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Re: Starting To Grind... And An Answer For Pablo

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Posted by Robert May on August 18, 1999 03:36:47 UTC

First of all, congratulations on finding the "washer" method of making a iron tool. The reason that pros use iron tools is that they can make thier curves in glass very fast with an iron tool. They want to make the curves accuratly so they make large iron discs with the curve turned into them. This method is great for mass production but the down side is the 5x or larger iron tool and the mechanism to turn it. The other thing that the pros do is use a diamond grinder to make the approximate curve into the glass. What I've been doing is using a piece of iron (you don't even have to make a curve on it) and do the rough grinding with it. A cheap piece of iron that's readily available is the end cap for iron pipe. You don't even have to use iron, one guy grinding brought in a bad piece of phospher bronze (about 2lb worth) from his outboard motor and has successfully ground a lot of his mirror in a little more than an hour. The secret here is to spend most of your grinding in the center of the mirror. After you have a sort of a center ground out and it sort of looks like a sphere then you can cast a tool with CERAMIC (not the cheaper glazed plaster ones)tiles. The tiles embedded in the plaster will not come loose like the epoxied ones and the plaster will wear away faster than the tiles and remember that before this, two pieces of glass were used. You can get materials from Willmann-Bell or from ASM Products (both of which are on the web) and you can get kits from them also. I might also note that with the iron tool the pros use, they can skip grades of grit with no problems. I haven't tried to skip grits but it's a possibility that I am going to explore in the future. For your first mirror, I would expect to start with 80 grit and you can skip to 220 without a problem if you so desire if you're just buying grit and a piece of glass from your glass shop or you've found a piece of glass. A note on portholes - newer ones are tempered and thus are not good to use as they will probably do the shatter thing somewhere along the line. There is an easy test using a polariad sunglasses and a windshield (for a polarized sun source) that shows tempered glass well. I've posted more on the grinding process elsewhere on this forum so go look. One of the things that I have threatened to do is write a mirror grinding article and it looks like it's time to get started. Below (if it comes out) is a picture of me with some tools of testing about me. The Foucault Tester is from another person in the telescope making group and is the tester used on the 200" mirror up there at Palomar. It's been modified since the testing with a slitless moving source and the batteries for it. After using it, I am amazed at some of the things that amateurs have overbuilt to do the job of testing thier optics.

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