A question on making ground glass...

[bvaughn4]

Just to add something that made my experience much easier when I made a couple of 8x10 screens - I used one of these suction cup handles on a 3"x3" piece of 1/4" plate glass. Much easier to grip and apply pressure at same time. Hope it hasn't already been referenced called myself looking at each comment.

http://t.harborfreight.com/suction-c...ase-46900.html

[Ari]

Hairspray, Dullcote, Staples magnifier: great ideas, thanks!
I'm keeping Scotch tape in my bag from now on.

[Robert A. Zeichner]

I have used an old 2"x2" Tiffen filter (flat and thick enough) with a golf ball cemented to it with JB Weld. The golf ball makes for a very comfortable handle. With this combination, I have gotten great results making both 4x5 and 5x7 ground glass. As a work surface, I use a slab of 3/4" plywood covered with a few layers of newspaper. When the paper gets wet from the grinding slurry, everything stays in place while I grind away.

[Nodda Duma]

I've been doing my homework getting ready to make a ground glass or two, and have a question. It's popular for people to use a smaller square of glass to grind the ground glass, using aluminum oxide as the grinding compound. But I was curious... if one used a 5x7 piece of glass to grind another piece of 5x7 glass, would one end up with two serviceable 5x7 ground glasses? Enquiring (and lazy) minds want to know...

Yes. Use normal center-over-center polishing strokes for maybe 10 minutes, then flip them over so that the glass on bottom is now on top (grinding surfaces still sandwiched in the middle), and repeat. The flipping over is important to maintain flatness. Otherwise you start grinding out a spherical surface. Continue these steps until the surfaces are satisfactorily ground.

Random link showing the grinding strokes: http://www.fvas.net/victor/mirror.htm

[Jim Jones]

Using two blanks the same size in hopes of getting two good ground glasses demands two very flat blanks. It may be faster to grind just one GG at a time. A smaller tool can conform to the deviations in flatness that are too slight to affect GG performance.

[Nodda Duma]

Using two blanks the same size in hopes of getting two good ground glasses demands two very flat blanks. It may be faster to grind just one GG at a time. A smaller tool can conform to the deviations in flatness that are too slight to affect GG performance.

Actually, when talking about grinding/polishing optical flats, using blank and tool (or two glass plates in this case) of the same size is what leads to perfectly flat surfaces. Undersized grinding / polishing tools just aren't used in an optical shop at the sizes we're talking. An optician can't afford to monkey around like that if he can just use the same size tool. Not to say the OP needs that precision of flatness but why make it harder? As soon as the whole surface frosts over he's done. That's not a function of how flat the plates start out.

The key in hand grinding (or polishing) a flat surface is to use the appropriate grinding stroke and a rigid base to set the glass on. That stuff can be looked up on the web. I've found that periodically reversing which glass piece is on top (maybe once every 10-20 strokes) makes it easier to grind flat as well.