N is the rpm of the work. SFM Surface Feet per Minute RPM x D 382. To determine this you might need to break out the calculator simply multiply the recommended cutting speed by the value 382 round up to 4 and then divide by the tool diameter.
In the British system cutting speed is expressed in feet per minute and diameter of the work in inches.
Cutting speed at the cutting depth border for oblique machining nSpindle Speedmin -1 Check the item you want to calculate input values in the four boxes and then press the Calculate button. Cutting feed IPR Cutting speed SFM 12 Tool diameter in p Spindle speed RPM Cutting feed IPR Spindle speed RPM Feed rate IPM. Cutting speed CS X 4 Diameter of cutter D This simplified version of the RPM formula is the most common formula used in machine shops. First calculate the cutting length per min.