

This has been going on for many years and it appears that there is no significant issue that has resulted from the change from Class W to Class X. The primary result of the change is lower cost STI screw thread plug gages. A possible secondary benefit is that, depending on where the STI screw thread gage falls in the Class X tolerance spectrum, it may have longer life before reaching the point of being worn-out. I do not expect a change in the standard anytime soon. These changes take years to incorporate into the official standards and usually require some compelling pressure to make them happen. I do not see anyone applying that pressure to make this change official, so it will really take a long time of occur.

There is only one small dilemma; this question may arise from time to time when someone takes the time to actually read and understand the standard, or when they use the ThreadTech and notice the discrepancy. Know that the ThreadTech software will not change until the standard changes, because the ThreadTech software must report the thread data as specified by the standard.

Initial publication date: 7/15/09
This data is accurate to the best of my knowledge as of 9/2/09.
Send comments or corrections to Wayne at:gageguy@gagecrib.com