Tips for cutting internal ACME threads?

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kingjamez
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Tips for cutting internal ACME threads?

Post by kingjamez » Sat Jul 12, 2014 7:08 pm

All of the tutorials I can find for ACME threads are for external threads. I want to make an adapter that fits onto my many YHM QD mounts. I found here, and measured my own mounts that the thread pattern is 8TPI with a major diameter of 1.030 inches and a thread depth of 0.045".

Any tips for threading this internal thread?

Any good sources for a pre-made threading bar that would work?

-Jim
My Form 1 Monocore build on YouTube:
http://www.youtube.com/playlist?list=PLH37sv2ySHmB1tTivFYIF8y2CW8rmiZRi

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john.t.little1
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Re: Tips for cutting internal ACME threads?

Post by john.t.little1 » Sat Jul 12, 2014 7:22 pm

I just bought a left hand threading bar that accepts inserts and got stub acme inserts for it. Just cut a relief and use it as start point put lathe in reverse and use thread chart for depths of each pass. It should be double start if I'm not mistaken. You'll need to figure out what numbers on your lathe to start fist thread and second I think on 8 tpi you'd use 1 for first and whatever's across from it. I do 10 stub acme threads on my qd mounts. And I had to do 1 and 6 I have 8 hashes on my threading dial tho. Are you sure it's 8? I know AAC uses acme threads but it's like 9.8 tpi or something screwy like that.
"A man is known by the silence he keeps. - Oliver Herford"
"Gunpowder, treason and plot. I see no reason why gunpowder, treason
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Historian
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Re: Tips for cutting internal ACME threads?

Post by Historian » Sat Jul 12, 2014 7:23 pm

kingjamez wrote:All of the tutorials I can find for ACME threads are for external threads. I want to make an adapter that fits onto my many YHM QD mounts. I found here, and measured my own mounts that the thread pattern is 8TPI with a major diameter of 1.030 inches and a thread depth of 0.045".

Any tips for threading this internal thread?

Any good sources for a pre-made threading bar that would work?

-Jim
Too hot to make a joke about asking Wylie Coyote. :)

<< http://www.youtube.com/watch?v=3Z7s6nXJfRo >>

Once tried it as a learning exercise. Like cutting any thread. It is the shape of
the tool that matters.

Checking my bookmarks here are some starting information:

<< http://www.smithy.com/machining-handboo ... -3/page/24 >>

<< http://www.homemodelenginemachinist.com ... php?t=8027 >>

<< http://en.wikipedia.org/wiki/Trapezoidal_thread_forms >>

My favorite is from Firearms Designer:
<< http://firearmsdesigner.com/?p=100 >>

My results were pretty crumby the first 4 to 5 times I tried with brass.
There after I did an acceptable job, checking off another box on my
quest to waste metal chips. :)


Have fun and let us know your results.

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kingjamez
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Re: Tips for cutting internal ACME threads?

Post by kingjamez » Sat Jul 12, 2014 8:26 pm

john.t.little1 wrote:I just bought a left hand threading bar that accepts inserts and got stub acme inserts for it. Just cut a relief and use it as start point put lathe in reverse and use thread chart for depths of each pass. It should be double start if I'm not mistaken. You'll need to figure out what numbers on your lathe to start fist thread and second I think on 8 tpi you'd use 1 for first and whatever's across from it. I do 10 stub acme threads on my qd mounts. And I had to do 1 and 6 I have 8 hashes on my threading dial tho. Are you sure it's 8? I know AAC uses acme threads but it's like 9.8 tpi or something screwy like that.
As I measured more tonight, I found that it's actually a trapezoidal thread and isn't truly ACME. It's single start, 8 TPI, with a thread depth of 0.045". It's thread depth is obviously not P/2 of a stock ACME thread, and it's not stub acme either at P/3. Looks like I'm going to have to grind my own internal threading bar... no fun...

-Jim
My Form 1 Monocore build on YouTube:
http://www.youtube.com/playlist?list=PLH37sv2ySHmB1tTivFYIF8y2CW8rmiZRi

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john.t.little1
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Re: Tips for cutting internal ACME threads?

Post by john.t.little1 » Sat Jul 12, 2014 9:52 pm

Almost make you want to re cap your cans and make your own brakes
"A man is known by the silence he keeps. - Oliver Herford"
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Re: Tips for cutting internal ACME threads?

Post by HandyMan » Sun Jul 13, 2014 11:27 am

Acme and stub Acme threads have substantial clearance built in to the thread form. On the order of .020". I believe YHM does use Acme threads. It is hard to measure with this much clearance.

The best way to measure is the three wire method. If you don't know what I am talking about, watch a couple of tutorial videos on Youtube.

The three wires measure the pitch diameter, not the major or minor diameters.

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L1A1Rocker
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Re: Tips for cutting internal ACME threads?

Post by L1A1Rocker » Sun Jul 13, 2014 1:04 pm

I'm in the process of gearing up to make my own QA/QD muzzle brake using ACME threads. I could not find any pre made HSS tool bits for cutting ACME threads - they are all the fancy insert type. Soooo, here is a gate guide I made for grinding 29* tool bits on my tool bit grinder.

Image

Image

Image

Note, the guide bar is too low. I plan on repositioning it so that the guide is MUCH closer to the grinding wheel. I've also made a guide for grinding 60* tools. They work very well.

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JohnnyO
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Re: Tips for cutting internal ACME threads?

Post by JohnnyO » Sun Jul 13, 2014 1:25 pm

I ground my own out of a HSS insert blank and used a boring bar. I profiled it to match the YHM mount then practiced on some delrin to get get it right. It is 8 tpi, I cut it just like I would cut any single point thread.
Image
Image

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john.t.little1
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Re: Tips for cutting internal ACME threads?

Post by john.t.little1 » Sun Jul 13, 2014 2:27 pm

I got a lh boring bar and 16er inserts that way I can use the same inserts with my external threading tool.
"A man is known by the silence he keeps. - Oliver Herford"
"Gunpowder, treason and plot. I see no reason why gunpowder, treason
Should ever be forgot..."

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Capt. Link.
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Re: Tips for cutting internal ACME threads?

Post by Capt. Link. » Sun Jul 13, 2014 3:19 pm

JohnnyO wrote:I ground my own out of a HSS insert blank and used a boring bar. I profiled it to match the YHM mount then practiced on some delrin to get get it right. It is 8 tpi, I cut it just like I would cut any single point thread.
Image
Image
Superb!
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john.t.little1
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Re: Tips for cutting internal ACME threads?

Post by john.t.little1 » Sun Jul 13, 2014 5:07 pm

I read on ar15 forum it's

ACME 8TPI
Major: 1.030
Minor: .940
"A man is known by the silence he keeps. - Oliver Herford"
"Gunpowder, treason and plot. I see no reason why gunpowder, treason
Should ever be forgot..."

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kingjamez
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Re: Tips for cutting internal ACME threads?

Post by kingjamez » Tue Jul 15, 2014 8:35 pm

Thanks for the tips all! I've been doing a lot of research on ACME threads lately. The wide tolerances does make it difficult to measure accurately. So far, I believe that it's 1.030 major diameter, 8 TPI, Stub ACME. At least, that's what I'm going to cut on my first try.

I've decided to try and thread mill this thread. My Mini-Lathe is a joke, and the 6AL-4V I want to tap will just laugh at that trapezoidal tool after it flexes away. So it's not going to be as simple as grinding a tool that fits the YHM threads.

Practical Machinist says I can't thread mill an internal ACME, and they are probably right. In this application, I don't think it needs to be perfect though, the chamfer around the muzzle device after the threads is what keeps it all straight. All the ACME needs to do is provide the linear motion... which a non-perfect thread will do just fine.... I hope. I'll come back if it works, or if I fail I suppose :-)

-Jim
My Form 1 Monocore build on YouTube:
http://www.youtube.com/playlist?list=PLH37sv2ySHmB1tTivFYIF8y2CW8rmiZRi

mhidpa
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Re: Tips for cutting internal ACME threads?

Post by mhidpa » Sat Aug 16, 2014 4:06 pm

Not muchh more to offer, but I made mine with an 8tpi ACME insert on a threading bar and used my mount as a go gage. I cut it slow and used a ton of cutting oil. Don't forget to set your compound to 14.5°. Image

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