Your idea sounds like a good one except I'm not sure I'll make it with
little runout. I made some spherical speaker enclosures using a
rounter mounted on a turntable setup, but don't have that anymore.
Though it may cost some, a metal spinner could make the tank. They'll
make it more precisely in terms of TIR than anything
I could do.

The stepping motor may work but I'm concerned with torque ripple
transmitted to the tank contents, although less thru a gear reduction.
It's unfortunate that servodisc motors are so expensive since they
have no cogging and are brushed dc motors. Well, it's just like
anything - it'll take some experimenting to see what effects things
like cogging have on the contents. Perhaps it won't be a problem due
to its mass, just like platters used for record turntables. I'm just a
bit paranoid about building this contraption, because it's then a lot
of trial and error for a certain pattern.
The only other possibility is using say, three small motors with
rubber tires around the circumfrence to turn a mixing bowl floating in
a larger bowl of water. Just thinking out loud....


""Paul E. Bennett"" wrote in message

Make yourself a wooden mould for the bowl shape, a wooden ring and
make the bowl in fibre-glass. The wooden ring would form the basis
for the shaft attachment. Keep application of the fibre-glass and
resin even around the shape. A rotating flat table (with suitable
clamps) and a sander held at a fixed point will assist in making
the outside smooth (the inner would be smooth from the mould form).

On the drive side, a stepper motor (at least 100 steps per rev) with
worm drive gearing to the base gear wheel would provide a quite
smooth
rotation which you have total control over. The drive electronics
would be quite simple.

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