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Machine tools are vital to modern technology, for they are capable of shaping metal to the precise dimensions required by the system of mass production and interchangeable parts. Machine tools are applied to metal pieces that have been forged, rolled, or cast. They remove unwanted metal and produce the exact size and shape desired. After machining, many articles are further finished by grinding.
The many different types of machine tools can be loosely grouped according to the types of surfaces on which they are particularly useful and by the chief operation they perform. Some machine tools are highly specialized, while others are multipurpose and can operate on a variety of surfaces. Surfaces that are shaped or finished by machine tools include cylinders, both external and internal; flat surfaces and surfaces made up of planes; curves and surfaces made up of a number of curves; spirals, such as internal and external screw threads; and gear teeth of various types.
Types of Machine Tools
Drilling Machines. Drilling machines are used to cut internal cylinders into metal parts. They are capable of drilling holes up to several inches in diameter or several feet deep. The usual tool in drilling machines is a twist drill with spiral grooves for the removal of waste metal. It is mounted either horizontally or vertically and is able to move into or away from the work-piece. If a hole is to be drilled at an angle, the table supporting the workpiece is adjusted. In radial drills the mechanism supporting the cutting tool can be moved to various positions over the work for drilling a series of holes. Some drilling machines use more than one cutting tool simultaneously.
Boring Machines. Boring is the enlargement of an existing hole. Boring is necessary to produce accurate holes because drilling is not a precise operation. Like drilling machines, boring machines employ a cutting tool with rotary motion. However, a boring tool is normally a single pointed cutter mounted perpendicularly to a bar. The bar carries the tool into the hole to be bored. Boring machines can cut holes several feet in diameter, and they can be adapted for working external cylinders.
Both drilling and boring machines may use reaming tools for finishing internal cylinders. Reaming tools have cutting edges along the axis of rotation.
Lathes. Lathes are versatile machine tools especially adapted for reducing the outside diameters of cylindrical work. They perform an operation called turning, in which the workpiece is rotated and a single-point cutting tool is brought against it. Lathes are also capable of drilling, boring, thread cutting, and other operations. Other uses include: turning the ends of a component by a facing operation; producing axially symmetric contoured surfaces with suitable copying or numerical control equipment; and thread cutting, axial drilling and boring, and other operations.
Planers and Shapers. These machines are used for machining plane surfaces. Planers are usually very large machines with a stationary cutting tool. The moving bed of a planer carries the work past the cutting edge. Shapers are smaller machines in which a single-edged tool moves back and forth, cutting into the work on the forward stroke. The work is carried past the cutter (mounted horizontally or vertically) by a worktable adjusted to various heights and angles. Such cuts are called flat cuts. In addition, planers and shapers can be used to cut grooves and bevels.
Milling Machines. Milling machines are the most versatile of all machine tools. They are widely used for machining plane surfaces, but with special tools they can drill, bore, and cut gears. The tools usually used on milling machines are many-edged rotary cutters. The tool is mounted horizontally or vertically over a table that can make many complicated motions. For machining curved contours the table movements are governed by a lever mechanism that traces a model of the surface to be cut. Shapers can also cut curves by a similar method. For machining two dimensional or three dimensional contours, special copying or numerical control equipment is required. Shapers can also cut curves by tracing a model.
Broaching Machines. Broaching tools, or broaches, are generally made to fit the particular surface to be worked. The broaching machine is particularly suited to the machining of irregular surfaces. Broaching machines normally operate by pulling a broach over or through a section of the workpiece. The broaching machine has parallel cutting edges with each edge slightly larger than the one preceding it, so that a single movement of the tool results in a series of cuts in the work-piece. Broaching machines are commonly employed to cut internal keyways or splines. They are used in the mass production of precision surfaces in the automobile and aircraft industries.
Special Machines. For the quantity production of gears and screw threads, specialized machines are used. Threads can be cut with single-point tools or by tools with many edges in the same size and pattern as the threads to be cut. Gears are usually produced either by form tools, which have cutting edges shaped like the spaces to be cut between gear teeth, or by a generating process, in which the cutting edges are of a different shape but are moved while cutting to generate a space of the required shape. One gear-cutting tool using the generating process is called the hobber.
Power Saws. Although most machine tools are capable of cutting a line entirely through a workpiece, there are power saws designed for this purpose. The power hacksaw, like the similar hand tool, has a toothed blade that is moved back and forth across the work. The circular saw utilizes a circular blade, which brings more teeth to bear on the work. Most versatile of all power saws is the band saw, whose blade is an endless loop. It can cut curves as well as straight lines.
Cutting Fluids. Most machine tool operations require the use of a cutting fluid to cool and lubricate the cutting tool and the workpiece. If cooling is the chief object, an oil-and-water emulsion is used. If more lubrication is necessary, a mineral oil compound serves as the cutting fluid.
Electric-Discharge Machining. In electric-discharge machines, metal removal is brought about by means of an electric arc discharge produced under carefully controlled conditions. The cutting tool is machined to the size and shape of the opening or recess that is to be made. This tool and the piece that is to be machined are then placed close together in a dielectric fluid, and an electric voltage is applied between the two. This causes a large number of small sparks to pass from the surface of the cutting tool to the surface of the work. The action of the sparks causes metal from the workpiece to be melted or vaporized. The tool advances slowly into the workpiece, producing a recess of the desired shape.
Electrochemical Machining. Electrochemical machining is a process in which a piece of metal is shaped by means of an electrochemical reaction. The cutting action is similar to electroplating, but in the reverse direction, the metal being dissolved away from the workpiece rather than deposited on it. Electrochemical machining is more rapid but less accurate than electric-discharge machining.
Ultrasonic Machining. Ultrasonic machining is machining in which the cutting tool vibrates in an abrasive slurry at a frequency above the range of human hearing, typically about 20,000 hertz (cycles per second). The method is especially valuable for machining complex shapes in nonconductive materials, on which the electrical methods just described cannot be used. The tool is formed to the same shape as the opening desired in the workpiece and an abrasive slurry flows between the tool and the workpiece. The resulting grinding action removes material from the workpiece.
Laser Machining. In laser machining, cutting is done by the action of a narrow beam of high-energy coherent monochromatic light. Although the method works well for drilling small holes in very hard or refractory material and for certain small precision-welding jobs, the equipment required is complex and expensive and the process is difficult to control. High powered lasers are now capable of cutting through several millimeters of steel at high speed. Laser machining is becoming routine in many industries.
Machine tools can be operated
automatically by means of a the object programmed in two or three axis (it used to be on paper tapes punched with numerical data that
control the machine, now it's transferred into the onboard computer via cables). This saves a great deal of labor in repetitive
work, and it has also proved to be economical for many small machining
The feed motions of drills, lathes, and milling and boring and other machines can also be controlled by a computer. The computer converts the required component dimensions into command signals that drive servo mechanisms attached to the feed motions. Numerically controlled machines can be linked to computer controlled work handling equipment to form machining systems. They can also be linked to computer aided design systems that specify the required shape of a workpiece, such as a turbine blade.