开云体育

Ralph,

The RPMs are not constant, but the surface speed is... or, more accurately, in this system it is approximately constant between two limits.

This is an extremely important concept to understand. The ratings of cutting tool technologies, etc. are all based on the surface speed, I.E., how quickly the material being machined moves under the cutter. In a lathe, this depends on how far from the centerline the tool is cutting, and is most visible when facing.

Assume a simple round 4 inches in diameter being faced at a rotation rate of 100 RPM. When you start cutting at 2 inches from the centerline, the surface speed is:

100 r/m X 4 inches X pi = 1257 inches/min (approx)

When you are 1 inch from the centerline, the surface speed is:

100 r/m X 2 inches X pi = 628 inches/min (approx)

And when you are 1/4 inch from the centerline:

100 r/m x 0.5 inches X pi = 157 inches/min (approx)

Please note that the distance from the centerline for a round part is the radius. The circumference of the part is the diameter X pi, so we multiply the radius X 2 in the calculation.

The purpose of a constant surface speed system is to increase the RPMs to keep the surface speed constant during cutting.

BTW, have you ever noticed when facing that when you are almost to the centerline the the chips come off so slowly that you can watch each one form? That's because the surface speed decreases rapidly as you approach the centerline, and is? actually zero at the centerline. This is a problem for CSS systems, because the RPMs should approach infinity as the tool approaches the centerline in order to maintain constant surface speed, and very few spindle bearings are rated for infinity RPM.
--
Regards,

Charlie
New Jersey, USA

SATTINGER’S LAW: It works better if you plug it in.
EDWARD'S LAW: It works even better if you turn it on.