Why Stickout Matters More Than Diameter Alone
Tool deflection under cutting load follows a cantilever beam model, where deflection scales with the cube of the unsupported length but only the fourth power of diameter in the denominator. In practice this means going from 2x to 3x diameter stickout roughly triples deflection for the same load, while a 25% increase in tool diameter cuts deflection by more than half.
What This Estimate Leaves Out
Real end mills aren't solid round bars, fluted geometry and helix angle reduce effective stiffness compared to this simplified model, and cutting force itself varies through the cut as chip load changes. Treat the result as a relative indicator for comparing setups rather than a precise prediction of finished part accuracy.
Reading the Stickout Ratio
A stickout-to-diameter ratio above about 4:1 is where deflection and chatter risk start climbing quickly for most general milling work; staying below that, or stepping up to a larger diameter or shorter holder, is usually the more reliable fix than just slowing down the feed.