In article
"BR" writes:

Must be a hemisphere with height = dia/2. A stainless steel mixing bowl has the
correct
shape (except small flat on the bottom), but trying to spin it by hand while
floating in
water makes it difficult to fine tune. If the spin is off center, especially
when trying
to reverse rotation, the bowl wobbles and there's no control. Only one attempt
was
successful in producing an especially interesting pattern in the bowl
consisting of water
and a measured amount of baking soda . So I'm trying to figure a way to gain
some control
in the process, and it needs to be precise. If it were possible to obtain a
direct drive
DC servo with a bearing for direct mounting of the tank, capable of fine
resolution over a
0-200 rpm range, that would ease the project considerably. Of course the one I
mentioned
before shown at etech4sale was overkill and happened to be too slow. Something
smaller
would do. Otherwise there's bearing blocks, pulleys, whatever. Positional
accuracy isn't
needed. Only change in velocity.

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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Paul E. Bennett
Forth based HIDECS Consultancy .....http://www.amleth.demon.co.uk/
Mob: +44 (0)7811-639972 .........NOW AVAILABLE:- HIDECS COURSE......
Tel: +44 (0)1235-811095 .... see http://www.feabhas.com for details.
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
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