Glossary & Reference
Axial depth of cut — ap — is how far the cutter reaches down into the workpiece measured along the tool's spindle axis. It's the "how deep" half of every milling pass, and it drives spindle torque, chip thickness, and tool deflection just as much as radial engagement does.
In milling, depth of cut is described in two directions at once: axial (ap) and radial (ae). Axial depth of cut is the engagement measured parallel to the tool's centerline — essentially, how much of the cutter's flute length is buried in the material on a given pass. Common alternate names are stepdown, cut depth, and ADOC. It's a direct input you set at the CAM or program level, not a value calculated from other parameters.
Axial depth of cut, radial depth of cut, and feed rate multiply together to determine material removal rate: MRR = ap × ae × Vf. Because ap enters that equation directly, doubling the axial depth roughly doubles the volume of metal removed per minute for the same feed and radial width — and with it, the torque and power the spindle has to deliver. In roughing passes, spindle torque and horsepower are usually the limiting factor; in finishing passes, tool deflection and vibration take over as the ceiling.
Maximum ap is capped mainly by insert or flute edge length — you can't cut deeper than the cutting edge itself reaches — and by available machine power and rigidity. A long axial engagement on a long, slender tool also increases deflection and chatter risk, since the tool behaves like a cantilevered beam whose deflection grows fast as unsupported length increases. That's why heavy roughing strategies often favor a large ap paired with a lighter ae (or the reverse) rather than pushing both to their maximum at the same time.