A discussion on what leadscrew nut options are available to the DIY CNC builder and a short how-to on a very low cost option (making your own ACME tap and (nearly) backlash free leadscrew-nuts).
Off The Shelf Options
Bronze ACME Nuts
The most widely accessible nuts are probably bronze nuts. These are general purpose nuts that slide very easily on the lead-screw. The main problem with these nuts is that they have backlash - when the lead screw changes directions there is a period of time that the nut doesn't move. Backlash and CNC's don't mix well - see how not to make a CNC (part 2) for details.
Commercially available anti-backlash nuts solve the problem of backlash, but this comes at a (sometimes significant) cost. These assemblies are usually tens of dollars each and are well worth it, if your budget allows for it. There are lots of variations, but most designs can be thought of as basically two nuts that are pushed apart by a spring. This spring keeps both edges in constant contact with the lead screw, so there's never a period of time when the lead-screw is moving and the nut is not moving as well.
If you already have a metal-capable mill, then creating a proper anti-backlash nut assembly is an excellent option. It's not out of the question to improvise something without a proper mill, but would be fairly involved. For those of us without a full fledged home machine shop, there must be another way!
Acetal (e.g. Delrin) is an engineering plastic that machines like a metal, but glides well on metals. This is an excellent combination for a lead screw nut. For the remainder of this article will cover how to make a simplified anti-backlash nut from Acetal. Since the main function of an anti-backlash nut is to transfer motion immediately upon a direction changes the lead screw, as long as the nut rests tightly enough against the threats of lead screw it should accomplish this goal. The anti-backlash nut we'll be constructing is simply a tight fitting lead screw nut (a single piece without any spring).
COTS ACME Taps
The nicer ACME taps available are tandem taps that will provide roughing and finishing cuts in one single motion - they also start at around $50 + shipping. If we were planning on building lead screw nuts in quantity, this wouldn't be so bad, but for a one-off it's a bit hard to swallow.
DIY ACME Tap
An alternative to buying a tap it to, of course, make one! Since we already have the lead-screw that we'll be using - we can cut a portion of this and mimic the features of a "real" tap. This certainly doesn't work as well as the real tap, but it works in a pinch. Here's what to do:
- Cut off a small lenth of the leadscrew
- Rough an off-center slot along the axis of the lead-screw. This should be done in such a way that a fairly sharp cutting edge is created. This slot can be roughed with pretty much anything, a standard mill, circular saw with ferrous cutting wheel, dremel, etc. The bigger the slot the better. If you can add more, do that as well - they'll help to clear the chips.
- Cut a piece of Acetal that will be the nut.
- Drill a (tap size) hole into the Acetal - diameter will vary depending on what size leadscrew you have, threads per inch, desired thread engagement, etc. We used 0.3" tap hole for a 3/8" 12 TPI leadscrew. This ACME Tap Drill Size chart might help. There are also formulas out there as well.
- Using the same beefy drill as before, tap the hole several times until a stock leadscrew will slide through without too much effort. Make sure not to let things overheat too much.
So, there you have it, next time you're in a pinch for a nearly backlash free nut, give this a shot!
Comments, or suggestions - give a shout in the comments or the forum!