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View Full Version : OT glass cutting



darryl
08-18-2009, 09:14 PM
I've been playing with this front surface mirror for a bit. I have this piece that I scrounged from a polaroid camera, it's 50 thou thick and odd-shaped, but there's enough surface area that I can get four pieces out of it to use in my telescopes. I tried a glass cutter, but I can't get enough force to score the glass without actually breaking it into shrapnel. I tried the reflective side up, then down (hoping to not mar the surface), but no go. Then I thought to bring it up to the cutoff disc and grind a weak spot in a line where I wanted the break. At first I ground on the back side, but that would then mean laying it front surface down to snap it, which I really didn't want to do. Besides that, I couldn't see the grind mark without bringing it away from the cutoff disc, so I couldn't tell if my mark was sufficient.

Then I figured what the heck, grind a line on the front surface then lay it on its back to snap it. Worked great! I could see the back side of the reflective coating through the glass as I ground on it, so it was easy to see where the mark was and roughly how deep and/or even it was as the coating ground away. A couple of times it literally fell into pieces in my hands, no snapping required. The front surface still looks beautiful.

So now that I know what I'm doing, it follows that I could mount these pieces, chuck in the lathe, then grind a perfectly round mark into the mirror- probably being able after that to 'tink' away the outside and be left with the round chunk of mirror. All without having to touch the front surface. I don't really need them to be round for this project, just to have enough area to cover the optical requirement, which is about 3/4 inch diameter. At least I know now how I can do round discs if the need arises.

Sure, I could have purchased front surface mirror discs, but it's just not the same as 'preparing something useful out of junk'. At any rate, I now have my mirror segments and can continue the 'cheap 'scope' project. Pictures will be posted once I have something to show.

RobbieKnobbie
08-18-2009, 10:49 PM
I've had excellent results shaping glass with a low rpm wet grinder I got from HF. The water and slow speed never heat up the piece, and I've never had a problem with chipping as long as i use reasonable pressure.

jdunmyer
08-19-2009, 09:01 PM
Making round pieces or cutting holes in glass (same thing) is easy:

Make a "bit" that looks like a hole saw w/o teeth, brass works well. Put the bit (core drill?) in a slow-speed drill press, build a dam out of putty around where you want to drill, then have at it, using valve grinding compound and water. It's best to drill half-way through from both sides, especially if the glass is thick.

When I worked in the glass plant, we used to do "speak holes" through 1/2" glass using this method.