Internal Threading

[rwm]

So external threads are easy to set the compound. You want just a half degree or so short of 30 deg.
What about internal threads when you are feeding the compound in reverse? Do you use the same compound setting or do you need to move it 1 degree the other way?
Obviously all of this is for feeding with the compound. If you use the cross slide it is not an issue but I get cleaner threads feeding the compound.

Robert

 

[benmychree]

The compound should be turned to the same angle, but in the opposite direction, that is with the dial end leaning towards the headstock, not away from it. Personally, I have never subscribed to the half degree business; I was taught to set on 30 degrees, all my associates during apprenticeship did likewise, and to my knowledge, all the journeymen did so too.

 

[rwm]

with the dial end leaning towards the headstock, not away from it.

I am not following that part of you explanation? Interesting, I will try just a straight 30 deg.

Robert

 

[benmychree]

I am not following that part of you explanation? Interesting, I will try just a straight 30 deg.

Robert

What I am saying is that if the compound is set 30 degrees off zero, set it 30 degrees on the OTHER side of zero, in other words swivel it to the left rather than to the right as is done with external threading, this causes the tool to feed into the cut rather than cutting on the back side as the successive cuts are made.

 

[rwm]

If I swivel the compound to the left it will interfere with the chuck and the workpiece? Do you mean swivel it to the far side of the carriage? That would make some sense? Then I could feed it the same way. The last time I cut internal threads I left the compound the same and fed it in reverse. That worked but not great.
Robert

 

[benmychree]

Leaving the compound in the same position would make the tool cut on the backside as I mentioned above, it indeed does not work well. I find that with my lathe and its tooling, I can cut threads that way without interfering with the chuck, but there is no reason you could not position it towards the back of the machine, as you suggest.

 

[rwm]

Yes I see. This is what you are suggesting:



One would then back the compound out during cutting.
But wait a minute....wouldn't one really want 31 deg in this picture to keep the heel of the tool from rubbing?
Robert

 

[benmychree]

Yes, exactly!

 

[rwm]

I edited while you were typing. Sorry.
And while we are at it- is the depth of thread the distance on the compound divided by cos 30? Or in other words, the Depth of thread is 2 times the compound advancement?
R

 

[benmychree]

I did notice the one degree off of 30, but chose not to mention it, as I said before, I chose to use 30 degrees even, and one can, is they want to take final cuts straight in if they think it would make a cleaner thread; another method, taught in school by an excellent teacher, was to slightly drag against the carriage handwheel rotation to clean up the backside of the thread if it looks like it would benefit.