Hi Ian,

As soon as you get the grinding all the way to the edge of the mirror, cast a new tool and face it with tile (vitreous tile only). As Bob May indicated, the tool you are now using is only used to rough hog out most of the glass. Continue with the same grit while using the new tile tool until all the tiles show some wear and that you have about the right sagitta depth. This will true up the new tool and get you very close to a spherical surface. Now you can change to the next finer grit and continue. Keep a close eye on the sagitta depth and if it gets to large, change to tool on top with the morror on the bottom. When the mirror is on top, the depth will get greater while with the tool on top it will decrease. As you go through the various girt sizes (finer and finer to complete grinding) you should continue to check the sagitta depth and alternate the mirror and tool position to maintain the sagitta that you want. One last tip, when you think that you have done enough with any grade of grit, do at least six more wets and if you're still going strong, add a couple more. It's not possible to overdo the fine grinding but if you don't do enough the pits will drive you crazy.

Jarvis Krumbein,

• Cast Tool Experiment - imacca - April 1, 2002 - 16:18 UTC
  • Cast tool - bobmay - April 1, 2002 - 19:37 UTC