Making Acetal leadscrew nuts the easy way

[AlleyCat]

Evan- Have you measured the backlash? I'm sure it's minimal but just wondering how much is there. Thanks for sharing the idea.

[Ken_Shea]

What a simple solution that is!

[Bob Farr]

Excellent technique and thank you for sharing it.

Bob

[lynnl]

That's a great tip.

But I'm a little puzzled by the heating process you described.

You say heat with a heat gun, while tightening the vise slowly. That suggests to me that the acetal is only going to heat up to a softened condition.
Yet I'm understanding that the two halves are going to melt and flow together at the interfaces. ...is that right?

What is the melting temp of acetal? Are we just going right up to the melting threshold with the heat?

[Evan]

AlleyCat,

There is zero backlash, nil, nada. It must be opened up slightly to make it easier to turn. As formed, it is difficult to turn by hand on the screw.

On further reflection the way to chase the threads is to grind the end of the screw to perhaps 2 or 3 threads of length as a D bit. That means removing half the material so that the end is half round. That will give a good cutting edge to take off the very small amount of material required. I will take some more pictures to illustrate this.

What is the melting temp of acetal? Are we just going right up to the melting threshold with the heat?

Short answers: 338F, Yes.

Long answer: Acetal is limited to a maximum working temperature by the heat deflection temperature. This is load dependent. At a couple of hundred psi ( easily applied by the vise) the deflection temp is about 250F. This is when it begins to thermoform and consequently the amount of heat transmitted from the screw to the acetal increases as the contact area increases. As the temperature continues to rise the material softens further until liquidus is reached. This is easily observed as the acetal will quite suddenly start to flow. At that point the vise can be tightened some more and the acetal will squeeze out into the kerf and out the ends. It is important to heat mainly the screw as we don't want to melt the entire mass, only the inner portion.

[Peter.]

Evan you have completely surpassed yourself!

I'm going to do this as soon as I can lay my hands on some material.

[Mike Hunter]

Confused

Evan…First ..Nice tip.

Now somewhat confused, are you making nuts for machine tools out of plastic???

Full size lathe/milling machine nuts?
Like in lieu of brass/bronze?

If so how do they hold up compared to Brass/Bronze?

Thanks
Mike

[Evan]

I will just copy and paste these replies to that question from a recent thread by DickieBird.
-----------------------------
Acetal or Delrin is the material of choice for that application. If you can find a chunk at a reasonable price the only thing better is one of the PTFE filled varieties such as Torlon or Delrin AF or Ertalon and so forth. Plastic is the way to go as it is self wiping and doesn't wear the screw or itself. The Y axis nut on my mill is the original that I made from Acetal and it now has perhaps .0005 backlash. I made the shell for it in such a way that I can compress the nut and it removes any trace of backlash. I have made that adjustment only once.

My plastic Y axis nut hasn't even completely worn off the black oxide finish on the lead screw yet. It has hundreds of hours of running time on it. I purposely used bronze nuts and linear bearings on the X axis with plastic and PTFE on the Y axis so I could compare the long term wear and reliability. The Y axis is nearly pristine even though it is much more exposed to all the "grit" while the X axis shows significant wear both to the nuts and the lead screw as well as the linear bearings and rails. I will take some photos this evening when I have time. I will have to replace the bronze nuts, screw and the linear rails and shells fairly soon on the X axis. It will be with plastic nuts and bearings this time.

------------------

I will take those photos and post quite a few more on this subject a little later today. Acetal isn't the answer to everything as it has a limited static load capability. For the small home shop machine it is a winner. It is surprisingly stong and the impact resistance is superior to steel. Briggs and Stratton has been using acetal oil pump gears and acetal cams in their small engines for many years. Acetal gears are commonly used in the transmission of washing machines. I used acetal bushings to repair the 2" cast iron trunnion pegs on the yoke that drives the ram on my 16" shaper and they are holding up just fine.

Note: Don't try this techniques with the PTFE filled materials. The melting point is very different and if you overheat PTFE it will release toxic fumes. In this instance I didn't notice any trace of formaldehyde that might be released if the acetal is overheated. Be aware that it is a possibility and take measures to ventilate if needed. As long as the temperature is kept below about 500 degrees F this won't be an issue.

[Mike Hunter]

Evan

Thank you.
I’m getting a bit more play than I would like in my 14 x40 lathe, and looking to replace my cross and compound nuts, wondering if “plastic” will work.

You technique would certanly save time

v/r

Mike

[The Artful Bodger]

Excellant Evan!

[thinks] "If I connect my arc welder to the ends of the screw will a burst of 150 amps heat the screw enough?" [/thinks]

[whitis]

From www.roton.com, operating loads of some plastic acme lead screw nuts:
1/2"x8 625lbs
3/4"x5 1400lbs
1"x4 2500lbs
However, this was dramatically reduced when the OD of the nut was turned down to make a flange nut (for example 1/2" was turned down to 3/4" OD with a 0.188" * 1.5" flange), but turning it down to 0.938"x16TPI did not. You need some material to back up the threads.

You may need to scroll right to see the plastic nuts on their pages.

[Peter.]

Well I've done mine!

I remembered I had some white acetal which was already drilled through. The hole was a little bigger than ideal so I cut the piece in half and then sanded another mm from one side. I then followed Evan's instructions and everything went as planned except I kept turning the screw all the while during heating thinking that it might make releasing the screw easier. This wasn't a good idea because I ended up with scruffy-looking threads where they had dragged a little on the screw when the nut melted (also bear in mind my screw is a 'modified square section', with just 10 degrees included angle, which I think will tear easier than an acme thread due to the wider crest). I bored my original cross-slide nut through, turned the acetal nut down, pushed it in and pinned it either side with a pair of 2mm pins.

The result is I have eliminated the backlash from my cross-slide. The handle is a little heavier to turn but I'm going to leave it alone for a while to see if it loosens, if it doesn't I'll put the nut in the freezer and use Evan's trick to chase it out with the screw.

[Liger Zero]

There is nothing very critical about the hole. The material will flow into place and when it is machined in place on the screw concentricity is automatically achieved. This is basically injection molding turned inside out. Or is it outside in?

More of a "thermoforming" or "compression molding" method.

Very impressive. I can see this being useful for some other applications.

[Evan]

Another thing I haven't mentioned is the best way to drill acetal. Instead of using a regular twist drill use a spade bit. No pilot hole is needed. The spade bit should be reground to eliminate any grain cutters on the OD and the rake should be made very low angle. This means that the cutting edge should be nearly square with only a few degrees of relief on either side of the cutting edge. This will prevent digging in and because there is so much room for chips there is no tendency to overheat or bind. It also drills very straight and to size even with a cheap bit. Also note that auger bits DO NOT WORK.

[dp]

The result is I have eliminated the backlash from my cross-slide. The handle is a little heavier to turn but I'm going to leave it alone for a while to see if it loosens, if it doesn't I'll put the nut in the freezer and use Evan's trick to chase it out with the screw.

I wonder how it's going to hold up to heavy loading like knurling. Guess I'll give it a try, too.