Making a long threaded screw

Die cutting would not be very accurate for a long thread. The die will want to wander and the concentricity will be bad. I missed the part about cutting a lead screw in the OP. Precision lead screws are usually ground. Although some ball screws and lead screws today are cut out of hardened blanks on thread whirling machines.

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precision lead screws (those not used in a machine tool) are for the most part thread rolled. Long threads like those on all-thread rod are through-feed rolled on a double roll type thread rolling machine. You can easily make a long machine screw to the accuracy required by cutting the threads in sections on a manual lathe. The problem with cutting a lead screw on a lathe is that all of the accumulated error in the lathes drive train, plus deflection, run out, slide wear, and thread form errors in your tool all stack up and prevent you from making a lead screw that's as good as the one in the lathe.

That is what the optical comparator was invented to do, check thread form and thread lead.

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manufacturers of ball screws and precision lead screws use special electronic gaging machines that check the lead, run out, and accumulated error.

Dan

Reply to
Dan
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| | It would depend on the quality of the die, compared to the accuracy | of re-aligning the stock for the later cut(s). I'd like to think | that a good die is going to give you the quality you'd need?

Seems to me that if you used a die you would likely get good threads, but if you pulled or pushed on the stock while it was passing through the die you would gain or lose a thread or more over the length of the screw. Again, it's a tolerance stackup, but over a long item it adds up to be a whole bunch, especially on a lead screw.

Reply to
carl mciver

Great information. Question though, is that an SAE "whole bunch", or a metric "whole bunch"?

Reply to
Dave Hinz

[ ... ]

If I had a choice for this, I would probably use a Geometric Die Head to do it. Make the first cut as far as the turret would feed, then (after the head released), reposition the stock through the spindle, and close the Geometric on the existing thread, perhaps an inch or two from the end of the cut thread. Then switch spindle on again, and feed until end of turret travel again.

If I were dealing with an Acme thread, or even a particularly coarse thread, I would use a Geometric Die Head with the rough/finish switch lever, and do two passes for each section.

Actually, I've never seen Geometric chasers for Acme threads, but I could imagine them existing.

Enjoy, DoN.

Reply to
DoN. Nichols

Like you, I've never seen the inserts for Acme, but I can't help but think that using a die head for a thread of any length of substance would be a mistake, not that chasing it wouldn't present its own problems. If you're seen Acme taps, you know that they're considerably longer than a typical V type tap, so I'd assume the chasers for a die head would have to have a corresponding increase in length.

Die heads were suspect enough when I worked at Sperry that they were not used. We single pointed all threads, some of which were really a tough job. One of them was a long manganese bronze shaft, 5/8" diameter, center distance, roughly 4" long, was cut 16 pitch, with the ends turned and ground for bearings, plus another thread or two. There were various configurations depending on the function. These went to the control system that fired the missile.

I have and use a die head, but I've yet to see a thread that came from a die head that rivaled one that was single pointed, chased, although I have seen some quite pretty threads from die heads.

Harold

Reply to
Harold & Susan Vordos

What I would think of doing is cut the thread from the headstock end toward the tailstock end, using no tailstock but supporting the long unthreaded part with a center rest and other outboard expedients. I would start by cutting a doot of thread normally and flipping the screw over and screwing it into a special 'nut' in the lathe jaws.

I would make a long tight fitting nut and slit one side and semi-permanently (glue?) it to the chuck so it could not move. Then screw in the just threaded part and clamp down and cut threads away from the chuck. When you need to move the screw just loosen the chuck jaws and screw in the section you just threaded.

There might be a small discontinuity where you flipped the screw, but that could be cut off later.

Reply to
Nick Hull

| > Seems to me that if you used a die you would likely get good threads, | > but if you pulled or pushed on the stock while it was passing through the | > die you would gain or lose a thread or more over the length of the screw. | > Again, it's a tolerance stackup, but over a long item it adds up to be a | > whole bunch, especially on a lead screw. | | Great information. Question though, is that an SAE "whole bunch", or | a metric "whole bunch"?

Neither. It's an "Oh, s**t!" whole bunch.

Reply to
carl mciver

[ ... ]

Agreed. I have (so far) *one* Acme tap, and would by preference single-point the thread in a single setup. But with the task stipulated -- cutting a thread longer than the travel of the carriage -- the requirement to pick up the previous part of the thread and continue with another section of the thread -- it strikes me that a die head would close nicely on the previously cut part of the thread to continue.

As for the longer thread cutting requirement for Acme -- consider that Geometric chasers are available either plain, or with a projection to allow work closer to a shoulder, and that normally only the first few threads do the cutting, allowing many re-sharpenings. With a larger than normal Geometric head, it should be possible to pick up enough extra length so it would at least duplicate the extra length of the tap -- at the cost of many fewer resharpenings.

One of the benefits of the die heads is that they can cut a full thread form -- and are intended to work from stock that is a few thousandths oversize, so you can mike over the crests to get a good idea whether they are oversized, undersized, or right on, without the need for the more expensive thread pitch diameter micrometer -- or juggling the thread wires and the tables or formulae to get the right measurement. It does make setup a bit easier at least.

I normally use the Die heads when making batches of parts with the turret. The typical thread that I am cutting is 5/8-27 (microphone mount thread), and I am cutting it in 360 brass. Yes, I could single-point the thread, but in a situation like that (cutting to a shoulder, piece after piece for an afternoon and an evening), the Geometric die head is the better choice for me. (It is also a *lot* quicker than single pointing it, even with the rough/finish lever and two passes.)

The other thread is a small metric -- for concertina endbox screws. M2.5x0.45 -- again done with the turret. A second lathe (A Taig, FWIW) is set up with a watchmaker's collet and a form tool to crown the heads, and then the Nichols horizontal mill with a fixture for holding twenty of them at a time in four rows for slitting with four slitting saws.

Neither of these parts have a long enough engagement relative to the diameter for drift of thread pitch to be a problem, so I don't know how good that would be with the Geometric heads.

But non-production threads I normally do single-point.

Enjoy, DoN.

Reply to
DoN. Nichols

Hmm ... how about soft jaws on the chuck, bored to the right radius, and single-point threaded? Your initial thread on the workpiece (cut before you mount and prepare the soft jaws) would have to be long enough so you could then spend the time to properly align the tool with the threads (perhaps by disengaging the tumbler gears until you get things in sync), but once that is done, your remaining problem is starting the thread without a lead-in groove or a shoulder. If you have a lever setup to feed the tool into engagement and back out, you could do pretty well, as long as you don't engage too far back after several thread passes have already been made.

If there is a discontinuity there, there will be one each time you shift the workpiece. It is better to tune things to eliminate the discontinuity from the start.

But certainly an interesting approach.

Enjoy, DoN.

Reply to
DoN. Nichols

I have absolutely no idea if this is going to work, but it is just an idea to save on 2nd, 3rd, etc, realignments (which probably saves nothing if you are making it for your own lathe).

1) Make first bit of threaded screw. 2) make threaded tube to match screw, tube extends through head stock, 3) make "lock nut" (or 2?) 4) mount threaded tube into jaws, 5) load lock nut onto thread part of screw 6) load threaded bit of screw into tube 7) lock threaded screw into threaded tube 8) align in jaws and tail stock 9) realign tool. 10) cut next section 11) unlock lock nut(s) 12) screw threaded rod through threaded tube and relock[1] 13) got to 10.

Theorectically you could make it as long as you like, but you need steadies on the long bit,

[1] each turn just advances the rod the size of the pitch.
Reply to
Terry Collins

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