Depth & Width of Cut
Depth of cut and width of cut define how much material the tool removes in a single pass. They are different from feed — feed is how fast the tool moves, depth/width is how deep and wide it cuts.
Turning: Depth of Cut (ap)
In turning, ap is the radial depth the tool cuts into the workpiece per pass. If your bar is 50 mm and you want 48 mm, one pass at ap=1 mm removes 1 mm from the diameter (0.5 mm radius each side).
Milling: Axial (ap) and Radial (ae)
In milling there are two:
- Axial depth ap (step down): how deep the cutter goes into the material (Z-direction).
- Radial width ae (step over): how wide the cutter engages in the cut (X/Y direction).
Typical Values
| Operation | ap (depth) | ae (width) | Notes |
|---|---|---|---|
| Turning roughing | 1–5 mm | — (full diameter) | Maximize ap to reduce passes |
| Turning finishing | 0.2–0.5 mm | — | Light cut for good surface |
| Face milling | 0.5–3 mm | up to 0.8× cutter dia | Wide, shallow cuts |
| End mill slotting | 1–2× tool dia | = tool dia (full slot) | High radial load |
| Trochodial milling | up to 3× tool dia | 10–20% of dia | Thin chip, high ap |
Rule of thumb: depth of cut has almost no effect on tool life compared to speed. So for roughing, take the deepest ap the machine can handle — this removes material fastest with the least tool penalty.
Tool Engagement
Radial engagement (ae/D ratio) matters for tool life. In slotting (ae = D, 100% engagement), the tool spends half its time cutting and half rubbing — heat builds up. In trochoidal milling (ae ≈ 10% of D), the tool cuts a thin arc and cools in between — tool life doubles or triples.