Re: [ATM] grinding and polishing machine

["Bob May" <rmay@nethere.com>]

Please don't get me wrong here.  HP is HP.  It is torque times
speed.  You'll get the same amount of torque with either a DC or
an AC motor of the same starting speed and torque.  Your gearhead
motor which puts out 15RPM is still doing the same work that an
AC motor which is geared down, whether with a gearhead or a
pulley system.  If you've got , lets say, a hundred foot pounds
of torque at the 15RPm, you';ll stil have that much torque at
5RPm while a pulley system will have three times thatat that same
lower  speed.  An eigth Hp motor is more than sufficient for the
typical sized turntale for both the turntable and the overarm so
you don't see any problems.
There's no magic in this, just the figuring out what you want.  I
like to keep things pretty simple so I'd probably get a washing
machine motor or other motor of about a quarter horse and gear it
down with pulleys, allowing me to change ratios at some point in
the drive so that I can get the different speeds desired.
Swapping the position on a multiple pulley ala a drill press is a
lot easier than op[ening a gear drive and changing the gears!
The DC motor trick of variable speed is nice but you don't get a
really wide range of speeds and keep up the necessary torque
changes that would otherwise come with keeping the motor at full
speed and changing the amount of stepdown with the puplley
system.
I've worked with DC motors in electric trains and fully realize
the amount of speed change that you can get with such a setup but
there, the torque needed to drive the train is pretty much a
constant.  You can easily get a 50:1 ratio of speed change but
you still have only that much torque to work with.  The needs of
a grinding machine is that as the speed slows down with larger
pieces of glass, you need more torque to do the work so you want
to change the drive ratio rather than the speed of the motor.  A
gearheasdd motor, whether DC or AC, will only mean that much less
in the way of a pulley system needed to do the job.
Bob May
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