Subjects
 
Optical Shop
Polishing Bowl
Edging Machine
Null Tester
Tube Fittings
Worm Drives
Optical Workshop Equipment
Spindle Nose & Adaptors
Slip Ring Seting Circle
Mirror Cell
Diagonals
Odds and Ends
 













 

MIRROR POLISHING MACHINE.

This machine was originally designed and built for polishing optical flats. However mirrors of 28" radius of curvature have been polished on it. I do not think it would be satisfactory for a Maksutov corrector lens because of the straight up and down motion of the stroke pin that would be required.

Originally the machine differed from the drawings. The bowl was stationary. As I wanted the machine to run unattended I used an automotive windscreen wiper pump to direct streams of polishing compound onto the mirror. This failed because the pump and tubing was too small and the latter clogged up. Next I fitted a Jabsco pump drived by one of the motors. This failed also and while I didn't investigate it I believe the cerium oxide wore it out. The next was a small bilge pump immerced in a bucket of polishing compound suspended under the machine. I was using this on the other machine to polish an 8" flat when it must have failed. I was not in attendance but heard a clatter and bang. It must have failed and the lap stuck onto the blank. The glass was ripped out of the machine onto the concrete floor and a piece broken out of it. If it had been a recently purchased piece of Pyrex I would have cried. However it was plate glass purchased thirty years ago. Compared to today it would have cost very little.

This resulted in two things. First I put down vynel floor covering. Then I designed and built the mirror holder and bowl as illustrated in Fig.3. The mirror is immerced in polishing compound so cannot run dry and also cannot be, ripped out. The bowl was made out of two aluminium frying pans. A larger one for 8" mirrors was made from a plastic tray used under pot plants. Sufficient polish compound is used so that the surface is level with the mirror top when spinning and the lap dips into it as it strokes back and forth.

I used two electric motors; one for the spindle and one for the stroke. These are American Baldor DC motors with KB Electronics, lnc. KBVC 24 electronic speed control so that spindle speed and stroke can be varied independantly. A belt drive to a worm reduction box was used. Top spindle speed is 70 rpm. and stoke 85 / min. I bought worms and wheels and built up the reduction boxes myself.

Though provision is made for all sorts of adjustment to stroke and position of stroke I have used a third stroke centre over centre. I canot saywhat spindle speed or rate of stroke I used. The same regarding speeds and feeds on a lathe and miling machine. I go by "feel". To date I have polished thirty-five 6 and 8 inch mirrors and they have come off the machine perfect spheres, no zones and true to the edge. I have come to the conclusion that too soft a lap is the cause of zones and turned edge. I used the one lap to polish and figure ten 8" mirrors. The channells were deepened when necessary. The nylon mesh used on door screens was periodically pressed into the lap to break up the surface. The machine would be set in operation of a morning before I went to work and stopped when I came up for lunch. There was no need to check to see if it was polished properly. After four hours,sometimes six, on the machine it couldn't be otherwise. Parabolising when you start from a perfect sphere is comparatively easy.

Fig. 1. Plan view of top of machine
Fig. 2. Front view of machine
Fig. 3. Polishing bowl a.
Fig. 4. Polishing bowl b.
Fig. 5. Motor drives
Fig. 6. Machine details.