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View Full Version : What is needed to cut left handed threads.



mikep379
09-13-2010, 09:28 PM
I have a Clausing 1500 lathe. I was asked to cut threads on a shaft.One end will get 1"-8 right hand thread and the other end will get 1"-8 left hand threads.So i have never cut left hand threads.Do i reverse the spindle and cut the threads from left to right? Thanks

Black_Moons
09-13-2010, 09:30 PM
No, reverseing the spindle won't cut left hand threads, You must reverse the leadscrew in relation to te spindle.

(Incidently, Lots of people cut right hand threads with the spindle reversed, because its typicaly easyer to cut threads away from the headstock then towards, Left hand threads are easyer to cut with the spindle going forward)

mikep379
09-13-2010, 09:34 PM
So i turn the spindle as i normally would but just reverse the leadscrew and cut from left to right?

dp
09-13-2010, 09:40 PM
If you're accustomed to setting the compound at 29.5 or 30 you will need to turn it the opposite orientation.

Black_Moons
09-13-2010, 10:39 PM
Mikep: Yes.

Jim Stabe
09-14-2010, 09:14 AM
You also need a 60* tool with relief going the opposite way to clear the threads. Do a search.

whitis
09-14-2010, 12:27 PM
If you have a tumbler reverse on your lathe, use it. If you have change gears, add any one gear that will fit in between the gears you would use to cut right hand threads to effect a reversal. If you have quick change, find a place to insert a gear in the non-quick change portion between the spindle and the gear box. Usually there were specific provisions for a reversing gear but if not you should be able to improvise a way to put one in though in some cases you might need to make an extra banjo.

I doubt it is really necessary to change the compound angle from 29.5 to -29.5. If you do that, you may run into problems with the compound being too close to the chuck/headstock. Sometimes that will be an issue, other times not. If you don't have clearance problems, by all means rotate it. You should be able to cut either flank with modified flank infeed. There is a little assymetry with the cutting direction and the spiral of the thread between cutting the left or right flank that slightly affects rake and clearance angles but basically the idea is to make a thick chip on one side and almost no chip on the other so the chip clearance is easier. You can also use radial infeed. In some cases, alternating flank infeed is used (cutting both flanks on alternate passes to wear both sides of the tool). It doesn't do quite as well as modified flank infeed but can give some extra tool life and illustrates that it is possible to cut either the left or right flank. Ultimately, it doesn't matter what direction the compound is pointing as long as it doesn't hit things and isn't close to parallel to the cross slide (in which case you can't generate the necessary offset from the lead screw position to cut flanks instead of radial), but making it almost parallel to the flank you want to cut simplifies the math and operation a bit.

For production use, rotating the compound might have some advantages but for a one off job that isn't needing the best possible surface finish, it is probably unnecessary.

It is also possible to cut the right flank without rotating the compound clockwise by doing some math and moving both the cross slide and the compound. The math is actually easier if the compound is parallel to the ways but then it might hit the tailstock.

Al Messer
09-14-2010, 12:38 PM
Just use an Armstrong type threading tool which is flat on top and cut towards the tailstock with the lathe spindle turning forward as normally. Since the Armstrong tool has relief ground into both sides, you can cut in either direction.

Blueskys
09-14-2010, 12:50 PM
If you're accustomed to setting the compound at 29.5 or 30 you will need to turn it the opposite orientation.

I cut a lot of threads on my lathe. About 40% left-handed threads.
I don't change the compound from the 30 degrees of the right hand threads and
still feed with the compound.
I can't tell the difference in the quality of the thread.

It isn't ideal I guess. But I'm still doing most of my cutting with one side of the
toolbit.

dp
09-14-2010, 12:58 PM
I cut a lot of threads on my lathe. About 40% left-handed threads.
I don't change the compound from the 30 degrees of the right hand threads

I suppose if your carriage and lead screw don't have much slack that can work. Some people don't offset the compound at all - St. John doesn't even use a compound, for example.

ahidley
09-15-2010, 06:39 PM
As stated above, TUMBLE GEARS