Is there some reason Acme nuts made out of something other than Delrin - say, aluminum or bronze - couldn't or shouldn't be used on a CNC machine? I see bronze and stainless for sale at McMaster-Carr and other places; just wondering why they aren't used.
Are the metal nuts anti-backlash? ACME screws have many more applications than just in linear motion ... generic fasteners for one, and then a good metal nut would be the way to go. I think that the threads on lifting weight barbells and the like are acme.
The Delrin nuts I've seen have a small O-ring groove milled into the OD of the nut. That wouldn't be hard to duplicate, I wouldn't think. The O-ring, if I'm not mistaken, is what aids in the "anti-backlash" department.
Hmmm. I always think of the DumpsterCNC nuts, and there the AB is via progressively spring-loading the ID of the nut towards the ACME shaft to reduce play between the nut ID and the shaft OD. I don't see how an OD O-ring on the nut would help reduce backlash, but it might reduce alignment issues of the nut with respect to the carriage.Originally Posted by adt2
Not all linear motion requires anti backlash. Some require very high loads like a vice. Bronze and steel fit this requirement. For perscision leadscrews delrin makes a lot of sense. There's a very low abrasive nature to delrin with good tensile strength so your leadscrews will last essentially forever while maintaining the same tollerances and accuracy.
The O-ring type squeezes threaded "fingers" downward against the ACME rod threads at 90 degrees to the rod's center line.
The DumpsterCNC coil spring pushes a free-moving threaded part against the ACME rod threads in-line with the rod's center line. You can pull the spring loaded part back against the coil spring and it is much easier to screw the nut assembly onto the rod. This is because it removes the constant force that the coil spring applies between the faces of the threads.
These could be made from brass or bronze, but the compression type would need a much stiffer O-ring to do the compression. The type of metal would need to be chosen for low friction (galling tendencies) against the steel lead screws. Lubrication would need to be applied unless Oilite bronze bushing material is used.
Search these forums, and the web, to find home-made nuts that people have used.
CarveOne
CarveOne
http://www.carveonecncwoodcraft.com
Fit has a lot to do with it...Here's some info from Mcmaster....
Precision Acme threads are broader, stronger, and more square than standard V-shaped threads, which means they're better suited for carrying loads. They have a 2C (self-centering; tight and precise) thread fit and a smooth finish that increases rod life by reducing friction. Lower-precision general purpose Acme threaded rods have a 2G (general) thread fit. The 2C and 2G thread fits are compatible, but when used together you will lose the precision of the 2C thread fit.
Very often...with multi start screws in particular....taps have to be homemade, so they can be made to fit a little tight. And with a little
lubrication over time can adjust themselves/wear-in to be very backlash
free for a very long time. Dependent on machine purpose/uses naturally....
John
Delrin is a very good all around material. It has a low coefficient of friction and a relatively high structural strength. It is commonly used for industrial machine parts. I have a dumpster CNC anit-backlash nut on my Z and it sees LOTS of use. So far it has shown no signs of wear and is very precise.
Scott
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