Tool Selection - Tool Length
If the cut chatters and reducing the tool length is practical, reduce the tool length. A 10% reduction in the length-to-diameter ratio results in a 25% increase in tool stiffness.
Unless absolutely necessary, the tool should not be more than 3 times longer than it is wide.
Tool Length and Deep Pockets
Deep pockets require deviation from the normal length-to-diameter ratio for tools. Lower the cutting load with program adjustments: reduce the depth-of-cut, the width-of-cut, feed, or change to a constant-engagement toolpath.
Use the highest-performance tool possible, ideally with variable helix and cutting angles. High-performance tooling uses tool geometry designed to dampen the vibrations that cause chatter.
Fragile workholding, a long, skinny part, overhangs, etc.—Lower the cutting load through adjustments to the tool selection or program. Stiffen the workholding with supports if possible.