[ATM] Making blanks round

[IAPDK@ADMIN.DRAKE.EDU]

After trepanning three 14-inch disks from a tabletop I was faced with
the problem of some uneven and broken edges.  After a little Googling 
for "centerless grinding" (thanks Richard!) I built a simple machine 
to do the work - and it worked great.

Basically the machine stands the disk on edge on two rollers - one a 
motor shaft (motor scavenged from a Selectric typewriter) and the 
other a rollerskate wheel.  The motor runs at a steady rate, turning 
the disk about 30-40 rpm.  The two rollers are probably about 90 
degrees offset.  

A grinding stone is pressed against the disk just above the skate 
wheel.  I used a 4-inch masonry grinding disk meant for an angle 
grinder, and bolted it so that I could adjust the pressure on the 
glass.  I set it out in the grass and ran it dry, then cleaned up
the accumulated dust by hosing it down with water.

Since the disk was pretty rough all round I started by tightening the 
stone up enough to raise the glass off the skate wheel about 1/8 inch. 
After some hours with the motor running it had ground down enough to 
ride on the wheel, which is where the centerless grinding effect
took over and produced a lovely round edge.  

Total run time per disk was about 48 hours.  There are some concentric 
cuts in them due to the coarse grain size of the stone (which shows 
nearly no wear) but those are ok.  And they have slight tapers because 
the stone was not square to the glass but that is ok too.

When I get the blank centered on my grinding table and spin it, a 
light touch with a pencil produces a mark all way round :-)  I 
processed two of the three disks this way and they both look great.
My disks were pretty rough, one had a huge taper on one side, sort
of like one end of an elliptical mirror, but everything was straightened
out.

The principle of centerless grinding is to never pinch the disk 
between the stone and a roller, which exaggerates your 
out-of-roundness, but to place the stone at some offset angle.  
Then when a high spot goes over a roller it won't grind down the 
opposite side.  But when a high spot goes over the stone it 
lifts the disk off the nearest roller which increases the pressure 
on the high spot.  

"the glass that sticks up shall be ground down" 

Paul K

Theories without evidence are as pestilent as fleas without a dog.
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