Question 1

How do I calculate spindle RPM from cutting speed?

Accepted Answer

Use RPM = (1000 x Vc) / (pi x D), where Vc is the cutting speed in metres per minute and D is the diameter in millimetres. The 1000 converts metres to millimetres so the units line up. For example, a cutting speed of 100 m/min on a 50 mm diameter gives (1000 x 100) / (3.1416 x 50), which is about 636.6 RPM. For turning use the workpiece diameter, and for milling or drilling use the tool diameter.

Question 2

What is cutting speed?

Accepted Answer

Cutting speed (Vc) is the speed at which the cutting edge moves across the surface of the material, measured in metres per minute. It is a property of the material and tooling, not the machine. Once you pick a cutting speed for a given material and tool, you convert it into RPM for the diameter you are actually cutting. Run too fast and the edge burns up, run too slow and you waste time and can rub instead of cut.

Question 3

What is the difference between cutting speed and feed rate?

Accepted Answer

Cutting speed (Vc) is how fast the edge travels across the material surface, and it sets the RPM. Feed rate is how far the tool advances into the work, either per revolution (mm/rev) or per minute (mm/min). Feed rate in mm/min equals RPM multiplied by the feed per revolution. Cutting speed mostly drives tool wear and finish, while feed drives how quickly you remove material and the chip load on each edge.

Question 4

What are typical cutting speeds for steel and aluminium?

Accepted Answer

As rough guidance, mild steel with a high speed steel (HSS) tool runs around 30 m/min, while carbide tooling on the same steel handles roughly 100 to 200 m/min. Aluminium runs faster, often 200 m/min and well above with carbide. These are starting points only. Always check the insert or tool maker's data, since the right speed depends on the grade, coating, coolant, depth of cut and how rigid the setup is.

Question 5

Why does the right RPM matter for tool life?

Accepted Answer

RPM sets the actual cutting speed at the edge, and cutting speed is the biggest driver of heat at the cutting zone. Run too high and the edge overheats and wears or chips quickly, so tool life drops sharply. Run too low and the tool can rub rather than cut, which work hardens the material and dulls the edge. Getting the RPM right for the material and tool keeps the edge in the range it was designed for, which protects both tool life and surface finish.

Cutting Speed & RPM Calculator

Cutting Speed to RPM: the formula

Feed rate from RPM

Worked example