What Is Surface Feet per Minute (SFM)?
Surface feet per minute (SFM), also called cutting speed, is the linear speed at which the surface of a rotating tool or workpiece moves past the cutting edge. It is a fundamental parameter in machining — turning, milling, drilling, and grinding — because each tool material and workpiece combination has a recommended SFM range. Running too fast burns up tools; too slow wastes time and can cause rubbing.
How to Use This Calculator
Pick what you want to solve for. Choose SFM (from RPM) and enter the tool diameter in inches plus the spindle speed to find the actual cutting speed. Or choose RPM (from SFM) and enter the diameter and your target SFM (from a speeds-and-feeds chart) to get the spindle RPM you should program into the machine.
The Formula Explained
The circumference of a circle is \(\pi \times D\). In one revolution the surface travels that distance in inches. Multiply by RPM for inches per minute, then divide by 12 to convert to feet per minute:
$$\text{SFM} = \dfrac{\pi \times D \times \text{RPM}}{12}$$ Solving for RPM gives $$\text{RPM} = \dfrac{\text{SFM} \times 12}{\pi \times D}$$
Worked Example
You have a 0.5-inch end mill in aluminum with a recommended cutting speed of 300 SFM. The required spindle speed is $$\text{RPM} = \dfrac{300 \times 12}{\pi \times 0.5} = \dfrac{3600}{1.5708} \approx \textbf{2292 RPM}$$ If instead you ran a 2-inch face mill at 1000 RPM, the actual $$\text{SFM} = \dfrac{\pi \times 2 \times 1000}{12} \approx \textbf{523.6 SFM}$$
FAQ
Why divide by 12? Diameter is entered in inches, so \(\pi \times D \times \text{RPM}\) is in inches per minute. Dividing by 12 converts to feet per minute.
Is this for metric (SMM)? No. This tool uses inches and surface feet per minute. For metric surface meters per minute the constant differs.
What diameter do I use? For milling and drilling use the tool diameter; for turning use the workpiece diameter at the cut.
