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Re: Spin Casting Mirrors?

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Posted by Jon Elliott/">Jon Elliott on February 22, 1999 17:40:21 UTC

:I have become interested in spin casting mirrors as well. To date I have "thrown" sixteen mirrors with various results and dubious success. I have figured out how to create some mirrors that are pretty close to 'optical' quality, but not quite good enough. what I have learned is you must spin cast it in several layers. the first may be thick but the successive ones shoudl be about an inch. You must cover the curing expoxy or wind currents will deform the surface. Use epoxy that is 100% solids and use no expoy thinner either in the material or on the surface. You must reduce the speed of a phonograph turntable. I purchased one at a thrift store for $10 and modified the pulley on the drive motor to run at various speeds. I worked out a spreadsheet showing the RPM required to generate a particluar focal length. 33 1/3 rmp makes a fl of 22.1 cm -- a little short for any mirror over 3 inches. By way of comparison, 18 RPM will generate about a 50 in focal lenghth and 25 rpm will generate about a 28 in fl. Mix the epoxy very slowly to prevent bubbles. The lower the viscosity and surface tension the better. Use High Mod (cures hard not flexible) A miniscus forms on the side of the spin casting dish which is not part of the parabolic shape. I simply cut this off with a band saw.

After the usual problems of turntable speed, vibration, epoxy selection, contamination of the surface, etc, the remaining issue seems to be that either surface tension or internal pressures created by the curing epoxy result in tiny, irregular wave-like formations on the surface of the parabola. The overall shape is parabolic and the surface is very smooth, but not optically smooth. These mirrors will focus an image that is clearly recognizable, albiet a bit fuzzy. I have experimented with numerous polishing compounds and techniques in an effort to perfect the surface. In one experiment, I removed the miniscus from the side of the mirror, the build a dam around the mirror and poured a 'blank' on top. This provided me a matching tool to use in polishing the blank. ( a layer of Saran Wrap pressed tightly against the mirror provided the 'release agent'. Mixed results. Basically, the blank is hard and can be used for finish griding (1500 and 2500 grit polishing compound worked well here.) Still looking for the a "pitch lap" for epoxy. Any ideas?

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