need to cut internal 1.125" x 6 ACME thread 2.2" inches long

[barts]

I'm getting back to my 1920's 16" x 8' South Bend restoration. The half nuts
are completely worn out (lead screw is pretty good). I've clamped the two parts together and bored 'em out on the Bridgeport. The plan is now to turn a spool-shaped piece of ductile iron w/ the threads in the bore; the half nuts get clamped to the spool and silver brazed in place. I then cut the two apart on the bandsaw and clean up the parting line on the Bridgeport.

Any suggestions or advice on cutting those internal ACME threads? Any idea how much slop should be added so that things work well?

- Bart

[SGW]

At least it's big enough so you can use a good-size boring tool. On the other hand, it's also big enough so you'll be needing to take a pretty hefty cut, which may chatter.

I've never done internal Acme threads, but obviously, you need an accurate tool shape to get the thread form.

Any chance of buying an Acme nut of the proper size and turning down the o.d. of that to make the insert? Finding one with 2.2" worth of thread might be a problem, but if you could get one, that would solve the thread-cutting.

[Evan]

Here is the spec for ACME leadscrew threads.

[JCHannum]

1-1/8" is large enough that you will be able to use a stout boring bar. The slop for clearance should not be too difficult to achieve, as the leadscrew probably will have enough wear that a properly sized nut will work well.

You will want to either put a spacer between the two parts when threading or mill a bit off the faces of the completed parts so they don't meet when closed.

[Evan]

Note that what isn't clearly stated on the diagram is that the major diameter allowance for external centralizing ACME thread is +0 -.05p (p=pitch). For a leadscrew halfnut it isn't critical. Allow .05p at the root and crest of the nut thread and it should do fine.

[tyrone shewlaces]

Wouldn't hurt to have one of these acme thread gages to grind & align your thread boring tool:


I made an acme nut for a kind of low-precision application a while back and once I got the nut to fit a bit snug on the screw, I applied some lapping compound then ran the nut back & forth on the screw (spinning it in the lathe) until it felt about right. Worked very well.

On a lathe leadscrew a purist might be apprehensive to do this. Then again, if the screw is used and a little worn in the usual place, it will help to even up the fit along the length.

Here's a link to an Acme thread specs. calculator: