Three-level diagnosis of lathe spindle noise: from bearing to taper bore accuracy

The operator reported a hum above 1200 r/min on the spindle and out-of-tolerance roundness on the workpieces. Grease had been changed and preload adjusted; the noise eased but did not go, and the roundness problem remained.

When I took over I first set up vibration measurement points — front, middle and rear of the headstock — and recorded spectra through the run-up.

The spectrum showed the dominant noise frequency matching the bearing outer-race characteristic frequency, confirming front-bearing wear. But on teardown the inner-race raceway was unevenly worn, pointing to slight deformation of the spindle taper bore. Checking the maintenance records revealed an unreported tool crash six months earlier.

The work came in three levels: replace the spindle bearings as a pair and preload to specification; lap the taper bore and check it with marking compound; then verify roundness by cutting a standard test piece. Nine days later roundness was back within 0.005 mm.

Diagnosing machine tool noise means reading the spectrum, not trusting experience. Characteristic frequencies do not lie; ears do.

A tool crash must be reported and the accuracy re-checked. Concealing one crash usually means ten unexplained out-of-tolerance jobs later on.

The usual answer to spindle noise is "bearing wear - replace the bearing." This case shows that replacing only the bearing leaves a hidden problem: once the new bearing is fitted, the slightly deformed taper bore makes it wear abnormally again, and the noise returns within months. Spectral diagnosis confirmed bearing wear; the wear pattern at teardown pointed to a deeper root cause; and the unreported tool crash six months earlier turned out to be the origin. Three levels of diagnosis, each building on the last, restored factory-level accuracy in nine days - and recovered a management rule along the way: tool crashes must be reported.