A few years ago, I had some stepper motors with a threaded rod passing through the rotor. So when the motor is stepped, the threaded rod will push or pull.
Now, I cannot find them, so I am trying to buy some replacements. But I cannot find anything with a google search. Does anyone know what these things are called? Or some place that sells them? I think I got them last time from either Jameco or Allelectronics, but neither seems to have them now.