Machining

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New Guinea in 1943. Mobile machine shop truck of the US Army with machinists working on automotive parts Mobile Machine Shop US Army 1943.jpg
New Guinea in 1943. Mobile machine shop truck of the US Army with machinists working on automotive parts

Machining is a process in which a material (often metal) is cut to a desired final shape and size by a controlled material-removal process. The processes that have this common theme are collectively called subtractive manufacturing, [1] in contrast to additive manufacturing, which uses controlled addition of material. Exactly what the "controlled" part of the definition implies can vary, but it usually implies the use of machine tools.

Contents

Machining is a part of the manufacture of many metal products, but it can also be used on other materials such as wood, plastic, ceramic, and composite material. [2] A person who specializes in machining is called a machinist. A room, building, or company where machining is done is called a machine shop. Much of modern-day machining is carried out by computer numerical control (CNC), in which computers are used to control the movement and operation of the mills, lathes, and other cutting machines. This increases efficiency, as the CNC machine runs unmanned therefore reducing labour costs for machine shops.

History and terminology

The precise meaning of the term machining has evolved over the past one and a half centuries as technology has advanced. In the 18th century, the word machinist simply meant a person who built or repaired machines. This person's work was done mostly by hand, using processes such as the carving of wood and the hand-forging and hand-filing of metal. At the time, millwrights and builders of new kinds of engines (meaning, more or less, machines of any kind), such as James Watt or John Wilkinson, would fit the definition. The noun machine tool and the verb to machine (machined, machining) did not yet exist.

Around the middle of the 20th century, the latter words were coined as the concepts that they described evolved into widespread existence. Therefore, during the Machine Age, machining referred to (what we today might call) the "traditional" machining processes, such as turning, boring, drilling, milling, broaching, sawing, shaping, planing, reaming, and tapping. [3] In these "traditional" or "conventional" machining processes, machine tools, such as lathes, milling machines, drill presses, or others, are used with a sharp cutting tool to remove material to achieve a desired geometry. [4]

Since the advent of new technologies in the post–World War II era, such as electrical discharge machining, electrochemical machining, electron beam machining, photochemical machining, and ultrasonic machining, the retronym "conventional machining" can be used to differentiate those classic technologies from the newer ones. In current usage, the term "machining" without qualification usually implies the traditional machining processes.

In the decades of the 2000s and 2010s, as additive manufacturing (AM) evolved beyond its earlier laboratory and rapid prototyping contexts and began to become common throughout all phases of manufacturing, the term subtractive manufacturing became common retronymously in logical contrast with AM, covering essentially any removal processes also previously covered by the term machining. The two terms are effectively synonymous, although the long-established usage of the term machining continues. This is comparable to the idea that the verb sense of contact evolved because of the proliferation of ways to contact someone (telephone, email, IM, SMS, and so on) but did not entirely replace the earlier terms such as call, talk to, or write to.

Machining operations

Making a shipboard manhole cover in the machine shop of the aircraft carrier USS John C. Stennis. US Navy 081008-N-9610C-039 Hull Technician 3rd Class Robert Paasch, from Parkdale, Ore., makes a shipboard manhole cover in the engineering department machine shop aboard the Nimitz-class aircraft carrier USS John C. Stennis (C.jpg
Making a shipboard manhole cover in the machine shop of the aircraft carrier USS John C. Stennis.

The three principal machining processes are classified as turning, drilling and milling. Other operations falling into miscellaneous categories include shaping, planing, boring, broaching and sawing. [5]

An unfinished workpiece requiring machining will need to have some material cut away to create a finished product. A finished product would be a workpiece that meets the specifications set out for that workpiece by engineering drawings or blueprints. For example, a workpiece may be required to have a specific outside diameter. A lathe is a machine tool that can be used to create that diameter by rotating a metal workpiece, so that a cutting tool can cut metal away, creating a smooth, round surface matching the required diameter and surface finish. A drill can be used to remove metal in the shape of a cylindrical hole. Other tools that may be used for various types of metal removal are milling machines, saws, and grinding machines. Many of these same techniques are used in woodworking.

More recent, advanced machining techniques include precision CNC machining, electrical discharge machining (EDM), electro-chemical machining (ECM), laser cutting, or water jet cutting to shape metal workpieces. [6]

As a commercial venture, machining is generally performed in a machine shop, which consists of one or more workrooms containing major machine tools. Although a machine shop can be a stand-alone operation, many businesses maintain internal machine shops which support specialized needs of the business.

Machining requires attention to many details for a workpiece to meet the specifications set out in the engineering drawings or blueprints. Beside the obvious problems related to correct dimensions, there is the problem of achieving the correct finish or surface smoothness on the workpiece. The inferior finish found on the machined surface of a workpiece may be caused by incorrect clamping, a dull tool, or inappropriate presentation of a tool. Frequently, this poor surface finish, known as chatter, is evident by an undulating or irregular finish, and the appearance of waves on the machined surfaces of the workpiece.

Basic machining process. Metal Cut diag.svg
Basic machining process.

Overview of machining technology

Machining is any process in which a cutting tool is used to remove small chips of material from the workpiece (the workpiece is often called the "work"). To perform the operation, relative motion is required between the tool and the work. This relative motion is achieved in most machining operation by means of a primary motion, called "cutting speed" and a secondary motion called "feed". [7] The shape of the tool and its penetration into the work surface, combined with these motions, produce the desired shape of the resulting work surface.

Machining operations

There are many kinds of machining operations, each of which is capable of generating a certain part geometry and surface texture.

In turning, a cutting tool with a single cutting edge is used to remove material from a rotating workpiece to generate a cylindrical shape. The primary motion is provided by rotating the workpiece, and the feed motion is achieved by moving the cutting tool slowly in a direction parallel to the axis of rotation of the workpiece.

Drilling is used to create a round hole. It is accomplished by a rotating tool that typically has two or four helical cutting edges. The tool is fed in a direction parallel to its axis of rotation into the workpiece to form the round hole.

In boring, a tool with a single bent pointed tip is advanced into a roughly made hole in a spinning workpiece to slightly enlarge the hole and improve its accuracy. It is a fine finishing operation used in the final stages of product manufacture.

Reaming is one of the sizing operations that removes a small amount of metal from a hole already drilled.

In milling, a rotating tool with multiple cutting edges is moved slowly relative to the material to generate a plane or straight surface. The direction of the feed motion is perpendicular to the tool's axis of rotation. The speed motion is provided by the rotating milling cutter. The two basic forms of milling are:

Other conventional machining operations include shaping, planing, broaching and sawing. Also, grinding and similar abrasive operations are often included within the category of machining.

Cutting tool

A "numerical controlled machining cell machinist" monitors a B-1B aircraft part being manufactured. B1 machining.jpg
A "numerical controlled machining cell machinist" monitors a B-1B aircraft part being manufactured.

A cutting tool has one or more sharp cutting edges and is made of a material that is harder than the work material. The cutting edge serves to separate chip from the parent work material. Connected to the cutting edge are the two surfaces of the tool:

The rake face which directs the flow of newly formed chip, is oriented at a certain angle is called the rake angle "α". It is measured relative to the plane perpendicular to the work surface. The rake angle can be positive or negative. The flank of the tool provides a clearance between the tool and the newly formed work surface, thus protecting the surface from abrasion, which would degrade the finish. This angle between the work surface and the flank surface is called the relief angle. There are two basic types of cutting tools:

A single point tool has one cutting edge and is used for turning, boring and planing. During machining, the point of the tool penetrates below the original work surface of the workpart. The point is sometimes rounded to a certain radius, called the nose radius.

Multiple-cutting-edge tools have more than one cutting edge and usually achieve their motion relative to the workpart by rotating. Drilling and milling use rotating multiple-cutting-edge tools. Although the shapes of these tools are different from a single-point tool, many elements of tool geometry are similar.

Cutting conditions

Relative motion is required between the tool and work to perform a machining operation. The primary motion is accomplished at a certain cutting speed. In addition, the tool must be moved laterally across the work. This is a much slower motion, called the feed. The remaining dimension of the cut is the penetration of the cutting tool below the original work surface, called the depth of cut. Collectively, speed, feed, and depth of cut are called the cutting conditions. [8] They form the three dimensions of the machining process, and for certain operations, their product can be used to obtain the material removal rate for the process:

where

Note: All units must be converted to the corresponding decimal (or USCU) units.

Stages in metal cutting

Machining operations usually divide into two categories, distinguished by purpose and cutting conditions:

Roughing cuts are used to remove large amount of material from the starting workpart as rapidly as possible, i.e. with a large Material Removal Rate (MRR), in order to produce a shape close to the desired form, but leaving some material on the piece for a subsequent finishing operation. Finishing cuts are used to complete the part and achieve the final dimension, tolerances, and surface finish. In production machining jobs, one or more roughing cuts are usually performed on the work, followed by one or two finishing cuts. Roughing operations are done at high feeds and depths – feeds of 0.4–1.25 mm/rev (0.015–0.050 in/rev) and depths of 2.5–20 mm (0.100–0.750 in) are typical, but actual values depend on the workpiece materials. Finishing operations are carried out at low feeds and depths – feeds of 0.0125–0.04 mm/rev (0.0005–0.0015 in/rev) and depths of 0.75–2.0 mm (0.030–0.075 in) are typical.[ citation needed ] Cutting speeds are lower in roughing than in finishing.

A cutting fluid is often applied to the machining operation to cool and lubricate the cutting tool. Determining whether a cutting fluid should be used, and, if so, choosing the proper cutting fluid, is usually included within the scope of cutting condition.

Today other forms of metal cutting are becoming increasingly popular. An example of this is water jet cutting. Water jet cutting involves pressurized water in excess of 620 MPa (90 000 psi) and is able to cut metal and have a finished product. This process is called cold cutting, which eliminates the damage caused by a heat affected zone, as opposed to laser and plasma cutting.

Relationship of subtractive and additive techniques

With the recent proliferation of additive manufacturing technologies, conventional machining has been retronymously classified, in thought and language, as a subtractive manufacturing method. In narrow contexts, additive and subtractive methods may compete with each other. In the broad context of entire industries, their relationship is complementary. Each method has its own advantages over the other. While additive manufacturing methods can produce very intricate prototype designs impossible to replicate by machining, strength and material selection may be limited. [9]

See also

Related Research Articles

Lathe Machine tool which rotates the work piece on its axis

A lathe is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, and turning, with tools that are applied to the workpiece to create an object with symmetry about that axis.

Machine tool Machine for handling or machining metal or other rigid materials

A machine tool is a machine for handling or machining metal or other rigid materials, usually by cutting, boring, grinding, shearing, or other forms of deformation. Machine tools employ some sort of tool that does the cutting or shaping. All machine tools have some means of constraining the workpiece and provide a guided movement of the parts of the machine. Thus the relative movement between the workpiece and the cutting tool is controlled or constrained by the machine to at least some extent, rather than being entirely "offhand" or "freehand". It is a power driven metal cutting machine which assists in managing the needed relative motion between cutting tool and the job that changes the size and shape of the job material.

Shaper Machine that uses linear motion to cut

A shaper is a type of 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 (archetypally) linear instead of helical.

Metalworking Process of making items from metal

Metalworking is the process of shaping and reshaping metals to create useful objects, parts, assemblies, and large scale structures. As a term it covers a wide and diverse range of processes, skills, and tools for producing objects on every scale: from huge ships, buildings, and bridges down to precise engine parts and delicate jewelry.

Machinist Technician

A machinist is a tradesperson or trained professional who not only operates machine tools but also has the knowledge of tooling and materials required to create set ups on machine tools such as milling machines, grinders, lathes, and drilling machines.

Broaching is a machining process that uses a toothed tool, called a broach, to remove material. There are two main types of broaching: linear and rotary. In linear broaching, which is the more common process, the broach is run linearly against a surface of the workpiece to effect the cut. Linear broaches are used in a broaching machine, which is also sometimes shortened to broach. In rotary broaching, the broach is rotated and pressed into the workpiece to cut an axisymmetric shape. A rotary broach is used in a lathe or screw machine. In both processes the cut is performed in one pass of the broach, which makes it very efficient.

Drilling Cutting process that uses a drill bit to cut a hole of circular cross-section in solid materials

Drilling is a cutting process that uses a drill bit to cut a hole of circular cross-section in solid materials. The drill bit is usually a rotary cutting tool, often multi-point. The bit is pressed against the work-piece and rotated at rates from hundreds to thousands of revolutions per minute. This forces the cutting edge against the work-piece, cutting off chips (swarf) from the hole as it is drilled.

Speeds and feeds Two separate velocities in machine tool practice, cutting speed and feed rate

The phrase speeds and feeds or feeds and speeds refers to two separate velocities in machine tool practice, cutting speed and feed rate. They are often considered as a pair because of their combined effect on the cutting process. Each, however, can also be considered and analyzed in its own right.

Countersink

A countersink is a conical hole cut into a manufactured object, or the cutter used to cut such a hole. A common use is to allow the head of a countersunk bolt, screw or rivet, when placed in the hole, to sit flush with or below the surface of the surrounding material. A countersink may also be used to remove the burr left from a drilling or tapping operation thereby improving the finish of the product and removing any hazardous sharp edges.

Tool and cutter grinder

A tool and cutter grinder is used to sharpen milling cutters and tool bits along with a host of other cutting tools.

Turning Machining process

Turning is a machining process in which a cutting tool, typically a non-rotary tool bit, describes a helix toolpath by moving more or less linearly while the workpiece rotates.

Milling cutters are cutting tools typically used in milling machines or machining centres to perform milling operations. They remove material by their movement within the machine or directly from the cutter's shape.

Metal lathe

A metal lathe or metalworking lathe is a large class of lathes designed for precisely machining relatively hard materials. They were originally designed to machine metals; however, with the advent of plastics and other materials, and with their inherent versatility, they are used in a wide range of applications, and a broad range of materials. In machining jargon, where the larger context is already understood, they are usually simply called lathes, or else referred to by more-specific subtype names. These rigid machine tools remove material from a rotating workpiece via the movements of various cutting tools, such as tool bits and drill bits.

Tipped tool

A tipped tool is any cutting tool in which the cutting edge consists of a separate piece of material that is brazed, welded, or clamped onto a body made of another material. In the types in which the cutter portion is an indexable part clamped by a screw, the cutters are called inserts. Tipped tools allow each part of the tool, the shank and the cutter(s), to be made of the material with the best properties for its job. Common materials for the cutters include cemented carbide, polycrystalline diamond, and cubic boron nitride. Tools that are commonly tipped include milling cutters, tool bits, router bits, and saw blades.

Boring (manufacturing)

In machining, boring is the process of enlarging a hole that has already been drilled by means of a single-point cutting tool, such as in boring a gun barrel or an engine cylinder. Boring is used to achieve greater accuracy of the diameter of a hole, and can be used to cut a tapered hole. Boring can be viewed as the internal-diameter counterpart to turning, which cuts external diameters.

Grinding (abrasive cutting)

Grinding is a type of abrasive machining process which uses grinding wheel as cutting tool.

Bar stock

Bar stock, also (colloquially) known as blank, slug or billet, is a common form of raw purified metal, used by industry to manufacture metal parts and products. Bar stock is available in a variety of extrusion shapes and lengths. The most common shapes are round, rectangular, square and hexagonal or hex. A bar is characterised by an "enclosed invariant convex cross-section", meaning that pipes, angle stock and objects with varying diameter are not considered bar stock.

Threading is the process of creating a screw thread. More screw threads are produced each year than any other machine element. There are many methods of generating threads, including subtractive methods ; deformative or transformative methods ; additive methods ; or combinations thereof.

Milling (machining) Removal of material from a workpiece using rotating tools

Milling is the process of machining using rotary cutters to remove material by advancing a cutter into a workpiece. This may be done varying direction on one or several axes, cutter head speed, and pressure. Milling 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 for machining custom parts to precise tolerances.

Workpiece Piece being processed into another shape

A workpiece is a piece, often made of a single material, that is being processed into another desired shape.

References

  1. "MAS.863/4.140J-P7". fab.cba.mit.edu. Massachusetts Institute of Technology. Retrieved 2016-08-22.
  2. "Machining Page". Archived from the original on 2018-09-08. Retrieved 2011-10-05.
  3. Machining: An Introduction
  4. Additive Manufacturing Advances Another Step
  5. Kalpakjian & Schmid (2008). Manufacturing Processes for Engineering Materials, 5th Ed. University of Notre Dame. p. 33 via https://web.archive.org/web/20160304045041/https://www3.nd.edu/~manufact/MPEM_pdf.html.
  6. "Precision CNC Machining". BMG. Retrieved 2016-08-22.
  7. "Machining". www.public.iastate.edu. Retrieved 2016-08-22.
  8. "Speeds and Feeds". staff.mica.edu. Retrieved 2016-08-22.
  9. ADDITIVE/SUBTRACTIVE MANUFACTURING RESEARCH

Bibliography

Further reading