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.
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:
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.