Cutting Speed Calculator
Calculate cutting speed in SFM or m/min from workpiece diameter and spindle RPM. Enter values for instant results with step-by-step formulas.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
V = (Pi x D x N) / 1000
Where V = cutting speed in meters per minute, D = workpiece or tool diameter in mm, N = spindle speed in RPM. Pi times the diameter gives circumference, multiplied by RPM gives surface distance per minute.
Worked Examples
Example 1: Turning Mild Steel on a Lathe
Problem:A 75mm diameter mild steel bar is being turned at 425 RPM. What is the cutting speed?
Solution:V (metric) = Pi x D x N / 1000 = 3.14159 x 75 x 425 / 1000 = 100.07 m/min\nV (imperial) = Pi x (75/25.4) x 425 / 12 = 328.3 SFM\nThe recommended range for mild steel with carbide tooling is 80-120 m/min, so 100 m/min is optimal.
Result:Cutting Speed: 100.07 m/min (328.3 SFM) - within optimal range for mild steel
Example 2: Finding RPM for Aluminum Milling
Problem:A 20mm end mill is used to machine 6061-T6 aluminum. The recommended cutting speed is 300 m/min. What RPM should be set?
Solution:N = (V x 1000) / (Pi x D) = (300 x 1000) / (3.14159 x 20) = 300000 / 62.83 = 4774 RPM\nRound to nearest available spindle speed: 4800 RPM\nActual cutting speed at 4800 RPM = Pi x 20 x 4800 / 1000 = 301.6 m/min
Result:Required RPM: 4774 (use 4800 RPM) - actual speed 301.6 m/min
Frequently Asked Questions
What is cutting speed and why does it matter in machining?
Cutting speed is the rate at which the cutting tool edge moves across the workpiece surface, measured in surface feet per minute (SFM) or meters per minute (m/min). It is one of the most critical parameters in machining because it directly affects tool life, surface finish quality, and material removal efficiency. Running too fast causes excessive heat buildup that destroys the cutting tool edge prematurely, while running too slow reduces productivity and can cause work hardening in certain materials. Every material has an optimal cutting speed range that balances tool life with production efficiency.
How do you calculate cutting speed from RPM and diameter?
Cutting speed is calculated using the formula V equals Pi times D times N divided by 1000 for metric units, where V is cutting speed in meters per minute, D is the workpiece or tool diameter in millimeters, and N is the spindle speed in revolutions per minute. For imperial units the formula becomes V equals Pi times D times N divided by 12, where D is in inches and V is in surface feet per minute. The formula derives from the fact that each revolution traces out one circumference of the workpiece, and multiplying by RPM gives the total distance traveled per minute.
What cutting speed should I use for aluminum?
Aluminum is one of the easiest metals to machine and allows very high cutting speeds compared to steel or titanium. For uncoated high-speed steel tools, recommended cutting speeds range from 200 to 400 m/min or 650 to 1300 SFM. With carbide tooling, speeds can reach 300 to 600 m/min or even higher depending on the alloy. Aluminum alloys with higher silicon content like 6061-T6 require slightly lower speeds than pure aluminum. Always use sharp tools with positive rake angles and apply cutting fluid or compressed air to prevent built-up edge formation on the tool.
What is the difference between cutting speed and feed rate?
Cutting speed refers to how fast the tool edge moves across the workpiece surface measured in m/min or SFM, while feed rate describes how quickly the tool advances into the material measured in mm/rev or mm/min. Cutting speed is primarily determined by the workpiece material and tool material combination, whereas feed rate depends on the desired surface finish, chip load, and machine rigidity. Together these two parameters along with depth of cut determine the material removal rate. Increasing cutting speed generally improves surface finish while increasing feed rate improves productivity but roughens the surface.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy