Multistart threading

[mod_eng]

I have a 12" x 36" Craftsman lathe made by Atlas Clausing. The lathe has a quick change box.
I need to make a multistart thead having three starts and a lead of 0.125". That means that
the thread will have the pitch of a 24 threads per ins screw thread. The lathe manual describes
methods for mutistart threading, and states that almost any multistart thread can be cut,
but restricted to certain division by 2 or 4. Slipping the gear on the headstock is the other
possibility but that gear's teeth are not divisable by three. I posed this question to ATLAS/CLAUSING
but they had no one that had operational knowledge on this lathe. Can anyone help?
I could use a four start thread but even that does not fit the guidlines of the quickchange box.

[gezr]

there is nothing wrong with using the cross slide to advance the tool to depth, it changes the way the tool cuts, but im sure you will find that a plung feed should be fine.
use the compound to change the tools position for the new thread by advancing it the required lead amount. keep your compound parallel to the work for this, and it should work just fine, just take some caution because the infeed method may be different then what your used to.

[Jacin]

Seems to me that for a triple lead the easiest method MIGHT be to turn between centers and use a drive dog off the jaws. Index the dog to the next jaw for each "next" thread.

Heck as I think about it I might be inclinded to turn a shoulder on my part (or clamp a temporary collar on as design dictates) once you are positioned you could scribe a mark on your part and line that up to a jaw. Un clamp and move to next jaw. Obviously there's some risk unclamping so that might depend on how repeatable your chuck is.

Lastly you could always move the compound to be 90 degrees to your work and simply "move" over for each thread. To do this would mean you'd be "plunge cutting" your threads which might require you to get tricky on your cuts - In another words you might want to jockey it around a bit so you're kinda duplicating a 29 degree plunge.
And lastly I think that if your were to just set it up like normal - crosslide at 29 - 29-1/2 degrees. You could calculate how much crossslide movement would end up moving over the .125. So in this case - plunge threading tool until it just barely touches OD of part - zero dials. Proceed to cut as normal. Next (with tools AWAY from part as you don't want to crash anything) you can advance the calculated amount (from the origional zero point - again move tool using compund until it just barely touches part OD now Rezero compound. Cut second thread as first. Repeat procedure for third thread.

And lastly you could "chase" the threads with a die chaser and basically either cut all three at once or use any previous method to get it close and finish up by chasing them.

Just some thoughts - I hope they make some sense

[david_F]

I don't know about a 3 start... but maybe it will work on some lathes...

On my Myford, I did a 2 start 18 tpi thread recently, on a 1/4" bar, to make a tap to tap a plastic insert for a sewing machine tension adjuster knob that had stripped. the knob itself was hard to chuck up.....

My threading dial has 8 lines, 4 of them labelled 1 2 3 4

I set the gears for 18tpi, then scratched the first cut onto the work with the threadign dial on '1'
I then started the next thread at the point 1/2 way between the 1 and 2 marks.
that was too far to the left, so I ran back and started again, this time engaged at 1/4 the distance between marsk 1 and 2. perfect scratch half way between the original scratch pitch.

I then did the usual few thou infeed per cut, but cut twice at each setting, once on '1', and again in '1 and a 1/4' on the thread in dial.

I think I'd be stuck for 3 start and possibly 4 start, but this worked for 18tpi and 2 start.