The science of milling sounds

Milling sounds range from loud to quiet, and experienced machinists often use those sounds to judge what is occurring in the cutting zone. It is difficult to position a sensor such as an accelerometer or a displacement probe close to the cutting zone, and it is often a challenge to see the cutting zone, but the sounds are pervasive.

Some of the sounds are normal—and expected—and some indicate trouble, but how can you tell which are which? The sounds are often sorted by their frequency, or pitch. Sound can be measured using a microphone and sorted into frequencies using a Fourier analyzer. This device takes a time-varying sound signal as an input and outputs a graph that shows the composition of the sound as a number of frequencies with different intensities.

Courtesy of All images: Manufacturing Laboratories Inc.

Figure 1. The red hatched area in this stability lobe diagram indicates chatter, and the white area indicates stable machining.

Figure 2. The red bull’s eye in the Harmonizer chatter control product, which detects chatter using the sound signal measured with a microphone, indicates the presence of chatter at a frequency slightly higher than 5,600 Hz when cutting with a 2-flute, 8mm-dia. tool. The top part of the figure shows sound magnitude (loudness) vs. frequency. The horizontal red line is a threshold, so that background noise does not trigger the system. The bottom part of the figure shows the sound signal vs. time, and the box surrounds the sound of the cut. The software is partway through an optimization process, and the most recent cut was at 40,167 rpm. The check in the harmonic filters checkbox indicates the “normal sounds” have been removed from this picture.

Normal situations include:

1. The sound of tool teeth hitting the workpiece. These impacts cause forces that make the cutting tool and workpiece vibrate, which produces the sound. The tooth-impact frequency is given by ft = n × m ÷ 60, where n is the spindle speed in rpm, m is the number of teeth on the tool and ft is the tooth-impact frequency in cycles per second, or Hz. For example, if the tool has two teeth and the spindle speed is 13,200 rpm, the tooth impact frequency is 440 Hz, which, for the musically inclined, is the note A above middle C.

2. Integer multiples of the tooth-impact frequency. Because the cutting force is not purely sinusoidal, there are frequency components that are exact integer multiples of the tooth-impact frequency (2 × 440 = 880 Hz, 3 × 440 = 1,320 Hz and so on for the previous example). These integer multiples of the tooth-impact frequency are called “harmonics,” and, because they are directly related to the tooth-impact frequency, the sound produced is very pure and “clean,” like a single note.