… [O]ur facility makes a number of cylindrical pins and shafts with I.D. Threads. After we tap the threads many parts will be induction heat treated or welded in an assembly. A number of times we will shrink the threads with the heat being applied and the threads will not gage. This will result in a rework op to retap and gage. Then we run into the issue of the NOGO going after being retapped. Does anyone have a better solution to this. … It seems like we have the worst luck with 1/8-27 NPSF threads. … [T]he threads at weld are caused by adjacent heating of the process.
Heat Treat normally requires a certain depth to be hard, so no matter what the threads will be affected because even if you’re not intending to go as deep as the threads the heat will have to affect the threads in order to meet the depth spec.
… You could try using a tap with a higher "H" value which will give a slightly larger thread and thus be more forgiving after heating.
Another thought is to use a thread forming tap for any retap operation. These will likely last longer since they are not "cutting" material, rather they will "Move" material so should be less likely to gage large after retap.
As is normal, I have several thoughts for you to consider.
1.
If the problem is being caused by weld spatter getting into the threads, then re-tapping is the best way to go, but use a smaller H-size for the re-tap so that you only knock off the spatter and not cut the thread larger. A second thought would to be to use worn-out taps from your manufacturing line. They have been worn smaller and might have the same effect as a smaller H-size. For the reader who does not know: Each H-size = 0.0005” increase in size with H0 being the basic screw thread pitch diameter.
2.
I notice that you are having problem with NPSF thread. This could be a serious problem as this thread is intended to have very specific dimensions held so that the connection made is a dry-seal connection. The dry-seal connection is specified when the chemical passing through the piping will eat away the seal material and then leak. The chemical passing through the piping is usually highly flammable or highly caustic, neither of which you want leaking. Being oversized could compromise the seal, and open up your company for a product liability law suit.
3.
It sounds like you are dealing with a thin-ish wall after tapping. I think JRKH is right; the material is being distorted not shrunk. It sounds like the process of tapping is forcing the tube back to round, while at the same time cutting some material out of the thread, making the final screw thread oversized. I really like JRKH’s idea of using a forming tap. This may solve your problem.
4.
I have some customers who, to compensate for heat treat distortion, cut the initial thread oversized. This again is the same as JRKH’s first suggestion. They use an oversized BHT (Before Heat Treat) GO/NOGO screw thread gages. This reduces the final rework, but the thread is really oversized and weaker. I would not advise this with an NPSF thread because it will not seal; but for a ¼”-20 UNC it might be an option.
5.
If weld spatter is not an issue, and you are only dealing with distortion, you may rewrite your acceptance requirements to require acceptance by standard GO/NOGO gages only pre-heat treatment. If you want to set a low limit to the amount of distortion, an undersized AHT (After Heat Treat) GO screw plug gage could be used to test the limit of the distortion. The final product will have distorted threads, but being that they are in a thin-ish walled part the thread will reform to round as the bolt is inserted. The NPSF is designed to have a tapered NPTF male thread inserted, so getting the male thread into the hole may be easy. The UN-series threads, if class 2B, are designed to assemble with 2A, which has an allowance (about -0.002”). The smaller size might allow it to enter the warped internal screw thread. This is something that should be tested in your facility before implementing.
I do hope you find some tidbit in the above pile of thoughts that will help solve your problem.
