Making a tool for a telescope mirror making project is easy. There are a number of ways to pull it off, and failure isnít going to happen because you can pick and choose whichever one suits your fancy.
Glass Tools.††† This is the most common type of tool. In fact many suppliers, such as Willmann-Bell, sell mirror making kits with two pieces of glass in the kit, one for the mirror and one for the tool. When the student is finished with their project, they can always make another mirror out of the other piece of glass, or sell it to another ATMer to make their mirror. If you use a glass tool, bevel the edges to a 45o angle so the glass wonít fracture out from the forces you will be applying to it. When that bevel reduces, re-bevel the edge. On the tool you should keep a fairly large bevel, possibly as much as 1/8Ē.
Dental Stone Tools.†††† For your grinding tool, you will mold the tool to match the curve of the hogged out mirror blank; that way you will not grind the edge tiles so thin that they break off. Cover the mirror with a piece of trash bag. Make a circular dam out of concrete expansion foam the diameter of your tool. Lay a mat of square glass tiles tiles on top of the plastic. These can be found at home repair stores in the counter tile area. I recommend glass, not tile. The tiles must be clean. Do not break the ligaments that hold the tiles together.
Mix up the dental stone [Not Dental Plaster]. If you use Buff Labstone, use 30 g water for every 100 g labstone. If your volume is very large, make it with more water to keep it workable a bit longer. The more water, the less strength the product will have. It is wise to mix up more dental stone than you think you need. If you find that you did not mix enough, you should mix some more and pour it into the mold as quickly as possible before the first pour gets stiff. Use dental stone; plaster of Paris will not work for this application because it will disintegrate when wet. Dental stone is superior since it will resist water well after it has cured. Work quickly with the dental stone. Pour immediately once you have it mixed. Itís kind of like plopping turds onto the mirror, but it will shake level. To level the mound and get rid of air bubbles, vibrate the entire mold on your workbench until there are no more bubbles. After a few minutes, your tool will start to harden and it will become warm. When it is hard enough not to flow, you may remove the dam and sponge the edge, and when it is cool, you may slide it off of the mirror blank and peel the garbage bag off of the face. Sponge the working face to clean it up. If sponging does not do the job, use a stiff brush. If there are any cavities around the tile ligaments, flow a creamy slurry of dental stone over the surface and sponge it when it starts to harden (it will harden rapidly). Once your tool is hardened, the back may not be flat enough for it to rest on your workbench without rocking. Yopu can rasp it flat or shave it with a razor [be careful].
Do not use your grinding tool for a pitch lap foundation! A common problem is the need, during polishing, to go back to fine grinding to eliminate a deep scratch or correct an inadequate fine grind that cannot be polished. You don't want to have to destroy your polishing tool for this! Once your grinding tool is in good contact with the glass and you have the desired curve, you can make your pitch lap foundation just as described above.
The procedure is the same, but you do not use tiles. Once you have a tool made, paint the contact surface with turpentine a few times to let it soak it up. It will now stick to the pitch. At our shop we place a lap mat onto the mirror, pour cooked pitch onto the lap mat, then place the tool onto the pitch. Once it has cooled to the point of being a bit brittle, but still warm to the touch, we pick the whole thing up, carefully peel off the lap mat, then place the pitch lap back onto the mirror. We place a plastic screen door screen material between the mirror and the pitch lap. We add some weight. This will press the pitch lap into the same shape as the glass. You can tell if you are in total contact by the crisscross patterns from the screen. Later you can chip off the pitch that has overhung the tool and add it back to the pitch supply.