Machining

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machining

[mə′shēn·iŋ]
(mechanical engineering)
Performing various cutting or grinding operations on a piece of work.

Machining

 

industrial processes of metalworking by chip removal; performed by cutters on metalcutting machine tools to impart the required shape, dimensions, and surface finish to metal parts. The basic varieties of machining are turning, planing, drilling, reaming, broaching, milling and gear-cutting, grinding, and honing. The principles of machining are regarded as the result of interactions in the machine-attachment-tool-workpiece system.

All types of machining are characterized by the cutting rate, which is the sum of the following main elements: cutting speed ν, cutting depth t, and feed s. The cutting speed is the speed of the tool or workpiece in the direction of principal motion, which results in removal of a chip; the feed is the speed in the direction of feed motion. For example, in the case of turning (Figure 1), the cutting speed is the rate of displacement of the blank with respect to the cutting edge of the cutter (circumferential velocity) in m/min, and the feed is the displacement of the cutting edge of the tool during each revolution of the blank in mm per revolution. The cutting depth is the thickness (in mm) of the layer of metal removed per pass (the distance between the work surface and the machined surface, measured along the normal). The cutting elements (physical parameters) considered in the cross section of the layer of metal that is removed (see Figure 1) are the thickness and width of the layer being removed; their magnitude at constants t and s depends on the side cutting-edge angle ø

Figure 1. Elements of cutting rate in the case of turning: (1) work surface, (2) cutting surface, (3) machined surface, (D) diameter of blank being machined, (d) diameter of part after machining, (a) and (b) thickness and width of the layer being removed

Among the Russian and Soviet scientists who have made major contributions to the development of the foundations of the mechanics of cutting processes are I. A. Time, K. A. Zvorykin, A. A. Briks, A. V. Gadolin, la. G. Usachev, A. N. Cheliustkin, I. M. Besprozvannyi, G. I. Granovskii, A. M. Danielian, N. N. Zorev, A. I. Isaev, M. V. Kas’ian, A. I. Kashirin, V. A. Krivoukhov, V. D. Kuznetsov, M. N. Larin, T. N. Loladze, A. Ia. Malkin, A. V. Pankin, N. I. Reznikov, and A. M. Rozenberg. Important contributions have also been made by foreign scientists, among them Merchant and Ernst (USA); W. Degner, R. Reinhold, and N. Jakobs (German Democratic Republic); H. Opitz (Federal Republic of Germany); M. Okoshi (Japan); and E. Skřiván (Czechoslovakia). Important practical contributions have been made by the Soviet workers and innovators G. S. Bortkevich, P. B. Bykov, V. I. Zhirov, V. A. Karasev, V. A. Kolesov, S. I. Bushuev, E. I. Lebedev, and V. K. Seminskii.

The chip removed by the cutting tool (cutter, drill bit, broaching tool, or milling cutter) during machining may be of the shearing, continuous, or discontinuous type, depending on the cutting conditions. The type of chip formation and deformation of the metal is usually analyzed for specific cases, depending on the cutting conditions, and also on the chemical composition and physicochemical properties of the metal being machined, the cutting rate, the geometry of the cutting part of the tool, the orientation of the cutting edges relative to the cutting velocity vector, and the lubricant-coolant. The deformation of the metal in various zones of chip formation is different, and it also involves the surface layer of the part being machined, which therefore becomes work-hardened, and internal (residual) stresses arise in it. This affects the quality of the machined parts as a whole.

The transformation into heat of the mechanical energy expended during machining generates thermal focal points in the zones of deformation of the layer being removed, as well as in

Figure 2. Temperature field on the surfaces of a drill (the workpiece is made of no. 45 steel, and the drill is made of high-speed steel; ν = 25 m/sec, s = 0.11 mm per revolution; no cooling)

the zones of friction at points of contact of the tool bit with the chip and the part. The focal points affect the resistance of the cutting tool (the operating time to a specified degree of bluntness), as well as the quality of the surface layer of the machined part. A description of the temperature field in the cutting zone (Figure 2) may be obtained experimentally, by calculation, or by computer simulation of the cutting process.

Thermal phenomena in machining cause changes in the structure and physicochemical properties of both the layer of metal that is removed and of the surface layer of the machined part; changes also take place in the structure and hardness of the surface layers of the cutting tool. The process of heat generation also depends on the cutting conditions. The cutting speed and the properties of the machined metal exert a substantial effect on the cutting temperature in the zone of contact between the chip and the leading edge of the cutting tool (Figure 3). The thermal and temperature factors of machining processes are elucidated by experimental methods of calorimetry, by means of thermocouples, according to changes in the microstructure (for example, of the surface of the tool), and by using thermocolors, optical methods, and radiation techniques.

Figure 3. Effect of properties of metal being machined on the cutting temperature: (1) St. 3 steel, (2) 40Kh steel, (3) cast iron, (4) brass, (5) aluminum

Friction of the chip against the surfaces of the cutting tool, as well as thermal and electrical phenomena during machining, causes wear of the cutting tool. The following types of wear are distinguished: adhesive, abrasive-mechanical, abrasive-chemical, diffusion, and electrodiffusion. One of the main factors determining the selection of the optimum geometry of the cutting part of a metalcutting tool is the nature of wear of the tool. The criterion of wear used in selecting a cutting tool depends on the material of the cutting portion of the tool, as well as other cutting conditions. The nature of wear of the back surface of a cutter is shown in Figure 4. The cutter must be reground after the working time T2 at the wear level hopt (before the onset of critical wear hcrit, which corresponds to T3).

Figure 4. Nature of wear of the back surface of a cutting tool: (OA) running-in, (AB) wear during operation, (BC) catastrophic wear

The system of forces that act during machining may be reduced to a single resultant force. Although the solution of practical problems does not require knowledge of the magnitude of this force, the components of the force are important. They are the cutting force Pz acting in the plane of cutting in the direction of the principal motion; the radial component Py, acting perpendicular to the axis of the workpiece (in the case of turning) or to the axis of the tool (during drilling and milling); and the feed force Px, acting in the direction of feed. The forces Pz, Px, and Py affect the operating conditions of the machine tool, cutter, and holding device; the precision of cutting; and the roughness of the machined surface of the part. The magnitude of the forces is affected by the properties and structure of the material being machined, by the cutting rate, by the geometry and material of

the cutter, and by the cooling method. The force Pz is usually greatest in magnitude and requires the expenditure of the most energy to overcome it. The methods for the determination of Pz, Py, and Px may be theoretical or experimental. The determination is performed using special dynamometers. Empirical formulas based on experimental determinations are frequently used in practice. In most machining processes, the expended power (in kilowatts) is

Ne = Pz · v/60 · 102

where Pz is the component of the cutting force (in newtons, or kilograms-force) in the direction of feed, and ν is the cutting speed (in m/min); the power consumption of the electric motor of the machine tool is Nm = Ne/η, where η is the efficiency of the machine tool.

The cutting speed permitted by the cutting tool depends on the same factors as the cutting forces and is related in a complex manner to the tool’s resistance to wear (Figure 5).

Figure 5. Resistance of a cutting tool as a function of cutting speed (t = 1 mm; s = 0.1 mm per revolution)

Machining is influenced to a considerable degree by the active lubricant-coolant fluids, which, if correctly selected and properly supplied, are capable of increasing the resistance of the cutting tool, increasing the permissible cutting speed, improving the quality of the surface layer, and decreasing the roughness of the machined surfaces, particularly in the case of parts made from tough heat-resistant and refractory steels and alloys, which are difficult to machine. The forced oscillations (vibrations) of the machine-attachment-tool-workpiece system, as well as self-oscillations of the elements of the system, cause deterioration of the results of machining. Oscillations of either type may be reduced by influencing the factors that cause the oscillations, such as discontinuity of the cutting process, imbalance of rotating parts, defects in the transmission elements of the machine, and insufficient rigidity and deformations of the blank.

The efficiency of machining depends on the establishment of efficient cutting rates, taking all pertinent factors into account. Computers are used to speed up calculations. Computation of the cutting rates in computers consists of preliminary selection of input information, development and specification of algorithms, entry of the input information on data cards, coding of information, and programming of algorithms.

An increase in labor productivity and a reduction in metal losses (chips) during machining are associated with the increasing use of methods for the fabrication of blanks whose shape and dimensions are as close as possible to those of the finished articles. This leads to a sharp reduction in—or complete elimination of—preliminary (rough-working) operations and to a predominance of finishing operations in machining.

Future trends in machining include intensification of cutting processes, introduction of techniques for the processing of new materials, increase of precision and quality of finishing, use of hardening processes, and automation and mechanization.

REFERENCES

Besprozvannyi, I. M. Osnovy teorii rezaniia metallov. Moscow, 1948.
Russkie uchenye—osnovopolozhniki nauki o rezanii metallov: I. A. Time, K. A. Zwrykin, la. G. Usachev, A. N. Cheliustkin. Zhizn’, deiatel’nost’ i izbrannye trudy. Moscow, 1952.
Rezanie metallov. Moscow, 1954.
Avakov, A. A. Fizicheskie osnovy teorii stoikosti rezhushchikh instrumentov. Moscow, 1960.
Pankin, A. V. Obrabotka metallov rezaniem. Moscow, 1961.
Razvitie nauki o rezanii metallov. Moscow, 1967.
Elektricheskie iavleniia pri trenii i rezanii metallov. Moscow, 1969.
Briukhov, V. A., and E. N. Pavlov. Raschet rezhimov rezaniia i normirovanie spomoshch’iu EVM. Moscow, 1969.
Roman, O. V., A. A. Leventsov, and I. F. Shelkovskii. Obrabotka metallov rezaniem i stanki. Minsk, 1970.

D. L. IUDIN

Machining

An operation that changes the shape, surface finish, or mechanical properties of a material by the application of special tools and equipment. Machining almost always is a process where a cutting tool removes material to effect the desired change in the workpiece. Typically, powered machinery is required to operate the cutting tools. See Production methods

Although various machining operations may appear to be very different, most are very similar: they make chips. These chips vary in size from the long continuous ribbons produced on a lathe to the microfine sludge produced by lapping or grinding. These chips are formed by shearing away the workpiece material by the action of a cutting tool. Cylindrical holes can be produced in a workpiece by drilling, milling, reaming, turning, and electric discharge machining. Rectangular (or nonround) holes and slots may be produced by broaching, electric discharge machining, milling, grinding, and nibbling. Cylinders may be produced on lathes and grinders. Special geometries, such as threads and gears, are produced with special tooling and equipment utilizing the turning and grinding processes mentioned above. Polishing, lapping, and buffing are variants of grinding where a very small amount of stock is removed from the workpiece to produce a high-quality surface.

In almost every case, machining accuracy, economics, and production rates are controlled by the careful evaluation and selection of tooling and equipment. Speed of cut, depth of cut, cutting-tool material selection, and machine-tool selection have a tremendous impact on machining. In general, the more rigid and vibration-free a machining tool is, the better it will perform. Jigs and fixtures are often used to support the work-piece. Since it relies on the plastic deformation and shearing of the workpiece by the cutting tool, machining generates heat that must be dissipated before it damages the workpiece or tooling. Coolants, which also acts as lubricants, are often used.

To increase the life and speed of cutting tools, they are often coated with a thin layer of extremely hard material such as titanium nitride or zirconium nitride. These materials, which are applied over the cutting edges, provide excellent wear resistance. They are also brittle, so they rely on the toughness of the underlying cutting tool to support them. Coated tools are more expensive than conventional tools, but they can often cut at much higher rates and last significantly longer. When used properly on sufficiently rigid machine tools, they are far more economical than conventional tooling. See Metal coatings