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Rethreading OD?


Feb 24, 2023
I have some parts, they are made of plastic, more specifically nylon. They are already out of machine and machined. The problem is they need to get dyed and when they do the OD threads dont work as they are too big. Does anyone have any tips of how they can be fixed or rethreaded? I highly doubt that I can put them back in the machine to be rethreaded unless you know some trick that I don't. Do those rethreading tools work for making minor ods on threads smaller, or do they just get rid of any gunk that may have accumulated on parts over time? Any other ideas that may work? I can use these parts as undyed pieces but I would rather fix them than hold onto them for a later order.
Perhaps the Nylon absorbed moisture during the dye job? Maybe heat & or vacuum would help?
I am not familiar with the dye process but to my understanding they use heat and im pretty sure they use vacuum as well. It seems to be adding about .005 to the part, which isn't a lot but I guess they were on the edge of the go.
You can pick the thread lead back up and rethread/chase the thread. It's just up to you if it's worth the time. Are you looking for a run down of this process? If so what machine to do this is available to you? CNC lathe, manual lathe? You could also make a die nut as just about anything will cut plastic, but you would have be able to securely drive them I don't know what your parts look like so it is hard to advise.

Edit: https://www.mscdirect.com/product/details/33124108
Let a manual lathe do the work, just need a warm body with half a brain to hold the wrench handle and stop the lathe when the die bottoms out.
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How many parts? Depending on the lot size you might be better off remaking them particularly as nylon can be cut pretty fast compared to metal.
I have re-threaded parts on a CNC lathe before. It's a little complicated, but it works and is worth it when you have a lot of high-value parts that you need to save.

You need to come up with a way to load the parts in the spindle so that the existing thread orients the same way each time. My method is to have a fixture that the part threads into and then bottoms out. Load the part into your collet or chuck so that the fixture hits your spindle to set the length of stickout. Then you also need to have a way to orient the rotation of the part. My method is to have a removable dowel pin that goes through the fixture and into a chuck jaw. Once you have the chuck closed, remove the dowel pin and unscrew the fixture.

Next, you need to align your tool path with the current thread. The best way I found is to just watch it and keep making adjustments until you get it. Move the tool up in X so that it won't actually cut anything. Use a low rpm so that you can see what is going on. Run it dry with the door open and look closely at how the point of the tool is lining up with the V of the threads. If it looks like it's lined up perfectly, move it down some and check again. If it looks like the point of the tool is right over the crest of the thread, move your tool's Z offset in or out by half the pitch. If it's somewhere in between, take an educated guess and try again. Once you get it so that the threading tool is cutting on both flanks, you are set. Dial in your diameter and go.

I think there are some machines with a special function to catch the thread on a part, but the method above will work on any machine.
I assume the parts were to size in the first place or did you not allow them to sit overnight. If they gauged ok after stabilising, I expect they will be ok when the moisture from the dye has evaporated.
if you machined with coolant it will be absorbed.