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

[twooldvolvos]

Well, I've given up for the day on cutting my first threads on my South Bend 10K. My objective was a 1/4 20 thread in a piece of brass stock. I took my OD down to 1/4 inch and set my lead screw speed.




I put my drive into back gears and set the RPM to about 150.



My compound was at 29 1/2 degrees. My tool is ground to 60 degrees. My first pass looked OK and measured 20 threads per inch as expected. But after several passes of about 5 thousandths, I kept coming up with a mess that looked like this.



The threads were rough and read at 40 TPI, not 20. I tried to engage my half nut on any of the 8 lines on my thread gauge since it was an even number of threads.


I did notice that my gauge does not allow me to engage exactly when the lines are on the mark. I can go a little before or a little after. Maybe that is where where I am making my mistake. The next time I can try always going a little before the mark. Does anyone know if I can adjust the threading gauge to engage closer to the mark?

Anyway, any advice is appreciated.

 

[T Bredehoft]

Does anyone know if I can adjust the threading gauge to engage closer to the mark?

I would expect some way of adjusting the disc relative to the mark. can you turn it by hand, twist the face?

 

[Brento]

A good practice you can try that i think alot of us do is try to stick to the same line when you engage. Are you going back to your zero everytime?

 

[epanzella]

An easy way to at least figure out if it's you or the machine is to lock the carriage as close as you can to any whole number then mark that spot with a sharpie. Go back to that same mark every time. If you find you can't lock the half nut at that exact spot, something is slipping. An easy way to get the same spot is to put some pressure on the half nut lever ahead of the spot so when you get there it'll drop in. If the thread comes out OK doing it this way then you were doing something wrong before. Use your lowest speed.

 

[jcp]

I had to adjust the dial of my 9A when I first got it. Pretty sure there's a set screw in the gear to shaft connection of the thread dial.
On a Southbend thread dial for any even numbered thread can you can close the half nuts on any line. For odd numbered threads close on any numbered line. This is not true on all lathes.

 

[mattthemuppet2]

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.

 

[RJSakowski]

It is important that your marks line up with the index mark. If they don't, you will experience what you have seen; that you can engage slightly before or slightly after. This will give you two different tool paths.

You should be able to rotate the dial relative to the gear so that when the half nuts are engaged one of the marks lines up perfectly.

Another cause of your problem with an older lathe is wear of the lead screw, half nuts, or thread dial dear. Any of these can cause enough backlash that you could jump a tooth when you engage. To minimize any issues make sure that you start your pass several inches before your thread and that you engage at the same position on the dial each time. It might be just before the dial mark or just after. You should be able to use any of the allowed marks for your thread pitch.

Rather than attempting to cut a thread while you are figuring this out, just repeat your initial pass without advancing the cross feed or compound. You should be tracking the same path. Once you are sure of what you are doing, go on to actually cutting a thread.

 

[markba633csi]

RJ makes a good point- practice engaging and disengaging till you get consistent tracking
-Mark

 

[tjb]

This may be a bad idea, but when I was first practicing cutting threads, it worked for me. 1/4-20 is a fairly fine thread to learn on. I started by trying much coarser threads like 12 or 14 TPI. I didn't even care what round stock I started from - I just wanted to see if I could cut the threads successfully. For me, it was lot easier to visually see what was happening with the coarse threads. And it was a lot easier to see if my subsequent tweaking was behaving as I anticipated.

Veterans, comment on this please. If it's a bad idea, I'd like to know that myself.

Regards

 

[AGCB97]

Like someone mentioned the sharpie mark on the dial works for me but if the TDI bolts to the right side of the carriage you may be able to shim it with a washer or 2.
All the above posts are good but mainly the practice till you got it.
Aaron