In article
"BR" writes:

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 fibreglass bowl was an idea to keep it light but strong. Naturally
some thought needs to go into the design details. However, if you know
a metal spinner who could do your task to your satisfaction then that
is an option to explore.

The stepping motor may work but I'm concerned with torque ripple
transmitted to the tank contents, although less thru a gear reduction.

Having done quite a bit of work with stepper motors over the past
couple of years (very special application where we were concerned
with the potential for forming striations within an extrusion process)
we found that having a large number of steps per revolution of the
stepper motor means that you have already minimised the ripple
factor. Our motor drives a linear translator which advances 5mm for
every rev. Our current motor is 400 steps per rev and the mechanical
output in the extrusion process is very smooth (couldn't see the steps
in the extrusion force measurement at close magnification).

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.

You can only try. I noticed that someone mentioned disc drives as a
source of stepper motors. Old hard drives are aslo quite good if the
use stepper motors in them for head movement or as the main drive.
I know I have obtained a couple of quite good motors that way which
had 160 steps per rev at reasonable torque.

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....

Synchronisation might be your problem with this last idea.

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