Help with thread finish on lathe, nothing I have ever seen before

Again, thank you guys for the replies. Some great advice, he has the link to this thread so I am not sure what all he tried like the left hand thread, etc but thats a good idea.

The problem is still ongoing. The newest is that he said sometimes when he takes a clean up / spring pass, the cutter will still cut about .003-.005 (His estimate) Said it does not that every time, only sometimes.

I had him indicate EVERYTHING I could think of, the compound is not moving, has a DRO on the cross slide so would know if that moved, also had him check the mail leadscrew, I had him put an indicator on the right end of the leadscrew and zero it out once the carriage was engaged to thread to make sure any backlash was out, and said to watch it on every pass, nothing is moving there, at zero every time. Its not in the threading dial, thats all tight, and its cutting just a little more, if it was the dial and engaging in the wrong spot, it would totally wipe out the thread.

Anyone have any opinions of this? Personally I have that happen all the time when threading, its either that maybe its catching up on the spring passes and I was cutting a little heavy before that, or maybe it kind of grabs the tool and pulls it in a slight bit. I find it is usually from going too deep on the last cut. If anyone has any other comments on that, please feel free to chime in. The ONLY thing I can think of on this if nothing is moving, and is that maybe on the pass before the spring pass, the half nuts are not 100% engaged, and then on the spring pass they are, so it pushes the carriage a little more to the left. Just a guess though.

He did have someone come look at the machine from a machine service company, did not seem to find anything. But he also said the person who worked on it was a machine repair guy, not a machinist. I have a message in for this repair person to call me back to see if there was anything at all, I have not heard back yet.

Of course he is getting frustrated, but I am still 99.9999% sure it is not in the machine, thats the hard part. I am still trying though, because I definitely do not know everything, but what else could it possibly be? I have also asked him to join here and post so that we can get this figured out, it may be a lot better for him to also post in case I am missing anything.
 
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Obviously he's not that interested in getting the issue resolved if he still hasn't bothered to join or post here. User error? machine error? We won't know without more pics/details.
 
I think he may be newer to forum world, but I will ask him again to join in so we can try to help him out.
 
Maybe he's running in reverse.
 
Looking at the picture again I noticed the work was not supported with a center. As previously mentioned he general rule of thumb id anything extending beyond 1 1/2 to 2 times the diameter should be supported with a tailstock center when threading. It might be a case (depending on the material) that the material itself is flexing causing the tool to cut on an angle to the stock.

You didn't mention what materials were giving the most problems. On 1018 commonly known as "cold rolled steel" is difficult to get a clean thread. To help determine if it's a problem with the material I'd take a fresh HSS die and run it over the threads. If you get the same results it's more likely a material problem than a machine or operator problem.
 
Yes he tried to move the tool back, and I believe the material is 4140. He also tried aluminum with a similar result.
 
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