Tile on plaster tool

Original article from 1996:

Traditionally a second glass disc of same or smaller diameter than the mirror is used to grind the curve into the substrate. Replacing that glass disk with a Tile on Plaster Tool offers several advantages, despite the little effort in making the tool.

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.

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!)

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.

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 removing 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 use 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.

Update 2010:

Since writing the original description of the process many years have passed and I have made plenty of tools in the mean time. Many new techniques and materials have become available and, among other things, it has become popular among ATMs to embed the tiles directly in the plaster thus bypassing the step to glue tiles on the plaster base. I will describe that process in continuation.

Other ideas include using glass shards instead of tiles and even going back to the original glass tool that has been previously channelled with a diamond file/tool.

New types of plaster have become available for the telescope maker and some even use solid granite tools.

Embedding tiles directly

Embedding the tiles directly into the plaster has the advantage that the tool can be used instantly after fabrication, as we bypass the gluing steps and curing times. It would seem that this technique is a great time saver and that might be true, if you do all your grinding by hand. However, I found that those tools require quite a bit of maintenance since by very nature of the process of embedding the channels between the tiles are closed and therefore the tool acts very much like a solid glass tool unless we open the channels between the tiles with a Dremel tool or similar.

You can maybe skip doing that for manual work, but if you grind with a grinding/polishing machine you will have to cut those channels, otherwise you will get problems at the finer grits when the tool and mirror tend to seize up. Scratches are also becoming an issue, as the plaster is in contact with the glass and the tool-rim has to be beveled very well to avoid scratching your mirror.

All in all, I found that time saved at production is offset by maintenance time and grinding problems and I don't use that technique anymore. However, I include a description of the process:

• 
• You start by covering your mirror with aluminum foil and preparing a round form conforming to the diameter that your tool will have. I use a variable size cake form that you can see in the photograph.
  Then you cut the tile mat to size so it fits into the form. If you use loose tiles that are not held together by a matrix, you should fix them to the alu-paper with a drop of glue, so they don't move when pouring the plaster.

• 
• The next step is to mix and pour the plaster and you are done.
  A word about mixing dental stone/plaster: You should slavishly adhere to the proportions given by the manufacturer of the plaster, as changing the proportions will impact the dimensional stability of the final product.
  The photograph shows a finished too with embedded tiles.

Tool with glued on tiles

The next example is a tool with glued tiles. I use marble, glass or porcelain bathroom tiles that are fixed to a matrix and are usually sold in squares 40cmx40cm. The glue of choice is slow (24h) curing water-resistant epoxy that has to be applied generously on the plaster surface.

• 
• We start by pouring the plaster and let the finished tool dry for a few days. Then we glue the tiles to the top generously applying epoxy resin to make sure that the tiles are embedded in the glue.

• 
• I usually remove corner tiles that have less than 50% of their surface area on the plaster base. The other ones, I let overhang while gluing and cut of later with an angle grinder. For operation it is important to maintain ALL tile edges well beveled at all times to avoid scratches.

Pitch on plaster lap

Obviously a plaster tool can be used perfectly as a pitch polishing tool. I usually pour the tools for laps after finishing grinding with #300 grit. The tool should be dry before pouring the pitch and rubbing it with turpentine will make the pitch stick better.

• 
• The 350mm (14") lap that can be seen on the photograph has been made with a pitch mat from Pitchlaps.com. I usually use the pitch mat for bigger laps than 8". For smaller laps I use the lap carving method of Carl Zambuto.
  After pouring the pitch, I carefully bevel the outer diameter with a razor blade and coat the entire tool with wax to avoid plaster coming off during polishing.
  On the photographs one can also see the micro facets on the surface that are achieved by wire brushing the lap under water.

• 
• For machine work I attach a plywood disk to the back of the plaster tool after pouring and while the plaster is still soft. That way it is easy to fit a socket for the machine spindle, as can be seen on the photograph.

Diamond pellet tool

Finally, I would like to present a special type of tool that I use for curve generating on the M-o-M grinding machine:

• 
• The tool is used with diamond pellets instead of tiles for curve generating or "hogging" with the M-o-M grinding machine. Diamond pellets are available through different sources. I buy mine from Gator diamonds.