The Difference: SFM vs RPM
Beginners often confuse Surface Speed (SFM) and Spindle Speed (RPM).
- SFM (Surface Feet per Minute): How fast the tool tip is moving across the material. This is a property of the material/tool combo (e.g., Aluminum + Carbide = 800 SFM).
- RPM (Revolutions Per Minute): How fast you spin the spindle to achieve that SFM at a specific tool diameter.
The Imperial Formula (Inches)
RPM = (SFM × 3.82) / Tool Diameter
Where does 3.82 come from?
It's actually 12 / π (12 divided by 3.14159). We need to convert feet (SFM) to inches (Diameter), so we multiply by 12, then divide by Pi used for circumference.
The Metric Formula (Millimeters)
RPM = (Vc × 318) / Tool Diameter
Where does 318 come from?
It's 1000 / π (1000 divided by 3.14159). We convert Meters (Vc) to Millimeters (Diameter), so we multiply by 1000.
Step-by-Step Example
Let's say you are milling 6061 Aluminum with a 1/2" End Mill.
- Find the Surface Speed: Check the tool manufacturer's chart. For Aluminum, let's pick 800 SFM.
- Plug into Formula: RPM = (800 × 3.82) / 0.5
- Multiply Top: 800 × 3.82 = 3056
- Divide by Bottom: 3056 / 0.5 = 6,112 RPM
So, you set your machine to S6112.
Why Smaller Tools Need More Speed
Look at the formula again. Diameter is in the denominator. As diameter gets smaller, RPM must get larger to maintain the same surface speed.
- 1.000" Tool @ 100 SFM = 382 RPM
- 0.100" Tool @ 100 SFM = 3,820 RPM
- 0.010" Tool @ 100 SFM = 38,200 RPM
This is why micromachining centers have 30,000+ RPM spindles.