The word mill has several meanings around the machine shop which describe a number of different tools and processes.
Mill n.
- A milling machine. A machine tool capable of shaping a piece of material in a precise fashion by cutting the material, never by grinding it. Milling machines are normally thought of as being able to shape prismatic parts as opposed to a lathe which shapes cylindrical parts. There are a number of different types of milling machines in common use, the two most common being the vertical milling machine and the horizontal milling machine.
The material to be shaped is clamped to the bed of the mill. A cutting tool, also referred to as a mill, is attached to the spindle of the mill and rotated at high speeds. Shaping is accomplished by bringing the cutting tool in contact with the material and moving the tool in relation to the material in a controlled manner to remove material. Positioning of the tool with respect to the material is accomplished by a series of cranks. These cranks have graduated dials which allow fine control. Common graduations are on the order of .001 in. The rate at which the cutting tool travels is called the feed rate. Feed rate can be controlled manually or it can be set precisely as apart of a power feed system which is part of most milling machines.
Feed rate is limited primarily by heat, as cutting metal generates a lot of heat. Softer materials (in general) are easier to cut, generate less heat, and therefore can tolerate a higher feed rate. Using sharp cutters and bathing the cutter and material in a cooling/lubricating fluid also increases the maximum feed rate.
Milling machines are sometimes classified by the number of degrees of freedom they have. Most common shop mills have three degrees of freedom (front/back,left/right, and up/down). A cartesian coordinate system is used and the directions of movement are usually referred to as X, Y, and Z. Milling machines with three degrees of freedom are referred to as a 3-axis mill. On the vertical milling machine the axis of the cutting tool is usually labeled Z. Moving the cutting tool into the material is called plunging. Moving the cutting tool across the face of the material is called facing. By plunging into the material the milling machine can be used to bore very accurate holes in a material. This can be done using a normal twist drill bit, or with an end mill. If an end mill is used to cut straight into the material it must be of the "center cutting" type (see end mill for more details]).
More complex milling machines have as many as 6 degrees of freedom and are referred to as 6-axis milling machines. The machines have the ability to rotate the tool in relation to material being cut about the X, Y, and Z axis. Machines this complicated are usually computer controlled (see CNC). In general, the more degrees of freedom a milling machine has the more complicated the parts it can machine. For example, a 6 axis mill is capable of machining both prismatic and spherical surfaces.
- A mill cutter. The most common type of mill cutter used with a vertical milling machine is the end mill. Mill cutters come in a great variety of shapes and sizes. They are most commonly constructed from high speed steel or with replaceable carbide cutting edges attached to steel tool holders.
Mill v.
In regards to the machine shop, milling is the process of using a milling machine to shape a piece of material. The material is shaped by cutting it with a sharp rotating cutter. The concept of milling is rather simple. In practice however things become more complicated especially when one wishes to achieve any sort of accuracy. The milling of complex pieces can be quite time consuming when done by hand. Today most complex, low quantity items are produced using computer controlled mills.
Mill adj.
A type of file having fine, straight cutting teeth.
Olivo, C. Thomas. Machine Tool Technology and Manufacturing Processes. Albany: C. Thomas Olivo Associates. 1987
Kibbe, Richard R., et al. Machine Tool Practices 5th ed. Englewood Cliffs: Prentice Hall. 1995