Amateur Telescope making

Tile on Plaster grinding tool

For grinding my 6" mirror, I used a Tile on Plaster Tool . instead of the second 6" glass that I got with my grinding kit.
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Using that kind of tool offers several advantages, despite the little effort in making the tool. Since I never ground a mirror on a glass tool myself, I will give a short summary of what people say, who tried both:

Advantages:

You can save your second blank to make another mirror.
The channels in the tool minimize the danger of seizing.
You can grind faster and more efficiently by using less abrasives.
For big mirrors, the weight of the tool can be reduced.
Even distribution of the abrasive across the tool.
No trapped air-bubbles.

Disadvantages:

You have to make the tool yourself, which might be bit of an effort the first time.
Cleaning the tool is more difficult.
Unbeveled tile edges are a potential danger for scratching the mirror.
Plaster or cement particles coming off the tool can cause scratches.

BUT!
Most of the disatvantages can be eliminated by taking precautions, while the advantages are tremendous!

Required material:

Dental plaster; or plaster of paris; or any kind of cement preferably without sand and as fine as possible. Dental plaster cures very fast, which makes it the best choice.
Slow (2-4 hours applying time) curing epoxy glue. I made good experiences with white Porcelain epoxy. Of course it should be waterproof.
Porcelain tiles cut in small squares (~2,5 cm) . Porcelain is best, because it is very hard. If you can only find big tiles, then cut them yourself with a tile cutter.
Masking tape and aluminium foil.
Carbo grinding stone. The kind that you use to sharpen carpentry tools. (Buy a cheap one!)

Required Skills:

NONE!

Procedure:

First you cover the mirror face with thin plastic kitchen foil and create a mould by wrapping aluminium foil and masking tape around the blank. The side which later on will become the mirror face looks up. Take care that there are no air bubbles between the plastic foil and the mirror.
(Click on the small icon to see a bigger photo)

To avoid that the plaster leaks out while pouring, you should put masking tape around the plastic foil-covered blank before attaching the aluminum-foil. On the upper side of the glass, let the tape overlap 1 cm and fold it down, so you get a sticking outer rim. On this rim you can easily attach the aluminum foil strip. To get a firm foil strip of the required mould broadness (min. 1 1/2 blank thickness), you can fold a length of foil until it matches the size.
Place the ready casting form on a straight table.

Now, mix the plaster. Don't make the mixture too thick. It should flow easily, without being too watery.
Pour it in the form and rattle the table for some time. The vibrations help the plaster to spread out evenly. If you use dental plaster, the tool will start getting firm after approx. 15 minutes. Wait until rattling does not move the plaster anymore. Then start carefully rolling of the mould. Remove the tool from the mirror and let it set on a plane surface for one hour. Afterwards let it dry completely for a week or two.
If you don't use dental plaster, the drying times will be somewhat longer. Since I only used dental plaster, I have no reference timings for the other!

While the tool is drying, cut the tiles. 2,5 cm is a good size for the tiles, but cutting them slightly bigger or smaller will not harm either. Don't waste your time trying to cut them perfectly symmetrical.
Mark the center of the tool, draw a line through it and place the edge of one tile on the center. By offsetting the center tile, you avoid zoning while grinding. Now place tiles on the tool until the whole surface, except the edge, is covered. Try to get the channels straight. That will make cleaning the tool easier. Once this is done cut the small edge pieces to obtain an approximate circle. To avoid confusion when you later have to glue the tiles, write a number on each tile and make a plan of the positions on a piece of paper.
Use the time, while the tool is drying, to bevel all edges and corners of each tile with the Carbo grinding stone . That is a lot of work, but will avoid scratches and chips while grinding.

Once the tool is dry, clean the tiles and tool with alcohol and cover the upper tool-side with epoxy (don't forget to somehow mark the center!) . Now start placing the tiles, according to your plan, on the tool. Move them a little around to make sure that all gaps are filled with epoxy. Let the epoxy cure for at least 48 hours, then bevel the outer edge of the tiles as much as you can. Especially if you do rough grinding yourself, you will have to rebevel at least once before fine grinding, and during the process of fine grinding.

To avoid plaster particles coming of the tool while grinding, I recommend to cover the rest of the tool with paint and to mask the sides with tape while grinding.

Now the tool is ready and you can start grinding.
Get a plastic spoon (metal spoons tend to loose small particles which can produce scratches while fine grinding) and fill all the channels with abrasive. Spray water on the tool until all the abrasive and the tool are wet and start grinding. Once you feel the grinding effect weakening, spray some more water on the tool. After repeating this a couple of times, the channels will be nearly empty. Now it is time for a wet. You will find that each wet will take about 15-20 minutes and grinding will show faster results. Once you reach #400 Grit, don't fill all the channels with abrasive. Instead mix some abrasive with water and 2 drops of liquid soap. This mixture can be applied directly on the tool.

Cleaning the tool:

First remove the masking tape from the side and rinse the tool in a wet-bucket. Now clean all the channels with a toothbrush under running water. Change the toothbrush after every second grinding stage.

Due to the easy procedure of casting a tool of plaster, it would be wise to make a second tool for polishing after reaching #300 grit. This tool will already have the approximate curvature.
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