making a tool for fine grinding

[dark star]

I am going to make a 15 inch diameter grinding tool for fine grinding my 20 inch mirror. I am spin grinding on a fixed post machine. I have picked up some scratches when figuring so I am going back to fine grinding. I poured the pitch for the pitch lap on to the original grinding tool so I cannot use this.

I have some Dentstone KD dental plaster. I wanted to double check that I have understood the mixing instructions correctly so I don't make a stupid mistake. The instructions say water to plaster ratio ( by weight) 30%. Plaster to Water Ratio (by weight) 3.33:1 This  is probably obvious, but I am afraid I don't have a very logical brain when it comes to anything with figures, however simple!

I assume this means that I need just over 3 times the weight of plaster compared to water, and not the other way round?

Also, I have looked at on on line calculator for mixing cement , and it appears that I should use around 12 kg = 26.45 pounds of dental plaster. Does this seem about right? I am planning for the tool to be around one and a half inches thick.

It appears that I should use 3.6 kg = 7.94 pounds of water?

In the past I have just guessed at the amount of dental plaster needed which has often worked. But once I made a tool so heavy it was not usable! So I want to stop guessing.

David

 

[Astrobits]

You are correct, 30% of 12Kg = 3.6Kg

Nigel

[dark star]

I made the grinding tool. Unfortunatley I made a silly mistake, one I have not made before. I need to make a hole for a socket in the middle of the grinding tool to hold a metal shaft from the fixed post machine. I forgot that the hole should not go al the way through the tool! So now I have a hole in the middle at the front of the grinding tool which as far as I can see makes it unusable.

I will have to make another tool. To look at the bright side, this time when making the tool I weighed the plaster and water. There was some left over when I poured the dental plaster, so next time I will know to use a bit less. I should have enough dental plaster left to make another tool.

 

[davidc135]

If the hole matters could part of it not be filled in?  David

[dark star]

I should have thought of that,! If I put the tool on to the mirror and pour a bit of plaster in to the hole that should fix it.

I use two penny peices attached to the tool with small blobs of pitch for grinding.

[maw lod qan]

I would love to see some images of your process! 

[Chriske]

If  the pitch has to go why put it in a freezer. When it has cooled enough, take it out of the freezer the pitch will slide of very slowly.
The trick is to wait until the pitch start making soft cracking sounds.
This will only work with a full glass tool, not with a tiled tool.