How to use the milling feeds and speeds calculator
- Choose inches or millimeters, and enter the cutter diameter, flute count and chip load.
- Start from a spindle speed in RPM, or from a surface speed (SFM for inches, meters per minute for millimeters).
- If you start from surface speed, you can enter the machine's top RPM and the speed is capped at it.
- Open More options to add a depth and width of cut for the material removal rate.
Formula
RPM = surface speed x 12 / (pi x diameter) for SFM and inches; = m/min x 1000 / (pi x diameter) for mm
Surface speed = pi x diameter x RPM / 12 (SFM) or / 1000 (m/min)
Feed rate = RPM x flutes x chip load
Material removal rate = depth of cut x width of cut x feed rate
Chip load is the thickness of material each tooth removes per revolution. The tool maker's chart gives surface speed and chip load for each material.
A router bit and an end mill
A 1/4 inch two-flute bit at 12,000 RPM with a chip load of 0.005 in has a surface speed of 785.4 SFM and a feed rate of 12,000 x 2 x 0.005 = 120 in/min. A cut 0.25 in deep and 0.25 in wide removes 7.5 cubic inches per minute.
A 1/2 inch three-flute end mill wanting 300 SFM needs 2291.8 RPM, and with a chip load of 0.002 in feeds at 13.751 in/min. A 6 mm two-flute router bit at 18,000 RPM and a chip load of 0.05 mm feeds at 1800 mm/min, with a surface speed of 339.29 m/min, and removes 32.4 cubic centimeters per minute at a 3 mm by 6 mm cut.
Speed and feed together
Spindle speed sets how fast the edges move through the material, and feed rate sets how much each tooth bites. Too slow a feed for the speed rubs and heats the cutter instead of cutting, while too fast a feed overloads it. Staying close to the chip load recommended for the cutter and material keeps the heat in the chips, not the tool.
Related shop tools include the drill bit size converter, the metal weight calculator, the miter angle calculator and the horsepower from torque calculator.
Where the numbers come from
The page does not hold material tables. Surface speed and chip load depend on the material, the cutter coating, the depth of cut and the rigidity of the machine, so take them from your tool maker's data and adjust by sound and finish. When a machine tops out below the wanted RPM, the page caps the speed and shows the new surface speed and feed, so the chip load stays what you chose.
Assumptions and limits
- Chip load is per tooth per revolution. Chip thinning at light radial cuts is not corrected for.
- Diameter is the cutting diameter, and flutes means the number of cutting edges.
- Material removal rate uses depth of cut times width of cut times feed, for a straight slot or pass.
- Values are for planning. Start low and increase by sound and finish. The page is not machine or safety advice.
Frequently asked questions
How do I calculate RPM from surface speed?
For inches, RPM = SFM x 12 / (pi x diameter). A 1/2 inch cutter at 300 SFM runs at about 2292 RPM. For metric, RPM = meters per minute x 1000 / (pi x diameter in mm).
How do I calculate feed rate?
Multiply the spindle speed by the number of flutes and by the chip load per tooth. At 12,000 RPM with 2 flutes and 0.005 in chip load, the feed is 120 inches per minute.
What chip load should I use?
Use the figure from your tool maker for your cutter diameter and material. It is usually a few thousandths of an inch for small cutters, and the right value changes with material.
What if my machine cannot reach the RPM?
Enter the machine's maximum RPM. The speed is capped, and the page recalculates the surface speed and feed so the chip load stays what you asked for.