You may have heard of a machinist who supposedly could optimize a cut by just listening to it.
Is this possible?
A stable milling cut has a pure sound. The tool point frequency (it’s back and forth vibration) matches the tooth impact frequency or one of its harmonics (more on that in another post) and therefore each tooth impact depth is equal. If there is a mismatch of these two frequencies, the tooth impact depths are unequal and this creates a feedback mechanism that, if large enough, will make the harsh sound we call chatter.
Here’s an example (from Dr. Scott Smith): a 2 flute endmill cutting at 15,000 RPM will have a tooth impact frequency of 30,000 per minute (2 teeth x 15,000) or 500 impacts per second (30,000/60 secs). That is expressed as 500 Hz. Now let’s say you hear chatter and then identified the frequency as 427 Hz. You would simply multiply 427 x 60 seconds and divided that by 2 teeth and you would come up with a stable new speed of 12,810 RPM. Problem solved.
But, can a human ear really hear and count an exact frequency that is in hundreds of cycles per second? There are people with “perfect pitch” but that requires them only to make a go/no go decision of a known frequency, such as a specific pitch for tuning a musical instrument.
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