CNC Cutting Speed Calculator
Cutting speed is the velocity of the cutting edge across the workpiece, measured in surface feet per minute (SFM). On a CNC machine, the control programs spindle speed as RPM, so you need a CNC cutting speed calculator to translate between the SFM in a tooling catalog and the RPM in your G-code. This tool handles CNC milling, CNC routing, and CNC turning with one equation and a library of material-specific SFM values.
CNC cutting speed formula
- SFM comes from a material table. Aluminum = 1500 SFM carbide, mild steel = 400, stainless = 220, titanium = 120.
- RPM = (SFM x 12) / (pi x D) where D is the diameter of whatever is spinning (tool on mill/router, workpiece on lathe).
- Metric: Vc (m/min) = SFM x 0.3048.
Example (CNC mill): 1/2 inch carbide end mill in 6061 aluminum at 1500 SFM. RPM = (1500 x 12) / (pi x 0.5) = 11,459 RPM. Vc = 457 m/min.
Example (CNC lathe): 2 inch bar of 1018 mild steel, carbide insert at 400 SFM. RPM = (400 x 12) / (pi x 2) = 764 RPM. Vc = 121.9 m/min.
Example (CNC router): 1/4 inch compression bit in hardwood at 2200 SFM. RPM = (2200 x 12) / (pi x 0.25) = 33,614 RPM. Most CNC routers top out at 24,000 RPM, so cap there and accept the lower actual SFM.
CNC cutting speed by material
| Material | HSS SFM | Carbide SFM |
|---|---|---|
| Aluminum 6061 | 600 | 1500 |
| Mild steel (1018) | 100 | 400 |
| Alloy steel (4140) | 70 | 280 |
| Stainless 304/316 | 60 | 220 |
| Cast iron | 80 | 260 |
| Brass | 250 | 500 |
| Bronze | 150 | 350 |
| Titanium Ti-6Al-4V | 40 | 120 |
| Tool steel (A2, D2) | 50 | 180 |
| Hardwood | 1200 | 2200 |
| MDF | 1500 | 2500 |
| Plywood | 1300 | 2300 |
| Plastic (Delrin, acrylic) | 800 | 2000 |
Why CNC cutting speed is set in SFM, not RPM
SFM stays constant across tool sizes; RPM does not. A 1/4 inch carbide end mill in aluminum wants 1500 SFM, same as a 1 inch end mill. Only RPM changes (22,918 vs 5,730 RPM). By specifying cutting speed in SFM, the Machinery's Handbook and tool vendor tables give you one value per material, and the CNC cutting speed calculator handles the per-tool RPM translation.
CNC cutting speed for turning (lathe)
On a CNC lathe, D is the workpiece diameter, not the tool diameter. G96 (constant surface speed) in the control tells the machine to hold SFM constant as the workpiece OD shrinks during the cut by raising RPM. This CNC cutting speed calculator returns the SFM for your material and the RPM at the current workpiece diameter. Program G96 S400 to hold 400 SFM throughout the job.
CNC cutting speed on a router
CNC routers spin 10,000 to 24,000 RPM, which caps the maximum SFM you can hit on small bits. For a 1/8 inch bit in hardwood at 2200 SFM target, the required RPM is 67,229, well out of reach; the actual SFM at 24,000 RPM is 785 SFM. Compensate by raising chip load (feed per tooth) to keep the feed rate productive. Wood forgives lower SFM more than metal does.