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From: "Jim Wilkins" <muratlanne@gmail.com>
Newsgroups: rec.crafts.metalworking
Subject: Re: grind flat surface on rocks / granite
Date: Fri, 20 Sep 2024 08:47:00 -0400
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"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

----------------------------
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