Member postings for Billy Mills

The torque depends on the method of microsteping. There are two main methods, full current to one coil then step the next coil incrementally to full current for the half step position so both coils are at max current in the half position and approximatly sine and cosine drive where maximal current is on the full step positions.

The torque is not just the current, the half step position has maximum air between the poles so the reluctance of the magnetic path is maximal and torque therefore lower as John said. The sine and cosine drive concept is just that- it does not determine the rotor angle, just a simple concept. Stepper motors are designed to step and hold, what happens between steps is not pecisely controlled at all which is why microstepping is not fine angle control but smoothing the motion between whole step positions. The drive manufacturers can only approximate to the required wave drive because they don't know the exact function needed which will vary between motors.

Billy.

I do a bit of spin casting of tin alloys in silicone rubber moulds using a centricast machine. As Jason has said, Tiranti's is a good starting point . There are many different alloys for different uses, For a ME low temp casting can be very useful for making small quantities of identical parts such as nameplates, bezels, cases and lineside models of all kinds. One route is to make an original in brass or plastic then make a two part silicone rubber mould then cast directly into the mould.

You can do most moulding tricks e.g. use nylon rods to make cast in holes, install bushes into the mould to be cast in and join complex parts by soldering. Imagination is the major limit. You don't have to use silicone rubber moulds or centrifical casting, simple jobs can be cast into alloy moulds, cuttlefish or whatever else that will withstand 300 degrees C for a few seconds but it is vital to be certain that the mould is completly dry before pouring the metal in.

Finally I would point out that most "Pewter" is tin, copper and antimony , it has been in use for thousands of years as drinking vessels and plates without mass extinction.

Billy.

ABCABC forwards ACBACB reverse i.e. swap B & C to reverse.

Billy.

Microstepping is not a precise angular subdivision of whole steps, it is an approximate subdivision which has the considerable advantage of smoothing the rotation of the rotor, reducing noise and vibration and avoiding some resonance issues. For most uses 8 or 10 steps is about the sensible maximum.

Most motors have about a 5% tolerance on whole step angle, however the microstep interpolation accuracy is probably much worse, the in-between steps depend on the variation of flux between whole steps which varies with different motors. Don't think of microstepping as an electronic gearbox, it is more an intermediate stage to make the stepper look like a two phase motor with quasi sine/cosine drive.

Billy.

Why not use a scrapyard car starter motor and car battery? You can vary the speed by connecting and disconnecting the battery using a starter solenoid ( relay) from some way away for safety when testing the thing out. A rigid support may not be needed, when you spin the cloth from it's centre it will tend to go flat.

Harmonographs are easy with a digital camera pointing upwards at the bottom of a stairwell. Suspend a torch minus the reflector ( or just a led) from a cord above the camera, swing the torch and open the shutter. More interesting patterns if you attatch a small weight to the cord between the hanging point and the torch to make a compound pendulum. You can increase the pattern range by putting the camera on another pendulum.

Billy.