A POLISHING LAP..
A chap who makes plastic patterns suggested that after spraying the
matrix with Silicon Mould Release I should sprinkle it with powder-
I set the matrix on a levelled cast iron plate. Around it a band of
aluminium to hold the resin. This was split so that it could be easily
removed later. It was held together with adhesive tape. Then it was
all sprayed with the release agent and powder sprinkled over to well
cover everything. Time was allowed for the powder to adhere to the silicon
and the lot to dry. This only took a few minutes. Then the excess powder
was carefully blown off to leave the matrix and side band with a thin
coat of powder.
The resin was melted and and then as it was allowed to cool it was stirred
until it thickened up. It was then poured over the matrix.to a depth
of about 6mm. An aluminium backplate which had been previously warmed
up was placed on top of the resin inside the sleeve. This was not pressed
down. A bubble level was placed on top of the backplate and pressure
applied where necessary to keep it level. This ensured that the lap
was of even thickness. When the resin had set the lot was held under
a tap to cool it down.
The tape holding the outside band was released and the band removed
. The back of the matrix was rapped with a piece of wood, the lot turned
over and the matrix fell away. Looking at the drawing it will be noticed
that there is a raised piece of resin around the outside. This is trimmed
off. The lap was then scrubbed with a nail brush to remove powder embedded
in it. The flat glass blank was placed face up, coated with cerium oxide,
and a nylon net laid over it. The Lap was warmed and placed on top and
weighted. After half an hour the lap was lifted off and the nylon net
peeled off. Success at last .It looked beautiful.
I don't know how many laps I could have made by the conventional method
while I was doing all this. I was determined to master it. Now making
a lap is quick and easy. For a lap for a spherical mirror I use a bottom
plate with a concave surface the same as the mirror I am making. The
matrix is slammed down on this and it being thin, conforms to the curve.
I make my laps hard as I want them for polishing many mirrors. After
several hours running on a polishing machine the lap flattens a little
and the grouves need widening. I use a 12 tooth per inch hand saw and
a thin sharp blade for trimming. This blade is a piece of HSS hacksaw
blade ground to a long tapering point and sharpened. The thinner it
is the less chance of chipping facets.
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