Chatter costs money when you reduce the productivity of the machine by slowing it down  to make the vibration go away.

Conventional wisdom states that there are two kinds of chatter- Forced and Self Induced. Some shop guys like to think its caused by the material.
Forced chatter is a result of alternating cutting forces  that result from
1) Interrupted cuts (milling);
2) Machine vibrations such as out of balance motors,  spindles, gear or shaft irregularities, bad couplings or bearings, (Loose motor mounts and weakened or stretched) couplings;
3) Load on tool / workpiece changing as a result of acceleration or decceleration;
4) Vibrations being transmitted through the machine and foundation from other equipment.
If forced vibration is what you have, confirming the integrity of  the machine tool and its power train is a critical first step. Reducing the feed per revolution is one way to determine if it is the variation in the cutting process that is forcing the vibration. Changing the SFM or RPM’s by at least 25% is also something to try (increase or decrease!)
Self excited chatter is induced by a change in the cutting forces themselves, and is where I place the chatter that may be caused by the material.  Self excited vibrations can be distinguished from forced vibrations in the machining system  because self excited vibrations stop when the cutting does. Forced vibrations are not dependent on the cutting process, and so continue even when the tool is not in the cut. Self excited chatter can be caused by:
1) Change in forces needed to cut caused by differences in the material-  Material characteristics (such as workhardening or microstructural differences) that result in variation in chip thickness.
2)  Unstable built up edge (BUE) forming then breaking off causing variation in the cutting forces
3)  insufficient Stiffness of the workpiece,  spindle,  tool   and tool holding (think deflection and too much length).
To eliminate self excited chatter decrease the length of the tool in the cut,  shorten the tool holder, or substitute more rigid tooling and support materials (a carbide boring bar  deflects less and can make three or more times heavier a cut  than one made of steel for example) .
Still think it’s the material? Here’s my Metallurgist’s tip:  Look and see if you have  changing build up edge conditions on the tool that exhibits chatter. If  the self excited chatter is due to material such as an unstable built up edge (BUE)  forming,  try increasing the RPMs / SFM. Spindle speeds that are too slow allow workpiece material to weld to the tool edge  (pressure weld) and build up.This creates higher forces until it sloughs off.  Then forces go back to normal, and build up again until…
Higher RPMs help to keep BUE stable and  under control. And  they allow you to run faster cycle times, contributing to profits.
Bottom line: Chatter doesn’t always mean you need to slow down.
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