Mark Twain is to have once said, "You have to know the true definition of a word before you can start misusing it."
We have no idea where this started, but the word "Harmonic" has been used in place of "Chatter", or more specifically, regenerative waviness. Perhaps because chatter is audible and someone wanted a more "scientific" sounding word. It is used in a phrase like this:
"Our __________ breaks up harmonics!"
Harmonic Frequencies are defined as integers of a fundamental frequency. In the case of milling, its fundamental frequency is the rate at which it vibrates that matches the tooth impact frequency. If the endmill's teeth are impacting 500 times a second (500 Hz), and it is vibrating back and forth 500 times per second (also 500 Hz), then 500 Hz is its fundamental frequency. If the endmill vibrates at 1000 Hz, it is still stable because it is going back and forth twice between tooth impacts. The same for 1500 Hz (3X between impacts), 2000 (4X), 2500 (5X) and so on. Those are "Harmonic Frequencies".
The frequencies that cause chatter are "non-harmonic". The timing of the tooth impacts would be off and create unequal waviness on the part surface. Each succeeding tooth impact will make that regenerative waviness worse and eventually chatter will develop.
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