I'm currently designing a new machine in the circuit board assembly industry. I've been asked to reuse a particular tool design from an obsolete product that contains a special collet. Unfortunately, I have absolutely no information, documentation, or even someone who knows much about this, so I need to reverse engineer it, which isn't terribly difficult. However, it has an odd brass insert in the nut that has us perplexed.
The nut has 2 opposing tapped holes through the sides. The threads appear to be perhaps M4x.5, but I've yet to gage it. In the tapped holes are 2 brass inserts, recessed and flush back with no means of screwing them in. Instead they appear to be knurled around the OD and pressed in. It appears to me that the brass inserts are pressed onto the nut flush with the ID prior to the threads being cut. Then the threads are cut into the nut and the insert together and they seem to have no effect at all. Everyone has come up with their own theories of functionality. Lubrication (it's a infrequently manipulated collet and they definitely don't appear to be oil filled sintered inserts), some sort of antibacklash mechanism (though there's plenty of slop in all 3 assemblies I have), and my favorite, the inserts could be pressed in after assembly by a special tool to provide some sort of semi-permanent lock. The threaded holes with a pressed in insert confuses me, but perhaps it was originally a set screw, but that changed it for some reason, maybe damage to the male threads.
Is this anything that anyone has seen before and for what purpose? Or is this some kind of kluge that someone came up with to solve some unknown problem.
My best guess is that it a design for locking the collet to the corresponding male screw thread; and, as you propose there would be a set screw used to tighten the brass plug against the male thread. Once the set screw is loosened then the soft brass insert would be unlikely to damage the male thread during collet removal and subsequent reassembly.