Cutting Multiple Lead Threads...how?

[Whyemier]

I bremember me many years ago while working in the Jax Shipyard Machine shop (non existent any longer, so that's how old I am), cutting double and triple threads on valve stems. I thought I remember doing this by which number/quadrant/etc on the thread dial I engaged the feed. I can't remember exactly how I did this and which 'number' on the dial I used.
I may be wrong but somebody out there must know.

With this method I did not have to rotate the workpiece, or move the compound or do any different set ups. Just remember which two or three spots on the dial I needed to engage.

 

[intjonmiller]

I have seen numerous videos on the subject on YouTube among my recommended videos list recently. I haven't actually watched them because I don't have a current need for multiple-start threading and I figured they will still be there when I do.

 

[derf]

If it's a double start thread, you go 180 degrees from the number on your dial for the second start.. Always use the same numbers, not just odds or evens.

 

[tertiaryjim]

Lathes of differing brands can have very different results when using the thread dials. Try experimenting with yours to see that it conforms to the book and gives the needed results.
If you have a faceplate with the needed number of precision divisions you could also turn between centers.
I need to do some experimenting myself.

 

[intjonmiller]

180 degrees on the thread dial will result in the same thread as previously cut for any normal integer thread pitch (32, 13, etc.) on any lathe I've ever seen.

 

[intjonmiller]

From what I'm reading at my desk, the simplest solution is to set your compound at 90 degrees (so its movement matches the movement of the tool by the leadscrew, along the bed ways), cut your first thread as needed, using the cross slide for depth. Then move the compound by the difference between two adjacent leads. If you have a 2-start lead then you would divide the single thread pitch in half. A 4-tpi 2-start would get the first one cut as usual, then move the compound 0.125" and start again, using the same setting on the dial as the first time.

Alternative ways include turning between centers and moving the dog to a different driving position on the plate, and counting off gear teeth. Clearly the compound method is easier.

 

[intjonmiller]

180 degrees on the thread dial will result in the same thread as previously cut for any normal integer thread pitch (32, 13, etc.) on any lathe I've ever seen.

To put this another way, a 2-start thread needs the threads to be cut at 180 degrees apart ON THE WORKPIECE, not on the threading dial. Typically threading dials indicate 2 or 4 inches of travel per revolution, so whole numbers mean whole inches. 180 degrees on the threading dial means the same position 1 or 2 inches down the workpiece, not 180 degrees apart on the workpiece. Depending on the thread pitch and the specifics of the lathe's leadscrew, the threading dial, the screw gear, and the number of starts to be cut, you may be able to accomplish this by dividing the threading dial appropriately. But it seems to me that it is more complicated to do it that way, with more opportunities for error.

 

[Whyemier]

Thanks for all replies, looks like I'll have to do some xperimenting for approach to this little task. Well that'll wait til I get back from Richmond to see the #2 daughter.

 

[BGHansen]

If you have a lathe with a D1-5 (like my Grizzly G0709) or D1-6 spindle, you can remove the chuck with the work in it and rotate the chuck on the spindle. I believe D1-4's have just three pins so you can't go 180 deg. like on the D1-5 or D1-6 that have 6 pins.

Another way might be the compound method mentioned above, but do the trig and move the compound an appropriate amount after your first thread is done to position the tool at the start of the next thread. When you cut the first thread (let's say a 10 tpi for each on a double lead thread), set the compound to 0 and your cross feed to 0 as the tool just touches the stock. Oh, set the compound to 30 deg. Cut the first thread to depth.

To offset for the second thread, back off the cross feed and set the compound to the start of the next thread. In the case of 10 tpi for each of the double lead threads, they will be 0.050" apart (10 tpi is 0.100" between threads, other helix starts at halfway or 0.050" in this example). The compound should be a 0.058" for the start of the 2nd thread. The movement in Z on your lathe at a 30 deg. compound setting is Z/cos 30 or in this case 0.050" / cos 30 deg. which is 0.0577". Formula is "Z movement on the lathe or longitudinal movement = compound movement x cos 30 (or whatever your compound angle is set to)". I've got a sketch below which hopefully makes sense.

Once you dial the compound to 0.0577", you could zero it out again, advance the cross feed until you touch the work and zero it out. Then cut the second thread to the same thread depth as the first.

Bruce

 

[DoogieB]