Delrin nuts for lathe?

[kong]

I realy think this is a no-no, but since I cant afford ball-screws, I am going for standard acme leadscrew and nuts. Am I going to have to fork out for a big acme tap, or is delrin a possibilty for the z-axis nut? I guess I will need to make it long, to resist the "squish" from the forces, but even then, will the squish be too much? FWIW, I am no precision engineer, I am just looking to make some money selling rc-car alloy wheels. An accuracy of 0.15mm would be nice.

[Swede]

Kong, I think your delrin nuts will work fine. There are some advantages to using delrin over bronze for ACME screws. The material itself is very cheap. It is somewhat self-lubricating, machines beautifully and to tight tolerances, resists oil, and best of all, with delrin, you can easily fabricate zero-backlash nuts which won't create a huge torque requirement. For 0 backlash, you can spring-load two nuts back to back, or even better, make a long nut, slit it lengthwise 2 to 4 places for ~ 75% of it's length, and use a hose clamp (low tech) or some other type of clamp to squeeze the delrin fingers into a tighter diameter. The threads in the delrin will push into the valleys on the acme screw and take out the slop without the signnificant clamping action which would happen with bronze.

Go for it! The accuracy you'll achieve will be tied in to the inherent accuracy of the screw, if you can knock the backlash down to below 0.15mm, then you're in good shape.

[kong]

Thanks Swede, that's just what I wanted to hear! I like the idea of squashing the fingers onto the leadscrew, (I have just been reading up on the commercial a-b nuts) in the past I would have said this method would create too much friction on the screw.

[Swede]

I think where guys end up with too much friction is when they simply clamp too hard in an attempt to REALLY get rid of every teeny bit of backlash. I think the idea is to elininate 97%. Getting that last 3% will really drive the torque requirements higher.

While a hose clamp would work OK, I think a much better solution would be to use some type of spring arrangement. This would push the nut fingers into the threads, but since the screw will have natural variations in pitch and diameter, as the nut travels, the spring will naturally squeeze the correct amount. It'll also decrease wear.

May I suggest when you are tooled and producing the nuts, you make a few spares... there's no denying that their lifespan will not match a bronze nut, and having some extras on hand will make the eventual swap less of a hassle.

Let us know how it goes!

[ger21]

I've seen a sketch a while ago of a similar nut, I think from Ballendo. Where your "fingers" are, turn a small groove and use either an o-ring, or maybe a snap ring to apply the pressure.

[RotarySMP]

I use Nylatron (some Molydisulphide impregnated nylon plastic) nuts on the Z axis of my mini-lathe conversion. I bought two nuts, any will set up an anti-backlash spring loaded arrangment, but for now the first nut was simply pressed into an undersized bore. The resulting back lash is 0.17mm, and works fine. Adding the second nut is pretty low priority for now.