From: "Bill T." 
To: atm{at}shore.net
Reply-To: "Bill T." 


For limited use, you can make massive, economical turntables out of
plaster, using the same techniques used for making tool bases.

The main thing is, you must pour while the plaster is still VERY liquid,
and the mold must be VERY parallel with the ground.  Otherwise you get a
bumpy and/or wedged surface.

Put a disc of chicken wire inside the plaster disc for rigidity.  You can
also embed a nut or other shaft attachment device inside the plaster.  You
may want to make a jig to keep the attachment truly at 90 degrees to the
disc.  You can also embed mirror constraints in the wet plaster.

I made a 20" x 2" table using Hydrostone.  It was very strong,
even without an interior wire frame.  I supported it on a surplus 6"
pulley gear with a 1" bore.  The pulley was epoxied after the plaster
had dried for about a week.  Plasters need at least 4 days to dry enough to
be glued or painted.

For a mold I cut a 20" disc of particle board (inherently very flat)
and built a dam around it out of aluminum flashing.  It took 35 lbs of dry
Hydrostone plaster to make the disc, cost about $12.  Hydrostone or other
water resistant plasters are to be preferred.  Using a whole 50 lb bag
would make a disc 20" x  3" weighing 75 lbs.

Here's a sort-of related link showing how to make a very symmetrical tool
base with shaft...

http://www.kupercontrols.com/tooltrick/

For use, cover the disc surface with a layer of fine textured bubble-wrap.
Paint if you wish, but this may not be necessary with good, well mixed
plasters like Hydrostone.  Plaster is subject to chips and edge fractures
in the same way as mirror blanks, so be careful to avoid both.  Otherwise
this is a good, cheap way to make a table for a limited number of uses.

My application was for a spindle intended for manual polishing.  I drove
the table with an ancient DC servo motor with analog tach feedback, powered
by an equally ancient Glentek servo amp.  This arrangement happily ran the
table over a very wide speed range with only a 4:1 gear reduction between
motor shaft and turnatable. The low RPM talents of a servo motor and amp
can often eliminate the need for a gearbox if the motor is strong enough. 
The speed control was just a pot wired up for velocity mode control.  I
used dirt cheap, hardware store pulleys, bearings, and a shaft intended for
swap cooler use.  The support frame was made out of 4" redwood beams
bolted together.

IMHO even a simple spindle is a great step forward over walking the barrel.
Among other things it is much easier to avoid astigmatism this way, most
especially when working on large mirrors.

Bill T.

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