Let the Grinding Begin!

(Day 1)

Well its time to put the grit to the glass. Here is a picture of the first time I ever poured grit on glass for the purpose of grinding a mirror (trepanning doesn’t count)

Under the mirror is a piece of plastic to help keep the grit from embedding in the surface of the plywood, I then cut 3 short pieces of thick walled rubber vacuum tubing , put a drywall screw thru the middle and secured these to the perimeter of the mirror. The great part about the rubber tubing was as I tighten the screw the rubber expanded, which allowed me to get just the right amount of pressure to hold the glass, but not enough to cause it to “potato chip” or warp the relatively thin glass.

Now it was time to start pushing glass J! The first goal was to dig out a curve in the center of the mirror, which also grinds down the edges of the tool. To do this I put the tool on the bottom and the mirror on top of it and did what’s called a cordial stroke, which looks something like this, but I later found that off centering the tool and mirror a little bit more than displayed in the picture made things go a little faster.

Since I’m building a Binocular telescope I need two mirrors. To make it more fun each mirror must be exactly the same! To do this I ground both mirrors on the same tool, which worked amazingly well! To implement this I would put down some grit on the tool, Put mirror A on top, do 2 laps around the barrel, with 20 cordial strokes per position, and 8 positions around the barrel to make 1 lap. After completing 2 laps I would set aside mirror A, add fresh grit to the tool, put mirror B on top and repeat.

The neatest thing is watching that curve start to form! Hard to describe the feelings, but it was just really cool!

After 2 hours of cordial grinding switching between mirrors after 2 laps, I stopped for the evening, cleaned everything up and measured the depth of the curve, which is called the sagita. To do this I put a straight edge across the center of the mirror and measured from the bottom of the straight edge to the top of the center curve of the mirror. After 2 hours, an hour for each mirror I had 26 thousandths of an inch of sagita, my ultimate goal was 58 thousandths of an inch, so I was dang near half way there! (whooo hoooo) OH yea, to measure this distance I used spark plug feeler gauges. Actually I used 2 typed of feeler gauges. I put a 15 thousands wire type feeler gauge on the center of the mirror, and then used the flat type feeler gauges between the wire feeler gauge and the bottom of the straight edge. Worked rather well.