In a machine shop, a mill is a machine that looks very similar to a drill press, except that

  1. It is used to grind metal into shapes, not drill holes in it.
  2. This means that the bit for a mill has a completely flat head, not one that is tapered. It also means that
  3. It uses a 3-axis mount to move the sample in the X, Y, and Z diections wrt the mill bit. You can also rotate the mount to get different angles, but I don't count this, since it should not be done while the motor is on. It also means that
  4. It usually uses a much more powerful motor.

The process of milling requires a lot of patience. Despite the similarities to a drill press, you should never try to drill straight down into a block of metal with a mill. And actually shaping a piece of metal requires the incremental shaving of layers of metal off the top of your sample with the very tip of the bit. If you try to force anything, then at best, you will ruin both your expensive bit and your carefully crafted sample. At worst, you will have shards of hot bit chip off and fly into your eyes (You forgot to wear your saftey goggles again, eh?). Not surprisingly, many professional machine shops use CNC mills nowadays., and leave the work to the machine.

The word mill has several meanings around the machine shop which describe a number of different tools and processes.

Mill n.

  1. 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.

  2. 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

Mill (mil), n. [L. mille a thousand. Cf. Mile.]

A money of account of the United States, having the value of the tenth of a cent, or the thousandth of a dollar.


© Webster 1913

Mill, n. [OE. mille, melle, mulle, milne, AS. myln, mylen; akin to D. molen, G. mühle, OHG. mulI, mulIn, Icel. mylna; all prob. from L. molina, fr. mola millstone; prop., that which grinds, akin to molere to grind, Goth. malan, G. mahlen, and to E. meal. √108. See Meal flour, and cf. Moline.]


A machine for grinding or comminuting any substance, as grain, by rubbing and crushing it between two hard, rough, or indented surfaces; as, a gristmill, a coffee mill; a bone mill.


A machine used for expelling the juice, sap, etc., from vegetable tissues by pressure, or by pressure in combination with a grinding, or cutting process; as, a cider mill; a cane mill.


A machine for grinding and polishing; as, a lapidary mill.


A common name for various machines which produce a manufactured product, or change the form of a raw material by the continuous repetition of some simple action; as, a sawmill; a stamping mill, etc.


A building or collection of buildings with machinery by which the processes of manufacturing are carried on; as, a cotton mill; a powder mill; a rolling mill.

6. (Die Sinking)

A hardened steel roller having a design in relief, used for imprinting a reversed copy of the design in a softer metal, as copper.

7. (Mining)


An excavation in rock, transverse to the workings, from which material for filling is obtained.


A passage underground through which ore is shot.


A milling cutter. See Illust. under Milling.


A pugilistic encounter. [Cant] R. D. Blackmore.

Edge mill, Flint mill, etc. See under Edge, Flint, etc. --
Mill bar (Iron Works), a rough bar rolled or drawn directly from a bloom or puddle bar for conversion into merchant iron in the mill. --
Mill cinder, slag from a puddling furnace. --
Mill head, the head of water employed to turn the wheel of a mill. --
Mill pick, a pick for dressing millstones. --
Mill pond, a pond that supplies the water for a mill. --
Mill race, the canal in which water is conveyed to a mill wheel, or the current of water which drives the wheel. --
Mill tail, the water which flows from a mill wheel after turning it, or the channel in which the water flows. --
Mill tooth, a grinder or molar tooth. - - Mill wheel, the water wheel that drives the machinery of a mill. --
Roller mill, a mill in which flour or meal is made by crushing grain between rollers. --
Stamp mill (Mining), a mill in which ore is crushed by stamps. --
To go through the mill, to experience the suffering or discipline necessary to bring one to a certain degree of knowledge or skill, or to a certain mental state.


© Webster 1913

Mill (mil), v. t. [imp. & p. p. Milled (mild); p. pr. & vb. n. Milling.] [See Mill, n., and cf. Muller.]


To reduce to fine particles, or to small pieces, in a mill; to grind; to comminute.


To shape, finish, or transform by passing through a machine; specifically, to shape or dress, as metal, by means of a rotary cutter.


To make a raised border around the edges of, or to cut fine grooves or indentations across the edges of, as of a coin, or a screw head; also, to stamp in a coining press; to coin.


To pass through a fulling mill; to full, as cloth.


To beat with the fists. [Cant] Thackeray.


To roll into bars, as steel.

To mill chocolate, to make it frothy, as by churning.


© Webster 1913

Mill, v. i. (Zoöl.)

To swim under water; -- said of air-breathing creatures.


© Webster 1913

Mill (?), v. i.


To undergo hulling, as maize.


To move in a circle, as cattle upon a plain.

The deer and the pig and the nilghar were milling round and round in a circle of eight or ten miles radius.


To swim suddenly in a new direction; -- said of whales.


To take part in a mill; to box. [Cant]


© Webster 1913

Mill, n.


Short for Treadmill.


The raised or ridged edge or surface made in milling anything, as a coin or screw.


© Webster 1913

Mill, v. t.

1. (Mining)

To fill (a winze or interior incline) with broken ore, to be drawn out at the bottom.


To cause to mill, or circle round, as cattle.


© Webster 1913

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