Simply a Run of the Mill Question

The owner of the metalworking machine sales company woke up predawn to his cell phone ringing. The number was from the Eastern U.S., a couple of time zones away, so he immediately roused himself and answered it. He assumed that the call was about the milling machine he had listed online, and he was eager to talk to all inquirers.

The mill was a fantastic find. The aerospace firm he had bought it from for several hundred thousand dollars had originally purchased it for $6M, so he figured to turn a tidy profit from it, especially since it had never been put into production. Administrations had changed and the aerospace sector had taken a hit, with several government contracts being cancelled. The company was trying to cut their losses by selling a couple of big multi-axis mills that ran on rails over areas the size of a large garage.

The voice on the other end of the line asked, “Are you the one selling the mill?”

“Yes, I am,” came the reply.

In all sincerity, the voice on the phone then asked, “Does it mill wheat?”

What’s In a Name?

The word “mill” has several different meanings, though they all stem from the same root. In Latin, the verb “molere” (to grind) was turned into the noun “mola,” meaning a mill or grindstone. In Old English the word became “mylen,” then “milne” and “mille” in Middle English, evolving to the simpler “mill” we use today. To mill (the verb) means to break a solid material down into smaller particles by grinding or a similar method, and generally done by using a mill (the noun).

Before it came to mean a process and type of machine for working with metal—the topic of this article—mills were better known historically for breaking down rocks or foods like grains. Prior to electrical power, mills have been operated by hand power and (literal) horsepower and other natural means, such as in the case of windmills and water mills. At some point, the buildings housing milling machinery started to be known as mills, with the word sometimes becoming synonymous with a factory with a high output of product, even if milling isn’t the main function of the facility. Examples include steel mills, sawmills, textile mills, and paper mills. The term is sometimes used figuratively in a derogatory sense to refer to businesses that churn out large volumes of the same product or service in a questionable manner, such as puppy mills, divorce mills, or diploma mills.

In metalworking, the term “milling” refers to a type of “machining,” or using a machine to shape metal by removing small pieces of it called “chips” or “swarf.” Unlike a lathe, where a workpiece is rotated at a high speed and a cutting tool is slowly applied to the surface, milling machines (often called “mills” for short) will apply high speed rotary cutters to a stationary workpiece to remove material.

History of the Metal Mill

By the middle of the 1500s, hand powered rotary files had been developed to cut gears for clock mechanisms. Two centuries later, lathe operators discovered that if they mounted a circular cutter into the headstock of a lathe, it could be rapidly spun to perform a filing action on a workpiece. The first actual milling machine was invented by Samuel Rehé in 1783 when he substituted true milling cutters instead of rotary file cutters. In the United States, Eli Whitney (best known for inventing the cotton gin) was credited with creating a milling machine for the mass-production of interchangeable gun parts around 1818. By this time, mills had become recognized as a separate class of machine tool from lathes.

As mills progressed in development throughout the 1800s, their basic design resembled drill presses and could only operate in a single axis. That changed in 1861 with the development of the Universal Milling Machine by the Brown & Sharpe company. This new type of mill could work in three axes and allowed spirals to be shaped in metal, such as the flutes found on a drill bit. In the 1930s the Bridgeport milling machine was invented, innovating a style of knee-and-column construction that remains the most popular design for vertical mills today. Numerical control (NC) milling machines were developed in the 1950s, followed by mills with a computer numerical control (CNC) by the 1970s.

Parts of a Vertical Mill

Mills, like all machine tools, have multiple parts with distinctive functions. While different configurations of mills vary in components and their arrangement, below are the main sections of the vertical, column-type knee mill, one of the more common milling machines used in metalworking.

  • Base. The foundation of a mill.
  • Column. Mounted on the base, it houses some of the drive components and supports the head, knee, and other parts.
  • Cutting Tool. The changeable tooling that mounts to the spindle to perform the actual milling operation on the workpiece.
  • Head. The upper section of a vertical milling machine, containing the spindle, drive motor and some controls.
  • Knee. The part of a mill that supports the saddle and table. It can be adjusted vertically.
  • Saddle. The part of the mill between the table and the knee. It allows the table to be moved horizontally left and right.
  • Spindle. The part of the mill that holds the cutter in place. It faces down from the head in a vertical mill and out horizontally from the column in a horizontal mill.
  • Table. The rectangular part of the machine that secures the workpiece. It sits on top of the saddle.

Types of Milling Machines

There are many different terms for describing types of metal mills, based on components and configurations. They are not exclusionary, meaning that a specific milling machine can be more than one type. For example, a knee mill is so named because it possesses a knee that extends from the main column of the machine. Such a mill is also a column mill and could be either a vertical or horizontal mill. Some of the names of metalworking mills include:

  • Arbor Milling Machine. Another name for a horizontal mill.
  • Bed Type Milling Machine. A vertical mill without a knee or saddle that has its table mounted to a rigid bed. The spindle of a bed mill is attached to a pendant that can be raised or lowered in relation to the workpiece. The spindle can move in one axis (a simplex mill), two axes (a duplex mill), or three axes (a triplex mil). It is also known as a fixed-bed type mill.
  • Boring Mill. Not a traditional milling machine, a boring mill is primarily used to enlarge existing holes, usually involving very long workpieces. A jig borer is a type of vertical boring mill.
  • Box Milling Machine. A bench-mounted, low-end mill used by hobbyists for simple milling processes. It gets its name because the head rides up and down on a box way.
  • Bridge Milling Machine. Another name for a gantry mill.
  • Bridgeport Milling Machine. A brand name, the term Bridgeport is often used in the industry to describe the style of vertical turret knee mill that has been replicated by other manufacturers since the Bridgeport company innovated the original in the 1930s.
  • C-Frame Milling Machine. A large, hydraulically powered mill used in industrial production. C-frame mills get their name from the configuration of the head, column and table looking like a letter “C.”
  • CNC Milling Machine. A multi-axis mill controlled by a programmable computer numeric control (CNC).
  • Column Milling Machine. Another name for a knee mill. It can be a vertical or a horizontal mill.
  • Drum Milling Machine. A mill that has a table called a drum that rotates a workpiece horizontally through three or four spindle heads (like the process with a rotary table mill).
  • Duplex Milling Machine. A mill that has a spindle that can move in two directions, both horizontally and vertically.
  • Five-Axis Milling Machine. A mill that has the standard X, Y, and Z linear axes, along with two rotational axes. Many 5-axis machines will allow all axes to work simultaneously to perform complex machining on a workpiece.
  • Floor Milling Machine. A mill with a horizontal pendant spindle mounted on a set of tracks that runs parallel to a row of rotary tables. Workpieces can be changed out while an operation is being carried out on a different table.
  • Four-Axis Milling Machine. A mill that adds an additional rotational movement to the standard X, Y, and Z axes.
  • Gang Milling Machine. A horizontal mill with two or more cutters mounted to its spindle for simultaneous cutting in the same operation.
  • Gantry Milling Machine. A large-scale mill with a head mounted on a structure that runs along two rails (a gantry) over a long table. Also called a movable bridge mill.
  • Horizontal Milling Machine. A mill with a horizontal spindle that is parallel to the table.
  • Jeweler’s Mill. Not a traditional milling machine, a jeweler’s mill uses a hardened steel roller to imprint a design into copper or another softer metal.
  • Knee Milling Machine. A mill that features an extension from the column called a “knee” that can move the workpiece up and down. It is also called a knee-and-column mill.
  • Machining Center. An advanced type of CNC mill that is usually enclosed and is equipped with an automatic tool changer (ATC).
  • Manual Milling Machine. Any mill that is run by hand—instead of a control like a CNC—is called a manual mill.
  • Manufacturing Type Milling Machine. Another name for a bed type mill.
  • Plain Milling Machine. Another name for a horizontal mill.
  • Planer Type Milling Machine. A large, heavy-duty mill that resembles a planer but has a milling spindle instead of a planing head. Planer-style mills are also called “plano millers.”
  • Ram-type Milling Machine. Any mill with a cutting head mounted on a sliding ram.
  • Rotary Table Milling Machine. A mill with a round table that rotates the workpiece around a center axis through a series of multiple cutters set at different heights. Workpieces can be loaded and unloaded continually throughout the operation.
  • Simplex Milling Machine. A mill that has a spindle that can only travel in a single direction, usually vertically.
  • Three-Axis Milling Machine. A mill that can perform an operation on a workpiece in three directions, milling in the X, Y, and Z axes.
  • Tracer Controlled Milling Machine. A mill that can replicate a part in a milled workpiece by tracing an original finished part.
  • Triplex Milling Machine. A mill with a spindle that can move in the X, Y, and Z axes.
  • Turret Milling Machine. A vertical mill with a stationary cutter and a workpiece that moves along the X and Y axes while being cut.
  • Universal Milling Machine. A versatile mill that features a table that can pivot at any angle up to 45° on either side.
  • Vertical Milling Machine. A mill that has a downward facing spindle.

Just Milling About

The advent of the milling machine brought about a true revolution in machine shop productivity. Mills allow machinists to produce—individually or in small batches—complex shapes that simply aren’t possible using only a lathe. The mass production of identical, complex metal parts that has been commonplace for the last two centuries in economies around the globe would never have come about without the invention of the metalworking mill. 

Milling machines are incredibly versatile, being able to not only mill, but also bore, cut, drill and shape. For the machinist who needs to produce slots, perform diesinking on a block of metal, or cut gears, a metal mill is an essential tool for the shop.