The Amateur Telescope Maker’s Page: A Grinding Rig (May, 1951)
The Amateur Telescope Maker’s Page
A Grinding Rig
WALKING around a barrel is undoubtedly a tedious procedure, but on the other hand it is the simplest method of grinding and polishing a telescope mirror. However, a number of our disciples have evidently gotten just a bit tired of this ambulatory procedure and have written to inqure whether there exists a more satisfactory and sedentary method of grinding said telescope mirrors. There is. As a matter of fact a number of such grinding rigs are described in Amateur Telescoping Making edited by Albert Ingalls.
The grinding stand illustrated and sketched on this page was made by a professional optician, Ben Miller of New York City, to grind and polish six-inch optical mirrors. The ultimate curve of the mirror is determined by the radius of the cast iron tools used for grinding. All of the tools used are turned on a lathe and are accurately checked over their face by means of a spherometer. The convex tool is then placed upon the spindle of the machine while the glass disk to be ground is held immediately above it by means of the mirror holder.
The ratio of speed between the main spindle and the eccentric arm can be varied in the initial stages of grinding and polishing. For most purposes a spindle speed of six times that of the eccentric cam will do for most work.
As a rule, the larger the mirror, the slower the speed. Under no circumstances should the spindle speed exceed 20 rpm.
For six-inch mirrors and under, a 1/3 hp motor will do. However, if you’re planning on anything larger, it may be wise to install a half-horse job. Inasmuch as most commercially available motors operate at 1750 rpm, some sort of a speed reducer will be required. The method indicated in the drawing will be perfectly satisfactory.
Grinding through the progressive carborundum and emery stages should take from 10 to 20 hours. As a general rule, polishing will take about 50 per cent more time. It is understood, of course, that this rig will only grind and polish, final parabolization must be done by handâ€”by walking around the barrel.