ATM Spin Casting Experiments

["Bill T." <twentiethwave@hotmail.com>]

I did some spin casting experiments recently.  While I was really trying to
develop a method to cast and press oversize pitch laps, this info may be
useful to those thinking about spin cast mirrors.

The setup is a micro-stepping motor driving a smooth, massive turntable
through a very elastic belt.  This is like an old audiophile belt drive
turntable where the belt provides isolation from motor vibration, and the
massive turntable keeps things nice and steady:

http://www.kupercontrols.com/misc/spindrive.jpg

On the turntable, the belt is simply held in place by friction.  I obtained
the belt and motor pulley from WM Berg: http://www.wmberg.com

The motor is epoxied to a weight, which is enough to keep it in position.  I
used 2.5lbs -- 5 or 10lbs would be better:

http://www.kupercontrols.com/misc/spinmotor.jpg

The turntable is also on a weight (25lbs), with three adjusting screws for
leveling.  This massive type of turntable is carried by ceramic supply
stores, and is used to put stripes on pots.  The bearings are very high
quality, very smooth.  I have found many other uses for this unit as a
general purpose leveling device:

http://www.kupercontrols.com/misc/spintable.jpg

I used a crude spherometer (three screws in a stick) to just the right
turntable RPM for the desired curve.  The disc is a good ol' Home Depot
particle board disc, with a aluminum flashing dam held in place with some
wire:

http://www.kupercontrols.com/misc/spinsphere.jpg

This first experiment failed.  You can see that most of the resin moved out
to the very edge, which is very steep, leaving a big flat zone in the
center.  I used only enough liquid to get an average depth of .25" on the
24" disc.

http://www.kupercontrols.com/misc/spinedge.jpg

The moral is: you need to have some minimum thickness to get a good curve.
I suspect about no thinner than roughly 18:1 is needed to get something like
a sphere.

In the end I wound up making Hydrostone bases with various radii on my
normal grinding/polishing turntable.  To make oversized pitch laps, I select
a disc with the desired radius, put the flexible lapmold on top, pour the
pitch, and plop on the base:

http://www.kupercontrols.com/misc/spinplaster.jpg

With a suitable coating these discs can also be used as a mirror substitute
for rough pressing oversized laps.  For a 24" disc of 1.25" depth, 14 RPM
gives about a 240" radius or curvature.  The required RPM for a given radius
is very dependent on the depth of the liquid material.

Bill T.