Depth of Cut (ap and ae)

Depth of cut describes how much material a tool engages in one pass. In milling, axial depth (ap) is measured along the tool axis and radial depth (ae) across it; in turning, ap is the radial depth removed from the diameter.

In milling, axial depth of cut ap is the engaged length along the tool axis, and radial depth ae, also called width of cut or step-over, is the engaged width perpendicular to the axis. In turning, depth of cut is half the difference between the diameters before and after the pass. Together with feed and cutting speed, depth of cut defines the chip cross-section and therefore cutting forces and power.

Roughing operations use large depths of cut to remove material quickly, while finishing uses small depths for accuracy and surface quality. Modern roughing strategies often combine a large axial depth with a small radial depth, spreading wear along the cutting edge and reducing heat, instead of shallow full-width passes.

Usable depth is limited by available spindle power and torque, machine and workholding rigidity, tool flute or insert edge length, chip evacuation and chatter stability. Stability lobe diagrams show the axial depth above which chatter is likely at a given spindle speed. Finishing passes also need a minimum depth so the edge cuts cleanly rather than rubbing or deflecting away from the surface.

Key points

ap is axial depth along the tool axis; ae is radial width of cut
In turning, depth of cut is half the change in diameter
Large ap with small ae is common in modern roughing
Limited by power, rigidity, edge length and chatter stability

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Related terms

Terms that refer to Depth of Cut (ap and ae)

Feed RateCNC & Precision Machining