Machining is a manufacturing process where a desired shape or part is created using the controlled removal of material, most often metal, from a larger piece of raw material bycutting. Machining is a form ofsubtractive manufacturing,[1] which utilizesmachine tools, in contrast toadditive manufacturing (e.g.3D printing), which uses controlled addition of material.
Machining is a major process of themanufacture of manymetal products, but it can also be used on other materials such aswood,plastic,ceramic, andcomposites.[2] A person who specializes in machining is called amachinist. As a commercial venture, machining is generally performed in amachine shop, which consists of one or more workrooms containing primary machine tools. Although a machine shop can be a standalone operation, many businesses maintain internal machine shops or tool rooms that support their specialized needs. Much modern-day machining usescomputer numerical control (CNC), in which computers control the movement and operation ofmills,lathes, and other cutting machines.
The precise meaning of the termmachining has changed over the past one and a half centuries as technology has advanced in a number of ways. In the 18th century, the wordmachinist meant a person who built or repairedmachines. This person's work was primarily done by hand, using processes such as thecarving of wood and the writing-forging and hand-filing of metal. At the time,millwrights and builders of new kinds ofengines (meaning, more or less, machines of any kind), such asJames Watt orJohn Wilkinson, would fit the definition. The nounmachine tool and the verbto machine (machined, machining) did not yet exist.[3]
Around the middle of the 20th century, the latter words were coined as the concepts they described evolved into widespread existence. Therefore, during theMachine Age,machining referred to (what we today might call) the "traditional" machining processes, such asturning,boring,drilling,milling,broaching,sawing,shaping,planing,abrasive cutting,reaming, andtapping.[4] In these "traditional" or "conventional" machining processes,machine tools, such aslathes,milling machines,drill presses, or others, are used with a sharpcutting tool to remove material to achieve a desired geometry.[5]
Since the advent of new technologies in the post–World War II era, such aselectrical discharge machining,electrochemical machining,electron beam machining,photochemical machining, andultrasonic machining, theretronym "conventional machining" can be used to differentiate those classic technologies from the newer ones. Currently, "machining" without qualification usually implies the traditional machining processes.
In the decades of the 2000s and 2010s, asadditive manufacturing (AM) evolved beyond its earlier laboratory and rapid prototyping contexts and began to become standard throughout all phases of manufacturing, the termsubtractive manufacturing became commonretronymously in logical contrast with AM, covering essentially any removal processes also previously covered by the termmachining. The two terms are effectivelysynonymous, although the long-established usage of the termmachining continues. This is comparable to the idea thatthe verb sense ofcontact 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 ascall,talk to, orwrite to.[6]
Machining is any process in which a cutting tool removes material from the workpiece (the workpiece is often called the "work"). Relative motion is required in traditional machining between the device and the work to remove material; non-traditional machining processes use other methods of material removal, such as electric current in EDM (electro-discharge machining). This relative motion is achieved in most machining operations by moving (by lateral rotary or lateral motion) either the tool, or the workpiece. The shape of the tool, the relative motion, and its penetration into the work, produce the desired shape of the resulting work surface.
Machining operations can be broken down into traditional, and non-traditional operations. Within the traditional operations, there are two categories of machining based on the shape they machine; being circular shapes that includes; turning, boring, drilling, reaming, threading and more, and various/straight shapes that includes; milling, broaching, sawing, grinding and shaping.
A cutting tool has one or more sharp cutting edges and is made of a harder material than the work material. The cutting edge serves to separate the 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 the newly formed chip, is oriented at a certain angle and 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 and flank surfaces is called the relief angle. There are two basic types of cutting tools:
A single-point tool has one cutting edge for turning, boring, and planing. During machining, the device's point penetrates below the work part's original work surface. The fact 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 work part by rotating. Drilling and milling use turning multiple-cutting-edge tools. Although the shapes of these tools are different from a single-point device, many elements of tool geometry are similar.
An unfinished workpiece requiring machining must 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 byengineering drawings orblueprints. For example, a workpiece may require a specific outside diameter. A lathe is a machine tool that can 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 remove the metal in the shape of a cylindrical hole. Other tools that may be used for metal removal are milling machines, saws, andgrinding machines. Many of these same techniques are used inwoodworking.
Machining requires attention to many details for a workpiece to meet the specifications in the engineering drawings or blueprints. Besides the obvious problems related to correct dimensions, there is the problem of achieving the right finish or surface smoothness on the workpiece. The inferior finish found on the machined surface of a workpiece may be caused by incorrectclamping, a dull tool, or inappropriate presentation of a device. Frequently, this poor surface finish, known as chatter, is evident by an undulating or regular finish of waves on the machined surfaces of the workpiece.
Relative motion is required between the tool and work to perform a machining operation. The primary action is at a specificcutting speed. In addition, the device 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, reaching the cut's depth. 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 thematerial removal rate for the process:
where
Machining operations usually divide into two categories, distinguished by purpose andcutting conditions:
Roughing cuts are used to remove a large amount of material from the starting work part as rapidly as possible, i.e., with a significant Material Removal Rate (MRR), to produce a shape close to the desired form but leaving some material on the piece for a subsequent finishing operation.Finishing cuts 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 – dinners 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.[9] Cutting speeds are lower in roughing than in finishing.
Acutting 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 the 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 over 620 MPa (90,000 psi) and can 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 andplasma cutting.
With the recent proliferation ofadditive manufacturing technologies, conventional machining has beenretronymously classified, in thought and language, as asubtractive 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 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.[10]
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