I had an old Makerbot stepper motor with a worm shaft on it that I wanted to use to make a linear actuator. It looks like this:
Worm shafts are typically meant to be coupled with gears and can give you good speed-reduction in little space. However, I really wanted to put a nut on it that could travel along the length of the shaft. They don’t really sell these (at least they don’t on McMasterCarr) so I needed to make one. Luckily I have access to a 3D printer (a Replicator 2).
I found this great tutorial on making nuts in Solidworks. I found it easy to follow, intuitive, and easy to change the features as I was tweaking things. The first thing I had to do was characterize the shaft. I couldn’t find any specs for it online; I could have called the manufacturer but it was a millimeter-standard part so I figured all the important dimensions would be in full or half millimeters. This allowed me to measure it with calipers somewhat inaccurately (you can’t really measure circular things that accurately with calipers) and round to their actual value.
The first thing to realize is that it is quadruple threaded. Your average screw and nut have a single, tight thread. Some shafts, such that things can move more quickly per revolution, have multiple, longer threads. You can tell this by looking at the top of the shaft– there are four starting points.
After that, follow the tutorial! Print your first nut aaaand it won’t screw all the way onto the shaft.
Close, but no cigar. It was just too tight. This is a common problem with average-low quality 3D printers: it’s hard to get your dimensions exactly right. I make four modified models, figuring it was faster to CAD them all and print them together than to do one after the other, having to print them in serial would be very slow.
Since it would go on partially on one side and not at all on the other, I figured the chamfer might be a problem– even thought the top and the bottom are the same in the CAD model, the 3D printed part would be different for the first layer printed than the last. However, it was also very hard to screw on, so I figured the threads were also too big in some dimension.
It had a little bit of wiggle room laterally, but was actually a very good fit: little friction *and* little wiggle room. I’ve been thinking about starting to post some CAD models I make to Thingiverse; at least, this one seems like it might be useful to others.