[ATM] Fine grinding with tile/plaster tools

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

James Lerch wrote:
> One item of interest, when the day comes that your ready to take both mirrors 
> past 25 micron grit, try and set aside enough time to finish fine grinding both 
> that day.  Stopping at 9 micron and restarting again the next day has never 
> worked out very well for me, and usually results with the tool getting stuck to 
> the mirror.  (BTW, when this happens, just stop and soak the mirror / tool for 
> several hours in a bucket of room temp water, they will eventually just slide 
> apart)

This has happenned to me if the tool is made of plaster and tile.

It happens because the plaster (in my case, Hydrostone) tool dries out 
when not in use.  When it is in use, it is being dunked in water, 
sprayed with water, and it will hold a good amount of moisture in the 
plaster itself because the plaster is porous.

When the tool is not in use, it dries out.  It can change shape 
slightly.  When you go back to fine grinding, you apply the abrasive 
slurry and put the mirror on, and almost immediately the tool (being 
quite dry and porous, like a dry sponge) absorbs the water, and voila, 
the mirror and tool glue themselves together!  Consequent brute-force 
attempts at separation can lead to scratches.

This is why soaking it in water allows them to be separated - the tool 
gets "re-hydrated", and the two can be slid apart.

The solution to the sticking problem is to keep the tool moist, by 
keeping it near, BUT NOT SUBMERGED IN, some water.  Put it on a stand 
in a bucket of water, and cover the bucket, to keep it in an 
environment of high humidity.  (I say NOT to submerge it because I 
have seen Hydrostone slightly "eaten away" by long-term submersion in 
water.  Granted, the stuff might not have been mixed perfectly.) 
However, exposure to water during general use over the typical 
lifetime of a grinding tool has no noticeable effect on the tool.

Alternatively, just dunk the tool in water for a few minutes before 
resuming grinding.  The tool will soak up the water (you can actually 
hear this!) and then it will be ready to use.

	Mike Lockwood

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