ACME threading questions

Well, here goes nuthin'. Seems like every time I do a google search PM is somewhere in the results. Lots of good info here, but this time I came up dry. Guess it's time to join and ask. I am not a machinist by any stretch of the imagination although I have been scratching along with my mill and lathe for more than a few years now.

To get to the point I have a project I'm working on, patent applied for and figuring out how I'm going to actually make the thing. It's basically a screw type puller, and although I did my prototyping with a normal (3/4 -16) thread it seems to make sense to use an acme thread for this, and I'm wondering how this is gonna work. I need a 3/4 inch shaft to handle the load, so going with the finest pitch tap (for mechanical advantage) I see commonly available I get 10 pitch. I'll have to tap two holes per unit through 1.5 inch steel bar. I'm thinking I should heat treat at least the threaded part for strength as I'd like to see a 50 ton capability (shared between 2 screws) with this device. All that brings up material choices and tap life questions. Something like 1045 should heat treat ok, 1145 would probably be easier to machine. Would something else fill the bill and be easier to tap a 3/4-10 acme thread into? How about tap life? They ain't cheap, I'd like them to last as long as possible. Don't know how many of these I might be making, so I might as well plan for longevity from the start. Aside from the tapped holes, the majority of machine work involves drilling 1/2" holes and milling some slots and flats. It's a fair amount of metal removal but I have an old Index Model 60 Horizontal mill (2 hp 3 phase) that's in almost pristine shape, or will be as soon as I finish making an overarm and arbor support for it. Any thoughts, suggestions, criticisms, etc. you guys have I'd love to hear it. Thanks!

Suggest you see if you can get a set, a rougher and a finish tap. The rougher will take a lot of wear off the finish tap.

Gonna be fun to turn that tap....

BTW, that 50 tons.... 3/4" acme 10 tpi, I'd need to look up the minor diameter, but I'm saying you have a good deal less than a square inch of material, and so you are probably going to need a high strength heat treated screw to get the yield up high enough to handle anything close to 50 tons, especially if you want any sort of margin. Then also, with a 10 tpi..... the torque might be pretty significant, 10 tpi is fairly steep.

Two screws... possibility of cocking off perfectly straight, which screws in threaded holes do not like. Added forces, reduction in allowable tension, etc, etc....

You have PAF status, so no need to be secret. Head off and invest in a PE to look over your plan and give some analysis of the structure and torque required, etc. Also advice on any changes to meet your requirements. You may not like it, but it has two nice features....

1) You will get something that works out of it, if there is a way to do it.

2) You also get a PE with E&O insurance responsible for the thing not breaking and damaging people etc. All you gotta do then is make it like he says. When dealing with that level of forces in a commercial product, you probably will want to do your due diligence. Your liability carrier will love you better for it, too.

50 tons for two screws - shared load? Have you looked at the numbers? 3/4 - 10 Acme has a root diameter of 0.520 corresponding to a stress area of 0.21 square inches. Multiply that by the safe yield strength of the heat treated material (1045 HT to RC 40 roughly 100,000 PSI yierld; then work in a 2:1 safety factor) 50,000 working PSI, that equals a safe load of roughly 10,600 lb per screw.

Here's a push in a safer direction: I think you are better advised to consider a 1 1/4 - 8 Acme which has a root diameter of roughly 1 1/8 making the stress area 0.994, probably a better choice for a 2:1 shared load of 100,000 lb or 50,000 on each screw..

This is bone basic statics. You also need to crank in the proportions of the strongback and other parts of the stress loop. It aint rocket science but the numbers do have to work out for all stressed parts. One weak link and -.kerblam! and something heavy falls - hopefully not on your foot.

Presses are typically very conservatively designed to reduce deflection energy. I saw a springy shop made 500 ton hydraulic puller fail with a bang and launch a 3" puller bolt 2 ft long about 50 feet. It pays to keep deflection to a low figure even when it makes the press massive, hard to move, and expensive in material

Any chance you can go with hydraulic rams? Much more compact and safe way to create this sort of motion.

Sent from my XT1072 using Tapatalk

Ave made a "100" ton press with 4, 1-8 threads.
Structural Failure! Home made press - YouTube

a lot of interesting problems displayed in this video..
he then tries again here
REMATCH: 1�� ton-ish press versus one point five inch punch - YouTube
i think you'll find it entertaining.

but two 3/4-10 acme screws would have to be made from a single crystal of silicon or some other commonly available element to handle the stress.. but i don't think you can justify trying to find 300,000 psi steel to grind the screws from. not to mention the stress risers that the acme thread form will create at the root of the thread, or your fixture distorting under the load.

you would have to cut the acme threads at an angle so that under the full load the plate you're applying these forces to when it bends would become inline with the screw, otherwise the screw will snap due to bending forces, or the threads will give out due to stress concentration.

and you would have to cut the acme threads at a different pitch from the screw (say, 9.993 turns per inch) so that when the screw is fully loaded, the load is spread across the threads evenly. (if the screw is stretched at 200,000 psi, then it will be stretched approximately 7 thousandths per inch.

interestingly i have only heard of pressure vessels being tapped with threads cut at a different pitch, i haven't found anyone mentioning that they did it.

allegedly helicoils are also a known, legitimate way to spread the thread load out across more threads.. but again, i asked about this on reddit (i know, i know) and the answer i got was something along the lines of the larger hole needed for the helicoil weakens the flange, just use a larger bolt instead, rather than a helicoil and a stronger, but smaller diameter bolt.


another known way of reducing the thread stress is you simply cut the threads at the same pitch as the bolt, but then you run a tapered reamer through the tapped hole and cut off some of the thread form. the taper might be on the order of 1 degree, you might cut off half the thread form of the first thread and none of the 8th thread.

50 tons on two 3/4" diameter threaded rods?

Death trap.

I saw a setup fail, two 1" diameter thread puller rods, made by OWT (Owatonna Tool Company) with a 50 ton hydraulic cylinder. One rod broke way before maximum pressure, a piece of rod with the nut flew back, hit a guy on the arm and broke his arm.

If anyone knows how to make quality puller rods, it is OWT. The was simply a case of misuse.

Look around in tooling catalogs, especially OWT and EnerPac. Chances what you want to do has been done before, nothing new in manual and hydraulic pullers.

And if your design uses only the threaded rods in some type of yoke, no one will buy it, they do as I and purchase pullers using hydraulic cylinders.

PS if you are thinking of mass marketing, don't forget the liabilities involved.

Regardless of all the other issues raised here, a single Unipass type of tap would work. We tap thousands of 1-5 tpi holes every year in 1045 using them. That is the way to go for production. If you just want a few, you will probably save money buying a rough and a finish tap. As stated above, you are going to need a lot of low rpm torque to make it work.

+1 on the fact that your thread diameters look a bit on the small side.

If I were you I'd look into (and at the very least consider) using a Buttress thread if you only want the force going in one direction.

Are there vibrations involved in what you're trying to do?