Cutting Speed (Vc)
Cutting speed — also called surface speed — is the speed at which the tool edge moves across the workpiece surface. It is measured in meters per minute (m/min) or surface feet per minute (SFM). It is the single most important parameter for tool life.
Why Cutting Speed Matters More Than RPM
RPM is a machine setting. Cutting speed is a physical reality. The tool edge sees the same surface speed whether it is a 3 mm drill or a 50 mm face mill. If you run both at the same RPM, the large tool's edge is moving much faster — and will wear out much quicker.
Typical Cutting Speeds
The right Vc depends on two things: workpiece material and tool material. Here are starting points for carbide inserts:
| Workpiece | Roughing Vc | Finishing Vc | Coolant |
|---|---|---|---|
| Aluminum (6061) | 200–400 m/min | 400–800 m/min | Air or flood |
| Mild steel (1018) | 120–180 m/min | 200–300 m/min | Flood |
| Alloy steel (4140) | 90–150 m/min | 150–250 m/min | Flood |
| Stainless (304) | 80–120 m/min | 120–180 m/min | High-pressure |
| Cast iron (GGG40) | 80–150 m/min | 150–250 m/min | Dry or air |
| Titanium (Ti6Al4V) | 40–60 m/min | 60–100 m/min | High-pressure |
| Inconel 718 | 20–40 m/min | 40–60 m/min | High-pressure |
Full reference: Material Property Reference
Tool Material Influence
The same workpiece can be cut at very different speeds depending on your tool:
- HSS (high-speed steel): slow, cheap, flexible — 20–50% of carbide speed.
- Carbide: the workhorse — 2–3× faster than HSS.
- Cermet / ceramic: very fast, high heat — for hardened steel and high volume.
- Diamond (PCD): only for non-ferrous (aluminum, carbon fiber) — never ferrous.
Cutting Speed and Tool Life
This is the famous Taylor tool-life relationship: doubling speed roughly halves tool life. A small increase in Vc causes a big drop in tool life.
Example: if Vc=150 m/min gives 30 minutes of tool life, running at Vc=180 m/min (20% faster) might reduce tool life to 12–15 minutes. This is why programmers carefully optimize speed, not just feed.
Calculate reverse: Cutting Speed Calculator · Plan tool life: Tool Life Planning