3 TPI Acme thread

Mine switches from 9 to 4-1/2 tpi by switching from a 24 tooth stud gear to a 48 tooth. If I could find a 72 tooth, that'd cut 3 tpi.
Pete Keillor

========== It depends on your lathe, and what change gears you have, can buy or can make. Even with a quick change box you can generally change some external gears between the spindle and box and get a whole other range of threads.
Bear in mind that your thread dial may not work with the new gears and you will have to leave the half nut engaged. Also note that cutting 3 TPI with a 8 TPI lead screw requires the lead screw revolve about 3 times [8/3s] faster than the spindle and this can be a considerable load/strain on the gear train. Some of the old old lathes had a set up to drive the lead screw to reduce the strain for very course pitch threads. Acme threads are tough anyhow so take it easy.
A good reference is @ $4.95
Let the group know how your make out and good luck. Unka' George [George McDuffee] ------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).

I suspect it may be metric since it seems that a lot of that type of equipment comes from metric countries. If he won't bring it for proper measurement, it can't be all that important to repair eh?
Can you change both pieces so you can use something like the 1.25x5 that Enco carries? Convert to pneumatic or hydraulic?

Come to think of it, are you sure it isn't a dual/multi lead screw?

How about casting a new nut from moglice, babbitt, zamak, etc. using the unworn (less worn) section of the existing screw as the core for the mold?
Wolfgang

Once you actually measure it, on the odd chance that it matches, I have a 1 1/8-3 lh acme unitap laying around.
You are welcome to borrow it.
If it doesn't match, good luck anyway.
Paul K. Dickman

Pete Keillor explained : I don't think I'd get into looking for different gears for a one off project such as this one.
Wayne D.

Could be metric since I think he said it came from the old country. He does have another one, so maybe it isn't all that important for him to get it fixed. All I hear is it's too heavy. A mill or lathe is heavy.
I missed that Enco had acme rods/nuts. Mcmaster's prices would put a repair at about half the cost of the unit. Enco's prices would be feasible.
I didn't even know about multi lead screws until I researched a mini acme rod for a small stepper motor. Wasn't an even number of TPI. If you look on the end, do you see 2 starting points? What's the purpose of them?
Wayne D.

I think I've heard about that stuff on RCM occasionally. I really don't know anything about them.
Wayne D.

Paul K. Dickman laid this down on his screen :
Wouldn't it be great if that was the size then? At least 3 tpi exists somewhere. I've only done an external single point thread once, with a couple of practice pieces prior. It was done in plastic for the bathtub drain plug. I was a 1" 20 TPI plug, which I couldn't find anywhere. The only internal tapping I've done is with taps.
Wayne D.

F. George McDuffee was thinking very hard : Thanks for the input. Everything I do seems to take a long time. I have started experimenting taking bigger cuts, but still am conservative. I'd never make it in the real world of machining. So if I tried an inside acme thread it'd probably be a day or 2 project for me.
Wayne D.

========== Although considered an obsolete accessory, a lathe spindle hand crank is almost a necessity for this type of project. see Also be reminded that if this is indeed a metric screw both metric Acme [29 degree included thread angles] and metric trapezoidal threads [30 degree include thread angle] exist.
Good luck and let the group know what you find out and how you make out.
Unka' George [George McDuffee] ------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).

========= for babbitt information see These books cover cast in place nuts, babbitt lathe half nuts and babbitt tips for travely/steady rests.
Unka' George [George McDuffee] ------------------------------------------- He that will not apply new remedies, must expect new evils: for Time is the greatest innovator: and if Time, of course, alter things to the worse, and wisdom and counsel shall not alter them to the better, what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman. Essays, "Of Innovations" (1597-1625).

If the 1.25" or so acme screw is doing the pressing part, I can't imagine the whole thing being more than a couple hundred pounds, what I'd consider pretty light.
Yes, the Enco 1.25x5 was ~$30 for a 3' length.
I'm not real sure the advantage of multi lead, but they do exist, and yes looking at the end you do see the multiple starts. I seem to recall that the TPI number counts all the leads, so the turns per inch is the threads per inch divided by the starts.

see During the last few years an article was published in Home Shop Machinist that described the casting of a new feed nut using the commercial epoxy-like material MOGLICE. The author swore by this stuff (not at it!) while commenting that it was fairly expensive.
I'd be inclined to try a filled epoxy to which I'd added some teflon powder.
Wolfgang

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The advantage is that you get more motion per turn.
The disadvantage is that you don't get as much force multiplication.
The reason to use a multi-lead thread in place of a coarser standard thread is that you don't have to cut as deep into the rod (thus weakening it) -- the threads are shallower for the travel per turn.
The primary example that I have here of multi-lead thread (three starts) is the leadscrew used to raise and lower my garage door. (Hmm ... another advantage is that it provides three locations in which you can snap the nut back into engagement after disengaging it for manual motion of the door. With a coarse single-start thread (which would require a larger diameter shaft and a larger aluminum extrusion in which the shaft moves) you would probably have to jiggle the door up or down a little to get it to snap into engagement.
That sounds reasonable -- though I think the term fro the latter is "the lead".
Enjoy, DoN.

Another advantage is that a multi-start thread on a small diameter is more efficient than a single start of equal lead on a diameter large enough to comfortably accommodate the deeper thread. More efficient because of the larger helix angle.

They're commonly used on fly presses, even 3 and 4 start, to get the downward speed for the strike.
Pete Keillor