Making Acetal leadscrew nuts the easy way

[A.K. Boomer]

Some time ago I repaired the trunnion pins on my shaper by bushing the worn pins using acetal bushings. It has had a lot of use since then and there has been no indication that the bushings have worn or are failing in any way. It is a pretty severe application and the bushings are not very thick so if they can take this sort of punishment then I see no reason why they won't work well on a larger machine in leadscrew service.

Very good Evan, I remember you doing that and wondered about that machine from time to time as I argued against it mainly for the fact that if you lost one side it would put the remaining components under a heavy awkward uneven bind, That plastic is tougher than I thought it would be - esp. because its always seemed a little crack happy when Iv machined it...

[mrpeja]

Leadscrew acetal nut

Hi,
That's a very good idea.
I have question: what do you think about making acetal nut for standard metric threaded rod? I can byu very cheap rod 1 meter lenght.
BTW, does anyone have an idea how to put together two rods, and get 2 meter lenght?
Mrpeja

[Peter.]

For a standard metric thread you would make a nut by drilling & tapping, surely?

[Evan]

Welcome to the forum Mr Peja.

Threaded rod will work but has a much lower load carrying capacity than acme thread.

To fasten two pieces of all thread rod together bore and thread each piece to fit a smaller threaded rod or cut off end of a bolt. Wind the rods together and check for proper alignment of the threads using a regular nut. As required shave off a tiny amount from the mating end of one rod until good alignment is achieved. Assemble with locking compound.

[mrpeja]

Leadscrew acetal nut

Hi Evan,
thanks a lot for the quick reply, this will help me.
To Peter:
Surely I can do drilling & tapping but Evan's method is without backlash.
Mrpeja

[Ken_Shea]

A second welcome to the forum.

Would be a good idea also to face off the end of each rod, generally the start thread is not particularly square to the rod due to the thread being roll formed.

[Kerry Harrison]

Just went to the Enco website to purchase some Acetal. Their website says Delrin is the same thing. Is that true?

Thanks for the great idea, should make CNC conversions a lot simplier.

Kerry

[Evan]

No, it is not the same thing. There are differences in the chemical resistance that matter a lot in some applications dealing with acids. The strength is different with Delrin being about 15 to 20 percent stronger but acetal has more uniform density in large extrusions. The melting temperatures are also slightly different but not enough to make a difference in this application.

[RKW]

I have never seen Delrin on their site only Acetal

Just went to the Enco website to purchase some Acetal. Their website says Delrin is the same thing. Is that true?

Thanks for the great idea, should make CNC conversions a lot simplier.

Kerry

[Toolguy]

Acetal works fine for this.

[alsinaj]

acetal worm gears

Would the following procedure work for forming an acetal worm gear:

Turn and gash the blank out of acetal.
mount the worm so it is spring-loaded against the blank, with a stop to prevent the worm from going beyond full depth.
Rotate the worm while heating it.
Continue until the worm hits the stop.

O-pinions?

[RKW]

Try it and see. That's how Evan found out and why he started the thread. It's all about experimentation. For many of us it's half (or more) of the fun.

Let us know what you find out.

Would the following procedure work for forming an acetal worm gear:

Turn and gash the blank out of acetal.
mount the worm so it is spring-loaded against the blank, with a stop to prevent the worm from going beyond full depth.
Rotate the worm while heating it.
Continue until the worm hits the stop.

O-pinions?

[sansbury]

Would the following procedure work for forming an acetal worm gear:
<SNIP>
O-pinions?

While I agree with RKW's advice to try it, my gut sense is less than fully optimistic.

What you're trying to hit is a point where the acetal is deformable enough to take on the shape of the worm, but not so malleable that the worm can slice right through it. My horse sense is that's going to be a very fine, possibly impractical, and perhaps even impossible line to stay on. But I could be totally wrong. Perhaps acetal has a very wide and gradual viscosity-temperature curve? One thing I know is that I don't know.

This isn't a problem with Evan's nuts because there's really no problem if the acetal becomes softer than necessary, short obviously of igniting it.

[jkilroy]

Please don't ignite Evan's nuts, I can't imagine the thread that would follow!

[Evan]

My horse sense is that's going to be a very fine, possibly impractical, and perhaps even impossible line to stay on.

That is basically correct. Acetal, like many plastics is a crystalline plastic. That means when it melts it behaves a lot like ice in that it undergoes a phase transistion from the solid to the liquid state. It happens at a specific temperature and is not a gradual process. While it does become easier to deform as it is heated it doesn't flow into another shape until molten.

However, acetal is very easy to machine so make a worm on the lathe and grind some cutting edges axially and use that to hob the acetal cold. That will work.