Cutting speed and feed rate are the two foundational parameters of CNC machining. Machinists who confuse them end up with broken tools, poor surface finishes, and scrapped parts. This guide explains what each parameter means, how they relate to each other, and how to choose them correctly for any material and tool combination.
Cutting speed — measured in Surface Feet per Minute (SFM) or meters per minute (m/min) — describes how fast the cutting edge moves through the material. It is a property of the material-tool interface, not the machine. The same cutting speed applies whether you are on a manual Bridgeport or a 5-axis machining center. Formula: SFM = (RPM × diameter_inches × π) / 12. Each material has an optimal SFM range based on its hardness, thermal conductivity, and abrasiveness. Running too fast burns the tool. Running too slow work-hardens stainless and causes built-up edge on aluminum.
Feed rate — measured in Inches Per Minute (IPM) or mm/min — describes how fast the tool moves through the workpiece. Unlike cutting speed, feed rate IS machine-specific because it depends on spindle RPM. Formula: IPM = RPM × chip load × number of flutes. Chip load (also called feed per tooth) is the thickness of the chip each flute removes per revolution. Chip load is determined by the tool manufacturer based on tool diameter and material. Too low a feed rate causes rubbing, heat buildup, and rapid tool wear. Too high causes chatter, tool deflection, and breakage.
The correct workflow is: 1) Select cutting speed for your material (SFM from reference or tool manufacturer data). 2) Calculate RPM: RPM = (SFM × 12) / (π × diameter). 3) Select chip load for your tool diameter and material. 4) Calculate feed rate: IPM = RPM × chip load × flutes. Example: Aluminum, 0.5" carbide 4-flute end mill. Target SFM = 800. RPM = (800 × 12) / (π × 0.5) = 6,112 RPM. Chip load = 0.003" per tooth. Feed rate = 6,112 × 0.003 × 4 = 73.3 IPM.
Mistake 1: Using the same RPM for different diameters. RPM must change with diameter to maintain the same cutting speed. A 1" tool at 1,000 RPM runs at 262 SFM. A 0.25" tool at 1,000 RPM runs at only 65 SFM — far too slow for most materials. Mistake 2: Ignoring chip load ranges. A 0.5" end mill may spec 0.002–0.004" chip load. Starting at 0.001" causes rubbing and heat. Mistake 3: Treating feed rate as independent. Feed rate must be recalculated any time RPM changes, flute count changes, or you switch to a different diameter tool.
Aluminum (6061): SFM 600–1,000 (carbide), chip load 0.002–0.005" per flute. Mild steel (1018): SFM 150–250 (carbide), chip load 0.001–0.003". Stainless steel (304): SFM 100–180 (carbide), chip load 0.0008–0.002". Titanium (Grade 5): SFM 40–80 (carbide), chip load 0.001–0.002". Brass: SFM 300–500 (carbide), chip load 0.002–0.004". These are starting points — always consult tool manufacturer data and adjust based on actual cutting conditions, rigidity, and coolant.
Cutting speed has a larger impact on tool life. Tool wear increases exponentially with cutting speed (Taylor's tool life equation: VT^n = C). Doubling the cutting speed can reduce tool life by 50–90% depending on the material and tool. Feed rate affects tool life too, but less dramatically. Most machinists find that running slightly conservative SFM with aggressive feed rate (high chip load) is more productive and economical than the reverse.
Running too slow a feed rate (too low chip load) causes rubbing instead of cutting. The cutting edge slides along the workpiece rather than shearing chips. This generates heat, work-hardens the material (especially stainless and titanium), causes built-up edge on the cutting edge, and dramatically reduces tool life. The minimum chip load for most end mills is around 0.001" per tooth — below that, rubbing becomes the dominant mode.
No. Carbide can run 3–5× faster than HSS for most materials. Using HSS SFM values with carbide tooling means running far below optimal speed, wasting productivity. Always use tool-material-specific SFM charts. Carbide in aluminum: 600–1,000 SFM. HSS in aluminum: 200–400 SFM.