Making Acetal leadscrew nuts the easy way

[Evan]

This is one of my favorite uses for Sketchup. It can be used to automatically make 3d models just from a single perspective view, especially if the image has very clear perspective lines. In this case I didn't bother going through the entire process although it is easy. By drawing an overlay of the acetal block I can get a good idea of just how big a block will fit. Then I can check it by doing a sketchy grid overlay in the correct perspective.

It is clear that a good sized block will fit. Make a nut somewhere near the centre of an oversized block, then machine the sides to locate the screw hole in the correct position. Leave enough material on the side that will be where the spigot goes and machine that on the lathe in a 4 jaw. Go a little past the correct depth so the nut can be accurately positioned in height using shims. If you are good enough you can skip that part but shims are easier to make than removing and replacing the nut in the four jaw to get it right.





To ensure the strength of the nut make the join line vertical. The underside has a couple of screws to hold it so all you need is a thin metal plate on top with a few flush screws on each side. (top is as in the photo)

[darryl]

Wouldn't it be good if you could cast the nut in place with the leascrew set up exactly in position-

You would have the option to make the nut using the Evanut process, but make it with a series of grooves or whatnot on the outside, then cast a resin around that and directly onto the table- again with the lead screw being set up in exactly the position it would be in when it's all asembled. The nut gets to be delrin or acetal, while the grooving or whatever makes sure it is firmly held in place in the casting. You would have to build up a wall around the nut to contain the resin, and this wall would be fastened 'permanently' in place.

[etard]

So Evan,
You are suggesting slicing the block down the middle so there would be one left and one right side? I was thinking a top and bottom scenerio would be easier to machine and also be easier to fit some clamping bolts around the edges.

[Evan]

It depends how much room you have and you are the best judge of that. It should work either way.

[Peter.]

I followed the original photos and then turned mine down on the screw and bored the nut to suit. Fixed it with 2 pins and it's been working for more than 2 years now.

[tarawa]

Great thread. I wish I saw this when I had to make a halfnut for my Colchester Triumph. I still may make one with your method.


half nut profile by tarawa1943, on Flickr


new halfnut by tarawa1943, on Flickr

[Ritch]

Do ya think the same method would work on ball screws? Might it be stronger? I would like to have ball screws, but the almost 0 backlash would be good, especially at the cost.

Thanks
Ritch

[darryl]

No reason why it wouldn't work- in fact it would probably mold easier to the screw since there is more width and less depth without sharp corners for the plastic to flow into. Would it be stronger- I don't think so. An acme thread has more surface area at a better angle to take the forces.

[kf2qd]

Consider that the shape of the groove on a ball screw is radius, so under load a fair amount of force would be directed into expanding the nut. An acme thread has a shallow angle so most of the force is inline with the central axis of the thread and results in movement of the nut along the thread.

[rocketsled]

Just wanted to let y'all know this thread is STILL generating interest. This time with folks in the 3d printer hobby. We're a frugal bunch and I've got a box of Delrin cutoffs that would make GREAT screws for the Z-axis on a printer....might try using it for bushings, too.

[bvd1940]

Thanks Evan for starting this thread, has been most informative.

[SteveP]

My experience/helpful hints...

Since my experience with making an EvaNut for my lathe's cross-slide was somewhat different from other people's experience I thought I'd share my findings.

First, I bought black Delrin from McMaster. It turned out to be from Quadrant, who actually provide a real datasheet for their products. This can be important, as you'll see in a moment.

Drilled the hole to Evan's spec: diameter (0.500") minus one thread depth. I had a twist drill of the right size so I ran the lathe at 125 rpm and just used that. Worked fine. Cut in half and proceeded to heat with my heat gun which has a nameplate rating of 1650 watts. Barely melted the Delrin even after 20 minutes, and, not knowing how much melt to look for, I removed the leadscrew only to have the nut break in half with very little sign of melting.

Retried with my propane torch (just the garden variety type used for plumbing) and got things too hot too fast--lots of melt and the lead screw just about melted itself all way through the nut! We're talking maybe 3-5 minutes. After it cooled, I managed to get it off the leadscrew but it took a good sized wrench to back it off (my blocks of Delrin were rectangular, not round). I tried freezing it and chasing the threads per Evan's directions but I found the thread chasing didn't remove any material that I could see, nor did it make a noticeable difference to the tightness.

Having nothing to lose, I tried various hacks to loosen things up but nothing worked.

Attempt #2: heated with torch more slowly (held farther away), squeezed less hard. Melting was more controlled but nut was still tight and couldn't be made looser. Drilled it out to exactly 0.500", cut it in half again, and remelted. It broke upon removal because not enough material had been melted--clearly Evan's spec for hole diameter is right on in that regard.

Attempt #3: Drilled to Evan's spec, but wrapped PTFE plumber's tape around the leadscrew. Didn't pay particular attention, but I probably had 3-4 wraps. Heated with torch and when cool, removed the nut which was clearly not as tight as the previous ones. I chased the threads several times and removed a lot of the PTFE tape (which had gotten stuck in the melted threads). I also applied a brush with very stiff plastic bristles to help clean out the PTFE tape--that helped. Lubricated the nut (as I had all the other nuts) and found that it went on much more easily.

With the leadscrew in the headstock I used the lathe (and a wrench to hold the nut) to run the nut back and forth on the lead screw a bunch of times and found that it got a bit looser. Finally, I could get it off by hand (no wrench). Although it was still pretty tight, I finished it up so it would fit in the cross-slide and tried it out. It is stiff but completely usable and I'm quite pleased with the result (especially compared with what was in there originally).

Along the way, I was puzzled as to why my results were so different from what others reported, so I looked into the specs for acetal. Not only is there a difference between acetal copolymer and acetal homopolymer, but Delrin (acetal homopolymer) itself comes in many flavors--the specs for coefficient of friction, deflection temperature, and coefficient of thermal expansion can vary quite a bit. The datasheet for my Delrin listed a CTE of 5e-5"/inch-degreeF which should have been enough to shrink the 1" nut by a couple of thou, but that didn't seem to be enough. The differences in deflection temperature might also explain why my heatgun didn't produce much melting (it could also just be a wimpy heat gun compared to what others have been using).

My suspicion is that people who easily got good results probably had acetal with lower friction and/or a larger CTE. I looked around to see if I could get some, but gave up after about an hour--most suppliers don't provide data sheets and the ones that do, provide only "typical" data sheets which often list ranges for the specs so you really don't know what you're going to get before you buy.

I think the principal outcome of my experimentation is that Evan's surmise that PTFE tape might be helpful was in fact correct. If I were to do it again, I'd probably try 5-8 wraps of tape just to see if I could get things to be a little less tight. Also, if you drill slowly, a twist drill will probably work fine--no need to grind your own unless you don't have the right diameter handy. If it's off by a few thou, it will be fine--so much melting happens that a few thou won't be noticed. If you use a torch, be aware that when melting starts it will happen fast and it will keep on melting--steel holds a lot of heat for a long time! Use a torch only if you can't get things hot enough with a heat gun.

Thanks to all, especially Evan.
--Steve

[+ or - Zero]

Thanks for that write up, Steve --interesting & useful info.

Zero.

[elf]

I tried using PTFE tape as well with only 1 wrap, but the threads were too loose. This was on 3/8 acme.

[+ or - Zero]

I tried using PTFE tape as well with only 1 wrap, but the threads were too loose. This was on 3/8 acme.

At a guess (pure guess), you had a different acetal --after Steve's write up it becomes obvious that there are different materials that act differently that are all "acetal".

This makes making an Evanut a more dicey proposition, but a bit of trail/error should show the way for any given "acetal" one might get. So I'd say the moral (Can a machining problem have a moral?), of this is buy more then you think you need --it seems redo's are possible, if needed.

Evan sorta touches on some of this starting at this post;

http://bbs.homeshopmachinist.net/threads/43645-Making-Acetal-leadscrew-nuts-the-easy-way?p=591424#post591424

It's a neat trick, even if it should take a couple of tries.

Zero.