3 tries and 3 fails to make a 1/4 20 thread

mattthemuppet2 said:
if it's 40tpi you're splitting the thread. I only ever use the same mark on the threading dial. It's slower but simpler.
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My scratch pass was right on 20. I found it too much of a coincidence that I measured 40 (2x what I wanted) when I stopped the operation. Would I get 1/2 a thread advance if I engaged the half nut one position late? I well could have engaged the nut just before the mark one time and just after the mark the next since my thread counter would not let me engage exactly on the mark.
 
benmychree said:
You need to reset the thread dial so that the lines line up with the half nuts is engaged, and then try to engage the half nuts when threading just slightly before the lines line up with the witness mark. No need to use the same mark for even threads, numbered or plain lines are all fair game, for odd numbered threads use any numbered line, opposite numbers for half threads, the same number for quarter threads; this works on every lathe that I have ever run.
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By reset the thread dial, I have read 2 possible ways to do this. How do you recommend?

1.) Shim the threading dial mount with washers.
2.) Somehow turn the face of the thread dial to make the marks line up better with the half nut.
 
Whatever works. For my mini-lathe, there's no dial adjustment, so all I can use are shims. You actually don't even need to shim the threading dial. All you need to do is to consistently hit your mark. If making the dial line up helps you hit the mark, everytime, by all means do it.

I made a screw like yours. It happens. Started out well, and went south from there. What irritated me was I had threaded the first end perfectly. But on the second thread I was not consistent hitting the same place. Pretty much looked like yours. I looked at it as me practicing making scrap...
 
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Either way will work. Shimming may be a bit of trial and error. It shouldn't take too much. If you have an 8 tpi lead screw, the most would be .125". You can get an idea as to the thickness required by engaging the half nuts close to a mark and driving in the direction that you will thread. Loosen the set screw on the pivot pin and carefully back the pin out while maintaining the gear mesh. When the mark is aligned with the index, lock the pon and measure the gap with a feeler gauge to get the thickness required.

If you can adjust the thread dial, you will want to drive the carriage with the half nuts engaged in the direction that you thread and and adjust the dial to match the index mark. I'm not familiar with your lathe but it doesn't appear that you have a screw holding the dial Someone above had mentioned a set screw in the gear which is a little more difficult.
 
You could just keep the half nuts engaged, stop machine at end of cut, retract, reverse back to past the beginning, advance cut, start cutting again.
This should show up if its the whole threading system slipping somewhere of if its just the threading dial.
All my threads are cut this way as they are mainly metric on an imperial lead screw.
 
I just ran 2 passes, one on the line and the next was one position late. This resulted in 40tpi so I think this is your problem. Loosen and adjust the gear on the bottom of the thread dial so the marks line up to make it easier to read.
 
martik777 said:
I just ran 2 passes, one on the line and the next was one position late. This resulted in 40tpi so I think this is your problem. Loosen and adjust the gear on the bottom of the thread dial so the marks line up to make it easier to read.
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Thanks for the test. That's good info. It also explains why a couple of passes were screechy. It was probably the tool trying to split the threads.
 
I found the embarrassing threads I made (described earlier) and a second attempt. There was some improvement between the attempts. It won't make the machinist's hall of fame, (still pretty rough) but the second one was good enough for what I needed. Long threads at 1/4-28 and short threads at 1/4-20. I did the 1/4-28 thread first, followed by the 1/4-20. 2nd attempt on top, 1st attempt on the bottom. Clearly was off in hitting the mark on the 1/4-20 side.
PXL_20210301_143235407.jpg
What threw me off was changing the gearing and the timing/cadence was slightly different between the two different threads. Once you get the hang of it, it will get much easier. For what it is worth, the second version was turned at a higher speed than the first. Got a lot of tearing at the lower (too low!) speeds. First threads for beginners are nerve wracking. After some successful threads, it is much less so.

Before you know it, your threads will be great! You might want to practice on 12L14 or 6061 rather than brass, since those materials are cheaper, just to get the timing down. Even 1/2 PVC conduit is fine for getting your timing right.
 
@Winegrower, anything that works is great. A new sharpie mark, a shim, remembering to hit just prior to the mark, leaving the half nut engaged, they are all equally valid. You just have to consistently be on the same "part of the lead screw".

I'm using "part of the lead screw" because I don't know what the correct terminology is. What is the correct term for "part of the lead screw" in this context?

Inspired by @RJSakowski's comments, I'm going to try to shim mine. It annoys me that the engagement doesn't line up with the markings on my mini-lathe.
 
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