Matching internal threads

[cdhknives]

I am machining a plug for a tube. Yes it is a suppressor and yes I have the approved ATF Form 1 in hand. I want to maximize the wall thickness under the tube threads so I cut the tube's internal threads JUST until I got sharp peaks, and now I want to cut the matching male threads. Yes I know there are calcs I could run and probably get just as close, but I was too eager to get started...another story. I'll probably do the second one different...

The plug/endcaps will are 17-4 annealed rod in a steady rest being turned by the 4 jaw. I need near perfect concentricity for the center bore, so I need to thread the OD and bore in one setup if possible, or else I'd turn between centers for the outside threads. Done that a few times but not the best option here I think. I have the rod centered to about .001" in the 4 jaw and it turns easy enough, I have it faced off, and now need to bore, turn down, and thread.

Anyway, my plan was to measure the tube nominal ID, calc threads for that dimension @ the root, and go up by a few thou. Cut the plug to those dims and start cutting threads...with a lot of test fits once the threads are about done. By sneaking up on it I should be okay...right? Yes it will be a lot of test fitting. I have more time than experience. Am I missing anything?

 

[AJB]

Sounds like a good plan. An alternate to consider would be thread the end caps and fit them to the tube. Then screw the end caps onto the tube, chuck the tube up and center it, and then bore the hole in the end caps. That should ensure that the end caps are concentric with the exterior of the tube. This method should work if you have uniform tube wall thickness and tube straightness.