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Milling (machining)

Full view of a Bridgeport clone.

Milling is the machiningprocess of using rotary cutters to remove material^[1]from a workpiece by advancing (or feeding) in a direction at an angle with the axis of the tool.^[2][3] It covers a wide variety of different operations and machines, on scales from small individual parts to large, heavy-duty gang milling operations. It is one of the most commonly used processes in industry and machine shops today for machining parts to precise sizes and shapes.

Milling can be done with a wide range of machine tools. The original class of machine tools for milling was the milling machine (often called a mill). After the advent of computer numerical control (CNC), milling machines evolved into machining centers (milling machines with automatic tool changers, tool magazines or carousels, CNC control, coolant systems, and enclosures), generally classified as vertical machining centers (VMCs) and horizontal machining centers (HMCs). The integration of milling into turning environments and of turning into milling environments, begun with live tooling for lathes and the occasional use of mills for turning operations, led to a new class of machine tools, multitasking machines (MTMs), which are purpose-built to provide for a default machining strategy of using any combination of milling and turning within the same work envelope.

Contents

 [hide]

• 1Process
  • 1.1Milling cutters
  • 1.2Gang milling
• 2Equipment
  • 2.1Types and nomenclature
    • 2.1.1Mill orientation
      • 2.1.1.1Vertical mill
      • 2.1.1.2Horizontal milling machine
      • 2.1.1.3Comparative merits
    • 2.1.2Alternative classifications
    • 2.1.3Variants
    • 2.1.4Alternative terminology
  • 2.2Computer numerical control
  • 2.3Tooling
  • 2.4Accessories
• 3CNC pocket milling
  • 3.1Linear tool path
    • 3.1.1Zig-zag tool path
    • 3.1.2Zig tool path
  • 3.2Non-linear tool path
    • 3.2.1Contour-parallel tool path
    • 3.2.2Curvilinear tool path
• 4History
  • 4.11810s–1830s
  • 4.21840s–1860
  • 4.31860s
  • 4.41870s to World War I
  • 4.5World War I and interwar period
    • 4.5.1Bridgeport milling machine
  • 4.61940s–1970s
  • 4.71980s–present
• 5Milling standards
• 6See also
• 7References
  • 7.1Notes
  • 7.2Bibliography
  • 7.3Further reading

Process[edit]

Face milling process (cutter rotation axis is vertical)

Milling is a cutting process that uses a milling cutter to remove material from the surface of a workpiece. The milling cutter is a rotary cutting tool, often with multiple cutting points. As opposed to drilling, where the tool is advanced along its rotation axis, the cutter in milling is usually moved perpendicular to its axis so that cutting occurs on the circumference of the cutter. As the milling cutter enters the workpiece, the cutting edges (flutes or teeth) of the tool repeatedly cut into and exit from the material, shaving off chips (swarf) from the workpiece with each pass. The cutting action is shear deformation; material is pushed off the workpiece in tiny clumps that hang together to a greater or lesser extent (depending on the material) to form chips. This makes metal cutting somewhat different (in its mechanics) from slicing softer materials with a blade.

The milling process removes material by performing many separate, small cuts. This is accomplished by using a cutter with many teeth, spinning the cutter at high speed, or advancing the material through the cutter slowly; most often it is some combination of these three approaches.^[2] The speeds and feedsused are varied to suit a combination of variables. The speed at which the piece advances through the cutter is called feed rate, or just feed; it is most often measured in length of material per full revolution of the cutter.

There are two major classes of milling process:

• In face milling, the cutting action occurs primarily at the end corners of the milling cutter. Face milling is used to cut flat surfaces (faces) into the workpiece, or to cut flat-bottomed cavities.
• In peripheral milling, the cutting action occurs primarily along the circumference of the cutter, so that the cross section of the milled surface ends up receiving the shape of the cutter. In this case the blades of the cutter can be seen as scooping out material from the work piece. Peripheral milling is well suited to the cutting of deep slots, threads, and gear teeth.

Milling cutters[edit]

Main article: Milling cutter

Many different types of cutting tools are used in the milling process. Milling cutters such as endmills may have cutting surfaces across their entire end surface, so that they can be drilled into the workpiece (plunging). Milling cutters may also have extended cutting surfaces on their sides to allow for peripheral milling. Tools optimized for face milling tend to have only small cutters at their end corners.

The cutting surfaces of a milling cutter are generally made of a hard and temperature-resistant material, so that they wear slowly. A low cost cutter may have surfaces made of high speed steel. More expensive but slower-wearing materials include cemented carbide. Thin film coatings may be applied to decrease friction or further increase hardness.

They are cutting tools typically used in milling machines or machining centres to perform milling operations (and occasionally in other machine tools). They remove material by their movement within the machine (e.g., a ball nose mill) or directly from the cutter's shape (e.g., a form tool such as a hobbing cutter).

A diagram of revolution ridges on a surface milled by the side of the cutter, showing the position of the cutter for each cutting pass and how it corresponds with the ridges (cutter rotation axis is perpendicular to image plane)

As material passes through the cutting area of a milling machine, the blades of the cutter take swarfs of material at regular intervals. Surfaces cut by the side of the cutter (as in peripheral milling) therefore always contain regular ridges. The distance between ridges and the height of the ridges depend on the feed rate, number of cutting surfaces, the cutter diameter.^[4] With a narrow cutter and rapid feed rate, these revolution ridges can be significant variations in the surface finish.

Trochoidal marks, characteristic of face milling.

The face milling process can in principle produce very flat surfaces. However, in practice the result always shows visible trochoidalmarks following the motion of points on the cutter's end face. These revolution marks give the characteristic finish of a face milled surface. Revolution marks can have significant roughness depending on factors such as flatness of the cutter's end face and the degree of perpendicularity between the cutter's rotation axis and feed direction. Often a final pass with a slow feed rate is used to improve the surface finish after the bulk of the material has been removed.. In a precise face milling operation, the revolution marks will only be microscopic scratches due to imperfections in the cutting edge.

Gang milling[edit]

Heavy gang milling of milling machine tables

Gang milling refers to the use of two or more milling cutters mounted on the same arbor (that is, ganged) in a horizontal-milling setup. All of the cutters may perform the same type of operation, or each cutter may perform a different type of operation. For example, if several workpieces need a slot, a flat surface, and an angular groove, a good method to cut these (within a non-CNC context) would be gang milling. All the completed workpieces would be the same, and milling time per piece would be minimized.^[5]

Gang milling was especially important before the CNC era, because for duplicate part production, it was a substantial efficiency improvement over manual-milling one feature at an operation, then changing machines (or changing setup of the same machine) to cut the next op. Today, CNC mills with automatic tool change and 4- or 5-axis control obviate gang-milling practice to a large extent.

Surface grinding

Surface grinding is used to produce a smooth finish on flat surfaces. It is a widely used abrasive machining process in which a spinning wheel covered in rough particles (grinding wheel) cuts chips of metallic or nonmetallic substance from a workpiece, making a face of it flat or smooth.

Contents

 [hide]

• 1Process
• 2Equipment
  • 2.1Types of surface grinders
    • 2.1.1Horizontal-spindle (peripheral) surface grinders
    • 2.1.2Vertical-spindle (wheel-face) grinders
    • 2.1.3Disc grinders and double-disc grinders.^[3]
  • 2.2Grinding wheels for surface grinders
• 3Lubrication
• 4Effects on work material properties
• 5See also
• 6References
  • 6.1Bibliography

Process[edit]

Surface grinding is the most common of the grinding operations. It is a finishing process that uses a rotating abrasive wheel to smooth the flat surface of metallic or nonmetallic materials to give them a more refined look by removing the oxide layer and impurities on work piece surfaces. This will also attain a desired surface for a functional purpose.

The surface grinder is composed of an abrasive wheel, a workholding device known as a chuck, and a reciprocating or rotary table. The chuck holds the material in place while it is being worked on. It can do this one of two ways: ferromagnetic pieces are held in place by a magnetic chuck, while non-ferromagnetic and nonmetallic pieces are held in place by vacuum or mechanical means. A machine vise (made from ferromagnetic steel or cast iron) placed on the magnetic chuck can be used to hold non-ferromagnetic workpieces if only a magnetic chuck is available.

Factors to consider in surface grinding are the material of the grinding wheel and the material of the piece being worked on.

Typical workpiece materials include cast iron and mild steel. These two materials don't tend to clog the grinding wheel while being processed. Other materials are aluminum, stainless steel, brass and some plastics. When grinding at high temperatures, the material tends to become weakened and is more inclined to corrode. This can also result in a loss of magnetism in materials where this is applicable.

The grinding wheel is not limited to a cylindrical shape and can have a myriad of options that are useful in transferring different geometries to the object being worked on. Straight wheels can be dressed by the operator to produce custom geometries. When surface grinding an object, one must keep in mind that the shape of the wheel will be transferred to the material of the object like a reverse image.

Spark out is a term used when precision values are sought and literally means "until the sparks are out (no more)". It involves passing the workpiece under the wheel, without resetting the depth of cut, more than once and generally multiple times. This ensures that any inconsistencies in the machine or workpiece are eliminated.

Equipment[edit]

Surface Grinder with electromagnetic chuck, inset shows a Manual magnetic chuck

A surface grinder is a machine tool used to provide precision ground surfaces, either to a critical size or for the surface finish.

The typical precision of a surface grinder depends on the type and usage, however ±0.002 mm (±0.0001 in) should be achievable on most surface grinders.

The machine consists of a table that traverses both longitudinally and across the face of the wheel. The longitudinal feed is usually powered by hydraulics, as may the cross feed, however any mixture of hand, electrical or hydraulic may be used depending on the ultimate usage of the machine (i.e., production, workshop, cost). The grinding wheel rotates in the spindle head and is also adjustable for height, by any of the methods described previously. Modern surface grinders are semi-automated, depth of cut and spark-out may be preset as to the number of passes and, once set up, the machining process requires very little operator intervention.

Depending on the workpiece material, the work is generally held by the use of a magnetic chuck. This may be either an electromagnetic chuck, or a manually operated, permanent magnet type chuck; both types are shown in the first image.

The machine has provision for the application of coolant as well as the extraction of metal dust (metal and grinding particles).

Types of surface grinders[edit]

Horizontal-spindle (peripheral) surface grinders[edit]

The periphery (flat edge) of the wheel is in contact with the workpiece, producing the flat surface. Peripheral grinding is used in high-precision work on simple flat surfaces; tapers or angled surfaces; slots; flat surfaces next to shoulders; recessed surfaces; and profiles.^[1]

Vertical-spindle (wheel-face) grinders[edit]

The face of a wheel (cup, cylinder, disc, or segmental wheel) is used on the flat surface. Wheel-face grinding is often used for fast material removal, but some machines can accomplish high-precision work. The workpiece is held on a reciprocating table, which can be varied according to the task, or a rotary-table machine, with continuous or indexed rotation. Indexing allows loading or unloading one station while grinding operations are being performed on another.^[2]

Disc grinders and double-disc grinders.^[3][edit]

Disc grinding is similar to surface grinding, but with a larger contact area between disc and workpiece. Disc grinders are available in both vertical and horizontal spindle types. Double disc grinders work both sides of a workpiece simultaneously. Disc grinders are capable of achieving especially fine tolerances.^[2]

Grinding wheels for surface grinders[edit]

Main article: Grinding wheel

Aluminum oxide, silicon carbide, diamond, and cubic boron nitride (CBN) are four commonly used abrasive materials for the surface of the grinding wheels. Of these materials, aluminum oxide is the most common. Because of cost, diamond and CBN grinding wheels are generally made with a core of less expensive material surrounded by a layer of diamond or CBN. Diamond and CBN wheels are very hard and are capable of economically grinding materials, such as ceramics and carbides, that cannot be ground by aluminum oxide or silicon carbide wheels.

As with any grinding operation, the condition of the wheel is extremely important. Grinding dressers are used to maintain the condition of the wheel, these may be table mounted or mounted in the wheel head where they can be readily applied.

Lubrication[edit]

See also: Cutting fluid

Lubricants are sometimes used to cool the workpiece and wheel, lubricate the interface, and remove swarf (chips). It must be applied directly to the cutting area to ensure that the fluid is not carried away by the grinding wheel. Common lubricants include water-soluble chemical fluids, water-soluble oils, synthetic oils, and petroleum-based oils. The type of lubrication used depends on the workpiece material and is outlined in the table below.^[4]

Types of lubricants used for grinding based on workpiece material^[4]

Workpiece material	Lubricant
Aluminium	Heavy duty oil
Brass	Light duty oil
Cast iron	Heavy duty emulsifiable oil, light duty chemical and synthetic oil
Mild steel	Heavy duty water-soluble oil
Stainless steel	Heavy duty emulsifiable oil, heavy duty chemical and synthetic oil
Plastics	Water-soluble oil, dry, heavy duty emulsifiable oil, light duty chemical and synthetic oil

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 Switzerland and the United States.^[1] It resembles a specialized species of milling machine that provides tool and die makers with a higher degree of positioning precision (repeatability) and accuracy than those general machines had provided. Although capable of light milling, a jig borer is more suited to highly accurate drilling, boring, and reaming, where the quill or headstock does not see the significant side loading as it would with mill work. The result is a machine designed more for location accuracy than heavy material removal.

A typical jig borer has a work table of around 400 x 200 mm, (15.748 x 7.87402 inch) which can be moved using large handwheels (with micrometer-style readouts and verniers) on particularly carefully made shafts with a strong degree of gearing; this allowed positions to be set on the two axes to an accuracy of 0.0001 inch (2.5 micron). It was generally used to enlarge to a precise size smaller holes drilled with less accurate machinery in approximately the correct place (i.e., with the small hole strictly within the area to be bored out for the large hole).

Jig borers are limited to working materials that are still soft enough to be bored. Often a jig is hardened; for a jig borer this requires the material to be bored first and then hardened, which may introduce distortion. The jig grinder was developed as a machine with the precision of the jig borer, but capable of working materials in their hardened state.^[2]

History[edit]

Before the jig borer was developed, hole center location had been accomplished either with layout (either quickly-but-imprecisely or painstakingly-and-precisely) or with drill jigs(themselves made with painstaking-and-precise layout). The jig borer was invented to expedite the making of drill jigs, but it helped to eliminate the need for drill jigs entirely by making quick precision directly available for the parts that the jigs would have been created for.^[3] The revolutionary underlying principle was that advances in machine tool control that expedited the making of jigs were fundamentally a way to expedite the cutting process itself, for which the jig was just a means to an end. Thus the jig borer's development helped advance machine tool technology toward later NCand CNC development. The jig borer was a logical extension of manual machine tool technology that began to incorporate some then-novel concepts that would become routine with NC and CNC control, such as:

• 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 inch) as a fast, everyday machine capability (whereas it had been the exclusive domain of special, time-consuming, craftsman-dependent manual skills); and
• circumventing jigs altogether.

Franklin D. Jones, in his textbook Machine Shop Training Course (5th ed),^[4] noted:

"In many cases, a jig borer is a 'jig eliminator.' In other words, such a machine may be used instead of a jig either when the quantity of work is not large enough to warrant making a jig or when there is insufficient time for jig making."

Several innovations in the development of the jig borer were the work of the Moore Special Tool Company. In particular, the adoption of hardened and accurate leadscrews, formed by grinding, rather than a soft leadscrew with a compensating nut.^[2]

The technological advances that led to the jig borer and NC were about to usher in the age of CNC and CAD/CAM, radically changing the way people manufacture many of their goods.

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