Delrin Lead nuts.

[artmin]

I know this is a resurrection of an old thread but it may be of use again.

The Z axis on my router has a trapezoidal thread with a delrin nut which managed to gain about 0.1mm backlash. I was finding it difficult to source the right flange nut to suit so I broke out the 'polymorph' (you can get it on ebay). Polymorph is a low melting point mouldable plastic. Here's a picture of the new polymorph nut.



I cast it in situ in the machine by clamping the z axis in position and then used the old nut screwed up form below to centralise the screw. Then using a blow torch, heated the machine frame & screw lightly to about 60degC (I did remove the limit switches first). I then spent about 5 mins kneading the polymorph down into the recess around the screw. Once I was happy I couldn't get anymore down I left it to cool. On cooling it does shrink back slightly so the new nut comes out of the aluminium housing quite easily & is quite tight on the shaft. I could screw it off though. Next step was using a craft knife, to slit the new nut. Then I inserted a slip of plastic into the cut to spread the nut and to make it a press fit into the aluminium housing. This made the nut quite a good fit to the screw, but I honed the two together using T-cut of all things. My Z axis has about 75mm of thread at the end which is never used so I used that part to hone. Once I had the perfect fit, I dropped the screw & nut into the ultrasonic tank to remove the cutting compound, oiled & rebuilt the machine. The results have been perfect but I will have to wait (hopefully much) longer to see how it wears over time. But certainly there is no backlash & the recess in the aluminium housing due to the shorter nut maintains a resevoir of thin oil.

[Jonathan]

Smart idea :whistling:

I will have to wait (hopefully much) longer to see how it wears over time. But certainly there is no backlash & the recess in the aluminium housing due to the shorter nut maintains a resevoir of thin oil.

Plus when it does wear you can just remould it. (Edit: Clearly taking it too far but theoretically you could build in a coil inside the nut to surround the screw then pass high frequency AC through that to induction heat the screw every so often (not whilst running!) causing the nut to remould itself to remove any wear. Self healing nut.)

I'd me more surprised if there was any backlash. Gravity causes there to be a constant force pushing down on the Z-axis nut, effectively pre-loading. So as long as the force due to static friction does not exceed the weight of the Z-axis and the component of cutting force parallel to Z-does not either you will not get any backlash. For this reason the backlash on my ball-screw driven Z-axis is smaller than I can measure. Most likely with the previous set-up the static friction was too great so the Z-axis didn't naturally 'fall' into position.

[artmin]

Good thought on the heating whilst in location, but I found that with my Nema 23s, the resistance was too great once the polymorph cooled. I had to slit them to reduce the friction. It was an easy job, about an hour all in. I doubt it would be a problem for larger motors though.

As for the backlash, the old nut was a rattley fit. Most of my work is cutting hard aluminium, clearing plateaus areas. What I noticed was as the cutter plunged in on Z, the head was being pushed up against the screw by the reaction to the plunge, and with no dwell time on Z down, the movement on X&Y was leaving a 0.1 ramp as the head slowly dropped down on the backlash whilst traversing. With the feedrate used the ramp was about 15mm long. It didn't look good after anodising.

All sorted now though

[artmin]

A wee update on the polymorph lead nut. Three years on and it is still in perfect condition and in daily use. No noticeable backlash at all (not measured, just feeling for it) and the aluminium machining shows it. Just a drop of oil daily