The Grinding Tool
Making a 4.25 Inch Dobsonian Reflector Telescope
For my first 12.5 inch mirror, I used an 8 inch piece of glass for the tool. It had already been used at least once, and was convex on both sides, although only slightly so. This is the classic method. This turned out to be bad for a number of reasons. Regardless of what type of tool you use, you really want one side of your tool to be very flat. That's the side you won't be grinding on. If it's not reasonably flat, when you rub the mirror blank against it, it will rock and that is very bad.
The Price of Glass
In case you are not aware, the Corning glass works recently closed down a foundry that was responsible for most of the borosilicate (trademarked Pyrex by Corning) glass blanks made in the US. For a while this seemed to be causing a scarcity, forcing the price of blanks and tools up. Recently I think I've been seeing a rebound, but I'm not at all sure. In any case, this caused me to investigate the other common option, a tool cast of plaster of Paris, covered with tiles. There are other options for the casting material. I've used Fixall successfully, and I've heard that the material they make dental casts from works well, cures in an hour and is waterproof although at a somewhat higher price. Some disadvantages with plaster of Paris are that it takes about a week to cure, and you probably want to coat it with epoxy to waterproof it. I found that this didn't seem to be necessary with the Fixall. While I've not seen it suggested anywhere, I think that a wooden tool should work equally well, and I plan to try this out on my next project.
I bought a grinding kit with blank and grits from Firsthand Discovery and separately their tool making kit. I had good luck with both kits, but the tool ingredients can be easily purchased separately. The tiles can be found on-line at Home Depot if you are willing to buy in quantity. The best type of epoxy seems to be the type that comes in a double cylinder plunger.
The Advantage to Tiles
There is an extra step in making the tool using tiles, however there turned out to be a few advantages other than the cost. When you get to the fine grinding, with a glass tool you will find a times a tendency for the tool and the blank to freeze together. This is somewhat un-nerving, although easy to fix if you are calm. You put the two in bucket of warm water and they slowly separate. Because the tile tool has small channels between the tiles, this doesn't happen.
The second advantage has to do with tradition. Typically when done grinding, you make a lap using the tool as your base. This makes it very difficult to go backward if you need to. But with a tile tool, just keep it. Make another slab for the base of your lap. When finished with your mirror, you can keep both tools for a future project.
I recommend that when you glue your tiles on, you put them as close together as possible. I don't know what I was thinking, but I put in spacers. I guess I was thinking ahead to the way you make a lap. This caused me to waste a lot of grit, because the spaces would fill in. Before my second go around on the fine grinding, I filled in the cracks with plaster of Paris. Some of this got on the tool, but came off as soon as I started grinding with 80 grit.
I tried to fill in the edges with broken tiles. I'm pretty sure this was a waste of time. I'd probably have lost less than 10% of the area without them, and for a bigger tool it will be even less.