Three Types of Vibration

There are three types of vibration in the physical world and this is how they apply to my favorite subject, Milling.

FREE vibration occurs naturally in an object and can dissipate over time. On a machine tool, a cold spindle may start off running a little rough, but as temperatures rise and lubrication gets to the bearings and the vibration subsides.

FORCED vibrations are those that are introduced to an object. Runout or unbalance in a tool assembly will create a once per revolution frequency. It is consistent, no matter what speed the tool assembly is rotating, it will always be once per revolution.

SELF-EXCITED vibration is the third and post challenging. It comes from an external force acting on an object. A tooth on a milling cutter strikes the workpiece and the force causes the tool to deflect. It rebounds back, but if the timing of the next tooth is off it either undercuts or overcuts changing the force and amplitude of each deflection. This modulation of the cutting depths of each tooth gets worse leading to chatter and tool failure. 

A great example of self-excited vibration is the infamous Tacoma Narrows bridge, known as "Galloping Gerdie".