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FINE GRINDING THE MIRROR

After generating the mirror curve by rough grinding it remains for us to smooth it by fine grinding and polishing before finally correcting it to a parabola of revolution.

Fine grinding is done in about the same way as the last wets of rough grinding, but with a series of finer and finer grits such as 100, 220, 320, and 500, and finishing with 5 micron (15 wets) before pouring a pitch lap for polishing. The function of each succeeding, finer grit is to grind out the pits left by the preceding, coarser grits. Usually eight wets with each grit will suffice. Success in removing the larger pits may be checked by washing and drying the mirror and holding it up to a single bright light (not the sun!). Pits remaining from the larger grits will stand out against the background of finer pits.

Care must be taken in cleaning up between grits to see that no coarser grit is carried over into the later stages. Before starting a finer grit, the grinding bench should be cleaned as well as the tool and mirror glass, and caution should be observed that coarser grits are not carried over by the clothing or the water supply.

It must be remembered that 100 grit (or even 220) is sufficiently rough to change the focal length considerably. It is best to test the focal length occasionally, and if it gets too short, lengthen it by grinding with the tool on top.

One must be careful also that the mirror and tool don't stick together during fine grinding, particularly in the later stages. Short, fast strokes nearly center over center with fresh grits tend to prevent sticking. If they do get stuck, they may be soaked in warm water or beaten apart with a rubber mallet or your fist.

One must also be careful to avoid glass scratches. Fine grits do not sprinkle like coarse grits, they tend to come out in lumps like flour and are needed in very small amounts. We smear them around with our fingers and squeeze out the excess water by pressing the discs together before starting to grind. Otherwise the upper glass may be tilted by resting on a pad of grits or by floating on the excess water, and the edge of one glass may collide with the face of the other glass producing scratches. Glass scratches may also result from drying at the edges. These scratches, though no serious, are worse than Carborundum scratches since they are wider.

We said earlier that the fine grinding is done in about the same way as the last wets of rough grinding (without very long strokes or excessive lateral overhang). It is not, however, done in exactly the same way. Minor defects in the curve are not important during rough grinding but they must not remain at the end of fine grinding or the polishing will not proceed as desired. Our fine grinding and polishing strokes will be about the same. During the fine grinding and polishing with the mirror on top, if a little extra pressure is exerted with the left hand while the mirror overhangs to the right, and a little extra pressure is exerted with the right hand while the mirror overhangs to the left, the generated curve tends to approach quite closely to a parabola of revolution. If the fine grinding and polishing is done with the tool on top, the extra pressure should be exerted where the edge of the tool overhangs the edge of the mirror. If the tool is much smaller than the mirror blank, the fine grinding and polishing should be done with the tool on top.

It is a good idea to use lukewarm water during the last wets of fine grinding, and also during polishing. Cold water may shrink the mirror face allowing us to grind or polish away the edge.