Crazy Threads...

Is it possible your lathe requires a specific number on the thread dial to do 10 tpi? Also check the change gear setup again. If it is running the wrong ratio it might mess it up. Try locking the compound and use the cross slide to adjust your cut. Try cutting .005" at a time. Is your chuck secure in reverse, or won't unscrew under load? If so try mounting the cutter upside down and cut from the shoulder out. Remember the feed works opposite in reverse so run it in air to test it first.
Dave
 
Have a look here in case this is the same problem. It solved my threading problems.
http://www.hobby-machinist.com/threads/stripped-half-nuts.44005/
Since I made a plinth for my lathe tool post I keep the tool at 90 degrees and just plunge in with the cross slide.
So far it has not caused any problems.
I dont count the crazy thread on my vertical slide lead screw as that metal just did not want to turn or cut nicely.
 
Are my old eyes seeing a double thread? maybe not. I would chuck up a new piece
stick a sharpie in the tool holder masking tape on the tread dial, move tail stock way
back, run carriage way back, turn chuck by hand so the half nut drops in, mark the
thread dial. Start lathe engage half nut at your dial " mark" let it run till it gets into
the work turning cross slide in enough for the sharpie to make a thread. Check
the sharpie thread. Hoping (its what you want). Repeat a few times eyeballing
the sharpie for tracking. Another words this is a tracking test. If it does this
once, it should do it all day. If it loses its tracking I first would take a good look
at your feed screw cause most lathes get worked to death near the chuck, meaning
you may engage on a good spot then get into a worn spot kind of retarding its
timing. I hope not. As far as half nuts as long as they are loaded and stay in
I wouldn't worry bout them. I know nothing about your lathe, but some
lathes use the half nut for everything, those are the ones capable of wear
on the lead screw. But try this test.
sam
 
talvare said:
In your photos, it looks to me like your compound is set at 60 deg instead of 29 deg. That may not be the cause of your threading problem, but it will give you a poor quality thread.

Ted
Click to expand...
Not true, threading may be done perfectly well from the cross slide without using the compound at all.

As far as the original posters question, lead screws will often have a clutch or shear pin that will prevent drive train damage in the event of a crash, ensure that the clutch is not slipping if so equipped, also a shear pain may not appear broken from the outside driving the lead screw by friction alone where it will slip under load, this sounds like a timing problem you are having.
 
Wreck™Wreck said:
Not true, threading may be done perfectly well from the cross slide without using the compound at all.
Click to expand...
True, but if you set the compound at 60 degrees and thread from the compound you get a mess. I think that is what Ted is getting at.
 
don't know if applicable, but my lathe was made as metric, and I cut gears to enable regular/imperial? threading. Since then, the threading dial is useless for the reasons described above.
 
Wreck™Wreck said:
Not true, threading may be done perfectly well from the cross slide without using the compound at all.

As far as the original posters question, lead screws will often have a clutch or shear pin that will prevent drive train damage in the event of a crash, ensure that the clutch is not slipping if so equipped, also a shear pain may not appear broken from the outside driving the lead screw by friction alone where it will slip under load, this sounds like a timing problem you are having.
Click to expand...
my lathe has a clutch and a half nut. the shear key is in the clutch... but the half nut is locked into the lead screw when engaged... regardless if the key was stripped, it is not used when the half nut is engaged, correct?
 
John Hasler said:
I'd investigate this if I were you. That's the difference between in the groove and on top of the threads.
Click to expand...
i was playing with this last night. i pulled some gears off to try to have a better look, and a flashlight to see under/inside the gear box. that slack seems to be in a union. much like a hydrolic pump union as if the nylon spacer had worn away between the cogs. HOWEVER, once the half nut is set with the machine is running, all the slack is taken up within the first half revolution... i cant see this being the issue. i could understand it causing some kind of chattering, but thats not the issue here. each pass is true and even, its just in a different spot each time.
 
i cant thank everyone enough for their comments.
im going to pull the apron off tonight after work/dinner. i will document along the way.
we'll get to the bottom of this... we have to! i cant afford another lathe..

edit - i will also try to take a video before i pull everything apart to show my process and the result. maybe it IS something im doing wrong... jeez, i hope it is anyways...
 
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