Choosing SFM. Surface speed depends on the material and tool coating as much as the operation — look up a starting SFM for your specific material and tool combination rather than relying on a single rule of thumb, then adjust from there based on tool wear and finish.

Tap drill size and thread engagement. The formula above gives roughly 75% thread engagement, a common target — going smaller increases engagement and the torque needed to tap, while going larger reduces both; most threaded holes do not need more than 75% engagement for full strength.

Stickout dominates deflection. Tool deflection scales with the cube of stickout length, so a tool hanging out twice as far deflects roughly eight times as much under the same load — shortening stickout is usually the single biggest lever for chatter and accuracy problems.