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Stub Acme, what is it?

Art K

Aluminum
Joined
Sep 2, 2004
Location
None, OR
I am a hobbyist so be gentle. :rolleyes:

I look in Machinery's Handbook and I can't seem to tell what the significance is between regular Acme and Stub Acme. Can someone educate me on any significant differences? When would I want one over the other?

Thanks in advance!
-Art K
 

toolmakerjim

Titanium
Joined
Jun 21, 2004
Location
titusville fl. usa
Art;
its been my experience that Acme threads are used for power transmission such as rifling machines, barrel presses, to straighten gun barrels.
now the stub acme threads that i have been associated were for warhead installation on anti personell bazookas (SMAW).
these threads were in aluminum and its my opinion that the stub acme is just a stronger application of a thread.
there is a great difference in the actual form of the 2 threads with the stub obviously being smaller in cross section.
hope this helps in some small way...jim
 

Art K

Aluminum
Joined
Sep 2, 2004
Location
None, OR
Thanks Toolmakerjim. Now I can take a closer look at Machinery's Handbook. I appreciate the reply.

Regards,
-Art K
 

bluchip

Stainless
Joined
Jun 11, 2005
Location
OH
Along the same lines as Toolmakerjim stated, I was under the impression that standard Acme was for power transmission or dynamic purposes, (adjustable tie bars on presses, vises, etc.) and stub was a more permanent situation...where assemblies are screwed together and not meant for disassembly. The stub form would be not only stronger but would allow a thread on thin walled material like a collar or cap threaded on a tube. Is a form of this used on well casings in the oil industry? EZ
 

Forrest Addy

Moderator
Joined
Dec 20, 2000
Location
Bremerton WA USA
The proportions of the stub Acme threads are the same as full depth Acme except in the depth calculation and the tool width. The stub Acme has 60% the depth of the general purpose Acme thread. In Machinery's handbook the illustration and formulae for full depth and stub Acme are combined. The designators, the tolerance and class of fit formulae are the same.

There's a regretable lack of general discussion of stub acme threads in the Handbook and I've cut very few myself in my long career. I venture to say without technical cooberation that stub acme threads are best used for thin walled items and where for rapid assembly under extreme conditions and high shock were factor but not necessarily motion transmission.
 

Norman Atkinson

Titanium
Joined
Feb 8, 2005
Location
TYNE AND WEAR, ENGLAND
Art,

As you will already have guessed your lathe and miller have Acme threads. So will your vice-vise. They are a stronger thread than the normal ones.
Having said that, stub acmes are used where it is not possible to fasten both ends down. One of the classic uses is in dividing heads where the thread drives a gear- usually a helical one.

It would be wrong to give you mental indigestion- and I am glad that other writers have given nice simple examples.
Keep them comin'. We are here to help.

Norman
 

BobB

Cast Iron
Joined
Dec 24, 2003
Location
United States of America
I cut stub ACME quite abit usaully for motor operated valve stems.

Forrest Addy got it right, the stub are basically 2/3 the thread deapth of the standard.

Having dealt with various factory reps over the years that build them, I've been told that the thread strength is basically the same as a standard ACME but quicker and easier to produce.

So you have the same thread format with the same strength, but easier to manufacture and it takes less time. If using a stub ACME tap, you'll use less horsepower to run it than you would for a standard.

25 years ago when I started as a Machinist at the Nuclear plant where I still work,everything was standard ACME. Its been in the last 15 or so years that the stub ACME had become popular.

Even so, it is not uncommon for a stub acme threaded valves stem to run in a standard acme bronze driver nut. I can think of a few cases where it is specified soley for the ability to be able to withstand higher clearences due to extreme length of a stem, or one that may be less than perfectley straight.

About a month ago a 12 foot long drive stem that was 2 inches in dia. that operated a 4'dia. butterfly valve was brought into the shop. That sucker was bent all over the place with about 3 different bends that deviated about 6 inches off plane. Beleive it or not, it still operated the valve perfectly. The mechanincs brought it to us to straighten, thinking that it was so ugly that they might as well get us to fix it while they were changing the rubber seating suface on the butterfly. I told them to bring me the drive nut as I was sure it had to be shot. I couldnt imagine that it couldnt be. As it was, there was nothing wrong with the drive nut and it actually looked pretty good.

We straightened out the shaft and put it back in operation. After looking at the tech manual AFTER the mechs put it in :eek: I noticed that is specified a Stub acme thread with a standard drive bushing. I have never heard that before but the manufacutuer confirmed that it was standard practice on some valves as regular threads on a regular drive nut would bind up and lock up the thread, rendering the valve inoperable.

Anyhow, in that case it worked. I have noticed that some stub acme taps come in a 2 tap set verses the standrd acme which is usually a 3 tap set.
 








 
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