Making Acetal leadscrew nuts the easy way

[elf]

you know I half wondered when responding if I was unwittingly playing the staight man lol

If it was bonze nut say, I'd buy that a bit of timesaver compound (stuff that's supposed to breakdown) might be a good solution, but figured in plastic it would be a mess. I've also never been 100% convinced that timesaver wouldn't at least partially imbed. I mean how would you know until years latter when its too late. I have done what you suggested, make a nut out of brass, split it, and used it as a lap on a screw. Its still a bit hack-ish though as who's the say the thread profile I made was perfect, but my logic was that I wanted a very smooth finish on the thread; that it would be the microscopic burrs, tearouts,ridges and aretes that would be increasing the rate of wear

I lapped a 6mm leadscrew with a cast polyurethane nut that was about 3 inches long before making the Evanut. The unit can hold tolerances under a micron.

[loply]

Hi folks,

Sorry to dredge up an old thread but I was reading something in Moore's book 'Foundations of Mechanical Accuracy' which made me think of this.

In Moore's book, he describes the process for creating the "Moore Master Leadscrew", which was a leadscrew+nut combination of ridiculous accuracy.

They make the nut too tight for the screw then set it up on some kind of rig which runs the nut back and forwards automatically for a long time, until it "burnishes" (his words) the nut to the screw. They decided it was right when the nut would rotate against gravity to some particular angle as the screw was rotated at a certain speed.

I made some acetal nuts a few years ago for a lathe and had good success, like many I found the main challenge that the nut was too tight initially. In the end I sorted it with the tapping technique but I thought the above was interesting never the less. Next time I will probably try that using a cordless drill.

[uhmgawa]

Now that my CNC conversion is done one thing I did learn when making these nuts ..I was able get smooth backlash free movement with nuts that are not too tight after just a few passes with the tap.

Following up to a long running thread and an old post. I have however been following this occasionally since a RF-45 mill appeared at my door a few years ago. It was supposed to be one of the Enco clones of a RongFu 45 but Enco couldn't seem to find one able to survive the trip after several attempts and shipped the Rong Fu instead.

Anyway my intention was to convert this machine for CNC operation and I'm still kicking around the choice of ball screws vs. the molded acetal nut described here. For the X-Y travel it seems a split acetal nut held in a sleeve able to adjust closure compression of the nut, tweaking for minimal drag vs. backlash should be straightforward. However the RF-45 (unlike the Enco version) has a rack/pinion for the Z-axis travel which in this case will be replaced with a screw/nut. That head is a fairly substantial hunk of grey iron so I'm wondering what sort of experience folks have had using a molded acetal nut for head travel in this type of mill? I suppose I could counterbalance the head which would also substantially reduce the needed motor driver torque. However I'd appreciate starting with some real data of what size/pitch head positioning screws have been successfully converted to use of an acetal nut in similar applications.

The ill-fated Enco according to my notes had an M30x3 lead screw which is about 8.5TPI. I had considered a 1" 10TPI acme screw for the conversion, but was concerned with constant dead static loading on that cross sectional thread area of an acetal nut. Any feedback would be much appreciated. Thanks!

[Evan]

It amazes me how this thread keeps coming up. My CNC mill is currently in storage but the last time I used it it was still on the original acetal nuts I made for it and still working fine. They have a very long lifetime it seems. It is looking like I may be able to get my mill and my SB9 back in operation, maybe next year. I have permission to park a storage trailer here which will be big enough for a very small but usable shop space. Will see how it goes next spring. It will cost me a lot less than the ridiculous amount I currently pay for storage and for that reason alone it is a good plan. I can run it all from battery power and already have everything I need to do that including big storage batteries and a big sine wave converter with a solar panel as well. It will be nice to get into some machining. I kept pretty much all the tooling I need for both machines.

[uhmgawa]

It amazes me how this thread keeps coming up.

I'd hazard because it is a considerably novel solution to an age old problem. Particularly to an audience which is in a position to experiment with the approach vs. say a production scenario. I my case conventional wisdom suggests to just yank out what's there and replace all three positioners with ball screws -- done deal. Well if it was an old mill with worn leadscrews I might do so. However as it hasn't yet generated its first chip and is still sitting on a pallet jack in my shop, I'd like to consider alternatives.

Yea I'll lose torque/speed to some extent with sliding frinction in acetal, but this is slated for experimental use vs. production. So I can trade off some margin. However I'm particularly interested in empirical results from folks who have converted the Z travel for a mill head with comparable mass. Even with a ballscrew, counterbalancing th head would lower the drive requirements and if done carefully result in more uniform wearing o the ways. Bit that is veering somewht off topic.

My CNC mill is currently in storage but the last time I used it it was still on the original acetal nuts I made for it and still working fine. They have a very long lifetime it seems.

Might you recall the type of mill and approximate dimensions of the head leadscrew? That's my greater question here.

I can run it all from battery power and already have everything I need to do that including big storage batteries and a big sine wave converter with a solar panel as well.

That's an interesting predicament. Along with the Enco (sadly RIP) saga surronding my mill resulting in the RF45, on the final attempt I'd asked if they could send a 3PH mill rahter than the former single phase versions. During the protraacted process of getting a mill delivered in one piece, I'd discovered the existence of downright cheap VFDs and picked up a 2HP VFD for around US$100. An internal bulk DC power rail is derived from the incoming AC such that it should functionally equally well from a suitable battery stack as it does from the rectified AC mains. So in this case you could factor out the need for an intermediate DC -> AC conversion step.

[Evan]

You don't lose any torque/speed at all after it has been in use a little while. The acetal nut will wear until it is a perfect fit with only a trace of friction and zero slop. There is a good reason they use acetal in washing machine transmissions. It just doesn't wear in the way metal parts do and it is far more impact resistant because it will give a tiny bit and then return to shape. There are a lot of applications where plastics are the superior choice.

[Tony Ennis]

You don't lose any torque/speed at all after it has been in use a little while. The acetal nut will wear until it is a perfect fit with only a trace of friction and zero slop.

For worn leadscrews, you'll be more challenged, as the acetal nut will be perfect in one spot and imperfect in others. But in this case you'd probably be challenged regardless of the nut's material.

[uhmgawa]

You don't lose any torque/speed at all after it has been in use a little while. The acetal nut will wear until it is a perfect fit with only a trace of friction and zero slop.

I wasn't aware this would be achieved in practice. I assumed some significant drag would be a non-negotiable trade off in eliminating backlash under load. But admittedly acetal is an impressive material.

There are a lot of applications where plastics are the superior choice.

I was looking for plastic blocks recently to machine as slide bearings for pole mounting a microscope boom stand. This was for 2" SS pipe and most of the blank would end up as swarf. I fished a large HDPE jug out of the trash, rinsed it out, and had at it with a heat gun. After working it into a rough slab form I'd clamped it flat to fully solidify. The ridgidity and density of this otherwise landfill material really needs to be witnessed to appreciate. While it's roughly half the modulus and tensile strength of acetal, the natural lubricity is far better and would probably make an even lower friction nut particularly for applications where lubrication wasn't possible and static loading was moderate.

[GEP]

I just returned from a visit to a molded gear manufacturer near Chicago.

http://www.winzelergear.com/

They do nothing but design and mold gears from acetal, specifically DuPont Delrin. (uniform quality, superior performance) Quite an eduaction.

To address the above question, they showed us that while a normal gear of any material should exhibit at least one point of contact or a little more, a Delrin gear under load may safely deform the teeth enough to achieve up to four points of contact with a similar mating gear. The additional points of contact then share the load. Such is not the case with a metal gear as the teeth cannot flex as much.

Similarly, in the case of the leadscrew nut, the acetel threads can flex a little to conform to the leadscrew to share the loading.

Weston Can one buy 1 or 2 gears from them

[Evan]

Been over this before. Ordinary acetal and Delrin are not the same plastic. Acetal is a copolymer and Delrin is a homopolymer. They have slightly and not so slight different characteristics. Melting temps are different and so is acid resistance. Also, in larger rod sizes Delrin tends to form porosity/weakness in the center section, acetal doesn't. Delrin is a bit stronger though, minus the porosity. Big plus is that acetal is cheaper, no brand name.

And yes, the ability of the plastic to slightly and temporarily deform makes it a lot less likely to crack and reduces wear. The load is spread out a lot better. That applies to both types. For most uses it really doesn't make much difference but it most certainly does when it comes to food processing and acid resistance. Delrin is less resistant to acids.

[Weston Bye]

Weston Can one buy 1 or 2 gears from them

No, they are a custom manufacturer and don't have any catalog listed. You might try contacting them directly...

http://www.winzelergear.com/contact-us/

[John Stevenson]

I have made quite a few Delrin nuts with a variation of the theme.


My method is slightly different in that the two previous methods you had to split the nut and force it round the screw.
I made mine with the bore of the nut the same as the OD but tight, in other words a pre-stripped nut. Cut the middle man out.

I think this sketch explains it better.



Basically you thread the 'new' nut over the screw, drop unto a tapered cup and fit a ram piece. This then goes onto a press and the cup and screw is heated with a hot air gun whilst pressure is applied.

This has the advantage of making it one piece and with a bit of experiment on how far you press the cup in you can choose what fit you want. Something the squeeze in the vise method can't do.

AFAIK this method is my own and I have not seen it repeated anywhere else.

Because at one time I had loads of ball screw thread offcuts from cutting long screws down i even made some Delrin nuts to fit these screws. Not as good as ball screws by any means but certainly better than Acme screws.
Again nothing new here Moore International make screws and nuts such as these and call them Ronda screws.
Any Google Child can find them.

[EDIT] There should be no gap between the ram piece and the top of the screw so as the ram piece is pressed down it carries the nut and screw with it so it cannot push the threads.

This is one made by the split method as it needs jacking screws for adjustment.



This is an 8 start thread off one of Mr Rollsie Royces test jigs in the crack detecting department.
Their guys couldn't make a new nut

[Evan]

That looks like a very nice way of doing it John. Once I get some sort of shop back in operation I will need to try that. It is probably about time I make a new nut for my mill. The way it looks now I may get a shop set up some time in the coming year. That will in part depend on just how busy I am at the university. Right now that is taking all of my time and I don't mind it a bit.

[alsinaj]

Re: the Stevonut method -- instead of using a heat gun, how about putting the delrin blank, the tapered cup, and the ram piece (and the screw, too, if it will fit) in a pot of boiling water. Then, quick like a bunny, put the lot on the press and have at it. The yield pressure/temperature curve for delrin (see below) shows that at 100 degrees C, it only requires about 6,000 psi to make delrin flow. In fact, the curves suggest that at 10,000 psi, the operation can be done at room temperature. That's easily achievable with a garden-variety hydraulic press. Evan, what sayest thou?



Another thought: Is the taper in the cup really necessary? Once the plastic starts flowing, does the shape of the mold make a difference?

[Evan]

The boiling point of water isn't quite hot enough. That is still below the glass transition temp. It must be at least 120C and preferably a bit higher. Below that temp the molecules are being sheared apart and that weakens the entire matrix. Above that temp the molecules are free to move without damage. Both Delrin and acetal are crystalline polymers although they don't look anything like a crystal.