DIAMOND MILLING.

A few years ago when I decided to get involved with telescopes and mirror making after a thirty year break. I purchased a 6" pyrex blank from a southern supplier, used this to design the mirror holding devices on my Polishing stand grinding machine, polishing machine, test bench and mirror cells. This cost me $45.00 When I was ready to start I ordered another four. These cost $90.00. The supplier did not have four blanks and had them flown out from America. I approached another southern supplier only to be told they were out of stock and would not be placing another overseas order for another eight months. It was then that I met by chance Peter Smith of Innisfail http://www.users.bigpond.com/pjifl/index.html who was involved in telescope optics and he put me on to Newport Glass in Los Angeles. It so happened that my son was going to the US and he brought back a sample six and eight inch mirror Diamond milled front, back and sides. I then ordered ten eight inch and twenty six inch mirror blanks.

When they arrived I could have cried. They were rough molded ups and downs back and front of 118" and thickness variations of the same amount. Even with a machine grinding with carborundum powder. I wondered if I would live long enough to true them up. The outside diameters, though cast were consistent and true. It was then that this chap mentioned above told me of an idea he had for making diamond milling cutters out of tile saws. Fig.1 shows two ways of setting up the saws.

I purchased a 4" diameter saw which had a segmented rim and made up the cutter as shown in the left hand drawing. This could only be used for concave surfaces. When one side got blunt, the saw could be turned over. Fig.2 & 3. and the photograph show the set-up on the Bridgeport Miller for milling the curve in the mirror blanks. It consists of a 12 volt transformer and electronic speed control, a 12 volt DC motor, a 20-1 worm reduction box, a belt and pulley reduction drive to a circular table which has a 40-1 reduction. The mirror fits ;n a recess on the rotating plate to locate it. Four clamps with nylon pads hold the mirror. The blanks do not need to be clamped hard. I found that Just letting them sit in the plate recess was sufficient to drive them. I used the machine coolant system to supply a copious amount of water to the diamond tool. The bowl and splash guard ( removed in photo ) stopped water and glass dust flying everywhere. The water drained into a settling tank before returning to the machine sump.