Acme threads

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jscarmozza
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Acme threads

Post by jscarmozza » Mon Dec 04, 2017 4:16 pm

When cutting Acme threads, is the compound rest still set at 29 degrees?

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GlennW
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Re: Acme threads

Post by GlennW » Mon Dec 04, 2017 4:23 pm

No, about 14 degrees.
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Harold_V
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Re: Acme threads

Post by Harold_V » Mon Dec 04, 2017 5:04 pm

The whole idea of setting the compound for chasing threads is to feed such that the tool takes the vast majority of material off one side of the thread, so there is little conflict of chip flow. Based on that theory, you choose somewhere between a half to one degree of angle less than the half angle of the thread in question.

It's also important (to prevent a drunken thread) to pick the proper attitude of the compound. In all cases, the angle chosen should be such that when the tool is advanced, it feeds in the same direction that the carriage travels in creating the thread. That way cutting pressure can't move the tool away from the thread. If fed in the wrong direction, that isn't true, and often yields an unacceptable thread.

Because manufacturers don't orient their markings from the same reference point, there are machines where the marking might read 60½° instead of 29½° (in this case, the example is for a 60° thread). For that reason, I prefer to reference compound location as one reads a clock (compound set @ 5:00 for a 60° thread, right hand class A (external)). That eliminates confusion.

H

Edit
One more comment. The slightly less than half angle serves a second purpose. By taking the majority of metal off one side, chips flow readily. However. to eliminate any possibility of a slight step in the trailing side, by choosing a slightly less than half angle, the backside of the thread is constantly wiped, assuring a better surface finish. By feeding at greater than the half angle, each pass of the tool results with an offset of the tip, leaving multiple steps on the backside of the thread.
Last edited by Harold_V on Tue Dec 05, 2017 1:07 am, edited 1 time in total.
Reason: To include another comment
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jscarmozza
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Re: Acme threads

Post by jscarmozza » Mon Dec 04, 2017 5:44 pm

Thanks Glenn and Harold. I cut a lot of threads but never did Acme threads. That being said, I plan to cheat a little, I'm going to get close on the lathe then chase it with a die. John

jscarmozza
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Re: Acme threads

Post by jscarmozza » Tue Dec 05, 2017 9:54 pm

My first try didn't turn out very well, the cutting pressure at 0.005" per pass, pushed the stock into the chuck and made a mess of the thread. I'm going to have to come up with a more secure setup.

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GlennW
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Re: Acme threads

Post by GlennW » Tue Dec 05, 2017 10:20 pm

Make sure that the tool bit isn't slightly above center. That could increase the cutting pressure considerably.

Also look at the tool bit closely and make sure it isn't rubbing below the cutting edge from not enough relief.
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John Hasler
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Re: Acme threads

Post by John Hasler » Tue Dec 05, 2017 11:50 pm

GlennW wrote:Make sure that the tool bit isn't slightly above center. That could increase the cutting pressure considerably.

Also look at the tool bit closely and make sure it isn't rubbing below the cutting edge from not enough relief.
That's very likely the problem.

A cheat for acme threads: cut a 60 degree thread to slightly less than the intended width of the acme thread first to get much of the metal out of the way.

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Re: Acme threads

Post by AllenH59 » Wed Dec 06, 2017 1:24 am

jscarmozza wrote:Thanks Glenn and Harold. I cut a lot of threads but never did Acme threads. That being said, I plan to cheat a little, I'm going to get close on the lathe then chase it with a die. John
I bet you can cut them to size without a die.. dies often cut loose threads.

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Re: Acme threads

Post by Harold_V » Wed Dec 06, 2017 2:55 am

Pushing the stock in the chuck leads me to believe you have an issue with clearance, as has already been suggested. The finish will suffer, and the tool won't cut well. Rarely can that condition exist and not leave tell-tale signs of rubbing, both on the part and on the tool.

Keep in mind, when cutting threads, especially coarse pitch threads, the helix angle is a problem, so the leading edge of the tool may require clearance (relief) that looks excessive. That makes sense because you have to overcome the angle of the helix, plus have clearance. The trailing edge requires very little, as the thread is moving away from the tool. The tool, then, may look rather strange.

If you are having difficulty, you can always use a slightly narrower tool, taking the thread to depth. Once to depth, using the compound set @ 90°, you can feed the tool side to side, taking skim cuts from each face until you have the required size.

If you make this a choice, make sure you snug the carriage lock when taking any cuts on the trailing edge of the thread, By doing so, cutting pressure won't be able to overcome the carriage, and will limit the machine's ability to cut a drunken thread. Keep your tool sharp.

This method can be used to create a square thread, first, then using the proper tool, cutting each side independently. That virtually eliminates chip flow issues, and decreases the amount of tool in contact with the cut by a huge margin.

H
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John Hasler
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Re: Acme threads

Post by John Hasler » Wed Dec 06, 2017 1:54 pm

Harold writes:
> Keep in mind, when cutting threads, especially coarse pitch threads, the helix angle is a problem, so the leading edge of the tool may require
> clearance (relief) that looks excessive.

That is a *very* good point.

Also, try alternating cutting passes with spring passes.

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Re: Acme threads

Post by Harold_V » Wed Dec 06, 2017 3:59 pm

John Hasler wrote:Also, try alternating cutting passes with spring passes.
That often works nicely for many materials, but if stainless is being machined, it can prove to be a mistake, especially if the tool isn't keenly sharp. Stainless has a nasty habit of work hardening, which, once established, is difficult, if not impossible, to overcome. In threads it is typically fatal, as one can't take a cut deep enough to get beneath the hardened surface. Key to success is in keeping the tool very sharp, so work hardening doesn't occur. Material choice can also be a factor. Choose 416 if magnetic stainless is acceptable, or 303 S or Se if it isn't. They machine freely, and don't display work hardening nearly as badly as most other stainless alloys.

H
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jscarmozza
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Re: Acme threads

Post by jscarmozza » Wed Dec 06, 2017 5:13 pm

Done...but not the best work I ever did. I was putting about 10" of 3/4x6 acme threads on the end of a 5 ft shaft, normally I would have used my SB lathe which is a little more rigid, but I have it shoehorned into a space that wouldn't accommodate the 5 ft shaft so I had to use my little Atlas lathe. Everything was going well until I lost my concentration and jammed the tool. I was close to the end, or so I thought, so I finished it with the die and it turned out a little rough...but it should work.

Thanks for the advice and tips, I did regrind the tool and carefully centered it on the work then took .0025" cuts, that made a big difference. I also added a collar with set screws so the work piece didn't push back into the chuck. I can also see the benefit of roughing out the blank before cutting the threads with the acme form tool, if I have to make anymore acme threads I'm going to try it. Thanks again, John

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