Speeds and feeds
Spindle speed (RPM) calculator
Enter cutting speed in SFM (surface feet per minute) or m/min and the tool or work diameter to get spindle RPM.
Technical review pending. Formulas and values are cited. This notice comes down after a machinist review.
Spindle speed
RPM
RPM = (SFM x 12) / (pi x D) = (100 x 12) / (3.1416 x 0.5) = 764 RPM
Starting point. 100 SFM is 30.48 m/min; 0.5000 in is 12.70 mm.
Source: Machinery's Handbook (cutting speed and spindle speed formulas)
RPM to SFM
Cutting speed
SFM
SFM = (pi x D x RPM) / 12 = (3.1416 x 0.5 x 764) / 12 = 100 SFM
30.48 m/min
How it works
SFM (surface feet per minute) is how fast the cutting edge moves across the work. The spindle speed that gives that surface speed depends on the diameter: a small tool has to spin faster to reach the same SFM.
Inch
RPM = (SFM x 12) / (pi x D)
| Symbol | Meaning | Unit |
|---|---|---|
| RPM | Spindle speed | rev/min |
| SFM | Cutting speed, surface feet per minute | ft/min |
| D | Tool diameter (milling, drilling) or work diameter (turning) | in |
| 12 | Inches per foot | in/ft |
| pi | 3.1416 | none |
Metric
RPM = (Vc x 1000) / (pi x D)
| Symbol | Meaning | Unit |
|---|---|---|
| RPM | Spindle speed | rev/min |
| Vc | Cutting speed | m/min |
| D | Tool or work diameter | mm |
| 1000 | Millimeters per meter | mm/m |
Shop shorthand for the inch formula is RPM = SFM x 3.82 / D, because 12 / pi = 3.82. Run backward, SFM = (pi x D x RPM) / 12, or Vc = (pi x D x RPM) / 1000 in metric.
Worked example: inch
A 1/2 in (0.500 in) end mill at a cutting speed of 100 SFM.
- Multiply the cutting speed by 12: 100 x 12 = 1,200.
- Multiply pi by the diameter: 3.1416 x 0.500 = 1.5708.
- Divide: 1,200 / 1.5708 = 763.9 RPM.
Result: 764 RPM. Treat it as a starting point and adjust by sound and chip.
Worked example: metric
A 12 mm drill at a cutting speed of 30 m/min.
- Multiply the cutting speed by 1,000: 30 x 1,000 = 30,000.
- Multiply pi by the diameter: 3.1416 x 12 = 37.699.
- Divide: 30,000 / 37.699 = 795.8 RPM.
Result: 796 RPM.
Shop notes
- Tool-maker SFM figures assume a rigid machine, sharp tools and good coolant. On a benchtop mill or a light lathe, start at the low end of the range and adjust by sound and chip.
- On a rigid CNC machine with a stiff setup, the upper part of the tool maker's range is where that data is meant to be used.
- Belt and gear head machines have fixed steps. Pick the step at or below the result.
- If the result is above your top spindle speed, run top speed and keep the feed per tooth. The feed rate in IPM (inches per minute) drops with it.
- Turning: D is the work diameter at the cut. When facing, surface speed falls toward the center; CNC lathes hold it with constant surface speed (G96).
- Rule of thumb from tool-maker data: carbide runs about 2 to 4 times the HSS speed in the same material. Use the range the tool maker gives for the grade you have.
FAQ
What is the formula for spindle speed?
RPM = (SFM x 12) / (pi x D) in inch units, or RPM = (Vc x 1000) / (pi x D) with Vc in m/min and D in mm. A 0.5 in tool at 100 SFM runs 764 RPM. A 12 mm tool at 30 m/min runs 796 RPM.
What SFM is right for aluminum, steel or stainless?
It depends on the tool material, the coating and the machine. Take the SFM from the tool maker's data or the Machinery's Handbook table for your material, then enter it above. On a light machine, start at the low end of the range.
Is SFM different for HSS and carbide?
Yes. Rule of thumb from tool-maker data: carbide runs about 2 to 4 times the HSS speed in the same material. Use the range the tool maker gives for the grade you have.
The result is faster than the mill can run. What now?
Run the spindle at its top speed and keep the feed per tooth (chip load) the same. The feed rate in IPM (inches per minute) drops in proportion to the lower RPM.
How do you convert RPM back to SFM?
SFM = (pi x D x RPM) / 12. A 0.5 in tool at 764 RPM is cutting at 100 SFM.
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