In production CNC routing, the choice between climb milling (down milling) and conventional milling (up milling) is the primary determinant of whether your endmill survives a high-feed roughing pass.
### Kinematic Chip Mechanics
In **conventional milling**, the tooth enters the workpiece at zero chip thickness. The cutting edge initially slides and rubs against the compressed wood fibers, burnishing the material until sufficient pressure builds up to begin shearing. The tooth exits at maximum chip thickness. This produces:
1. Severe upward lifting forces that pull warped workpieces off vacuum tables.
2. High rubbing friction that generates extreme heat at the cutting edge.
3. Rapid work-hardening of fibers, accelerating carbide flank dulling.
In **climb milling**, the tooth enters at maximum chip thickness and exits at zero thickness. The cutter shears cleanly upon immediate impact and presses the workpiece down firmly against the machine bed.
### Why Bits Snap on Conventional Cuts
When an endmill experiences chip rubbing during conventional milling, the instantaneous cutting force vector points away from the solid stock into the freshly machined surface. As the tooth approaches maximum thickness, the tool flexes backward.
When it exits, that stored elastic potential energy releases like a plucked bowstring. This cyclical mechanical deflection induces high-frequency chatter ($2.0 - 3.5\text{ kHz}$), triggering fatigue micro-cracks in brittle tungsten carbide binders and snapping the tool at the collet line.
**The Golden Rule:** Always program climb milling for roughing and finishing unless your machine has excessive backlash (
gt; 0.1\text{ mm}$) on worn lead screws.