TECHNICAL TIP #32; END MILLS; REDUCING VIBRATION & CHATTER:
When chatter arises it tends to be self-sustaining until the problem is corrected. This condition causes poor finished on the part and will damage and significantly reduce the life of end mills. Carbide end mills are peculiarly susceptible to damage.
When experiencing chatter problems, the basic reflex action is the reduction of cutting forces. This can be done by:
(1) Reducing the number of flutes.
(2) Decreasing the chip load per tooth by reducing the feed or increasing the speed or RPM.
(3) Reducing the axial or radial depth of cut.
Even though these steps can and will reduce chatter, slowing down the cutting process is not always the best course of action and reducing the chip load can be detrimental to the cutter.
Better first steps are to improve rigidity and stability:
(1) Use a larger end mill with a larger core diameter.
(2) Use end mills with reduced clearance or a small circular margin.
(3) Use the shortest overhang from spindle nose to tip of tool.
(4) Use stub length end mills where possible.
(5) Use balanced tool holders.
(6) Rework fixture to hold the work piece more securely.
(7) Reprogram the cutter path shifting cutting forces into stiffer portions of the work piece.
(8) Look for sweet spots in spindle speeds then adjust feed accordingly.

A common source of chatter is the machining of corners. As the end mills enters the corner the percentage of engagement increases the number of teeth in the cut. This drastically increases the cutting forces, causing chatter. Using circular interpolation and producing a bigger corner radius then the part print calls for then going back and removing the remaining stock with a smaller end mill using circular interpolation will reduce the tendency to chatter.
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