Speeds and Feeds Calculator (CNC Machining)
Calculate CNC machining speeds and feeds: RPM from surface footage and diameter, feed rate in IPM from chip load and flutes.
Covers milling, drilling, and turning for common materials.
What Are Speeds and Feeds? In CNC machining, “speeds and feeds” refers to two fundamental cutting parameters: Speed: the rotational speed of the cutting tool, measured in RPM (revolutions per minute). Feed: the rate at which the tool advances into the workpiece, measured in IPM (inches per minute) or mm/min. Getting these parameters right determines cut quality, tool life, and machine productivity. Too slow: wasted time and potential work hardening. Too fast: tool breakage, poor surface finish, burned material.
The Speed Formula (RPM) RPM = (SFM × 3.82) / Cutter Diameter (inches) Or in metric: RPM = (1,000 × Vc) / (π × D) Where SFM = Surface Feet per Minute (recommended cutting speed for the material/tool combination). Vc = cutting speed in m/min. D = cutter diameter in inches or mm. The constant 3.82 = 12 / π (converts from feet to circumference relationship in inches).
The Feed Rate Formula IPM = RPM × Chip Load (inches per tooth) × Number of Flutes Chip load (also called chip thickness or feed per tooth) is the amount of material removed per cutting edge per revolution. Too low chip load: rubbing instead of cutting → heat buildup, poor tool life. Too high chip load: excessive cutting force → tool breakage.
Recommended SFM by Material (Carbide Tooling) Aluminum: 600–1,200 SFM — the fastest to machine. Brass: 200–400 SFM. Mild steel (1018): 150–300 SFM. Stainless steel (304): 100–200 SFM — challenging due to work hardening. Tool steel (D2/H13): 50–150 SFM. Titanium (Grade 5 / Ti-6Al-4V): 50–100 SFM — very slow due to poor thermal conductivity. Hardened steel (60 HRC): 50–100 SFM — use ceramic or CBN tooling.
Recommended Chip Load by Cutter Diameter (4-flute carbide endmill) 1/8" (3.18 mm): 0.0008–0.001" per tooth. 1/4" (6.35 mm): 0.001–0.002" per tooth. 1/2" (12.7 mm): 0.002–0.004" per tooth. 1" (25.4 mm): 0.004–0.008" per tooth.
Depth of Cut and Width of Cut Axial depth of cut (DOC): how deep the tool plunges. Typically 0.5–1.5 × cutter diameter for slotting. Radial width of cut (WOC): how wide the step-over is. For side milling: 10–50% of cutter diameter for best tool life. Reducing WOC allows higher speeds and feeds — a technique called High Speed Machining (HSM).
Tool Life and Taylor’s Equation Tool life follows Taylor’s Tool Life equation: V × T^n = C Where V = cutting speed, T = tool life (minutes), n and C are constants for the tool/material pair. Doubling cutting speed typically reduces tool life by a factor of 6–10 for carbide in steel. Flood coolant can increase tool life by 50–200% in steel but is less critical for aluminum.
HSS vs Carbide High Speed Steel (HSS): slower speeds, less expensive, more flexible. Good for plastics and non-ferrous metals. Carbide: harder, more brittle, 3–5× faster cutting speeds. Dominant in production machining. Ceramic: extremely fast for cast iron and hardened steel. Very brittle — only used in rigid setups.