: I do know how to grind a mirror, but I don't know how it can grind out the concave shape in the glass? This is not that important, but I would like to know for my own sake. Also, is a ronchi test better than a focault test? And lastly, is a ten inch mirror a good start? : I apreciate all of your help!!!! THANK YOU!!!!

John:

Lets see if I can paint a mental picture of how grinding works. If you are to watch the two disks move back and forth over each other you would see - the closer you get to the center of the upper disk, the more time that glass is in grinding contact with the bottom disk so it gets ground out more. Away from the center, that glass progressivly spends more time "hanging" out in thin air. (I know, I know, you could say the same for the bottom disk but..) Now, for the bottom disk, picture you're pushing down with 10lbs of pressure as you grind. As the top disk progressivly overhangs the bottom, that 10lbs is being concentrated more and more on the edge of the bottom disk and the center of the top disk. These forces combined with the fact you are not doing anything to hold the two faces parallel as they grind has a natural tendancy to hollow out the upper disk. If you could mount the disks in a machine that held them rigidly (so the pressure is exactly even with no "rocking" tendancy and you ground them back and forth, they would not hollow out. You may not think you are trying (trying is the key word here) to rock the disks when grinding, but if you picture the pressure "curve" across the stroke it would be a curve, less pressure when the two are centered, and more as they overhang. Combine the two and you see as the pressure/time grinding is greatest for the center of the top disk and edge of the bottom disk.

Phew! hope that helped instead of confusing. Translating the picture in my mind to words that will paint the same picture in yours...

It was a great question, and one that has been asked by many before you - myself included.

Tom