Metalcutting Machine Tool
Metalcutting Machine Tool
a machine for working metal and other materials, semifinished products, and blanks by cutting off chips with cutting tools to produce finished articles.
Metalcutting machine tools are the main type of equipment in machine building and instrument-making. The improvement of metalcutting machine tools facilitates scientific and technical progress, as well as the development of the technology and organization of machine-building production.
Historical survey The machining of metals has been known since ancient times: articles were rotated manually and were machined by a flint cutter. Hand-driven turning lathes and drills appeared in the 12th century, and machines powered by water-wheels, in the 14th century. Mechanical lathes were produced mainly in Italy and France, from which they were imported to Russia. The St. Petersburg master craftsmen were famous for their medal lathes. A lathe made by the master craftsman Singer was brought to Russia in 1711 from Florence. Singer was invited into Russian service by Peter I. The court machine shop produced lathes according to designs prepared with the participation of A. K. Nartov, who also contributed to their production. Nartov later built other lathes, such as engraving and copying lathes, and also guillotine machines, and he was the first to make a screw-cutting lathe with a mechanical scale and interchangeable gears (1738). The main industrial types of metalcutting machine tools were developed later (H. Maudslay and others) in Great Britain, which was the first country to enter the path of capitalist development. The designs were further improved in Germany, France, and Switzerland (precision lathe building) and later (in the second half of the 19th century) in the USA (in particular, automatic lathes for mass production). In 1712–14, at the Tula Firearms Factory, the gunsmith la. Batishchev created a prototype of the modern standard unit lathe for the drilling of 24 gun barrels simultaneously. V. I. Gennin built a multiposition lathe at the Olonetsk Works in 1714. A considerable contribution to the design of metalcutting machine tools was made by M. V. Lomonosov, who in the mid-18th century designed and used original grinders in his shops. Contributions to the generation of new designs of lathes were also made by the Russian engineers and inventors I. Osipov, M. Sidorov, I. Polzunov, I. Kulibin, P. Zakhavo (the first automatic thread-cutting lathes, 1810), V. Ignatov, and G. Gorokhov. However, in spite of a certain number of outstanding inventions, the development of machine-tool building in tsarist Russia proceeded slowly.
Machine-building enterprises began receiving new machine tools only after the Great October Socialist Revolution, during the process of industrialization. The Krasnyi Proletarii Plant produced the first modern thread-cutting machines in 1932. The Experimental Scientific Research Institute of Metalcutting Machine Tools (ENIMS), where the preliminary design of new types of machines and the fabrication of assortments of turning lathes, turret lathes, drills, milling machines, and other types of metalcutting machines were begun, was founded in 1933. As of 1970, 1,817 types and sizes of metalcutting machine tools had been adopted in the USSR. Annual production was 230,000 units.
Valuable contributions to machine-tool construction in the USSR have been made by the Soviet scientists V. I. Dikushin, N. S. Acherkan, D. N. Reshetov, A. P. Vladzievskii, B. S. Balakshin, G. M. Golovin, G. A. Shaumian, V. S. Vasiliev, A. S. Pronikov, V. A. Kudinov, A. S. Britkin, and B. L. Boguslavskii and the designers N. A. Volchek, V. N. Kedrinskii, I. A. Rostovtsev, and lu. B. Erpsher.
The improvement of metalcutting machine-tool production is proceeding along several paths. An increase is becoming apparent in the production of unitized automatic and semiautomatic metalcutting machine tools, which are used to automate industrial processes in large series and mass production (the production of such metalcutting machine tools in the USSR increased by 22.6 percent from 1966 to 1970, as against an overall increase of 12 percent in the production of metalcutting machine tools). Production in 1973 was 211,000 units. The adoption of precision machine tools, which provide high precision of working and high quality of products, is promising.
A further increase is planned in the production of metalcutting machine tools with numerical control to achieve automation of the mechanical processing of parts in individual and series production. In 1968–70, 23 standard types and sizes of these machines were adopted in series production; in 1970, 15 experimental prototypes were adopted. Production of such machines
|Table 1. Classification of metalcutting machine tools|
|2Automatic and semiautomatic 2Semiautomatic 3Planing 4Cutting-off lathes|
|Number of group||Name of group||1||2||3||4||5||6||7||8||9|
|1||Lathes||Single-spindle1||Multiple1||Turret||Cutting off/drilling||Vertical||Screw-cutting; facing||Multiple-cutter||Specialized for profiles||Various lathes|
|2||Drilling and boring||Upright drilling||Single-spindle2||Multiple2||Jig-boring||Radial drilling||Boring||Diamond boring||Horizontal drilling||Various drilling machines|
|3||Grinding and honding machine||Circular grinding||Internal grinding||Rough working grinding||Specialized grinding||—||Tool grinding||Face grinding||Lapping and polishing||Various abserve machines|
|5||Gear-and thread-cutting machine||Gear planet for spur gears||Gear-cutting for bevel machines||Gear-milling for spur gears and splined shafts||Gearmilling for worm gears||For gear and face processing||Thread milling||Gear finishing||Gear- and thread-grinding||Various gear-and cutting machines|
|6||Milling machines||Upright bracket milling machines||Continuous milling machines||—||Copying and engraving machines||Vertical without brackets||plane milling machines||All purposes||Horizontal bracket machines||various milling machines|
|7||Planers, slotters, broaches||Single-pedestal3||Double-pedestal3||Shaping||Slotting||Horizontal broaching||—||Vertical broaching||—||Various planing machines|
|8||Cutting machines||With lathe cutting tool4||With abrasive disk4||With smooth or notched disk4||Straight cutting4||Band saws||Disk saws||Blade saws||—||—|
|9||Various||Sleeve and pipe processing||Saw hatching||Straight and off center rough working||Balancing||For testing tools||Dividing machines||—||—||—|
in 1973 was 3,800 units. The introduction of metalcutting machine tools with adaptive control systems opens new ways for increasing productivity and precision of processing. To satisfy the requirements of the national economy, an increase in the number of heavy unique machines is planned. About 500 such machines had been produced as of 1970.
Classification Metalcutting machine tools are classified according to specialization as universal tools, for the performance of a variety of operations on a wide range of products; multipurpose tools, for the performance of a limited number of operations on a wide range of articles; special-purpose tools for processing articles of the same type and various sizes; special-purpose tools for processing articles of the same type and size; and standard-unit tools, which are special types consisting of standardized parts, elements, and power packs.
Metalcutting machine tools may be manually controlled (loading and placement of blanks, starting, changing the mode of operation, return stroke, and removal of the article are performed manually), or they may have various degrees of automation: semiautomatic machines (manual placement of blanks, starting, and removal of the article; remaining parts of the production cycle are automatic); and automatic machines (automatic performance of all working and return strokes, with human monitoring of the production cycle). Metalcutting machine tools may be combined into automatic lines (groups of automatic lathes connected by a common system for the transport of blanks between them). They may be equipped with numerical control, in which case all working and return strokes are achieved through the use of a previously prepared coded program, which is fed into the machines and sends transformed pulses to the actuating and controlling mechanisms. Five classes of metalcutting machine tools are distinguished according to precision: N—normal precision (for example, most universal tools); P—increased precison (based on N); H—high precision; A—superhigh precision; and S—highest precision, or master machines.
Metalcutting machine tools are classified according to weight as light (up to 1 ton), medium-weight (up to 10 tons), heavy (more than 10 tons) and unique (more than 100 tons).
A uniform system of classification and conventional nomenclature for metalcutting machine tools (see Table 2), developed at ENIMS and based on the nature of the processing to be performed and the type of cutting tool used, has been adopted in the USSR. All metalcutting machine tools are divided into groups, which are subdivided into types. According to this classification, each series-produced metalcutting machine tool carries a code (index), which usually consists of a three- or four-digit number. The first digit indicates the group, the second indicates the type, and the third and fourth characterize the most important dimensions of the tool or the articles worked on it. For example, the code 2150 denotes an upright drilling machine with a maximum drilling diameter of 50 mm. A letter is inserted after the first digit if the tool has been modernized. For example, the code 1K62 denotes a modernized screw-cutting lathe with a center height 200 mm. Modifications of the basic model are denoted by a letter at the end of the code. For example, 6N12K stands for the modification of a modernized upright bracket milling machine.
Kinematics During processing on a metalcutting machine tool, the outline and shape of the part (generatrices) are produced as a result of correlated rotary and reciprocating motions of the blank and the cutting edge of the metalcutting tool. The motions, called the working motions (strokes), may be simple or complex. Four methods are used in metalcutting machine tools for the production of generatrices: copying, generating, and the tracing and contact methods. During copying, the shape of the cutting edge of the instrument coincides with the shape of the generatrix (Figure 1, a and b). During generating, the generatrix has the form of the envelope of a series of successive positions of the edge of the cutting instrument, which moves with respect to the blank (Figure l,c). In the tracing method, the generatrix is the track of motion of the cutting edge of the tool (Figure 1, d and e). In the contact method, the generatrix is tangent to a series of geometric auxiliary lines formed by a real point (the apex) of the moving cutting edge of the tool (Figure l,f).
The working motions of metalcutting machine tools are the principal and feed motions. The principal motion, which is in the direction of the vector of the cutting speed, results in the separation of the chip from the blank. The feed motion consists of successive penetrations of the tool into the blank—that is, the “bite” into new, as yet untouched areas. Depending on the type of tool, the principal motion may be performed by the blank (turning lathes and planers) or the tool (drills, shapers, slotters, broaches, milling machines, and grinders). The motion may be rotary (turning lathes, grinders, drills, and milling machines) or reciprocating (planers, slotters, and broaches). In addition to the working motions, metalcutting machine tools also perform setup and indexing motions, which are not used in the cutting process but are nevertheless required for performance of the complete processing cycle.
All motions in a metalcutting machine tool are produced by the corresponding mechanisms, which use various types of trans-missions, such as belt, gear, worm-gear, rack-and-gear, screwgear, cam, and friction types. The transmissions are coupled in a particular sequence, forming kinematic chains, which together make up the kinematic scheme of the tool. Conventional designations of the elements and mechanisms based on All-Union State Standard (GOST) 3462–61 are used. Kinematic diagrams show the diameters of the pulleys (D1, D2, and so on), the number of teeth on gears and worm gears (Z1, Z2, and so on), the pitch of screw threads, the number of starts of worm gears and screws, moduli (m) of some gears (usually those engaged with racks), transmission ratios of levers, and characteristics of the adjusting elements.
The cutting speed of machines with rotary principal motion is determined from the equation
V = (πDn)/l,000m/min
where D is the maximum diameter of machining (or the maximum tool diameter) and n is the number of revolutions of the spindle per minute. In a particular machine, the blank or tool may be of various diameters, and blanks of various materials may be processed using tools with cutting elements made of various tool materials (which leads to the selection of the corresponding permissible cutting speeds). Therefore, the drive for the principal motion must be capable of controlling the speed of revolution of the spindle. The control may be continuous or stepped. In the former case, any value of n may be produced within a certain interval by using friction, hydraulic, or electric drive. In the latter case there is a fixed, finite series of values of n. This is achieved by the use of transmissions with switchable gears.
In 1876 the Russian scientist A. V. Gadolin developed and substantiated the theory for the construction of series of numbers of revolutions according to the law of geometric progression. Such a relationship leads to minimum losses in the cutting speeds being adjusted and to the best performance characteristics of the machine. According to this law, all numbers of revolutions of the spindle of the machine per minute make up a geometric series, from the initial (minimum) n1 = nmin min to the final (maximum) nz =nmax in which the denominator ф in the geometric progression is determined from the equation
|Table 2. Standard values of speed change (A) in Soviet machine tool industry|
|Note: Permissible rounded-off values are given in the second row|
where D is the range of control of the number of revolutions of the spindle per minute and z is the number of control steps.
The values of ф and the corresponding speed changes A have been standardized in Soviet machine-tool building (see Table 2).
The basic characteristic of any kinematic chain is the final gear ratio:
Ufin = nf/ni = U1 . U2 . U3...
where nf and ni, are the speeds of rotation of the final and initial elements, respectively (in rpm) and U1, U2, and U3, are the gear ratios of the individual pairs of the links in the kinematic chain. The value of Ufin makes possible determination of the values of the final displacements of the links connected by the kinematic chain (that is, the blank and the cutting tool). The corresponding functional relationships are called the equations of kinematic balance. These equations were derived in the 1920’s and 1930’s by the Soviet scientist G. M. Golovin, who proposed unified tooling formulas for all machines.
The kinematic balance equation for rotating final links is nf = ni . Ufin The corresponding equation for the rotating initial link and the reciprocating final link is ni. Ufin. H = Sm mm/min, 1 rev. Ufin. H = s mm/rot, where H is the length of travel of the kinematic pair that converts the rotary motion into reciprocating motion and is equal to the displacement of the link moving in a straight line during a single revolution of the rotating link (for turning lathes, milling machines, and other machines of this type).
In the case of metalcutting machine tools with rectilinear principal motion (planers, slotters, and broaches), a distinction is made between the working stroke, during which cutting takes place, and the idle (return) stroke, during which the moving components of the machine return to the initial position. The speed of the return stroke Vr is equal to Vw.X, where Vw is the speed of the working stroke, and X = 1.5–2.5 is a coefficient selected as a function of the type and size of the machine.
The working and return strokes make up a double cycle, the length of which is
where L is the length of the stroke (in mm). The number of double cycles per minute is n = 1/T.
For a turning lathe with a simple kinematic diagram of a stepped main drive (Figure 2), the following numbers of revolutions of the spindle per minute are possible according to the equation of the kinematic balance:
Thus, 12 speeds are possible (n is a coefficient that takes into account slippage in the belt transmission).
A graphic-analytic method is used to facilitate the kinematic calculation of transmissions. The dependence of the speeds of revolution on the gear ratios is represented in the form of graphs and structural grids.
Design features All kinematic chains and working elements of metalcutting machine tools are made in the form of structural assemblies (mechanisms) consisting of various parts. The units and parts of metalcutting machine tools may be divided into two groups. The mounting and guide system provides the proper rectilinear and circular motions of the assemblies holding the parts and cutting tools. This group includes frames and bases, parts and assemblies for mounting and for providing rectilinear motion of articles (arms, table slides, and tables), components and assemblies for supporting and providing rectilinear and rocking motion of cutting tools (rests, slides and crosspieces of supports, and turrets), parts and assemblies that provide rotation of articles and cutting tools (spindles, spindle bearings, chucks, rotating columns, and tailstocks), and parts and assemblies for support and orientation of rotating components (housings of transmissions, feed boxes, and spindle heads).
The drive and control group performs the shaping and control operations. This group includes mechanisms for the principal, feed, and indexing motions; mechanisms for auxiliary motions (transporting, clamping, positioning, and chip removal); control mechanisms (starting and stopping and control of speed and reversal of uniform motions); and copying, programming, adaptive, and self-adjusting systems. The design of metalcutting machine tools of various types may be very diverse according to the classification given previously.
Trends in the development of designs for assemblies of metal-cutting machine tools include optimum use of the capabilities of mechanical, electrical, and hydraulic drives and their combinations, development of precision assemblies and mechanisms; reduction of friction, use of control and automation devices, achievement of high static and dynamic rigidity, increase in service life through the selection of optimum materials and methods for the hardening of parts, and the use of standardization and unitizing.
Reliability The reliability of a metalcutting machine tool is its capability to perform the required functions—that is, to process articles—with preservation of performance characteristics within the required limits (mainly precision and productivity) during the required time span (running time). Reliability is determined by trouble-free operation, service life, ease of repair, and retention of quality.
The reliability of metalcutting machine tools is influenced, above all, by the modes and methods of processing, which determine the precision and quality of the processed surfaces and, therefore, the performance characteristics of the products. An increase in reliability is achieved through an increase in the precision of manufacture, the design of special devices to increase the precision of processing, and the use of automatic control systems for the restoration of precision, which is decreased by the action of processes taking place at various rates —that is, the design of metalcutting machine tools with automatic fine adjustment of the mode of processing.
Automatic control systems are the most modern means for the production of highly reliable metalcutting machine tools. Automatic control may be simple (according to a preset program), direct (taking into account factors leading to deviations from the program), or of the closed-loop type, with feedback. The last method leads to the generation of adaptive self-controlling (self-adjusting) systems, which provide the highest reliability.
Adaptive control systems of metalcutting machine tools are divided into the following groups: systems for stabilization of the cutting parameters being controlled, for automatic changing of the control program, for balancing the dynamic and temperature stresses of the machine-attachment-tool-part system, and for optimization of the modes of processing with respect to precision and productivity. The use of adaptive systems makes it possible to decrease (or even eliminate) failures caused by overloads, to decrease the dependence of the result of processing on the operator, to simplify the programming of processing, to achieve automatic control of the dimensions of the products, to increase the economy of processing, and to facilitate the adoption of new production methods.
Metalcutting machine tools with numerical control Numerical control of metalcutting machine tools is economically advantageous in series production, in which there is a fairly frequent change of the products being processed, as well as in the production of parts of large dimensions and parts with curved profiles and surfaces. Numerical control makes possible automation of the processes of preparation for production and rapid readjustment of the machine. In metalcutting machine tools with numerical control, data on the required motions of the cutting tools with respect to the blank are transmitted to the control mechanisms in the form of a coded program, which consists of an arbitrary system of numerical symbols. The program is fed to the reader of the machine tool, which converts it into the corresponding command pulses (electrical signals), which are transmitted to the actuating members (supports, slides, tables, and so on) by means of control elements. All actions performed by the assemblies of a metalcutting machine tool according to signals from the numerical control system may be divided into two groups: (1) switching on and off to change the mode of cutting or to change the cutting tools in use, and (2) movement of the actuating members.
The numerical control systems used in metalcutting machine tools are classified as follows: according to purpose (positional, stepped, and functional control), the number of data flows (open, closed, and self-adjusting), the type of program carriers (internal, using switching panels, push-button panels, or pin boards; or external, using punched cards and tapes, magnetic tapes, or motion-picture film), the principle of limitation of the motion of the actuating members (pulse, analog, track, or time principles; coincidence circuits), and the physical principle of control of the motion of the actuating members (with mechanical, optical, electrical, or mixed measuring devices). A cyclical system of program control is also used, in which the operational cycle of the machine tool, as well as the modes of processing and of changing tools, is completely or partially programmed.
Numerical control systems usually consist of the following basic automatic elements: a device for program input, which “reads” the program and converts it into control signals; an intermediate “memory,” which “remembers” the control signals received and stores them for the required period of time; a comparator (active control assembly), which uses a feedback system to compare the motions specified by the program with those actually attained by the metalcutting machine tool (if a difference is detected, the comparator generates an additional signal to correct the error); and an actuating mechanism, which through the corresponding control linkages (hydraulic cylinders, screw pairs, and step motors) converts the incoming signals into the required movements of the actuating members of the machine tool.
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