- use a ring shaped tool (an off-cut from a thick walled pipe, old ball bearing housing, even cup shaped grinding wheel). The diameter is not critical, but somewhere between 1/4 and 1/2 D is ideal. Also, it is useful to fit sime sort of handle to it - I always end up with blisters on my hands
- bevel the blank WELL. There is lot of forces involved, and it is easy to chip it if there is no good wide bevel on the blank.
- lay down a mirror blank, sprinkle it with carbo #80 or coarser, and using a lot (and I mean a LOT) of force start grinding with a ring. Use the body weight (which means keeping the work much lower that usually - sort of waist level). Keep the strokes concentrated to central 1/2 to 2/3 area at the beginning. As you progress (and you will very quickly), widen the stroke, but don't go beyond the edges of the blank
- the wet will last very short. Because of forces involved, carbo breaks very quickly. Nothing to worry about, because glass gets ground out even faster !
- measure the saggita often. It's good for keeping the spirit high, and it's easy to overshoot (I can count on digging an f/5 curve in a 14.5" plate glass blank - 3.5mm saggita - in less than 4 hours ELAPSED time, i.e. easily in one afternoon)
- use concave blank to cast the matching convex tool. Prepare the tool after it dried well (if using plaster; this dental stuff sounds promising!) - cover it with a good layer of epoxy. That will make it stronger, and will keep the moisture away. Epoxy the tiles then later (the rough back ones, with sort of a fiberglass mesh holding them together are the best - the tiles NEVER came unstuck on me)
- continue with #80 until tiled tool gets nearly spherical. Then switch to #120 and you're on the familiar grounds from then on !
Last warning : tiled tool is a bit more difficult to clean than usual glass tools, because of channels. Use old toothbrush os something similar and wash THOROUGHLY. Don't cry on my shoulder if that grain of #80 that was hiding behind the tile decided to jump out and play a bit while you are at the finishing stages of 3 micron !
Bratislav