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The main rotating component of a milling machine that transfers power to the cutting tool is called a milling spindle. Milling is a machining process in which a work piece is moved relative to a rotating tool with cutting edges in order to remove material. As the milling spindle rotates the cutting tool, the work is fed into it from various directions to form the desired shape.
Cutting tools generally have straight or tapered portions on one of their ends while the cutting edges are at the other end of the tool. The end opposite the cutting end is mounted in the milling machine spindle to align and support the tool and to provide power and rotation. A variety of spindle adapters, collets, and other tooling are available to mount the tool in the milling spindle.
Milling spindles may be positioned either horizontally or vertically, depending on the particular mill in use. On a vertical machine, the spindle is mounted parallel to the support column of the mill in a sliding head that can move up and down. Many modern vertical mills have heads that can be rotated to position the milling spindle for cutting on an angle.
The milling spindle in a horizontal machine is usually in a fixed position with its motor housed in the column of the mill. For a horizontal machine, the spindle typically cannot be reoriented as some vertical machines allow. A horizontal machine may, however, have a table that can be rotated to change the angle of the spindle relative to the work piece. On some machines, a vertical spindle attachment may be used to convert a horizontal mill to a vertical configuration.
The speed of a milling spindle must be set appropriately for safe operation of the machine and to avoid damage to the tool or work piece. Mill manufacturers may provide recommended values, or they may be calculated for the particular tool used and material being machined. Speed is generally set using a dial or hand crank on the machine. Modern machines may allow continuous adjustment through a range of speeds while older models may have a limited number of specific settings.
Milling machines may suffer from excessive vibration, called chatter, under certain conditions. Chatter may make precision machining difficult or impossible as well as damage the tool in use. A common cause of chatter is when the spindle speed is set too high. When lowering the speed does not correct the condition, the problem may be due to unbalanced tooling, a loose fixture, or some other problem.
A simple mill may have just one milling spindle while more complex machines may incorporate multiple spindles. More than one spindle allows cutting in two or three dimensions without repositioning the work piece. Modern computer numerical controlled (CNC) machines used in complex milling operations often incorporate three or more milling spindles for faster manufacturing and increased throughput.