Varying The Workpiece Rpm: Design & Engineering

Dear Doc: Our cylindrical grinding machine has the option of varying the workpiece rpm. Is this legit? How does it work?

The Doc replies: Yes, it’s definitely legit. Here are the two main reasons it’s helpful.

Reason 1: When you begin to grind, the wheel/spindle assembly bounces up and down – ever so slightly – at its natural frequency. This can lead to self-excited workpiece- regenerative chatter. As the workpiece rotates, the slightly bouncing wheel creates small lobes on the workpiece. As these lobes come around again, they create force pulsations that excite the wheel/spindle, causing it to bounce slightly more, which creates bigger lobes. These bigger lobes come around again and excite the wheel/ spindle even more, causing higher bouncing, which creates bigger lobes, which come around again and excite the wheel/spindle even more, which creates even bigger lobes. Next thing you know, the wheel/spindle is bouncing like crazy – i.e., chatter is out of control.

For the lobes to excite the bouncing of the wheel/spindle, the wheel/ spindle bouncing and the workpiece lobes have to be in-sync, with no phase shift between the two.

This depends on the ratio of wheel/ spindle natural frequency and workpiece rpm. If the bouncing/lobes are in-sync, chatter gets worse. If the bouncing/lobes are out-of-sync, chatter is attenuated – i.e., it dies down.

Sometimes bouncing/lobes are in-sync, sometimes they’re not. There’s no good way to predict when that happens. It happens mostly by chance.

If you vary the workpiece rpm, the bouncing/lobes are going in-sync and out-of-sync. Chatter may build up for a second or two, but then a second or two later it is being attenuated.

The result? Chatter doesn’t self-excite drastically.