Re: grind flat surface on rocks / granite

"Richard Smith"  wrote in message news:m134lug3zn.fsf@void.com...
...
Anyway, I was thinking how it might be possible to produce a flat
surface.
In the steelworks labs. there was the "swing-grinder" which had a
vertical spindle and you swung it back and forth over a sample in the
chuck, lowering it a bit per pass to produce a flat surface to start
going through the finer and finer emery grits with until you could
diamond-polish it to mirror finish.
I thought of base and column of a bench-drill, clamp a collar on the
pillar at height of finishing plane, and have angle-grinder on an arm
you swing back and forth.  Letting the grinder ride-up for light
"cuts", but eventually stopping at the plane dictated by the collar
locked to the pillar (column).
Anyone got a better idea / know how it should actually be done - if at
all?
Regards,
Rich S
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I've ground surfaces quite close to shape freehand with an angle grinder by mounting them free to rotate and controlling the rotation speed vs grinding rate by the position of the grinding disk. An example is the front motorcycle tire for my sawmill which needed a wider crowned flat for the bandsaw blade. The sawmill frame helped hold the grinder steady enough to avoid high spots. I used the rear wheel as-is since the previous owner had worn it to the cords burning rubber in the street, and the crown was perfect. The blade was tracking centered after less than 15 minutes of adjustment.
When grinding the end a straightedge shows if you are grinding concave or convex. Touching the disk to both sides somewhat off the center of the work makes it rotate slowly and guides the disk to cut pretty flat despite its wear.
A variation is grinding the broken end of a drill bit or endmill square freehand by rotating it against the wheel before adding the drill point angle. I buy high quality dull or chipped endmills very cheaply second-hand and regrind them for roughing, saving the new ones for finish cuts.
Holtzapffel mentioned a cup filled with hide glue or pitch as a chuck for irregular objects, or actually high precision clock repair because the work can be precisely centered as the melted glue hardens. This describes using modern hot melt glue:
http://www.woodturned.co.uk/html/glue_chuck.html
For the metal sample grinder we placed the sample face down in a ring that we filled with casting resin. The resulting disk fit the grinder holder.
jsw