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Breaking Up Harmonics

Last Updated: Oct 08, 2015 08:29AM EDT
I do not like "breaking up harmonics". I have also heard that we are "dialing in the harmonics", which I also do not like. 

A "harmonic" in vibrations and acoustics is a signal, or a wave, or a sound whose frequency is an exact integer multiple of a fundamental frequency. If a vibration occurs at 712 Hz, then the harmonics are 1424 Hz, 2136 Hz, 2848 Hz, ... so what exactly is the harmonic that is being being broken up? I think the confusion that "vibration" = "harmonic" contributes to the low quality of discussion on this topic. It is like confusing mass and weight.

I'd say that the reason that the variable pitch design works it that it disturbs the regeneration of waviness mechanism that is responsible for chatter. If necessary, I'd explain regeneration of waviness: 

Any (even small) vibration between the tool and the workpiece leaves a wavy surface. Subsequent teeth encounter the wavy surface, which causes a variable force. The variable force causes a vibration that leaves another wavy surface. When the vibrations and surface waves get lined up right, the vibration grows and chatter happens. Variable pitch makes sure that the waves don't get lined up right.

Dr. Scott Smith

I agree with Scott.  We need to come up with a 1 phrase description that we can all expand on when necessary.  It is a pain to have to start every conversation by explaining why the current and widely accepted description is wrong.  

So what can we substitute that would be acceptable.

"Disruption of regenerative vibrations???"
"Interruption of the repetitive tool/workpiece interactions"

Nothing good above but we need to get something that is at least better than this harmonic issue. 

Dr. Tom Delio

I put together a "simple" description of frequency content with and without variable pitch teeth. Please see attached and let me know what you think. It certainly refutes the notion of "breaking up harmonics".

Dr. Tony Schmitz


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