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[ATM] Making blanks round
>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
Paul can you post for us a picture of this machine and maybe a glass
piece on it , Please ?
I did the google thing and saw a few sites but would like to see your
implementation of it. i need to fix up these 2 Frankenstein looking
blanks of mine.
thanks
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