Cutting Multiple Lead Threads...how?

[dave2176]

I've been thinking about this for an upcoming project. I would do it as described above using the compound to advance the bit to achieve the desired number of starts. One thing that occurs to me is to keep in mind or decide the bulk or size/pitch of the thread I need in advance. For instance if I set up to cut 10 tpi I can only cut the depth of a 20 tpi thread so room is left for the second start. This will provide me the motion of 10 tpi but the physical size and appearance of a 20 tpi thread.
Dave

 

[george wilson]

I wanted to cut a 10 part multiple thread to make a mechanism for a FERGUSON breech loading FLINTLOCK rifle. These were used by an outfit commanded by a British officer named Ferguson in the American revolutionary war. They could be loaded quickly,and WITHOUT STANDING UP! The mechanizmo had a 10 lead threaded rod connected to the trigger guard. A 180 deg turn would lower the threaded rod enough to clear the end of the barrel. Then,dump on a lead ball followed by powder. An interesting gun,and no one knows hor Mr. Ferguson funded the building of these VERY EXPENSIVE rifles. Probably through a wealthy donor. They were tricky to make in the 18th. C.,OR NOW!

I made a gadget that could just be held in the chuck. It had a snug fitting spindle in it(whole thing was just a few inches long),that could be rotated. I made an index wheel with 10 holes in it. There is a snug fitting dowel pin that can be pulled out to rotate the disc 1 hole at a time. Can't recall how I spaced the holes. Probably by hand as I can do very accurate hand work,but easily done with a rotary table if you aren't up to this job by hand. Since this is a single purpose jig,the end of the spindle was bored out to accept about a 3/4" dia, piece of drill rod. There is a set screw to keep the drill rod from rotating loose under the pressure of cutting those threads.

This is a very QUICK thread,angled about 45 degrees. So,I had to grind a special threading tool with the side angles to fit the inclined angle of the threads. It worked beautifully,producing an accurately indexed 10 start thread.

 

[mce5802]

A simple way is to put the shaft between centers and use a four jaw to hold the center, putting the dog against a jaw and cutting your first thread. Then back off the tail stock, leaving the dog clamped in place, and rotate the work 180* so the dog rests against the opposite jaw and cut your second thread. You can use a three jaw for a three start thread, etc. hope this helps

 

[GadgetBuilder]

Martin Cleeve's "Screwcutting in the Lathe" has a chapter devoted to multi-start threading. He describes a procedure which cuts all the starts automatically.

To do this he uses left and right carriage stops, where the left stop is a "soft" stop (an indicator would work nicely for this). He cuts a runout groove and sets the left stop to coincide with this groove. Assuming an inch (not metric) leadscrew, close the half nuts and move the carriage so it's against the left stop (or indicator zero). Do this by closing the half nuts and stopping the lathe when the carriage is at the left stop; turn the chuck forward or back to get the tool at the center of the runout groove. Release the half nuts and move the carriage to the right by (n+1/2) inches (for a 2 start thread) and engage the half nuts. Set the right stop against the carriage and lock it in position. For a 4 start thread move the carriage by (n+1/4), etc. for other number of starts).

To cut the multi-start thread proceed normally, stopping the lathe at the left carriage stop before opening the half nuts each time, crank the carriage to the right stop and re-engage the half nuts. For a 2 start thread make 2 passes, between adding infeed since a pass is needed for each start. It's difficult to stop exactly on the left runout groove so err on the side of going SLIGHTLY too far - then you can move the carriage to the right stop and back away a tiny bit to get the half nuts to engage -- but if you go too far (more than 1 leadscrew thread) it will damage the thread being cut so try to be precise (not a problem with coarse leadscrews but with a 16tpi it could be). A nice feature of this method is all starts are cut to the same depth as you go along so the nut can be test fitted as you approach final depth.

There are some wrinkles to the above method, e.g. it doesn't work if the thread pitch being cut is a multiple of the leadscrew pitch. But Cleeve has other ideas to deal with the wrinkles.

Cleeve's book is modest cost and worth having if you do much threading.

John

 

[Whyemier]

Once again well appreciated thanks to all who commented to my question. I think I know how I'll proceed but a trial piece seems to be inorder to make sure of the procedure.

 

[RJSakowski]

.......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.

The slots on a faceplate are meant for driving and not for indexing. They are not necessarily equally spaced, as I found out last week when I had to rotate a workpiece to properly match the slots. For my Grizzly faceplate the angular spacing of the eight slots is 44º 10', 44º 9',43º 56', 45º 58', 40º 26', 47º 14', 44º 25', and 49º48'.

 

[intjonmiller]

Good point. I was repeating what I read elsewhere. I'm glad you said something.

 

[machinistmarty]

I cut a double lead every couple of years and i split the thread with the chase dial. Its a 4 pitch thread which would be a 2 pitch lead. On our old monarch at work, I catch any line, then for the second lead I catch what would be a quarter line. I just mark the start points with a sharpie. I always catch this mark before a line. never tried it after a line. I used to turn the compound 180 degrees and dial off the pitch, but its much faster to split it. I came across this method by accident one night. I was threading an even pitch thread and not watching my chase dial and I accidentally split the thread. I just stopped the lathe and looked to see what I did wrong, I didnt think you could split an even thread. An odd thread should be even easier, you could catch a numbered line then a non numbered line i would think. On our tool-mex lathes, they have 8 numbered lines and 8 half lines. when I make the same thread on it, I catch 2 opposing lines then i catch what would be a quarter line just ahead of the line I caught. I mark theses with a sharpie also. Some where on here I posted a pic called Double Lead Left Hand Acme. The Pic is the test piece I made so I could make the nut that was needed. Hope this helps!

 

[Wreck™Wreck]

I cut a double lead every couple of years and i split the thread with the chase dial. Its a 4 pitch thread which would be a 2 pitch lead. On our old monarch at work, I catch any line, then for the second lead I catch what would be a quarter line. I just mark the start points with a sharpie. I always catch this mark before a line. never tried it after a line. I used to turn the compound 180 degrees and dial off the pitch, but its much faster to split it. I came across this method by accident one night. I was threading an even pitch thread and not watching my chase dial and I accidentally split the thread. I just stopped the lathe and looked to see what I did wrong, I didnt think you could split an even thread. An odd thread should be even easier, you could catch a numbered line then a non numbered line i would think. On our tool-mex lathes, they have 8 numbered lines and 8 half lines. when I make the same thread on it, I catch 2 opposing lines then i catch what would be a quarter line just ahead of the line I caught. I mark theses with a sharpie also. Some where on here I posted a pic called Double Lead Left Hand Acme. The Pic is the test piece I made so I could make the nut that was needed. Hope this helps!

View attachment 130790

Excellent, last year I did 30 shafts with a 2 1/2-4 thread on a lathe that I had never used before, fussed with the thread dial for the first few until realizing that the machine had a 4 TPI lead screw. Every position on the dial will make a 4 TPI thread in that case, I felt stupid at best.

 

[derf]

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.

Yes, I stand corrected.... it's been 15 years since I last cut a double pitch thread, and it was a left handed acme to boot. I just checked my thread dial, and it's actually about 150 degrees. Instead of using the numbers on the dial, I filed another index point on the dial housing to engage on. The point that I filed was right at 12 o'clock on the dial, the factory mark is at 5 o'clock. All I can really remember is that I took the phone off the hook and locked the doors so nobody could distract me.