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Machining an Internal Acme Thread...Mill? Or Tap? Simple? Or Tricky?

10/21/2021 12:22 PM

Have any of you fine folks ever worked with Acme threads? I'm going to have to get some 3/4-6 Acme internal threads put through a small 1" thick 304 SS block. Several dozen.

I'm curious if these will likely be thread milled on a (5-axis?) CNC, or if an Acme tap would do the trick. Considering my options. I might buy a tap and try to do in-house, maybe on a Bridgeport. If you think I might break a tap ($145) doing it myself, I might sent it to be tapped by one guy.

Or if it's better to mill it I might send it over to another guy.

Any thoughts or experiences with cutting Acme threads out there? In Stainless?

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#1

Re: Machining an Internal Acme Thread... Mill? Or Tap? Simple? Or Tricky?

10/21/2021 12:30 PM
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#2
In reply to #1

Re: Machining an Internal Acme Thread... Mill? Or Tap? Simple? Or Tricky?

10/21/2021 12:40 PM

Ah... a lathe. Of course! It's a small part. 2x3.

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#3

Re: Machining an Internal Acme Thread... Mill? Or Tap? Simple? Or Tricky?

10/21/2021 7:20 PM

304 is not the best candidate for hand or machine tapping because of its work-hardening, generic tap edges will wear fast and because of the increasing needed force you may get a broken tap after a number of threads. Taps specific for SS must have a bit higher rake angle to minimize the hardening but that makes edge less sturdy, so better, more expensive tap material is used and you still are on thin ice. On the lathe it should be pretty easy (if you don't need precision threads that is). Now what's happening? Too long out of the box? (LOL) S.M.

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#6
In reply to #3

Re: Machining an Internal Acme Thread... Mill? Or Tap? Simple? Or Tricky?

10/22/2021 8:23 AM

Good info. That being the case, BruceFlorida offered an interesting suggestion that could alleviate the issue you brought up.

And... I live out of the box. Don't even remember what the inside looks like any more.

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#4

Re: Machining an Internal Acme Thread...Mill? Or Tap? Simple? Or Tricky?

10/22/2021 7:25 AM

Here is an "out of the box" suggestion. Could you machine a hex shaped pocket and press fit or weld in a commercially available, off the shelf 3/4-6 SS Acme hex nut?

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#5
In reply to #4

Re: Machining an Internal Acme Thread...Mill? Or Tap? Simple? Or Tricky?

10/22/2021 8:03 AM

Interesting. That might not be cost effective, But I'll look into it.

In my situation, drilling these blocks to tap will be less expensive than usual because I already have them tapped with 5/8-16 UNC holes. So just opening the existing holes up to a size needed to change it to a 3/4-6 Acme threaded hole will be much easier than starting from scratch.

On second thought... it would be pretty easy to mill a hex hole with an existing hole too. Hmmm...

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#7
In reply to #5

Re: Machining an Internal Acme Thread...Mill? Or Tap? Simple? Or Tricky?

10/22/2021 9:25 PM

If they don't have to be pretty you could weld them into a round hole.

There are other options besides hex nuts. I'm not sure what McMaster Carr offers but your ideal solution might be (if available) Acme coupling nuts that are long enough to be cut to your desired full length. Within the world of CNC router parts and milling table parts there might be something that could be installed or cut to work. These additional ideas are probably only worthwhile if custom tapping your 304 SS parts becomes problematic.

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#8
In reply to #7

Re: Machining an Internal Acme Thread...Mill? Or Tap? Simple? Or Tricky?

10/25/2021 3:16 PM

Right, I was looking at coupling nuts. And you just mentioning that it could be welded into a round hole... yes, a light press fit to the diameter of the hex corners, to center, and then a few welds... it sounds closer and closer to the final solution.

Yeah, it's totally buried inside a hollow tube. No need for pretty.

That's what I'm going to try.

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#9
In reply to #8

Re: Machining an Internal Acme Thread...Mill? Or Tap? Simple? Or Tricky?

10/25/2021 8:31 PM

Now that I got you all excited we might have a little buzz-kill to deal with. Be aware that you will probably have more backlash with "off the shelf" coupling nuts than you will with carefully cut threads.

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#10
In reply to #9

Re: Machining an Internal Acme Thread...Mill? Or Tap? Simple? Or Tricky?

10/26/2021 8:29 AM

Backlash. Do you mean as you release the wrench/handle after tightening, it will move back some amount? That doesn't matter. Super high precision isn't needed.

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#11
In reply to #10

Re: Machining an Internal Acme Thread...Mill? Or Tap? Simple? Or Tricky?

10/26/2021 12:15 PM

If you were using a stepper motor to position something then going clockwise to a specific angle would not be quite the same position as if you went counterclockwise to that same angle. It sounds like you are not doing that so my revised response is .

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