"Jim Wilkins" wrote in message
...
"Pete Keillor" wrote in message
...
On Wed, 6 Aug 2014 12:48:44 -0400, "Jim Wilkins"
wrote:

"Pete Keillor" wrote in message
...
I'm to the point of turning the drive wheel for the belt grinder
I'm
building for my son. It'll be 6061 Al, 6" dia. x 2" wide,
running
on
a 3/4" keyed shaft, and turn up to 3800 rpm. I figure I'll put a
setscrew on the key and another at 90 deg., which will require
drilling from the circumference. I can drill holes opposite to
help
balance.

Any suggestions on how to balance this for a hobbyist? I'm not
buying
a balancer, but could build something. Or just spin it up and
see
how
it does? Thanks.

Pete Keillor

If you haven't already cut the shaft to length you could turn the
other end to 1/4" and spin the shaft and pulley with a fast drill,
a
variable speed one to accelerate it slowly and carefully. If the
shaft
is long enough the pulley will rotate smoothly around the combined
center of gravity of it and its end of the shaft. You could
carefully
touch a laundry marker to the shaft to mark the light, high side
and
feel the amount of vibration.

-jsw

Good idea. Since the shaft is cut, I can adapt your idea by
chucking
in a 3/4" 5C collet in my Hardinge mill, set it up for high speed,
and
use the VFD to bring it up slowly. Should be able to get a sense
of
an out of balance condition. More rigid, but with enough overhang,
it
should work.

I also thought of not drilling the opposing holes completely, and
deepening as indicated.

Thanks.

Pete

The lathe is NOT a good idea. I couldn't find a clear, short article
on Critical Speed to explain what happens. A loosely restrained hand
drill will let the spinning shaft and pulley find their own center
of rotation while the shaft and pulley may whip around dangerously
in the lathe.

Or mill, I posted fast to stop you. Don't spin it with anything until
I find a better explanation.