Cutting Speed Calculator — RPM from Cutting Speed & Diameter (CNC Turning & Milling)
Calculate spindle RPM from cutting speed and diameter, feed rate from chip load, and machining time. For turning, milling, and drilling.
Calculator
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Spindle speed: 955 RPM
Formula
RPM = (1000 × Vc) / (π × D), where Vc is cutting speed in m/min and D is cutter/workpiece diameter in mm. Feed rate (mm/min) = RPM × chip load per tooth × number of teeth. Machining time = cutting length / feed rate.
Example calculation
Milling 50 mm cutter in aluminium, Vc = 200 m/min: RPM = (1000 × 200) / (π × 50) ≈ 1,273 RPM. 4-flute cutter, chip load 0.05 mm/tooth: feed rate = 1,273 × 0.05 × 4 ≈ 255 mm/min. Cutting 300 mm path: time = 300/255 ≈ 1.18 min.
Engineering notes
Cutting speed depends on tool material and workpiece. HSS in mild steel: 20–30 m/min. Uncoated carbide in mild steel: 100–200 m/min. Coated carbide in mild steel: 150–300 m/min. Carbide in aluminium: 200–600 m/min. Carbide in stainless 316L: 60–120 m/min. Carbide in titanium Ti-6Al-4V: 30–60 m/min. Always start at the lower end; increase after verifying tool life and surface finish. Use coolant for steel; dry or MQL for aluminium. Reference: Sandvik Coromant, ISCAR, and Machining Data Handbook (3rd Ed).
When to use this calculator
- CNC turning programs — calculate RPM and feed per rev before programming a lathe G-code cycle
- CNC milling — set spindle speed and table feed rate from cutter diameter and material specification
- Manual lathe or mill — convert recommended cutting speed to the nearest available gearbox RPM
- Tool life optimisation — find the optimal cutting speed that balances tool wear and cycle time
- Cycle time estimation — compute machining time per operation for production planning and quoting
Frequently asked questions
- How do I calculate RPM from cutting speed (m/min)?
- Use the formula: RPM = (1000 × Vc) / (π × D), where Vc is the recommended cutting speed in m/min and D is the cutter or workpiece diameter in mm. Example: 10 mm end mill in mild steel, Vc = 150 m/min (coated carbide): RPM = (1000 × 150) / (3.14159 × 10) = 150,000 / 31.4 ≈ 4,775 RPM. For imperial: RPM = (12 × SFM) / (π × D_inches) or RPM = (3.82 × SFM) / D_inches.
- What cutting speed should I use for different materials?
- Recommended surface cutting speeds (coated carbide insert): mild steel (IS 2062, CK45): 150–250 m/min; stainless steel 304/316: 80–130 m/min; cast iron (grey): 100–180 m/min; aluminium (6061, LM6): 300–600 m/min; brass: 150–300 m/min; copper: 100–200 m/min; titanium Ti-6Al-4V: 40–70 m/min; Inconel 718: 20–40 m/min. For HSS tooling, use approximately 30–40% of the carbide speeds above. Always check the specific tool manufacturer chart for the grade you are using.
- What is chip load and how do I calculate feed rate from it?
- Chip load (also called feed per tooth or fz) is the thickness of material removed by each cutting edge per revolution, measured in mm/tooth. Feed rate (mm/min) = RPM × fz × number of flutes/teeth. Typical chip loads for end mills in mild steel: 0.02–0.05 mm/tooth (2–12 mm diameter), 0.05–0.12 mm/tooth (12–25 mm diameter). Too low: rubbing, work hardening, short tool life. Too high: tool breakage, vibration, poor finish.
- What is the difference between cutting speed and feed rate?
- Cutting speed (Vc, m/min or SFM) is the surface speed of the tool edge relative to the workpiece. It is a material property — determined by the tool-workpiece combination and primarily controls heat generation and tool wear rate. Feed rate (mm/min or mm/rev) is how fast the tool advances into or along the material. It controls chip thickness, surface finish, and material removal rate. Both must be optimised together — correct speed with wrong feed causes poor tool life or surface quality.
- How do I convert cutting speed from SFM to m/min?
- 1 SFM (surface foot per minute) = 0.00508 m/min. To convert: m/min = SFM × 0.3048 / 1000 × 1000 = SFM × 0.3048. To go from m/min to SFM: SFM = m/min / 0.3048. Common conversions: 100 SFM = 30.5 m/min, 200 SFM = 61 m/min, 500 SFM = 152 m/min, 1000 SFM = 305 m/min. Most modern machining data is published in m/min (metric) — use the SFM-to-m/min conversion when working from US or UK tooling catalogs.
- Why does RPM decrease when I use a larger diameter cutter?
- From RPM = (1000 × Vc) / (π × D): for a fixed cutting speed Vc, RPM is inversely proportional to diameter D. A 100 mm face mill rotates at half the RPM of a 50 mm end mill to achieve the same cutting speed. This is correct — the outer tip of the larger tool already travels farther per revolution, so it needs fewer revolutions per minute to achieve the same surface speed. Running a large cutter at the RPM calculated for a small cutter will dramatically overheat the tool and workpiece.
- How do I calculate machining time for a turning or milling operation?
- Turning: Time (min) = Length of cut (mm) / Feed per revolution (mm/rev) / RPM. Or: Time = Length / Feed rate (mm/min). Milling: Time = (Length of cut + Approach + Overtravel) / Feed rate (mm/min). Approach allowance for end mills = Cutter radius for slotting, or = √(D×d − d²) for side milling (d = depth of cut). Include a setup factor (1.2–2.0× cutting time) for realistic job time including tool changes, measurement, and loading/unloading.
- What RPM should I use for drilling in steel with an HSS drill?
- For HSS drills in mild steel: Vc ≈ 20–25 m/min. RPM = (1000 × 20) / (π × D_drill). Examples: 5 mm drill: RPM = 20,000/(3.14×5) ≈ 1,273 RPM; 10 mm drill: ≈ 637 RPM; 20 mm drill: ≈ 318 RPM; 25 mm drill: ≈ 255 RPM. For carbide drills in steel: Vc = 80–120 m/min → approximately 4× the HSS RPM. Feed rate for drilling: 0.1–0.2 mm/rev for small drills (below 6 mm), 0.2–0.4 mm/rev for 6–25 mm drills in mild steel.