Cutting Internal Threads 101

I am reading various sources for information on threading in the lathe. A lot is said about external threads, but not near as much about internal threading.

For the moment, I am interested in cutting right-hand internal v threads.

Various sources say to turn the compound 29.5 - 30 degrees to the left. Disadvantage: No room to enter hole. Or leave compound to the right and turn tool upside down. Or put compound in rear right corner. Disadvantage: Have to lean over lathe to adjust. Other permutations of these exist when the lathe is run backwards. Disadvantage: Danger of unscrewing the chuck.

Why can't you just leave the compound set the same way as when cutting external threads (29.5-30 to the right) with the tool in the normal right side up position? The tool is still only going to cut on one side. This time it will be mostly cutting on the rear side of the tool and not the front as when cutting external threads.

Any thoughts would be greatly appreciated. Thank You

Reply to
John Albers
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With the feed going the same way as the pressure from the cut, the tool may tend to jump ahead a bit. Better to have the feed forcing the tool into the cut.

The easiest way to thread is with a cross slide stop, but the only way you can use one while internal threading is if you cut on the far side of the axis, whether upside down or right side up.

John Martin

Reply to
JMartin957

snipped-for-privacy@aol.com (JMartin957)

And if you have loose gibs, loose carriage, there's even more bounce of the tool bit out of the cut--the tool bit is forced to the center of the bore--is what you are saying, if I read you right?

Frank Morrison

Reply to
Fdmorrison

Not to the center of the bore, but toward the headstock. You want the cutting force to be acting against the feed, not with it. Otherwise, the feed may not be steady.

The same thing, really, as climb milling. OK if you have good leadscrews and tight gibs, but pretty jumpy if you don't. With threading, the jumps wil give you a wobbly thread.

John Martin

Reply to
JMartin957

Sorry 'bout that. My cross slide stop, which I made, works on the infeed direction only. I'd temporarily forgotten that some can be set to work on either infeed or outfeed. I agree with you about the usefulness of the stops in threading especially.

John Martin

Reply to
JMartin957

Other approach for fine pitch threads, simply set the compound in line with the lathe bed axis, and feed only with the crossfeed screws. Disadvantage, the tool will be cutting on both sides simultaneously. This works for finer pitch threads.

When I do this, I put the compound at the right rear quadrant. You don't need to lean far to see what the dial is reading.

Jim

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Reply to
jim rozen

snipped-for-privacy@aol.com (JMartin957)

I understand, and I like the comparison to climb milling. What I was thinking (prompted by your post) was that the internal thread cutter bounces not only ahead, but--because it's on a shaft behind it, rather than on an arbor--moves also up and out of the cut. But I like the comparison you've made. FM

Reply to
Fdmorrison

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