Metal Cutting Speeds

There are three parameters that define how fast material is removed:

1. Surface Speed: Rate at which metal comes into contact with the tool at the cutting point. Usually expressed in SFM: surface feet per minute.
2. Feed Rate: Rate at which the cutting point moves transverse to the stock, like an autofeed rate.
3. Cut Depth: How much material is being removed in a given pass.

The chart of cut rates will make more sense after reading the following.

Surface Speed
Surface speed is how fast the metal comes into contact with the tool at the cutting point. On a lathe, it is the rate at which the surface of the stock passes the cutting tool. This takes into account the diameter of the stock. On a mill, it is the rate at which the end-mill, for example, moves past the stationary stock. This takes into account the diameter of the endmill. Surface speed is usually measured in Surface Feet per Minute (SFM).

For example, turning a piece of 1" stock on a lathe at 260 RPM yields a 68.0 SFM (surface feet per minute) surface rate:


The general formula for a surface speed is as follows:

where
D = diameter of stock (lathe) or diameter of cutting tool (mill), in inches
RPM = speed of the rotating part or tool, in revolutions per minute

Note: when facing a part on a lathe, to maintain a uniform surface speed, the RPM would need to go to infinity. When facing, select the speed based on the diameter at the outside surface. This will yield a slower speed than a diameter measured halfway through the stock, or at the center(!).

Feed Rate
Feed rate is how fast the tool moves relative to a fixed, stationary part. It is the autofeed rate when autofeed is used, and is usually measured in inches per minute on a mill, or inches per revolution on a lathe.

To translate between from inches per revolution to inches per minute on a lathe feed rate, multiply the inches per revolution by the spindle speed in RPM