threading problems

[Charles Spencer]

He pulled the thread-dial away from the lead screw when he wasn't threading and pushed it in to make full contact when he wanted to cut threads. If the thread dial wasn't fully seated onto the lead screw, the dial would turn but it wasn't accurate or repeatable.

Yeah, what he said.

 

[Fixnair]

I Am certainly no expert (very far from it), but I figure I can get the ball rolling for you. You need to give the experts a lot more info so that they can help. T Bredehoft asked some very good questions, but there are more. Basically, you need to describe EVERYTHING you do from the time you put the work in the chuck, until the time when the tool tears through the thread you cut on the first pass. I guarantee they will find the problem, once you do that. Good luck, JR49

Thanks for responding JR. I mounted my part in a 4 Jaw and indicated it in to within .001". Then I set the quick change gearset to produce 20 TPI. I then engaged my back gear ta the slowest speed. Then set my compound @30 degrees & backed it out alacthe way. My headstock has about .003 clearance. I then turned down my part to 1/2" & made a 1/8" cwide undercut 3/4" from end of part about .100" deep. Touched off & set my crossfeed to make a .005" cut. Engaged the half nut on #3 and at the end of the cut I disengaged the half nut. backed out the crossfeed & returned the carriage back past the end of the stock. Measured with a thread pitch gauge and it looked OK. Returned the crossfeed to zero & advanced the compound .010"and engaged the half nut at #3 again. disengaged the half nut, backed out the crossfeed, returned the carriage to beyond the end of the part & shut the lathe off. Looked at the thread and it looked as if it was a 40 pitch instead of a 20 pitch. I wised I could forward a pic to you but that's not possible.

 

[Fixnair]

Have you tried cutting some threads leaving the half nuts engaged?

Sorry but I cannot reverse my lathe. My chuck is threaded on & I don't want it unscrewing in me. thanks for the reply though.

 

[Fixnair]

Also not an expert on threading, but, here is a possible test of the machine. Try threading on a setting that isn't worn out much in the QCGB. Say, 36 TPI or higher on. Any diameter. 1" would be fine. If you find a gear setting that isn't badly worn, and the machine still wipes the thread, then either the lead screw/half nuts are really buggerrd or perhaps something in your process isn't as consistent as it should be.

Good idea Glenn. There is some wear in the lead screw but I don't know how to quantify it. The half nuts are new and I just replaced the driving key in the lead screw. I do have a question regarding the half nut indicator, is it supposed to engage exactly in a mark? I engage mine just before the dial reaches 3 but it doesn't engage 'till something after 3. I'll work on it some more during the weeek. Thanks.

 

[Fixnair]

Could one gear be missing a keyseat and be slipping? I can't think of anything else. Can you back out the tool, reverse the lathe keeping the halfnuts engaged, and try another pass? If it still wipes the threads something must be slipping. You are using the backgears, aren't you?
Mark S.

Yes Mark, I'm in back gear. Maybe I'll make sure my chuck is very tight and try that backing up trick.

 

[Fixnair]

One more ask.
Is your compound set at 29 1/2º or at 60 1/2º? Not all compounds are numbered the same. The compound should be not as much as 45 º to the work.

Well, it's set at 30. Thanks for asking

 

[Fixnair]

I don't see how a worn gear in the QCBG could cause the wiping of the previous thread as you describe. Can anyone enlighten me on how this would occur? I am thinking that the first pass using the "worn" gear in the QCGB would cut the thread and all subsequent passes would follow the same path.

From the info you provided it sounds like you are doing everything correctly; do you have a copy of SouthBend's How to Run a Lathe? While I haven't gone looking for it on this site, I'd bet a nickel that you can find a copy. Years ago I watched a guy cut threads on his SB, on his machine the thread dial was on a pivot. He pulled the thread-dial away from the lead screw when he wasn't threading and pushed it in to make full contact when he wanted to cut threads. If the thread dial wasn't fully seated onto the lead screw, the dial would turn but it wasn't accurate or repeatable. Is your SB setup this way?

Mike

Yes Mike. Mine is set up the same way and I do disengage it when not threading. I consider your suggestion later this week when I get back on this. I do have the South Bent book on how to run a lathe.

 

[markba633csi]

Starting to sound as if there is something wrong with the threading indicator- ? If you can run the spindle back by hand (kind of a hassle but possible) and the recut thread is ok then that's the only thing left...
Mark S.

 

[coffmajt]

I have read about the lower gear on the threading dial not being pinned or worse, shearing the pin . That could cause some problems, It sounds like the rest of your process is good except I try not to take more than .003 when threading, use lots of cutting oil, and if possible support the end of the shaft on a live center == Keep us posted -- Jack

 

[brino]

I do have a question regarding the half nut indicator, is it supposed to engage exactly in a mark? I engage mine just before the dial reaches 3 but it doesn't engage 'till something after 3.

Mine engages slightly after the numbers too. It always has and it works just fine.

Any chance that a previous owner changed the lead screw to a metric one?
That might mean the thread dial gear is not meshing properly.

-brino