Wednesday, July 4, 2012

Router lathe: New leadscrew

bottom of x-axis carriage showing new nut and diy tap
Upgraded lead screw. The screw that originally drove the x-axis was a standard hardware store 1/2" x 13 threads per inch rod. It had the advantage of being cheap, easy to work with and with standard threads, easy to connect to. The disadvantage was that it was inaccurate, not straight, and really slow: it took 13 revolutions to travel one inch. I upgraded to an acme lead screw with 5 threads per inch, 2 starts. This gives me faster travel at lower motor RPMs, where they have more torque. It also speeds up the rapid traverse -- it no longer takes a full minute to return the carriage to zero.

The screw was pretty expensive so I just couldn't bring myself to spend another $40 on a nut, and decided to make my own out of UHMW. It only took about 5-6 hours to come up with one that worked... (how cheap do I work? You do the math)

First, I tried to heat form the nut but wasn't happy with the results, so I decided to make a tap and cut the treads instead. I ground a couple of grooves in a length of the acme rod, tapered the ends, made sure that all the thread starts were sharp. Next, a hole was drilled in the UHMW blank a few thou over the thread minor diameter and it was chucked in the metal lathe. With the home brew tap in a chuck in the tail stock, I started the threads by hand. After a few turns, it bound up and started to spin, so I took it out and finished with a vise and a pair of channel lock pliers. That actually worked well because I was able to flex the nut and squeeze the sides in, making the cut deeper. By the time I had gripped the nut by all sides to turn it, it had cut enough clearance to spin fairly freely. Then, I ran the tap back and forth through the nut with  a hand drill a few dozen times until it was running smooth. A little teflon lube and it's running smooth with with very little resistance and almost zero backlash.

A couple of observations:

UHMW has a nasty habit of flexing away from the cutter and springing back, so your cuts and holes tend to be undersize.

This approach worked on UHMW because the stuff is so soft and easy to cut. It might work on acetyl, nylon or acrylic, but I doubt it would work so well. My "tap" was very crude -- I think you'd need to make something much more refined to cut a more rigid material.

I left the burr on the cutting edge, so when I was running it back and forth with the drill, it was shredding off a little bit of material with each cut. It did not leave a clean cut, but it helped to overcome UHMW's tendency to cut undersize -- each time through, it removed a little more material.


  1. Someone just sent me a link to your blog today. I can't tell you how much I've enjoyed reading about your project. I have been working on a very similar idea, but with all the trade-offs and decisions, I have gotten stalled numerous times along the way. It is great to see you did this with an Uno and no issues.

    BTW, now that you spent your 5-6 hours instead of $40, do you know about the parts that sells?

    Great blog. Thanks for taking the time to put it all out there!


    1. Jon, thanks! I'd love to hear about your project. I know what you mean -- I keep finding limitations caused by earlier decisions that seemed quite reasonable at the time.

      I'll probably buy acme nuts from dumpstercnc once I'm done prototyping (if that ever happens!) -- they make great stuff at a great price, but in the heat of the moment, the last thing I wanted to do was wait days for parts. Plus, I get a twisted kick out of making my own parts. Look at that ridiculous drive gear on the spindle. Particle board gear, but it actually works way better than I expected.

  2. Chris -- I'm hooked an esoteric kind of woodturning called 'ornamental turning' and I've been sorting out the issues to do way more than spirals. I'd say to look at the forum/gallery at but the permissions recently got changed and visitors can't see much anymore. You can get the idea and examples from here and computerized directions here and here and here Send me an email at my name above (e.g. and I'll send you a couple articles.



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