Milling machine
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A milling machine (also see synonyms below) is a machine tool used to machine solid materials. Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main spindle. 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 cutter, which cuts on its sides as well as its tip. Workpiece and cutter movement are precisely controlled to less than 0.001 in (0.025 mm), usually by means of precision ground slides and leadscrews 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 fluid is often pumped to the cutting site to cool and lubricate the cut and to wash away the resulting swarf.
Contents
Types and nomenclature
Basic nomenclature
A milling machine is often called a mill by machinists. The term miller also used to be common (19th and early 20th centuries), although it is typically not used today in reference to modern machines. (The term "miller" is one that people today are still familiar with from historical usage, but they generally don't use it anymore unless they are referring to machines built during the term's heyday, which is similar to the way that people today treat terms such as "motor car", "horseless carriage", or "phonograph".)
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.
Basic classification
There are many ways to classify milling machines, depending on criteria. The most important way, in some respects, is horizontal versus vertical, but 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 NC/CNC |
In the CNC era, a very basic distinction is manual versus CNC. Among manual machines, a worthwhile distinction is non-DRO-equipped versus DRO-equipped |
| Control (specifically among CNC machines) | Number of axes (e.g., 3-axis, 4-axis, or more); Within this scheme, also:
|
|
| Purpose | General-purpose versus special-purpose or single-purpose | |
| Purpose | Toolroom machine versus production machine | Overlaps with above |
| Purpose | "Plain" versus "universal" | A distinction whose meaning evolved over decades as technology progressed, and overlaps with other purpose classifications above; more historical interest than current |
| Size | Micro, mini, benchtop, standing on floor, large, very large, gigantic | |
| Power source | Line-shaft-drive 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 |
Comparing vertical with horizontal
In the vertical mill the spindle axis is vertically oriented. Milling cutters 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 bedmill and the turret mill. Turret mills, like the ubiquitous Bridgeport, are generally smaller than bedmills, and are considered by some to be more versatile. In a turret mill the spindle remains stationary during cutting operations and the table is moved both perpendicular to and parallel to the spindle axis to accomplish cutting. In the bedmill, however, the table moves only perpendicular to the spindle's axis, while the spindle itself moves parallel to its own axis. 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. These are frequently of lower quality than other types of machines, however.
A horizontal mill has the same sort of x–y table, but the cutters are mounted on a horizontal arbor (see Arbor milling) across the table. A majority of horizontal mills also feature a +15/-15 degree rotary table that allows milling at shallow angles. 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, 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 gears on a horizontal mill.
The vertical-vs-horizontal distinction seems trivial from some viewpoints; after all, changing the mounting of a machine part, accessory, or workpiece by 90° is often a straightforward matter. Yet the distinction has recurrently held more importance than one might expect, for similar reasons that the horizontal-lathe-vs-vertical-lathe distinction has mattered. The shape and size of workpieces and the number of sides that they require machining on can make one type of machine more practical than another.
In the pre-NC era, horizontal milling machines appeared 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. Work in which the spindle's axial movement is normal 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 most diesinking work has always favored a vertical mill. The heavier the workpiece, the more likely one is to want it to sit directly on the table rather than being mounted indirectly on an angle plate (or rotary table or indexing head perpendicular to the table), just as short, heavy workpieces are easier to set up on a vertical lathe or boring mill (and remove later) than on the headstock of a horizontal-axis lathe. 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.
More specific variants and nomenclature
- 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 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 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 form factor can be classified as a vertical-head ram-type mill. Van Norman specialized in ram-type mills through most of the 20th century. Since the wide dissemination of CNC machines, ram-type mills are still made on the Bridgeport form factor (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.
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 2.5D surfaces such as relief sculptures. When combined with the use of conical 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-engraving 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 machine, 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 eccentric turning. 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 CAM.
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 noun "tooling". There is a high degree of standardization of the tooling used with CNC milling machines, and a lesser degree with manual milling machines.
Milling cutters 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. of Peoria, Illinois, 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 Technology (formerly the National Machine Tool Builders Association (NMTB)) Taper size 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 drawbar 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, Connecticut. 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.
History
1810s-1830s
Milling machines evolved from the practice of rotary filing—that is, running a circular cutter with file-like teeth in the headstock of a lathe. 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. Rotary filing long predated milling. A rotary file by Jacques de Vaucanson, circa 1760, is well known.[1][2] 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. Between 1912 and 1916, Joseph W. Roe, a respected founding father of machine tool historians, credited Eli Whitney with producing the first true milling machine.[3][4] By 1918, he considered it "Probably the first milling machine ever built—certainly the oldest now in existence […]."[5] However, subsequent scholars, including Robert S. Woodbury[6] and others, suggest that just as much credit belongs to various other inventors, including Robert Johnson, Simeon North, Captain John H. Hall, and Thomas Blanchard. (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.) The two federal armories of the U.S. (Springfield and Harpers Ferry) and the various private armories and inside contractors that shared turnover of skilled workmen with them were the centers of earliest development of true milling machines (as distinct from lathe headstocks tooled up for rotary filing).
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 (Fox, Murray, and Roberts) were developing the planer, 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 Nasmyth 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 indexing 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 work flow 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 toolroom 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. Pratt, Elisha K. Root, and others. (These same men during the same era were also busy developing the state of the art in turret lathes. 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 & Sharpe.) The most successful milling machine design to emerge during this era was the Lincoln miller, 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 form factor over several decades. It took its name from the first company to put one on the market, George S. Lincoln & Company.
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 & Sharpe 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 (XYZ) travel much more elegantly than had been done in the past, and it allowed for the milling of spirals using an indexing head 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 sharpening 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 & Sharpe and the Cincinnati Milling Machine Company 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 I.[7][8]
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 borer 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 servos that worked the machine leadscrews or hydraulics. They also spurred the development of antibacklash leadscrew nuts. 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.
In 1938, a new knee-and-column vertical mill arrived on the market that would become so popular that its name would come to connote an entire form factor, and many companies would build clones. This was the famous Bridgeport, often called a ram-type or turret-type mill because its head has sliding-ram and rotating-turret mounting. The runaway success of the Bridgeport probably stemmed from a multitude of factors. 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. And the world at the time was ready for decades of "manual milling by the masses", as it were. For several generations of SME machinists, the Bridgeport's form factor has been the first thought in manual milling machines.
1940s-1970s
By 1940, automation via cams, such as in screw machines and automatic chuckers, had already been very well developed for decades. Beginning in the 1930s, ideas involving servomechanisms had been in the air, but it was especially during and immediately after World War II that they began to germinate (see also Numerical control > History). These were soon combined with the emerging technology of digital computers. 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 guidance—other applications in which humans wished to control the kinematics/dynamics 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 transfer.
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,[9] 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 service. 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.[10] 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 computer 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
References
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- Noble, David F. (1984), Forces of production: a social history of industrial automation, New York: Knopf, ISBN 978-0-394-51262-4, LCCN 83-048867.
- Roe, Joseph Wickham (1916), English and American Tool Builders, New Haven, Connecticut, USA: Yale University Press, LCCN 16-011753. Reprinted by McGraw-Hill, New York and London, 1926 (LCCN 27-024075); and by Lindsay Publications, Inc., Bradley, IL, USA (ISBN 978-0-917914-73-7).
- Roe, Joseph Wickham (1918), The mechanical equipment: Volume 3: Factory Management Course, 3, New York, NY, USA: Industrial Extension Institute, Inc, LCCN 18-006648.
- Woodbury, Robert S. (1972) [1960], History of the Milling Machine. In Studies in the History of Machine Tools, Cambridge, Massachusetts, USA, and London, England: MIT Press, ISBN 978-0-262-73033-4, LCCN 72-006354. First published alone as a monograph in 1960.
Further reading
- Hounshell, David A. (1984), From the American System to Mass Production, 1800-1932: The Development of Manufacturing Technology in the United States, Baltimore, Maryland, USA: Johns Hopkins University Press, ISBN 978-0-8018-2975-8, LCCN 83-016269.
- Rolt, L.T.C. (1965), A Short History of Machine Tools, Cambridge, Massachusetts, USA: MIT Press, LCCN 65-12439. Co-edition published as Rolt, L.T.C. (1965), Tools for the Job: a Short History of Machine Tools, London: B. T. Batsford, LCCN 65-080822.
External links
| 40x40px | Wikimedia Commons has media related to Milling machines. |
ar:فارزة cs:Frézka de:Fräsmaschine es:Fresadora eo:Frezmaŝino fa:فرزکاری fr:Fraiseuse gl:Fresadora it:Fresatrice he:כרסומת nl:Freesmachine (houtbewerking) ja:フライス盤 pl:Frezarka pt:Fresadora ru:Фрезерный станок sk:Frézovačka sv:Fräsmaskin uk:Фрезерна група верстатів
zh:铣床- ↑ Woodbury 1972, p. 23.
- ↑ Roe 1916, p. 206.
- ↑ Woodbury 1972, p. 17.
- ↑ Roe 1916, caption of figure facing p. 142.
- ↑ Roe 1918, p. 309.
- ↑ Woodbury 1972, p. 16-26.
- ↑ Woodbury 1972, pp. 51–55.
- ↑ Woodbury 1972, pp. 79–81.
- ↑ Pease, William (1952), "An automatic machine tool", Scientific American, 187 (3): 101–115, ISSN 0036-8733
- ↑ Noble 1984, throughout.
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