RPM to Feet Per Minute (SFM) Calculator
Calculated Surface Speed:
0 ft/min (SFM)
Formula used: (RPM × π × Diameter) / 12
Understanding RPM to FT/MIN (SFM)
In machining, metalworking, and manufacturing, converting RPM (Revolutions Per Minute) to FT/MIN (Feet Per Minute), also known as Surface Feet Per Minute (SFM), is a critical calculation. While RPM tells you how fast the spindle is spinning, SFM tells you how fast the cutting edge of the tool is actually moving across the material.
The Physics Behind the Calculation
A tool with a larger diameter covers more distance in a single revolution than a smaller tool. To maintain the same cutting speed (SFM) for different tool sizes, you must adjust the RPM accordingly. If you run a large drill at the same RPM as a tiny drill, the outer edge of the large drill will travel much faster, potentially overheating or breaking the tool.
The Formula
SFM = (RPM × π × Diameter) / 12
- RPM: The rotational speed of the spindle.
- π (Pi): Approximately 3.14159.
- Diameter: The diameter of the tool or workpiece in inches.
- 12: This constant converts the circumference (measured in inches) into feet.
Practical Example
Suppose you are using a 0.5-inch end mill spinning at 2,500 RPM. To find the SFM:
- Multiply RPM (2,500) by π (3.14159) = 7,853.97
- Multiply that by Diameter (0.5) = 3,926.98
- Divide by 12 = 327.25 FT/MIN
This value allows you to check your material manufacturer's recommendations to ensure you aren't running too fast (causing heat) or too slow (reducing efficiency).