Cutting speed is not RPM
Cutting speed, written Vc or SFM, is how fast the edge of the tool travels through the material. It is a property of the material and the tool, not of the machine. RPM is what you set on the machine, and it depends on the diameter: a 6 mm cutter has to spin far faster than a 50 mm one to reach the same edge speed.
The formula
RPM = Vc × 1000 ÷ (π × D)
with Vc in metres per minute and D the diameter in millimetres. In imperial units, RPM = SFM × 12 ÷ (π × D) with D in inches, which is often shortened to 3.82 × SFM ÷ D.
A 10 mm carbide end mill in mild steel at 120 m/min wants 120 × 1000 ÷ (π × 10) = 3,820 RPM. The same cutter at 10 mm in aluminium at 400 m/min wants 12,700 RPM, which is more than many mills have.
Which diameter to use
On a mill or a drill, D is the diameter of the tool. On a lathe, D is the diameter of the workpiece where the tool is cutting, so the RPM for a constant surface speed has to rise as a facing cut moves towards the centre. Machines with constant surface speed control do that automatically.
Feed rate
Speed alone does not cut anything; the tool also has to advance. For a milling cutter:
feed rate = RPM × feed per tooth × number of teeth
That 3,820 RPM cutter with four flutes at 0.05 mm per tooth gives 3,820 × 0.05 × 4 = 764 mm/min. For drilling and turning the feed is per revolution instead, so feed rate = RPM × feed per revolution.
Typical cutting speeds
Treat these as starting points; the tool maker's figure for the exact grade and coating beats any general table.
- Mild steel: 20 to 30 m/min with HSS, 100 to 200 m/min with carbide.
- Stainless steel: slower, often 15 to 25 m/min with HSS and 60 to 120 m/min with carbide.
- Aluminium: 70 to 120 m/min with HSS, 200 to 500 m/min or more with carbide.
- Brass: similar to aluminium, and it cuts cleanly dry.
- Cast iron: 15 to 25 m/min with HSS, 60 to 150 m/min with carbide.
- Hardwood: speeds are far higher, which is why router bits run at 10,000 RPM and up, and why large diameter bits have to be slowed down.
Reading the cut
Too fast and the edge wears or burns, and the chips come off blue. Too slow with too light a feed and the tool rubs instead of cutting, which also kills it. Chips that leave as firm curls of the right colour, and a tool that stays cool enough to touch after a stop, mean the numbers are about right.
Tools
The Cutting Speed Calculator converts between cutting speed, diameter and RPM, and gives the feed rate. The Torque Calculator links the spindle power to the torque at the cut, and the Screw Thread Chart gives the tapping drill when you follow a cut with a thread.