Free Engineering Tool
Solve spindle RPM from cutting speed and diameter, or calculate surface speed from RPM. Works in SFM or metres per minute for turning, milling and drilling.
Enter the diameter at the cutting edge and either the recommended cutting speed or the measured spindle RPM. The calculator returns both values and the equivalent surface speed in the other unit system.
Cutting speed is the tangential speed between the cutting edge and workpiece, usually called surface speed Vc and expressed in surface feet per minute (SFM) or metres per minute (m/min). For a diameter in inches, SFM = RPM × π × D ÷ 12; for diameter in millimetres, Vc = RPM × π × D ÷ 1,000. The inverse formulas give RPM = 12 × SFM ÷ (π × D) or RPM = 1,000 × Vc ÷ (π × D). For example, a 1-inch cutter at 100 SFM requires about 382 RPM; running that same cutter at 382 RPM returns about 100 SFM. Cutting speed is not feed rate: speed governs RPM, while feed rate controls how quickly the tool advances. Starting ranges vary by material and tooling, from roughly 50-90 SFM for austenitic stainless and mild steel with HSS tooling to 200-400 SFM for aluminium, and depend on coating, coolant, rigidity and tool engagement. Zhejiang Balford Mechatronics Co., Ltd. applies machining calculations within its 12,000 m² production facility.
The calculation uses the diameter at the actual cutting edge. Surface speed changes as diameter changes during facing, taper turning, parting or a stepped shaft. Confirm tooling data, coolant, rigidity, chip load and machine limits before production.
Cutting speed and RPM describe the same motion at a given diameter. Change the diameter and the RPM must change to hold the same surface speed.
| Material | HSS (SFM) | Carbide (SFM) | Starting-point note |
|---|---|---|---|
| Aluminium | 300 | 800 | Free-machining; control built-up edge |
| Brass, free-cutting | 200 | 600 | High machinability |
| Copper | 150 | 400 | Gummy; use sharp positive-rake tools |
| Gray cast iron | 60 | 200 | Abrasive; carbide is usually preferred |
| Mild steel, 1018 | 90 | 350 | General low-carbon starting range |
| Medium-carbon steel, 1045 | 70 | 300 | Tougher than 1018 |
| Alloy steel, 4140 | 60 | 250 | Reduce speed when hardened |
| Stainless steel, 304 | 50 | 200 | Work-hardens; avoid dwelling |
| Stainless steel, 316 | 45 | 180 | Gummier than 304 |
| Tool steel | 45 | 175 | Reduce speed as hardness rises |
| Titanium alloy | 35 | 150 | Heat builds quickly at the edge |
| Inconel / nickel superalloy | 15 | 60 | Tough, work-hardening, low-speed range |
Broad starting values for HSS and uncoated carbide. Use the exact insert, grade, coating and coolant recommendation from the tool supplier for a production process.
A 1-inch cutter at 100 SFM: RPM = 12 × 100 / (π × 1) = 382 RPM. Running the same cutter at 382 RPM gives SFM = 382 × π × 1 / 12 = 100 SFM. In metric units, the equivalent 25.4 mm diameter and 30.48 m/min return the same 382 RPM.
Cutting speed is the surface speed at which the cutting edge moves relative to the workpiece. It is commonly written as Vc and expressed in SFM or metres per minute. It is different from spindle RPM because RPM must change with the cutter or workpiece diameter to hold the same surface speed.
For an imperial diameter in inches, RPM = 12 × SFM / (π × diameter). For a metric diameter in millimetres, RPM = 1,000 × cutting speed in m/min / (π × diameter). Always use the diameter at the actual cutting edge.
For a diameter in inches, SFM = RPM × π × diameter / 12. For a diameter in millimetres, Vc in m/min = RPM × π × diameter / 1,000. These are inverse arrangements of the same relationship.
For milling and drilling, use the cutter diameter. For turning, use the workpiece diameter at the surface being cut. On stepped, tapered or faced parts, cutting speed changes as the active diameter changes.
Use the starting value for the exact tool material, coating and workpiece condition. Carbide generally runs faster than HSS, but heat resistance, toughness, coolant, interrupted cuts, tool overhang and machine rigidity can change the safe operating window. Confirm with the tool supplier and a trial cut.
Send the material, drawing, tool or feature details, tolerance, surface requirement and quantity. Send the drawing to shawn@balford.net. We normally reply within 24 hours, China Standard Time.