MIRROR GRINDING MACHINE.

I made my first mirror grinding and polishing machine over thirty years ago. It used the working principle of the machine illustrated at "A" Fig.3, Page 163, "Amateur Telescope Making" Book 1. A bowl which rotated was fitted to the spindle. I also built another machine of the same type. Also a Hindle type machine for a southern firm to grind and polish 12.5" mirrors. Later I sold them my two machines.

When I returned to telescope making I had to build another machine. I based my design on the machine "B" illustrated on the same page above. This I considered the simplest. What speed should the spindle rotate at. What stroke rate should I use. According to "Prism and Lens Making" by Twyman for a 6" diameter block of lenses the speed should be 40 -50 rpm. and the stroke 80 -100 per minute. After reading all I could find I decided to provide a range of speeds.

This machine and the polishing machine were fabricated from 1-1/2" square steel tubing. The table tops were 8 mm. aluminum plate. On top of this is a sheet of 1/8° rubber.

These are the spindle speeds and stroke rates I settled on, or should I say what I finished up with after the pulleys were made. Actually I had the stroke faster than the spindle at first but changed it to slower. While it may be all right at slow spindle speeds, at the speed I finished up using the mirror was being slammed back and forth too fast for my liking. Also I don't think it makes much difference what ratio is used when grinding. Polishing may be a different story.

I finished up using a spindle speed of 72 rpm. and a stroke of 56 per minute and used this for grinding both 6 inch and 8 inch mirrors.

The splash tray is made from a 15" diameter pizza tray. Along the front of the machine is a bracket holding jars of carborundurn powders. I used peanut butter bottles.

I had a 1/4HP 1350 rpm. single phase motor lying around and used this. I obtained a 27 to 1 worm and wheel and built it into a reduction box. This was on the spindle with a vee belt drive across to the crank spindle. Maximum stroke is 6 inches. The position of the stroke can be adjusted either way of centre over centre, A stabiliser bar holds the stroke arm in position.

The following drawings show the design in detail.

Fig. 1. Plan view of top of machine.
Fig. 2. Side view of top of machine.
Fig. 3. Front view of top of machine.
Fig. 4. Plan view of drive arrangement.
Fig. 5. Side view of drive arrangement.
Fig. 6. Details of stroke crank.
Fig. 7. Details of Stabalizer bar.
Fig. 8 Showing Lap & Mirror Holding Details
Fig. 9. Spindle Nose Details