Friday, May 30, 2008

Wire Wound Stepper Motor

I'd like to make my own stepper motors. This should be much cheaper than just buying three motors for every Darwin RepRap that gets made, and matches the replication philosophy. The idea is to wind the rotor with with 17 gauge steel fencing wire, the kind that can be purchased in 1/4 mile spools, this would be instead of using a cast iron rotor.

I'm not set up to run an iron foundry, and I suspect that most people involved with rapid prototyping aren't set up to cast iron. In my browsing the internet it does seem to be possible for people to advance from aluminum casting to iron casting, but it involves several levels of difficulty beyond what the average back yard tinkerer would be capable of. There are a few folks who might be able to machine suitable rotors from bar stock, but in order to get 0.9 degree steps the rotor needs 100 grooves (0.9*4 = 3.6, 360/3.6 = 100). That would be fairly fine control of a manual lathe/mill to get even grooves every 3.6 degrees.

Of course, since I don't have a working Darwin I can't really test this concept out yet, I suspect there are weaknesses in the design that will become more apparent as the first prototype is put together. Here's a few I would look for.
  • Bending of the wound steel wires at higher speeds.
  • Difficulty winding the steel around the plastic without cracking the plastic.
  • Securing the loose ends of the steel wire.
  • Alignment of the bearing clips while gluing them to the rotor.
  • Alignment of the caps to the magnet support, the rotor must not rub, but the bearings rest against the caps.
  • Clearance to wind the copper around the steel core wires. I was envisioning the magnet core wires as being wound figure eight, but it's hard to tell if there will be enough clearance.
  • The opposite magnets should be wound to have opposite NS alignment, but since the wires will be connected after securing the cap it might be easy to reverse them to be fighting each other.
  • The linear actuator bearings need to be somewhat larger to fit over the cylinder and still have the lead rod clear, I've yet to price the bearings, there needs to be two, one for the top cap and one for the bottom cap, they might be more costly than skate bearings.
  • Heating might be an issue, or warping, it's plastic after all. I did put some vents in the caps though.
(Old file links removed)