Acme Threading

JohnG said:
I'm confused here. My understanding is that Acme screws are designed to transmit motion, and so they develop less friction than V threads. This makes them less useful for clamping. Also, I've read that an Acme screw and nut should be dissimilar materials to minimize wear--steel screw and bronze nut is common.
I turned this left hand Acme screw to replace the worn cross feed screw on a surface grinder. I bought a pair of bronze Acme nuts and turned external threads on them to assemble a double nut with more bearing area and a slight tension between the nuts to minimize backlash.
Turning Acme threads develops much higher lateral force. If you are turning a left hand Acme screw between centers, tighten the tailstock extra tight. A single point tool for turning small diameter inside Acme threads is the hardest lathe tool to grind for my money.
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Correct, ACME is for motion and do not have the holding properties that a 60 degree thread would.
 
Acme threads do not have the wedging self locking tendencies of a standard 60 degree thread. Thus they can still turn while under high load. You can apply more clamping load for a given screw force, and more importantly, you can loosen it back up without a large hammer.
 
So inconclusion, take your pick. If you buy a V thread & Coupling nut try to get a grade 8. Better precision, stronger and the screw will have been rolled rather than cut. You can do the threading in tool steel and then harden and temper. O1 is supposed to be less inclined to warp than W1. Air hardening might be a good bet if you can cover it to cool slowly.
As to why right or left hand, depends on which part the nut is in. A drill press vice takes a right hand because the nut ins in the frame. A mill vice like Kurt has the nut in the moving jaw.
 
If you're worried about thread strength on v threads make the screw a larger diameter and the nut longer( since it's your design you can make it as hefty as you want) The nut on a 6" kurt vise is over 5" long.
With a 2' cheater bar on my 8" kurt I am pretty sure the pressure generated is well over 5 tons ( from the vise testing video on you tube it may be between 30.000 and 40,000 pounds) no bending or movement at all,rock-steady.
 
And I can see we are well down in the weeds now. It's a drill press vise, not for a horizontal mill or shaper.

devils4ever said:
How tight a fit is required so it doesn't spin?
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If you are going for a press fit, I'd go .0003 to .0008 interference. @markba633csi 's suggestion for a slip fit with Loctite is a good one. The fit will be less critical, assembly will be easier and the holding power more than sufficient.
 
I was going to comment that this if for a drill press. So, I'm not looking for maximum clamping force.
 
I think I might try using the lathe to create the internal Acme thread in the steel block. It sounds like a good experience to have.

If I have trouble, I have a few options:
  1. finish the hole with a tap
  2. bore it out and use a premade bronze nut/flange
  3. bore it out and make a bronze insert (I'm assuming cutting a thread in bronze is easier than steel?)
  4. use a V-thread instead since I already have 1/2-13 and 1/2-20 taps and dies
 
The angle of the thread contact face couples bolt tension to hoop stress in the nut. Shallower angles than 60 degrees mean the nut stretches less. With highly loaded shallow threads in thin nuts (think oilfield downhole tools) this can end in unexpected nut popping off thread failure.

For our purposes, acme, square, and buttress threads all have a bit less friction, are less self-centering, more wear resistant, more power efficient, and more irritating to cut ;-)

Buttress threads are the ultimate for one-directional loading, but super irritating to cut and gauge. And there are no taps or dies... there are, however, inserts available! In the oilfield, the suction covers on frac pump fluid ends are held on with buttress threads. BIG ones.

Sent from my SM-G892A using Tapatalk
 
A few comments, if I may. Nothing to add, just a few points to clarify things. First off, there are several thread forms for motion and load carrying. 60* such as bolt threads have the highest torque capacity. ACME is the most common motion transmission, with moderate load carrying. Square threads are the strongest in that respect but are hard to cut on external threads and a bear to cut internally. Butress threads are sort of a mix between ACME and square. They are essentially square on the load carrying side and ACME on the "off" side. Most often seen on big screw jacks, like a house jack or a railroad jack. For Square and Butress threads there are no "common" taps for doing inside threads. Although there must be somewhere for commercial producers.

Tandem taps now, I have a few. At one time I was building 5/8x8 Left Hand followers for a friend in the commercial ceramics industry. That has gone by now, but I still have the taps. I used a tap because my lathe is fairly light and holding the piece to be tapped was near impossible. The tandem tap for 5/8" is about 3-1/2 inches of a "V-sharp" 60* thread, followed by about 3 inches of the ACME. The taper for the starter portion is long, like a "starter" tap for bolts. The transition from V-sharp to ACME is shorter, sort of like a "plug" tap.

Tandem taps are pricey! I would hesitate to suggest one for one or two uses. I have both 1/2-10 and 5/8-8, right and left hand pitch. I bought them when times were flush, so the price didn't hurt too much. But they do exist, albeit costly. I have an Atlas lathe, MF-C milling machine and an S-7 shaper, that was the reason for acquiring the 1/2-10s. I haven't used them much, haven't needed them. I also have a few smaller sizes, down to 1/4 inch. They were salvage from an aircraft repair source. Never used them but they were cheap and I could foresee uses "down the road" for them.

In use, I would start the starter thread in the lathe to maintain concentricity. When the brass started to slip in the chuck, about 4-5 pitches, I would remove the entire assembly to a vise and cut the rest by hand. With a very large tap wrench. They do not cut easily, even in soft metal like brass. Precutting with a boring bar and then finishing with the tap would take a lot longer but much less work.
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I decided to order a Tandem tap to cut this thread. It is very $$$, but I think my chances for success are greater.

I've read that cutting internal Acme thread on the lathe with a small diameter (<= 1/2") is tricky because the tool can't be thick/stiff enough to cut well.

I may start the thread in the lathe and cut a 1/2-10 V-thread first. It may help remove some metal first and start the tap squarely. The largest tap wrench I have is a Starrett 91B which is 9" long. Do I need a larger one?

Also, remember this is being tapped in 1.2" of 1018 steel. If I run into trouble, I may bore it out and use a brass or bronze insert.
 
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