grinding glass perfectly flat

I've only seen the skill of grinding glass discussed in the context of grinding lenses. In particular, the glass is supposed to acquire a curved surface as a result of one's efforts.

What if you want to grind a piece of glass so that it is instead perfectly flat? Does this involve essentially different techniques, both in grinding and in testing, or do they belong to a common context?

I realize nothing is perfect, so it is also necessary to ask how flat one can expect hand ground glass to be?

There are lots of books on grinding lenses for telescopes. Where can one read about how to hand grind glass perfectly flat?

Ignorantly, Allan Adler snipped-for-privacy@zurich.ai.mit.edu

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Reply to
Allan Adler
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There was a thread on here a while back on this topic after someone asked about the same question and flat for that person turned out to be EXTRAORDINARILY flat - like quite an area flat to 0.00001" or some such. If you are talking about several square inches and flatness measured with Newton rings - basically you use very fine grit with stuff that is basically flat to start with and reverse the techniques of mirror grinding - instead of using force on the edge revolved around to make the basic spherical shape - you apply even force and use one pretty flat surface to take the high points off another pretty flat surface. Float glass is rather flat for the purposes of people who want to grind a flat surface on steel cutting tools.

Reply to
Mike Firth

You need to decide how flat 'perfect' is. .001"? .0001"? Give yourself a tolorance. Then you can decide how to get there.

Tool and die makers use something called a surface plate. They are made of black granite and are relatively cheap (around $20 for 9" by 12") Flatness depends on the size and grade purchased. Tool room grade is good down to

+/- .0001". Inspection grade down to +/-.00005".

There are also cheap (

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Reply to
Bill Browne

You are posting this from MIT? Don't you have a library there?

Reply to
Moonraker

We attained flatness on carbon rings in the machine shop by the use of lapping machines. We also ground the matching hard faces of tungsten carbide. We measured the flatness with an "optical flat", which is an extremely flat slab of glass. The optical flats were able to detect variations of one light wave, a particular light wave, which on our instrument IIRC was .000016" I assume that if the optical flat could measure 16 millionths of an inch it must have been at least that flat itself. I will also assume that the optical flat was ground in the same manner. They are quite expensive and are stored in velvet lined cases.

Read more here:

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Jack

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Reply to
nJb

My experience with float glass is that it is noticeably wavy although amazingly smooth.

Reply to
Parallax

Greetings and Salutations.

On 13 Nov 2003 01:33:58 -0500, Allan Adler wrote:

There is a wonderful trilogy of books titled "Amateur Telescope Making", edited by a fellow named Ingalls that covers all aspects of optics - including how to make optical flats. Go to your public library and see if they have a copy. The articles there will tell you more about the process than you want to know. However, in short...take three thickish pieces of glass and a series of grits, ranging from fairly coarse, to jeweler's rouge. Starting with the coarsest grit, grind all three plates on each other, until all three have an even scratch pattern on the ground face. Clean everything off. Goto the next finest grit. Repeat above steps for each grit until polished. It helps if you have a monochromatic light source, say, a sodium arc lamp, or, a candle burning salt for that bright, yellow flame. That is how one tests the surfaces to see how "flat" they are getting. By putting two of the plates together, face to face, and illuminating with the monochromatic light, one gets a series of interference fringes. The shape of the fringes and the number of them give an idea of how parallel the surfaces are. In a "perfect" world, you would have one, wide fringe that covers the entire contact surface...that would mean the plates are "perfectly" parallel, and, if you have used three plates, perfectly flat. (remember that it takes three points to define a plane, mathematically...works with glass too). Needless to say it is a TAD more tedious than this explanation, but, this hits the high points. (haw, haw, haw). Regards Dave Mundt

Reply to
Dave Mundt

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