Analyzing regenerative chatter: Design & Engineering

Dear Doc: We cylindrical-OD-grind camshafts with a CBN wheel. We start to see chatter after about 80 camshafts, so we dress the wheel every 60 camshafts — 0.002 mm × eight passes. What’s happening? And is there a way not to dress so often and consume less wheel?

The Doc replies: It sounds like you’re getting wheel regenerative chatter as opposed to workpiece regenerative chatter. What’s the difference? Let’s start with workpiece regenerative chatter.

Chatter measurements are given by Japanese grinding guru Ichiro Inasaki. Image courtesy of J. Badger

During grinding, the wheel bounces up and down at its natural frequency. (All wheels bounce at least a little.) This creates lobes on the workpiece. When they come around again on the next workpiece revolution, they cause a variation on the depth of cut, which causes a variation in the force. This variation in the force excites the system, and the wheel starts bouncing up and down even more. This creates bigger lobes, which cause bigger depth variation, which causes bigger force variation, which excites the system even more, creating even bigger lobes. The maddening cycle continues until chatter is out of control.

Because these large lobes require only a few workpiece revolutions to develop, workpiece regenerative chatter typically begins quickly — within a few seconds.

Wheel regenerative chatter is a different animal. Here, the wheel bounces up and down a little. (Again, all wheels bounce at least a little, even with a stiff spindle.) This causes the wheel to wear unevenly, creating lobes on it. They excite the system, which causes the wheel to bounce up and down even more, which creates bigger lobes on it, which excites the system even more. Next thing you know, chatter is out of control.