Until recently, it was unfeasible to machine precision, complex parts at high spindle speeds. Spindle vibration would cause rough part surfaces and short tool life. A new technology called the Adaptive Balancer from machine-tool builder Mori Seiki in Rolling Meadows, Ill., (www.moriseiki.com) solves this problem by compensating for spindle unbalance as the machine tool runs.

For example, when performing rectangular-side milling, a multi-axis, mill-turn machine cuts material from only one side of the part. This changes the part's center of gravity, thereby causing spindle vibration. In turning operations, on the other hand, the part's center of gravity stays the same, and the spindle remains vibration-free.

A quick look at what makes up the Adaptive Balancer helps understand how it works. The mechanism contains balancer and coil assemblies. The balancer assembly attaches to the chuck and rotates with the workpiece. Inside the balancer are two stacked rings, each with embedded counterweights. Permanent magnets are also embedded around the circumference of each ring. When the counterweights on one ring are directly opposite those on the other (at 180°, as they would be when turning) the balancer is neutral, that is, balanced, and does not affect the spindle.

When the workpiece becomes eccentric (such as with milling), a sensor picks up spindle vibrations and sends the signals to the Adaptive Balancer controller along with phase signals from a sensor on the balancer assembly. The controller powers the coil assembly, generating a magnetic field. This pulls on the permanent magnets and rotates the rings. The counterweights position with respect to each other then cancels out workpiece unbalance, providing part rotation without spindle vibration.

The mechanism also lets machines cut different-sized workpieces with the same chuck. A good example comes from machining titanium hip implants, which must vary in size. The job would typically run with a spindle speed of about 8,000 rpm. Adding the Adaptive Balancer lets the job run at 20,000 rpm.

Additionally, the mechanism is effective when cutting diverse materials. For instance, there are differences in the center of gravity of cast and forged materials. With the Adaptive Balance, operators need not adjust chucks to compensate. Instead, the NC program makes necessary adjustments.