A
milling machine is a
machine toolA machine tool is a machine, typically powered other than by human muscle , used to make manufactured parts in various ways that include cutting or certain other kinds of deformation...
used to
machineConventional machining is a form of subtractive manufacturing, in which a collection of material-working processes utilizing power-driven machine tools, such as saws, lathes, milling machines, and drill presses, are used with a sharp cutting tool to physical remove material to achieve a desired...
solidSolid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
materials. Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main
spindleIn machine tools, a spindle is a rotating axis of the machine, which often has a shaft at its heart. The shaft itself is called a spindle, but also, in shop-floor practice, the word often is used metonymically to refer to the entire rotary unit, including not only the shaft itself, but its bearings...
. Both types range in size from small, bench-mounted devices to room-sized machines. Unlike a drill press, which holds the workpiece stationary as the drill moves axially to penetrate the material, milling machines also move the workpiece radially against the rotating
milling cutterMilling cutters are cutting tools typically used in milling machines or machining centres . They remove material by their movement within the machine or directly from the cutter's shape .-Features of a milling cutter:Milling cutters come in several shapes and many sizes...
, which cuts on its sides as well as its tip. Workpiece and cutter movement are precisely controlled to less than 0.001 in (0.00254 cm), usually by means of precision ground slides and
leadscrewA leadscrew , also known as a power screw or translation screw, is a screw designed to translate turning motion into linear motion...
s or analogous technology. Milling machines may be manually operated, mechanically automated, or digitally automated via computer numerical control (CNC).
Milling machines can perform a vast number of operations, from simple (e.g., slot and keyway cutting, planing, drilling) to complex (e.g., contouring, diesinking).
Cutting fluidCutting fluid is a type of coolant and lubricant designed specifically for metalworking and machining processes. There are various kinds of cutting fluids, which include oils, oil-water emulsions, pastes, gels, aerosols , and air or other gases. They may be made from petroleum distillates, animal...
is often pumped to the cutting site to cool and lubricate the cut and to wash away the resulting
swarfSwarf, also known as turnings, chips, or filings, are shavings and chippings of metal — the debris or waste resulting from metalworking operations including milling and grinding. It can usually be recycled, and this is the preferred method of disposal due to the environmental concerns regarding...
.
Types and nomenclature
Mill orientation is the primary classification for milling machines. The two basic configurations are vertical and horizontal. However, there are alternate classifications according to method of control, size, purpose and power source.
Vertical mill
In the vertical mill the spindle axis is vertically oriented.
Milling cutterMilling cutters are cutting tools typically used in milling machines or machining centres . They remove material by their movement within the machine or directly from the cutter's shape .-Features of a milling cutter:Milling cutters come in several shapes and many sizes...
s are held in the spindle and rotate on its axis. The spindle can generally be extended (or the table can be raised/lowered, giving the same effect), allowing plunge cuts and drilling. There are two subcategories of vertical mills: the bed mill and the turret mill.
- A turret mill has a stationary spindle and the table is moved both perpendicular and parallel to the spindle axis to accomplish cutting. The most common example of this type is the Bridgeport, described below. Turret mills often have a quill which allows the milling cutter to be raised and lowered in a manner similar to a drill press. This type of machine provides two methods of cutting in the vertical (Z) direction: by raising or lowering the quill, and by moving the knee.
- In the bed mill, however, the table moves only perpendicular to the spindle's axis, while the spindle itself moves parallel to its own axis.
Turret mills are generally considered by some to be more versatile of the two designs. However, turret mills are only practical as long as the machine remains relatively small. As machine size increases, moving the knee up and down requires considerable effort and it also becomes difficult to reach the quill feed handle (if equipped). Therefore, larger milling machines are usually of the bed type.
Also of note is a lighter machine, called a mill-drill. It is quite popular with hobbyists, due to its small size and lower price. A mill-drill is similar to a small drill press but equipped with an X-Y table. These are frequently of lower quality than other types of machines.
Horizontal mill
A horizontal mill has the same sort of
x–
y table, but the cutters are mounted on a horizontal arbor (see
Arbor millingArbor milling is a cutting process which removes material via a multi-toothed cutter. An arbor mill is a type of milling machine characterized by its ability to rapidly remove material from a variety of materials, this milling process is not only rapid but also versatile.-Process...
) across the table. Many horizontal mills also feature a built-in rotary table that allows milling at various angles; this feature is called a
universal table. While endmills and the other types of tools available to a vertical mill may be used in a horizontal mill, their real advantage lies in arbor-mounted cutters, called side and face mills, which have a cross section rather like a circular saw, but are generally wider and smaller in diameter. Because the cutters have good support from the arbor and have a larger cross-sectional area than an end mill, quite heavy cuts can be taken enabling rapid material removal rates. These are used to mill grooves and slots. Plain mills are used to shape flat surfaces. Several cutters may be ganged together on the arbor to mill a complex shape of slots and planes. Special cutters can also cut grooves, bevels, radii, or indeed any section desired. These specialty cutters tend to be expensive. Simplex mills have one spindle, and duplex mills have two. It is also easier to
cut gearsGear cutting is the process of creating a gear. The most common processes include hobbing, broaching, and machining; other processes include shaping, forging, extruding, casting, and powder metallurgy. Gears are commonly made from metal, plastic, and wood.-Broaching:For very large gears or splines,...
on a horizontal mill. Some horizontal milling machines are equipped with a power-take-off provision on the table. This allows the table feed to be synchronized to a rotary fixture, enabling the milling of spiral features such as hypoid gears.
Comparative merits
The choice between vertical and horizontal spindle orientation in milling machine design usually hinges on the shape and size of a workpiece and the number of sides of the workpiece that require machining. Work in which the spindle's axial movement is
normalA surface normal, or simply normal, to a flat surface is a vector that is perpendicular to that surface. A normal to a non-flat surface at a point P on the surface is a vector perpendicular to the tangent plane to that surface at P. The word "normal" is also used as an adjective: a line normal to a...
to one plane, with an endmill as the cutter, lends itself to a vertical mill, where the operator can stand before the machine and have easy access to the cutting action by looking down upon it. Thus vertical mills are most favored for diesinking work (machining a mold into a block of metal). Heavier and longer workpieces lend themselves to placement on the table of a horizontal mill.
Prior to
numerical controlNumerical control refers to the automation of machine tools that are operated by abstractly programmed commands encoded on a storage medium, as opposed to controlled manually via handwheels or levers, or mechanically automated via cams alone...
, horizontal milling machines evolved first, because they evolved by putting milling tables under lathe-like headstocks. Vertical mills appeared in subsequent decades, and accessories in the form of add-on heads to change horizontal mills to vertical mills (and later vice versa) have been commonly used. Even in the CNC era, a heavy workpiece needing machining on multiple sides lends itself to a horizontal machining center, while diesinking lends itself to a vertical one.
Alternate classifications
In addition to horizontal versus vertical, other distinctions are also important:
| Criterion | Example classification scheme | Comments |
| Spindle axis orientation |
Vertical versus horizontal;
Turret versus non-turret |
Among vertical mills, "Bridgeport-style" is a whole class of mills inspired by the Bridgeport original, rather like the IBM PC spawned the industry of IBM-compatible PCs by other brands |
| Control |
Manual;
Mechanically automated via cams;
Digitally automated via NCNumerical control refers to the automation of machine tools that are operated by abstractly programmed commands encoded on a storage medium, as opposed to controlled manually via handwheels or levers, or mechanically automated via cams alone...
/CNC |
In the CNC era, a very basic distinction is manual versus CNC.
Among manual machines, a worthwhile distinction is non-DROA digital read out is a small computer usually with an integrated keyboard and some means of numeric representation. It reads the signals generated by the linear encoder installed to several machine's axes used to keep track of workpiece position or the tool's position...
-equipped versus DRO-equipped |
| Control (specifically among CNC machines) |
Number of axes (e.g., 3-axis, 4-axis, or more);
Within this scheme, also:
- Pallet-changing versus non-pallet-changing
- Full-auto tool-changing versus semi-auto or manual tool-changing
|
|
| Purpose |
General-purpose versus special-purpose or single-purpose |
|
| Purpose |
Toolroom machine versus production machine |
Overlaps with above |
| Table design |
"Plain" versus "universal" |
A distinction whose meaning evolved over decades as technology progressed, and overlaps with other purpose classifications above; Commonly, a "plain table" means the table is fixed in place on the machine and cannot be rotated. A "unversal table" means that the table may be rotated to various angles. |
| Size |
Micro, mini, benchtop, standing on floor, large, very large, gigantic |
|
| Power source |
Line-shaft-drive A line shaft is a power transmission system used extensively during the Industrial Revolution. Prior to the widespread use of electric motors small enough to be connected directly to each piece of machinery, line shafting was used to distribute power from a large central power source to machinery...
versus individual electric motor drive |
Most line-shaft-drive machines, ubiquitous circa 1880-1930, have been scrapped by now |
| Hand-crank-power versus electric |
Hand-cranked not used in industry but suitable for hobbyist micromills |
Variants
- Bed mill This refers to any milling machine where the spindle is on a pendant that moves up and down to move the cutter into the work. These are generally more rigid than a knee mill.
- Box mill or column mill Very basic hobbyist bench-mounted milling machines that feature a head riding up and down on a column or box way.
- C-Frame mill These are larger, industrial production mills. They feature a knee and fixed spindle head that is only mobile vertically. They are typically much more powerful than a turret mill, featuring a separate hydraulic motor for integral hydraulic power feeds in all directions, and a twenty to fifty horsepower motor. Backlash eliminators are almost always standard equipment. They use large NMTB 40 or 50 tooling. The tables on C-frame mills are usually 18" by 68" or larger, to allow multiple parts to be machined at the same time.
- Floor mill These have a row of rotary tables, and a horizontal pendant spindle mounted on a set of tracks that runs parallel to the table row. These mills have predominantly been converted to CNC, but some can still be found (if one can even find a used machine available) under manual control. The spindle carriage moves to each individual table, performs the machining operations, and moves to the next table while the previous table is being set up for the next operation. Unlike other mills, floor mills have movable floor units. A crane drops massive rotary tables, X-Y tables, etc., into position for machining, allowing large and complex custom milling operations.
- Gantry mill The milling head rides over two rails (often steel tubes) which lie at each side of the work surface.
- Horizontal boring mill Large, accurate bed horizontal mills that incorporate many features from various machine tools. They are predominantly used to create large manufacturing jigs, or to modify large, high precision parts. They have a spindle stroke of several (usually between four and six) feet, and many are equipped with a tailstock to perform very long boring operations without losing accuracy as the bore increases in depth. A typical bed has X and Y travel, and is between three and four feet square with a rotary table or a larger rectangle without a table. The pendant usually provides between four and eight feet of vertical movement. Some mills have a large (30" or more) integral facing head. Right angle rotary tables and vertical milling attachments are available for further flexibility.
- Jig borer
The jig borer is a type of machine tool invented at the end of World War I to make possible the quick-yet-very-precise location of hole centers. It was invented independently in the United States and Switzerland...
Vertical mills that are built to bore holes, and very light slot or face milling. They are typically bed mills with a long spindle throw. The beds are more accurate, and the handwheels are graduated down to .0001" for precise hole placement.
- Knee mill or knee-and-column mill refers to any milling machine whose x-y table rides up and down the column on a vertically adjustable knee. This includes Bridgeports.
- Planer-style mill Large mills built in the same configuration as planers
A planer is a type of metalworking machine tool that uses linear relative motion between the workpiece and a single-point cutting tool to machine a linear toolpath. Its cut is analogous to that of a lathe, except that it is linear instead of helical...
except with a milling spindle instead of a planing head. This term is growing dated as planers themselves are largely a thing of the past.
- Ram-type mill This can refer to any mill that has a cutting head mounted on a sliding ram. The spindle can be oriented either vertically or horizontally. In practice most mills with rams also involve swiveling ability, whether or not it is called "turret" mounting. The Bridgeport configuration can be classified as a vertical-head ram-type mill. Van Norman
The Van Norman Machine Tool Company was an American manufacturer of milling machines and other machine tools from late in the 19th century until the mid-1980s. The company was based in Springfield, Massachusetts, USA.-Early history:...
specialized in ram-type mills through most of the 20th century. Since the wide dissemination of CNC machines, ram-type mills are still made in the Bridgeport configuration (with either manual or CNC control), but the less common variations (such as were built by Van Norman, Index, and others) have died out, their work being done now by either Bridgeport-form mills or machining centers.
- Turret mill More commonly referred to as Bridgeport-type milling machines. The spindle can be aligned in many different positions for a very versatile, if somewhat less rigid machine.
Alternate terminology
A milling machine is often called a
mill by
machinistA machinist is a person who uses machine tools to make or modify parts, primarily metal parts, a process known as machining. This is accomplished by using machine tools to cut away excess material much as a woodcarver cuts away excess wood to produce his work. In addition to metal, the parts may...
s. The archaic term
miller was commonly used in the 19th and early 20th centuries.
Since the 1960s there has developed an overlap of usage between the terms
milling machine and
machining center. NC/CNC machining centers evolved from milling machines, which is why the terminology evolved gradually with considerable overlap that still persists. The distinction, when one is made, is that a machining center is a mill with features that pre-CNC mills never had, especially an automatic tool changer (ATC) that includes a tool magazine (carousel), and sometimes an automatic pallet changer (APC). In typical usage, all machining centers are mills, but not all mills are machining centers; only mills with ATCs are machining centers.
Computer numerical control
Most CNC milling machines (also called
machining centers) are computer controlled vertical mills with the ability to move the spindle vertically along the Z-axis. This extra degree of freedom permits their use in diesinking, engraving applications, and
2.5DIn machining, 2.5D refers to a surface which is a projection of a plane into 3rd dimension - although the object is 3-dimensional, there are no overhanging elements possible...
surfaces such as
reliefRelief is a sculptural technique. The term relief is from the Latin verb levo, to raise. To create a sculpture in relief is thus to give the impression that the sculpted material has been raised above the background plane...
sculptures. When combined with the use of
conicalA cone is an n-dimensional geometric shape that tapers smoothly from a base to a point called the apex or vertex. Formally, it is the solid figure formed by the locus of all straight line segments that join the apex to the base...
tools or a ball nose cutter, it also significantly improves milling precision without impacting speed, providing a cost-efficient alternative to most flat-surface hand-
engravingEngraving is the practice of incising a design on to a hard, usually flat surface, by cutting grooves into it. The result may be a decorated object in itself, as when silver, gold, steel, or glass are engraved, or may provide an intaglio printing plate, of copper or another metal, for printing...
work.
CNC machines can exist in virtually any of the forms of manual machinery, like horizontal mills. The most advanced CNC milling-machines, the
multiaxis machineMultiaxis machining is a manufacturing process, where computer numerically controlled tools that move in 4 or more ways are used to manufacture parts out of metal or other materials by milling away excess material, by water jet cutting or by laser cutting...
, add two more axes in addition to the three normal axes (XYZ). Horizontal milling machines also have a C or Q axis, allowing the horizontally mounted workpiece to be rotated, essentially allowing asymmetric and
eccentricIn mechanical engineering, an eccentric is a circular disk solidly fixed to a rotating axle with its centre offset from that of the axle ....
turningTurning is the process whereby a single point cutting tool is parallel to the surface. It can be done manually, in a traditional form of lathe, which frequently requires continuous supervision by the operator, or by using a computer controlled and automated lathe which does not. This type of...
. The fifth axis (B axis) controls the tilt of the tool itself. When all of these axes are used in conjunction with each other, extremely complicated geometries, even organic geometries such as a human head can be made with relative ease with these machines. But the skill to program such geometries is beyond that of most operators. Therefore, 5-axis milling machines are practically always programmed with
CAMComputer-aided manufacturing is the use of computer software to control machine tools and related machinery in the manufacturing of workpieces. This is not the only definition for CAM, but it is the most common; CAM may also refer to the use of a computer to assist in all operations of a...
.
With the declining price of computers and open source CNC software, the entry price of CNC machines has plummeted.
Tooling
The accessories and cutting tools used on machine tools (including milling machines) are referred to in aggregate by the
mass nounIn linguistics, a mass noun is a noun that refers to some entity as an undifferentiated unit rather than as something with discrete subsets. Non-count nouns are best identified by their syntactic properties, and especially in contrast with count nouns. The semantics of mass nouns are highly...
"tooling". There is a high degree of standardization of the tooling used with CNC milling machines, and a lesser degree with manual milling machines. To ease up the organization of the tooling in CNC production many companies us a
tool managementTool management is needed in the metalworking so that the information regarding the tools on hand, can be uniformly organized and integrated in the manufacturing environment and process flow. The information is stored in a database and is registered and applied using the tool management software...
solution.
Milling cutterMilling cutters are cutting tools typically used in milling machines or machining centres . They remove material by their movement within the machine or directly from the cutter's shape .-Features of a milling cutter:Milling cutters come in several shapes and many sizes...
s for specific applications are held in various tooling configurations.
CNC milling machines nearly always use SK (or ISO), CAT, BT or HSK tooling. SK tooling is the most common in Europe, while CAT tooling, sometimes called V-Flange Tooling, is the oldest and probably most common type in the USA. CAT tooling was invented by
Caterpillar Inc.Caterpillar Inc. , also known as "CAT", designs, manufactures, markets and sells machinery and engines and sells financial products and insurance to customers via a worldwide dealer network. Caterpillar is the world's largest manufacturer of construction and mining equipment, diesel and natural gas...
of
Peoria, IllinoisPeoria is the largest city on the Illinois River and the county seat of Peoria County, Illinois, in the United States. It is named after the Peoria tribe. As of the 2010 census, the city was the seventh-most populated in Illinois, with a population of 115,007, and is the third-most populated...
, in order to standardize the tooling used on their machinery. CAT tooling comes in a range of sizes designated as CAT-30, CAT-40, CAT-50, etc. The number refers to the
Association for Manufacturing TechnologyThe Association for Manufacturing Technology is a trade association based in McLean, Virginia in the United States founded as the National Machine Tool Builders Association in 1902...
(formerly the National Machine Tool Builders Association (NMTB))
Taper sizeA machine taper is a system for securing cutting bits and other accessories to a machine tool's spindle.- Explanation :Machine tool operators must be able to install or remove tool bits quickly and easily. A lathe, for example, has a rotating spindle in its headstock, to which one may want to mount...
of the tool.
An improvement on CAT Tooling is BT Tooling, which looks similar and can easily be confused with CAT tooling. Like CAT Tooling, BT Tooling comes in a range of sizes and uses the same NMTB body taper. However, BT tooling is symmetrical about the spindle axis, which CAT tooling is not. This gives BT tooling greater stability and balance at high speeds. One other subtle difference between these two toolholders is the thread used to hold the pull stud. CAT Tooling is all Imperial thread and BT Tooling is all Metric thread. Note that this affects the pull stud only, it does not affect the tool that they can hold, both types of tooling are sold to accept both Imperial and metric sized tools.
SK and HSK tooling, sometimes called "Hollow Shank Tooling", is much more common in Europe where it was invented than it is in the United States. It is claimed that HSK tooling is even better than BT Tooling at high speeds. The holding mechanism for HSK tooling is placed within the (hollow) body of the tool and, as spindle speed increases, it expands, gripping the tool more tightly with increasing spindle speed. There is no pull stud with this type of tooling.
For manual milling machines, there is less standardization, because a greater plurality of formerly competing standards exist. Newer and larger manual machines usually use NMTB tooling. This tooling is somewhat similar to CAT tooling but requires a
drawbarA drawbar or spindle drawbar is a clamping mechanism for toolholders on machine tools. The toolholder or machine taper itself is held by the drawbar and applies force to the spindle, especially when spinning at high speeds.-External links:*...
within the milling machine. Furthermore, there are a number of variations with NMTB tooling that make interchangeability troublesome. The older a machine, the greater the plurality of standards that may apply (e.g., Morse, Jarno, Brown & Sharpe, Van Norman, and other less common builder-specific tapers). However, two standards that have seen especially wide usage are the Morse #2 and the R8, whose prevalence was driven by the popularity of the mills built by Bridgeport Machines of
Bridgeport, ConnecticutBridgeport is the most populous city in the U.S. state of Connecticut. Located in Fairfield County, the city had an estimated population of 144,229 at the 2010 United States Census and is the core of the Greater Bridgeport area...
. These mills so dominated the market for such a long time that "Bridgeport" is virtually synonymous with "manual milling machine". Most of the machines that Bridgeport made between 1938 and 1965 used a Morse taper #2, and from about 1965 onward most used an R8 taper.
1810s-1830s
Milling machines evolved from the practice of rotary filing—that is, running a circular cutter with
fileA file is a metalworking and woodworking tool used to cut fine amounts of material from a workpiece. It most commonly refers to the hand tool style, which takes the form of a steel bar with a case hardened surface and a series of sharp, parallel teeth. Most files have a narrow, pointed tang at one...
-like teeth in the headstock of a
latheA lathe is a machine tool which rotates the workpiece on its axis to perform various operations such as cutting, sanding, knurling, drilling, or deformation with tools that are applied to the workpiece to create an object which has symmetry about an axis of rotation.Lathes are used in woodturning,...
. Rotary filing and, later, true milling were developed to reduce time and effort spent hand-filing. The full story of milling machine development may never be known, because much early development took place in individual shops where few records were kept for posterity. However, the broad outlines are known, as summarized below.
Rotary filing long predated milling. A rotary file by
Jacques de VaucansonJacques de Vaucanson was a French inventor and artist who was responsible for the creation of impressive and innovative automata and machines such as the first completely automated loom.-Early life:...
, circa 1760, is well known. It is clear that milling machines as a distinct class of machine tool (separate from lathes running rotary files) first appeared between 1814 and 1818. The centers of earliest development of true milling machines were two federal
armoriesAn armory or armoury is a place where arms and ammunition are made, maintained and repaired, stored, issued to authorized users, or any combination of those...
of the U.S. (
SpringfieldThe Springfield Armory, located in the City of Springfield, Massachusetts - from 1777 until its closing in 1968 - was the primary center for the manufacture of U.S. military firearms. After its controversial closing during the Vietnam War, the Springfield Armory was declared Western Massachusetts'...
and
Harpers FerryHarpers Ferry Armory, more formally known as the United States Armory and Arsenal at Harpers Ferry, was the second federal armory commissioned by the United States government located in Harpers Ferry, West Virginia , the first federal armory being the Springfield Armory located in Springfield,...
) together with the various private armories and
inside contractorsInside contracting is the practice of hiring contractors who work inside the proprietor's factory. It replaced the putting out system, where contractors worked in their own facilities. Inside contracting was the system favored by the Springfield and Harper's Ferry Armories...
that shared
turnoverIn a human resources context, turnover or staff turnover or labour turnover is the rate at which an employer gains and loses employees. Simple ways to describe it are "how long employees tend to stay" or "the rate of traffic through the revolving door." Turnover is measured for individual companies...
of skilled workmen with them.
Between 1912 and 1916, Joseph W. Roe, a respected founding father of machine tool historians, credited
Eli WhitneyEli Whitney was an American inventor best known for inventing the cotton gin. This was one of the key inventions of the Industrial Revolution and shaped the economy of the Antebellum South...
(one of the private arms makers mentioned above) with producing the first true milling machine. By 1918, he considered it "Probably the first milling machine ever built—certainly the oldest now in existence […]." However, subsequent scholars, including Robert S. Woodbury and others, have improved upon Roe's early version of the history and suggest that just as much credit—in fact, probably more—belongs to various other inventors, including Robert Johnson of
Middletown, ConnecticutMiddletown is a city located in Middlesex County, Connecticut, along the Connecticut River, in the central part of the state, 16 miles south of Hartford. In 1650, it was incorporated as a town under its original Indian name, Mattabeseck. It received its present name in 1653. In 1784, the central...
; Captain John H. Hall of the Harpers Ferry armory;
Simeon NorthSimeon North was a Middletown, Connecticut, gun manufacturer, who developed one of America's first milling machines in 1818 and played an important role in the development of interchangeable parts manufacturing.North was born in Berlin, Connecticut, into a prosperous family able to provide all...
of the Staddle Hill factory in Middletown; Roswell Lee of the Springfield armory; and
Thomas BlanchardThomas Blanchard was an American inventor who lived much of his life in Springfield, Massachusetts, where in 1819, he pioneered the assembly line style of mass production in America, and also invented the major technological innovation known as interchangeable parts. Blanchard worked, for much of...
. (Several of the men mentioned above are sometimes described on the internet as "the inventor of the first milling machine" or "the inventor of interchangeable parts". Such claims are oversimplified, as these technologies evolved over time among many people.)
Peter Baida, citing Edward A. Battison's article "Eli Whitney and the Milling Machine," which was published in the
Smithsonian Journal of History in 1966, exemplifies the dispelling of the "
Great ManThe Great Man Theory was a popular 19th century idea according to which history can be largely explained by the impact of "great men", or heroes: highly influential individuals who, due to either their personal charisma, intelligence, wisdom, or Machiavellianism utilized their power in a way that...
" image of Whitney by historians of technology working in the 1950s and 1960s. He quotes Battison as concluding that "There is no evidence that Whitney developed or used a true milling machine." Baida says, "The so-called Whitney machine of 1818 seems actually to have been made after Whitney's death in 1825." Baida cites Battison's suggestion that the first true milling machine was made not by Whitney, but by Robert Johnson of Middletown.
The late teens of the 19th century were a pivotal time in the history of machine tools, as the period of 1814 to 1818 is also the period during which several contemporary pioneers (
FoxJames Fox,fl 1780-1830, machine tool maker, was originally a butler in the service of the Rev. Thomas Gisborne, of Foxhall Lodge, Staffordshire. He had a strong interest in handicraft and his employer not only encouraged him, but enabled him to set up in business on his own account.The growth and...
,
MurrayMatthew Murray was an English steam engine and machine tool manufacturer, who designed and built the first commercially viable steam locomotive, the twin cylinder Salamanca in 1812...
, and
RobertsRichard Roberts was a British engineer whose development of high-precision machine tools contributed to the birth of production engineering and mass production.-Early life:...
) were developing the
planerA planer is a type of metalworking machine tool that uses linear relative motion between the workpiece and a single-point cutting tool to machine a linear toolpath. Its cut is analogous to that of a lathe, except that it is linear instead of helical...
, and as with the milling machine, the work being done in various shops was undocumented for various reasons (partially because of proprietary secrecy, and also simply because no one was taking down records for posterity).
James NasmythJames Hall Nasmyth was a Scottish engineer and inventor famous for his development of the steam hammer. He was the co-founder of Nasmyth, Gaskell and Company manufacturers of machine tools...
built a milling machine very advanced for its time between 1829 and 1831. It was tooled to mill the six sides of a hex nut that was mounted in a six-way
indexingIndexing in reference to motion is moving into a new position or location quickly and easily but also precisely. After a machine part has been indexed, its location is known to within a few hundredths of a millimeter , or often even to within a few thousandths of a millimeter , despite the fact...
fixture.
A milling machine built and used in the shop of Gay & Silver (aka Gay, Silver, & Co) in the 1830s was influential because it employed a better method of vertical positioning than earlier machines. For example, Whitney's machine (the one that Roe considered the very first) and others did not make provision for vertical travel of the knee. Evidently, the
workflowA workflow consists of a sequence of connected steps. It is a depiction of a sequence of operations, declared as work of a person, a group of persons, an organization of staff, or one or more simple or complex mechanisms. Workflow may be seen as any abstraction of real work...
assumption behind this was that the machine would be set up with shims, vise, etc. for a certain part design, and successive parts did not require vertical adjustment (or at most would need only shimming). This indicates that early thinking about milling machines was as production machines, not
toolroomThe term toolroom can refer to three related concepts. The concepts have evolved over the past two centuries as technology itself has evolved.- Storing tools :...
machines.
In these early years, milling was often viewed as only a roughing operation to be followed by finishing with a hand file. The idea of
reducing hand filing was more important than
replacing it.
1840s-1860
Some of the key men in milling machine development during this era included Frederick W. Howe,
Francis A. PrattFrancis Ashbury Pratt was a Connecticut mechanical engineer, inventor, and co-founder of Pratt & Whitney.Pratt was born in Peru, New York. In the early 1850s, he designed a milling machine for George S...
,
Elisha K. RootElisha K. Root was a Connecticut machinist and inventor.Root was born on a Massachusetts farm and worked as a bobbin boy in a cotton mill before switching, at the age of 15, to working in a machine shop in Ware, Massachusetts. At age 24 he was hired by Connecticut industrialist Samuel W...
, and others. (These same men during the same era were also busy developing the state of the art in
turret latheThe turret lathe is a form of metalworking lathe that is used for repetitive production of duplicate parts, which by the nature of their cutting process are usually interchangeable...
s. Howe's experience at Gay & Silver in the 1840s acquainted him with early versions of both machine tools. His machine tool designs were later built at Robbins & Lawrence, the Providence Tool Company, and
Brown & SharpeBrown & Sharpe is a division of Hexagon Metrology, Inc., a multinational corporation focused mainly on metrological tools and technology. During the 19th and 20th centuries, Brown & Sharpe was one of the most well-known and influential firms in the machine tool industry...
.) The most successful milling machine design to emerge during this era was the , which rather than being a specific make and model of machine tool is truly a family of tools built by various companies on a common configuration over several decades. It took its name from the first company to put one on the market, George S. Lincoln & Company (formerly the Phoenix Iron Works), whose first one was built in 1855 for the
ColtColt's Manufacturing Company is a United States firearms manufacturer, whose first predecessor corporation was founded in 1836 by Sam Colt. Colt is best known for the engineering, production, and marketing of firearms over the later half of the 19th and the 20th century...
armory.
During this era there was a continued blind spot in milling machine design, as various designers failed to develop a truly simple and effective means of providing slide travel in all three of the archetypal milling axes (X, Y, and Z—or as they were known in the past, longitudinal, traverse, and vertical). Vertical positioning ideas were either absent or underdeveloped. The Lincoln miller's spindle could be raised and lowered, but the original idea behind its positioning was to be set up in position and then run, as opposed to being moved frequently while running. Like a turret lathe, it was a repetitive-production machine, with each skilled setup followed by extensive fairly low skill operation.
1860s
In 1861, Frederick W. Howe, while working for the Providence Tool Company, asked Joseph R. Brown of
Brown & SharpeBrown & Sharpe is a division of Hexagon Metrology, Inc., a multinational corporation focused mainly on metrological tools and technology. During the 19th and 20th centuries, Brown & Sharpe was one of the most well-known and influential firms in the machine tool industry...
for a solution to the problem of milling spirals, such as the flutes of twist drills. These were usually filed by hand at the time. (Helical planing existed but was by no means common.) Brown designed a "universal milling machine" that, starting from its first sale in March 1862, was wildly successful. It solved the problem of 3-axis travel (i.e., the axes that we now call XYZ) much more elegantly than had been done in the past, and it allowed for the milling of spirals using an
indexing headAn indexing head, also known as a dividing head or spiral head, is a specialized tool that allows a workpiece to be circularly indexed; that is, easily and precisely rotated to preset angles or circular divisions...
fed in coordination with the table feed. The term "universal" was applied to it because it was ready for any kind of work, including toolroom work, and was not as limited in application as previous designs. (Howe had designed a "universal miller" in 1852, but Brown's of 1861 is the one considered a groundbreaking success.)
Brown also developed and patented (1864) the design of formed milling cutters in which successive sharpenings of the teeth do not disturb the geometry of the form.
The advances of the 1860s opened the floodgates and ushered in modern milling practice.
1870s to World War I
In these decades,
Brown & SharpeBrown & Sharpe is a division of Hexagon Metrology, Inc., a multinational corporation focused mainly on metrological tools and technology. During the 19th and 20th centuries, Brown & Sharpe was one of the most well-known and influential firms in the machine tool industry...
and the
Cincinnati Milling Machine CompanyThe Cincinnati Milling Machine Company is the name of a machine tool building company which was incorporated in 1889. The company was formed for the purpose of building and promoting an innovative new concept for a machine tool. The principals in forming the company were Frederick V. Geiger and...
dominated the milling machine field. However, hundreds of other firms also built milling machines at the time, and many were significant in various ways. Besides a wide variety of specialized production machines, the archetypal multipurpose milling machine of the late 19th and early 20th centuries was a heavy knee-and-column horizontal-spindle design with power table feeds, indexing head, and a stout overarm to support the arbor. The evolution of machine design was driven not only by inventive spirit but also by the constant evolution of milling cutters that saw milestone after milestone from 1860 through
World War IWorld War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
.
World War I and Interwar Period
Around the end of World War I, machine tool control advanced in various ways that laid the groundwork for later CNC technology. The
jig borerThe jig borer is a type of machine tool invented at the end of World War I to make possible the quick-yet-very-precise location of hole centers. It was invented independently in the United States and Switzerland...
popularized the ideas of coordinate dimensioning (dimensioning of all locations on the part from a single reference point); working routinely in "tenths" (ten-thousandths of an inch, 0.0001") as an everyday machine capability; and using the control to go straight from drawing to part, circumventing jig-making. In 1920 the new tracer design of J.C. Shaw was applied to Keller tracer milling machines for die-sinking via the three-dimensional copying of a template. This made diesinking faster and easier just as dies were in higher demand than ever before, and was very helpful for large steel dies such as those used to stamp sheets in automobile manufacturing. Such machines translated the tracer movements to input for
servosthumb|right|200px|Industrial servomotorThe grey/green cylinder is the [[Brush |brush-type]] [[DC motor]]. The black section at the bottom contains the [[Epicyclic gearing|planetary]] [[Reduction drive|reduction gear]], and the black object on top of the motor is the optical [[rotary encoder]] for...
that worked the machine leadscrews or hydraulics. They also spurred the development of
antibacklash leadscrew nutsIn mechanical engineering, backlash, sometimes called lash or play, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established...
. All of the above concepts were new in the 1920s but became routine in the NC/CNC era. By the 1930s, incredibly large and advanced milling machines existed, such as the Cincinnati Hydro-Tel, that presaged today's CNC mills in every respect except for CNC control itself.
Bridgeport milling machine
In 1936, Rudolph Bannow (1897–1962) conceived of a major improvement to the milling machine. His company commenced manufacturing a new knee-and-column vertical mill in 1938. This was the
Bridgeport milling machine, often called a ram-type or turret-type mill because its head has sliding-ram and rotating-turret mounting. The machine became so popular that many other manufacturers created copies and variants. Furthermore, its name came to connote any such
variantA genericized trademark is a trademark or brand name that has become the colloquial or generic description for, or synonymous with, a general class of product or service, rather than as an indicator of source or affiliation as intended by the trademark's holder...
. The Bridgeport offered enduring advantages over previous models. It was small enough, light enough, and affordable enough to be a practical acquisition for even the smallest machine shop businesses, yet it was also smartly designed, versatile, well-built, and rigid. Its various directions of sliding and pivoting movement allowed the head to approach the work from any angle. The Bridgeport's design became the dominant form for manual milling machines used by several generations of small- and medium-enterprise machinists. By the 1980s an estimated quarter-million Bridgeport milling machines had been built, and they (and their clones) are still being produced today.
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1940s-1970s
By 1940, automation via cams, such as in
screw machines and automatic chuckersAn automatic lathe is a lathe whose actions are controlled automatically. Although all electronically controlled lathes are automatic, they are usually not called by that name, as explained under "General nomenclature"...
, had already been very well developed for decades. Beginning in the 1930s, ideas involving
servomechanismthumb|right|200px|Industrial servomotorThe grey/green cylinder is the [[Brush |brush-type]] [[DC motor]]. The black section at the bottom contains the [[Epicyclic gearing|planetary]] [[Reduction drive|reduction gear]], and the black object on top of the motor is the optical [[rotary encoder]] for...
s had been in the air, but it was especially during and immediately after
World War IIWorld War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
that they began to germinate (see also Numerical control > History). These were soon combined with the emerging technology of digital
computerA computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s. This technological development milieu, spanning from the immediate pre–World War II period into the 1950s, was powered by the military capital expenditures that pursued contemporary advancements in the directing of gun and rocket artillery and in
missile guidanceMissile guidance refers to a variety of methods of guiding a missile or a guided bomb to its intended target. The missile's target accuracy is a critical factor for its effectiveness...
—other applications in which humans wished to control the
kinematicsIn physics and engineering, kinetics is a term for the branch of classical mechanics that is concerned with the relationship between the motion of bodies and its causes, namely forces and torques...
/
dynamicsIn classical mechanics, analytical dynamics, or more briefly dynamics, is concerned about the relationship between motion of bodies and its causes, namely the forces acting on the bodies and the properties of the bodies...
of large machines quickly, precisely, and automatically. Sufficient R&D spending probably would not have happened within the machine tool industry alone; but it was for the latter applications that the will and ability to spend was available. Once the development was underway, it was eagerly applied to machine tool control in one of the many post-WWII instances of
technology transferTechnology Transfer, also called Transfer of Technology and Technology Commercialisation, is the process of skill transferring, knowledge, technologies, methods of manufacturing, samples of manufacturing and facilities among governments or universities and other institutions to ensure that...
.
In 1952, numerical control reached the developmental stage of laboratory reality. The first NC machine tool was a Cincinnati Hydrotel milling machine retrofitted with a scratch-built NC control unit. It was reported in
Scientific American, just as another groundbreaking milling machine, the Brown & Sharpe universal, had been in 1862.
During the 1950s, numerical control moved slowly
from the laboratory into commercial serviceCommercialization is the process or cycle of introducing a new product or production method into the market. The actual launch of a new product is the final stage of new product development, and the one where the most money will have to be spent for advertising, sales promotion, and other marketing...
. For its first decade, it had rather limited impact outside of aerospace work. But during the 1960s and 1970s, NC evolved into CNC, data storage and input media evolved, computer processing power and memory capacity steadily increased, and NC and CNC machine tools gradually disseminated from an environment of huge corporations and mainly aerospace work to the level of medium-sized corporations and a wide variety of products. NC and CNC's drastic advancement of machine tool control deeply transformed the culture of manufacturing. The details (which are beyond the scope of this article) have evolved immensely with every passing decade.
1980s-present
Computers and CNC machine tools continue to develop rapidly. The
personal computerA personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
revolution has a great impact on this development. By the late 1980s small machine shops had desktop computers and CNC machine tools. After that hobbyists began obtaining CNC mills and lathes.
See also
- Arbor milling
Arbor milling is a cutting process which removes material via a multi-toothed cutter. An arbor mill is a type of milling machine characterized by its ability to rapidly remove material from a variety of materials, this milling process is not only rapid but also versatile.-Process...
- Cryomilling
- Milling cutter
Milling cutters are cutting tools typically used in milling machines or machining centres . They remove material by their movement within the machine or directly from the cutter's shape .-Features of a milling cutter:Milling cutters come in several shapes and many sizes...
- Multiaxis machining
Multiaxis machining is a manufacturing process, where computer numerically controlled tools that move in 4 or more ways are used to manufacture parts out of metal or other materials by milling away excess material, by water jet cutting or by laser cutting...
- Photo chemical milling
- Unimat
The Unimat was a commercially sold machine intended for machining and metalworking for model making hobbyists manufactured by the Emco company...
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