Cutting Acme threads

[flutedchamber]

I have to cut some external Acme threads, 5 per inch. I have cut threads before, but never Acme.

Do I just go straight in with the cross feed or in at a 14.5* angle with the compound

Thanks for your help.

 

[Tony Wells]

You can do it either way. Straight in will give even support to the tool on both sides, and minimize pushing from the heavy cut side if you compound in. If you have a heavy enough lathe, and it's rigid enough, compounding will yield a good thread, and requires less torque. If you have a neutral grind on the tool, straight in works just fine, but if you have any top clearance, it won't cut on the trailing side very well and you should compound in.

I never use the compound method myself. But then, I never use HSS for threading.

Are these full acme or stub acme threads?

 

[flutedchamber]

author=Tony Wells link=topic=3873.msg28364#msg28364 date=1317743494
You can do it either way. Straight in will give even support to the tool on both sides, and minimize pushing from the heavy cut side if you compound in. If you have a heavy enough lathe, and it's rigid enough, compounding will yield a good thread, and requires less torque. If you have a neutral grind on the tool, straight in works just fine, but if you have any top clearance, it won't cut on the trailing side very well and you should compound in.

I never use the compound method myself. But then, I never use HSS for threading.

Are these full acme or stub acme threads?

They are stub Acme threads in aluminum. It's for a paintball gun barre. My lathe is heavy enough to use the compound. I'll check the toolbit to see how it's ground.

Thanks everyone for your help.

 

[george wilson]

Do you have an Acme threading tool grinding gauge? Lets you establish the correct angles,AND gives you a gauge to get the flat tip of the threading tool the right width.

If you have at least a piece of Acme threaded rod of the correct spec.,you could use it to help grind the tip the right size.

 

[flutedchamber]

author=george wilson link=topic=3873.msg28381#msg28381 date=1317751465
Do you have an Acme threading tool grinding gauge? Lets you establish the correct angles,AND gives you a gauge to get the flat tip of the threading tool the right width.

If you have at least a piece of Acme threaded rod of the correct spec.,you could use it to help grind the tip the right size.

Yes, the gauge I have. The toolbit is ground and fits perfectly in the gauge. My only concern is if there is enough clearance so the bottom of the bit does not drag.

The piece I am threading uses a profile and size of a 4 tpi cutter but is threaded at 5 tpi. I guess the paintball gun manufacturer wanted to make things as difficult as possible to duplicate.

 

[flutedchamber]

I'll post a pic as soon as I thread the beast, which will probably be tomorrow morning. ::thumbzup::

 

[Tony Wells]

Stub acme is shallower in thread height, and the minor diameter is larger. Do you have all the pertinent dimensions? If not, I have a program that will calculate all you need to cut it. If you are using an acme thread gage, it won't be correct for stub. The root width is also different on a stub vs full acme.

What is the nominal size of the thread, or lacking that, if you are duplicating something, what is the measured OD?

 

[flutedchamber]

author=Tony Wells link=topic=3873.msg28405#msg28405 date=1317759925
Stub acme is shallower in thread height, and the minor diameter is larger. Do you have all the pertinent dimensions? If not, I have a program that will calculate all you need to cut it. If you are using an acme thread gage, it won't be correct for stub. The root width is also different on a stub vs full acme.

What is the nominal size of the thread, or lacking that, if you are duplicating something, what is the measured OD?

The bit profile for 4tpi matches exactly the width of the stub threads on the barrel..which are threaded @ 5tpi. I ground the cutter to match the 4tpi profile on thread gauge, which matches the threads on the old barrel perfectly. The OD on the threads from the top of my head is just under one inch..perhaps .985 give or take, one thread every .200, which would make it 5 tpi.

I'll be wandering out to the shop after dinner and will have exact measurements for you. ::thumbzup::

I checked everything by my Machinery Handbook (I'm old fashioned..that's the way I learned) and it all seems to fall in place. I'll cut a practice thread just to see if it turns out OK.

 

[flutedchamber]

A short pause while I secure a longer piece of aluminum. My grandson now wants an 18 inch barrel, not a 12 inch like he originally wanted

I will post pics as soon as I get my material. I was wrong about the depth of the thread on the barrel. It's not only a bastard OD and TPI, but the rood diameter is larger than standard. It's not a problem to cut...the threads just aren't as deep as the standard set for them.

Because I have to cut to a shoulder and there's only 1 1/2 threads I'll put the toolbit in upside down and spin the chuck in reverse, cutting from the shoulder out to the end. ::thumbzup::