Overcoming waviness: General Industry Coverage

Dear Doc: I cylindrical grind nitrided shafts with 20 spark-out revolutions. Sometimes I can spark-out forever and still never remove the waviness, and other times it takes only a few spark-out passes. I spark-out at 200 rpm with a constant wheel speed of 40 m/sec. (8,000 sfm) and a wheel diameter from 500mm to 350mm (19.7 ” to 13.8 “). The level of waviness seems to depend on wheel diameter. Why is that?

The Doc Replies: No grinding wheel is perfectly true, and that imperfect roundness on the wheel inevitably gets put into the workpiece in the form of waves. The key is not to “catch the waves” when the workpiece makes successive spark-out revolutions.

Take the wheel rpm and divide it by the workpiece rpm. That’s the rpm ratio. If the rpm ratio is an integer value, such as eight, then the workpiece catches the wheel in the exact same wave and exacerbates the waviness (see Figure). If the rpm ratio is a fractional value, such as ¼, 1⁄3, ½, 2⁄3 or ¾, the situation is a little better as the previous wave is obliterated by the next wave, providing a series of scallops.

Courtesy of J. Badger

The height of waves on the workpiece depends on where the wheel hits during successive spark-out passes.

The ideal situation is a long-decimal value, such as 7.84923785. Here, you never “catch a wave,” but just keep obliterating previous waves. I’ve seen unbalanced, out-of-true wheels generate little waviness because they found such a value.