Thread: High to Low RPM on a cnc lathe

What is better for the machine? I have a max spindle of 1800 rpm. (Big Lathe) and I am doing some parts that only require OD turning. 1 rougher and 1 finisher. The thing is when I'm roughing, my surface speed is 1400 so of course my 2.75 diameter part is running at max rpm. When it goes for a tool change, it comes out Z10. and X 10. The spindle slows down during the tool change then back up to max for the finisher. Is it better for me to leave my X under 2.75" when tool changing to keep the spindle at a constant high or is it ok to let it wind down then right back up again?

This isn't something Id ever worry about to be honest unless your machine has very slow acceleration on the spindle in which case it may mean a faster cycle time if you keep the speed up.

Originally Posted by Hertz

What is better for the machine? I have a max spindle of 1800 rpm. (Big Lathe) and I am doing some parts that only require OD turning. 1 rougher and 1 finisher. The thing is when I'm roughing, my surface speed is 1400 so of course my 2.75 diameter part is running at max rpm. When it goes for a tool change, it comes out Z10. and X 10. The spindle slows down during the tool change then back up to max for the finisher. Is it better for me to leave my X under 2.75" when tool changing to keep the spindle at a constant high or is it ok to let it wind down then right back up again?

Its better to keep the spindle revving, particularly with a big machine having a large chuck where there is a fair amount of inertia to contend with and the associated power requirements during acceleration/deceleration.

You don't actually have to do the tool change with the tool at the small diameter, particularly if its safer to get the tool well out of the way when tool changing. Just before you send the tool to X10.0, command constant RPM with G97. You haven't identified the control, but I assume G97 will be constant RPM with your control.

Its quite common to calculate the RPM the spindle will be revving at for the Next Tool at its starting diameter, and invoke those RPM prior to sending the Current Tool to the Tool Change Position. When the Next Tool reaches its start position, Constant Surface Speed is activated with the spindle already at the correct RPM for the CSS and the current diameter. This procedure avoids the repeated Slowing Down and Revving Up as the turret is moved to the Tool Change Position and back to the cutting zone.

On machines equipped with Spindle Load Meters, you will see these Max out when changing spindle RPM abruptly, typically when the X slide is moving rapidly between greatly varying diameters with CSS active.

Regards,

Bill

IF you have just a straight diameter cut, I would use G97 S1800 instead of G96, that way the spindle will stay constant. The other option, since you are already on top speed for the lathe, would be to use a higher value for your surface speed so that the machine will calculate near 1800 RPM's at X10., i.e.: (1800 RPM x 10")/3.82= 4712 SFM. To be honest, I rather use that G97.

HTH.

Edit: I see angelw beat me to it and explained it a lot better than I did!

Awesome. Thanks a lot. I will do the G97