RZ-axis
On the end of a SCARA arm there's usually an RZ tool. The RZ tool has a rotary ball spline that gives you independent rotation (R) and vertical translation (Z) using a single rod.
A rotary ball spline model downloaded from McMaster-Carr.
The rotary ball spline has threads (rotary) and straight grooves (splines). One actuator turns a threaded nut and the other turns two spline shims:
The COTS rotary ball spline is great, but it's too big and it costs $1300.
Also, if you want to route wires throught the center of a rotary ball spline, you'll have a very prominent wiring harness on the top of the robot. And you can't detach the axis from the robot.
I don't want this. Image from igus.
OK, so what's the simplest, most fabricatable version of a rotary ball spline? I modeled my own version of it, oversized so that I could easily 3D print the threads:
It works!
| Actuation | Result |
|---|---|
| Turn regular nut | Pure translation |
| Turn straight nut | Spiral motion (not in the video) |
| Turn both nuts | Pure rotation |
Todo
Order pogo pins (and maybe carbon brushes too).
SLA print the smaller version of the RZ axis along with the nuts, glue the steel rods in place, insert pogo pins into the spline nut and see if I get interruptions in the connection when turning the nuts.