George wrote:
Uh, not really. Read it again. Guess I threw you with my "reality" point.

I use it with kids in class because they learn math with calculators, and if
they plug the wrong value in, they get trash - unless they have an estimate
of valid range or "reality" point.

Motor speed times ratio of motor to spindle pulley (diameter,
circumference, no matter) equals spindle speed.

Thus if 1.5 on motor and 4.5 on spindle - 1/3 times 1725 = 575 - pretty
common.

"Bob Pritchard" wrote in message
...

To find a minimum speed the smallest pulley would be on the motor.
With a 4" pulley on the headstock and a 2" pulley on the motor the ratio

is 2:1

A normal motor of 1725 divided by 2 would be approx 862.5.
Your theorie suggests a high speed setting with the large pulley on the

motor.

Or: Measure your pulley sizes - outside OK, since you're interested in a
ratio. If the Pulley on the motor is 4", lathe 2", the ratio is 2:1.
Multiply by the motor speed.

If I were a betting man, I'd say minimum is 600.

Your guess is probably accurate, and the math fine, but they're counter to each
other. A 2/1 ratio on a 1750 motor would be a speed of 3500; a 1/2 ratio (2"
motor pulley, 4" headstock) would halve the speed. If the kids don't think
through the problem to arrive at a reasonable reality point, what they expect to
get and what they get will be way off and probably stump them for a while until
they do think it through.

In other words, your reality point was for the low end, and the example for the
high end. That tends to throw folks...

....Kevin
--
Kevin & Theresa Miller
Juneau, Alaska
http://www.alaska.net/~atftb