Multi-start threads on a lathe, working through how to do it

[WobblyHand]

I don't have an ELS, but I think you could do a multi start thread by setting the lead screw TPI to (whatever TPI you want to measure on the final part) / (the number of leads). Then, use the half nut like always, cut one of the leads, then advance the compound by the pitch amount (1/(the final TPI)) and cut another lead.

This automatically clocks the start of each lead appropriately.

Doesn't this work?

I'm not sure. And I'm having a lot of trouble thinking it through. Geometry wasn't my strength.

If you clock the spindle, it would work, and I think if you advance the lead screw. I'd guess moving the compound might work, but you would have to back off in X to prevent the thread from getting too deep. Moving the compound moves you both in X and Z. Unfortunately I do not have a DRO or angle measurement for the compound, only Z and X on the saddle. Basically, I really don't know the position of the compound that well - the compound lead screw has a lot of back lash. But, it could work, I think. I could try it out without writing a single line of code, that's a plus!

 

[Winegrower]

but you would have to back off in X to prevent the thread from getting too deep

Yes, but you have to keep changing X as you cut any of the leads, just as in single lead threading, taking multiple passes per lead, a little deeper each time. Moving X does not change any relationship about where the tool hits the part. But for example, and hopefully proof, when the compound is advanced a few thousandths for each pass, and the compound angle is set for normal threading (30 or 60 or 29.5, etc) the cut is only on one side of the thread. Now move the compound down a full (pitch/n) using the compound lead screw and dial, and a new lead gets cut, X moving out and in as needed to cut the thread.

Note that the compound needs to move a pitch amount in X, which will be the cos of the compound angle off center times the compound movement along the hypotenuse. Harder to say than figure.

Edit: The compound lead screw only has to move in one direction, so backlash would not be a factor.

 

[rabler]

Would be nice to actually solve this for the metric case, but for now, I'd be happy implementing the above example for imperial threads in a test version of my ELS.

I don't think you can solve the mechanical thread dial for the metric case.
Basically with imperial threading, everything is periodic on an inch boundary.
The thread dial is taking advantage of that periodic nature. The thread dial measures the angle modulo that period. When you shift to metric you change that period to a millimeter base (depending on the least common multiple of a metric leadscrew lathe's pitches in mm). That no longer matches the 1 inch period on a imperial leadscrew, so the threading dial is shot.

You could track the one turn on the leadscrew that is properly aligned. That was why I got into an electronic threading dial and Zexpected.
Maybe you're just saying it isn't worth worrying about the metric case for now.

There is some theoretical common multiple given 10 inches = 254 mm. But it would require counting total turns of the mechanical threading dial to get to that common multiple.

 

[WobblyHand]

Yes, but you have to keep changing X as you cut any of the leads, just as in single lead threading, taking multiple passes per lead, a little deeper each time. Moving X does not change any relationship about where the tool hits the part. But for example, and hopefully proof, when the compound is advanced a few thousandths for each pass, and the compound angle is set for normal threading (30 or 60 or 29.5, etc) the cut is only on one side of the thread. Now move the compound down a full (pitch/n) using the compound lead screw and dial, and a new lead gets cut, X moving out and in as needed to cut the thread.

Note that the compound needs to move a pitch amount in X, which will be the cos of the compound angle off center times the compound movement along the hypotenuse. Harder to say than figure.

Edit: The compound lead screw only has to move in one direction, so backlash would not be a factor.

Think I understand what you saying. As I'm going along here, I'm thinking of creating a way to do this that is simple and nearly identical to normal single start threads. By that I mean the actions the user takes wouldn't be different than normal threading, but the ELS would do any of the special stuff. It really for me - I want to make it easy on myself, because this will reduce the likelihood of me messing up. Eventually, I'm going to open source this. Advancing the lead screw for me is easy, and precise with a closed loop stepper. Since I already control the stepper, it's not a big deal.

As I wrote about the backlash, I realized, it didn't matter.

 

[rabler]

I don't have an ELS, but I think you could do a multi start thread by setting the lead screw TPI to (whatever TPI you want to measure on the final part) / (the number of leads). Then, use the half nut like always, cut one of the leads, then advance the compound by the pitch amount (1/(the final TPI)) and cut another lead.

This automatically clocks the start of each lead appropriately.

Doesn't this work?

I think you're assuming the compound is at 90 degrees to the cross slide.

edit: Ok, that basically came out in follow ups

 

[WobblyHand]

I don't think you can solve the mechanical thread dial for the metric case.
Basically with imperial threading, everything is periodic on an inch boundary.
The thread dial is taking advantage of that periodic nature. The thread dial measures the angle modulo that period. When you shift to metric you change that period to a millimeter base (depending on the least common multiple of a metric leadscrew lathe's pitches in mm). That no longer matches the 1 inch period on a imperial leadscrew, so the threading dial is shot.

You could track the one turn on the leadscrew that is properly aligned. That was why I got into an electronic threading dial and Zexpected.
Maybe you're just saying it isn't worth worrying about the metric case for now.

There is some theoretical common multiple given 10 inches = 254 mm. But it would require counting total turns of the mechanical threading dial to get to that common multiple.

I would think there is a way to unravel this somehow. Assuming the half nut engaged once, and we knew the Z, the lead screw angle of rotation and the spindle angle, when we first started threading, why can't we get back to that point again, after we released the half nut?

 

[WobblyHand]

I think you're assuming the compound is at 90 degrees to the cross slide.

That definitely would work. Not so sure it is a good solution for my light weight lathe though. On a hefty Monarch, yes.

 

[rabler]

I would think there is a way to unravel this somehow.

You can. You just can't rely on the mechanical thread dial to get you there. The threading dial "repeats" expecting the period to be 1 inch (or some imperial number). If you open the half-nut, shift the carriage 1", and close the half-nut again, you will pick up any thread that is integer number of TPI. So the thread dial only indicates fractions of an inch. (This may be adjust by some multiple of inches). On a metric thread, shifting the carriage by 1" doesn't get you to a multiple of the thread turns. If you get close to the original position, the thread dial will still lock you into the right turn.

Assuming the half nut engaged once, and we knew the Z, the lead screw angle of rotation and the spindle angle, when we first started threading, why can't we get back to that point again, after we released the half nut?

Yes, that is my basis for Zexpected. You can't do it by using the same position on the mechanical thread dial, across multiple turns of the thread dial, for metric, (unless you count those multiple turns and go back to zero on the count).

 

[rabler]

That definitely would work. Not so sure it is a good solution for my light weight lathe though. On a hefty Monarch, yes.

Wouldn't be my preference either. When set to 90, my compound tends to interfere with the tailstock (or chuck) when working on small parts. I can compensate by using extreme tool stick out, but that defeats the "hefty". Ran into this a couple days ago setting the compound to very near 90 to cut a morse taper.

 

[RJSakowski]

If you know the absolute orientation of the spindle why can't you just offset that position by the required amount? e.g, multiples of 72º for a five start thread.