Speeds and feeds

The spindle speed is the RPM of the drill bit or milling cutter when used in a milling machine or drill, or the speed the workpiece rotates in a lathe, turning center, or other rotary machine.

For a lathe, correct spindle speed is very important. Excessive spindle speed will cause tool wear, possibly produce inaccurate cuts, can cause tool chatter, and a possibly dangerous condition, while a low spindle speed will lengthen the process, also possibly produce inaccurate cuts, or cause other problems. Using the proper spindle speed for the material and tools will also greatly affect the quality of the surface finish.

For milling, spindle speed and feed rate are highly interrelated. A higher spindle speed can allow a higher feed rate, while a low spindle speed often requires a lower feed rate. Too high of a spindle speed will cause overheating and damage to the tool, while too low of a spindle speed will cause a rough cut and possible bind in the material. The ratio of the spindle speed and the feed rate controls how agressive the cut is, and the nature of the swarf formed.

Calculating the required spindle speed on a milling machine is a function of the diameter of the cutter, number of teeth on the cutter, desired feed rate, material being machined, and a variety of other factors. Most metalworking books have tables of spindle speeds and feed rates for different cutters and workpiece materials; similar tables are also likely available from the manufacturer of the cutter used.

Some materials, such as machinable wax, can be cut at a wide variety of spindle speeds, while others, such as stainless steel require much more careful control, especially to avoid overheating the cutter if cutting fluid is not used.