[HELP] Threading Troubles

[EdK]

OK, this is embarrassing but I've hit a brick wall so it's time to swallow my pride. I've cut a fair number of threads with my lathe, not a ton, but a decent amount. It's been awhile since I've cut one but it's not rocket science so I'm at a loss as to why I'm having this problem. I'm cutting a 1/4-20 thread and I do the first scratch cut to check that the TPI is set up properly, and it is. Then the next pass is off a little bit even though I engaged the half nuts on the same number as the first pass. According to the chart on the thread dial housing, for a 20TPI thread I can engage the half nuts on any of the 8 numbers. I tried number 2 the first time and assumed I screwed up on engaging the half nuts so I started over and paid attention to engaging the half nuts on the number 7. Same problem. The second pass is off by a bit but it's still 20TPI. I'm perplexed.

In case it might help, the lead screw is 8TPI.

Ed

   

       

[doubleboost]

Try leaving the lead screw engaged I do on short threads
Just revers the lathe
John

[EdK]

(10-22-2013, 03:18 PM)doubleboost Wrote: Try leaving the lead screw engaged I do on short threads
Just revers the lathe
John

John,

I have no doubt that would work but it shouldn't be necessary cutting an imperial thread pitch with an imperial lead screw. I've never kept the half nuts engaged while cutting threads with this lathe unless I'm cutting a metric thread. The perplexing part is that I've cut this exact thread before with no problems. I'll go ahead and try keeping the half nuts engaged. I'll be even more perplexed if that doesn't work.

Ed

[doubleboost]

The only part that can "slip" is the job in the chuck / collet
Check all the key ways ect in the gear train
John

[chucketn]

Ed, looks to me (a real novice) that there is some backlash or slop somewhere in the gear train driving your leadscrew. Possibly a sheared key? I too, would advise leaving the half nuts engaged and reverse the lathe. I do it for all threading, metric or imperial, and my lathe has a 16 tpi imperial lead screw. Just try it and see if you're results improve.

Chuck

[Highpower]

You have the correct change gears loaded up for doing imperial threads, yes?
Swapping them to do metric threads and forgetting to put them back will do that to you.

Might want to check that the gear on your threading dial hasn't come loose as well. Been there - done that.

[DaveH]

Ed,
Just wondering if you set the compound slide over to 29/30 degs to do your screw cutting, or do you go straight in?

DaveH

[EdK]

John,

Nothing I can see is slipping, but obviously something has to be and if it is, it's doing it very precisely and consistently. I'll investigate that some more.

Chuck,

All lathes have some backlash but it would be taken up once the tool starts moving towards the headstock.

Willie,

The change gears are correct, otherwise I wouldn't be getting the correct TPI. I did triple check them though. My lathe does not have separate gears for imperial and metric. You swap places of two large gears to change from imperial to metric and I did check those. I also checked the threading dial gear, that's one of the first things I suspected.

I did try cutting the threads with the half nuts engaged at all times and it did cut the thread correctly, but I already knew it would. So I'm still perplexed as to what is going on. I think I'll sleep on it and maybe I'll have an answer come in a dream.

Ed

[stevec]

I hate to say this but, If the threading dial sports an "INDICAOR TABLE" any number of other things could be wrong.

Willie's thought about the thread indicator gear having loosened is worth checking out.

[EdK]

(10-22-2013, 04:25 PM)DaveH Wrote: Ed,
Just wondering if you set the compound slide over to 29/30 degs to do your screw cutting, or do you go straight in?

DaveH

Dave,

I have it set at about 29 degrees. Angled or straight in shouldn't cause it to cut two threads though.

Ed