Feed
Feed is how fast the tool advances into the workpiece. It is the second primary cutting condition (after speed), and it controls chip thickness, surface finish, and cutting force.
The Three Feed Numbers
There are three ways to express feed, and they are related by RPM and tool teeth:
| Name | Symbol | Units | Used in |
|---|---|---|---|
| Feed per tooth | fz | mm/tooth | Milling (programming input) |
| Feed per revolution | f | mm/rev | Turning, drilling, tapping |
| Feed rate | Vf | mm/min | The number you type in CNC |
The Formula
Worked Example
You have a 4-flute end mill (z=4), running at N=3,000 RPM, with a feed per tooth fz=0.05 mm. What feed rate do you program?
Try it: Feed Rate Calculator · Chip Load Calculator
How Feed Affects the Cut
| Increasing feed... | Effect |
|---|---|
| Chip thickness | ↑ thicker chips (better chip breaker engagement) |
| Surface finish | ↓ rougher surface (bigger feed marks) |
| Cutting force | ↑ higher (more material per tooth) |
| Tool wear | ↑ slightly more (mostly flank wear) |
| Productivity | ↑↑ much faster (biggest lever for MRR) |
Beginner rule: for roughing, maximize feed (it barely affects tool life). For finishing, reduce feed (it directly improves surface finish). Speed is adjusted last — it has the biggest effect on tool life.
Feed and Surface Finish
In turning, the theoretical surface roughness is approximately:
Example: feed=0.2 mm/rev, nose radius=0.4 mm → Ra ≈ 0.031 µm theoretical. In practice, expect 1.5–2× higher due to tool wear and vibration.