Some Conventional Machine Tools

Conventional machine tools prepare workpieces for further fitting and use. Drills, grinders, punch presses, surface grinders, and boring machines are used extensively in industry. Particularly useful in large-scale production, these power tools produce uniform holes and smooth surfaces far faster and more accurately than they could be produced by hand.

Hole-making machine tools are used to drill a hole where none previously existed; to alter a hole in accordance with some specification (by boring or reaming to enlarge it, or by tapping to cut threads for a screw); or to lap or hone a hole to create an accurate size or a smooth finish.

Drilling machines vary in size and function, ranging from portable drills to radial drilling machines, multispindle units, automatic production machines, and deep-hole-drilling machines.

Boring is a process that enlarges holes previously drilled, usually with a rotating single-point cutter held on a boring bar and fed against a stationary workpiece. Boring machines include jig borers and vertical and horizontal boring mills.

Grinding is the removal of metal by a rotating abrasive wheel; the action is similar to that of a milling cutter. The wheel is composed of many small grains of abrasive, bonded together, with each grain acting as a miniature cutting tool. The process produces extremely smooth and accurate finishes. Because only a small amount of material is removed at each pass of the wheel, grinding machines require fine wheel regulation. The pressure of the wheel against the workpiece can be made very slight, so that grinding can be carried out on fragile materials that cannot be machined by other conventional devices. See Grinding and Polishing.

Commonly used power-driven saws are classified into three general types, according to the kind of motion used in the cutting action: reciprocating, circular, and band-sawing machines. They generally consist of a bed or frame, a vise for clamping the workpiece, a feed mechanism, and the saw blade.

Because cutting processes involve high local stresses, frictions, and considerable heat generation, cutting-tool material must combine strength, toughness, hardness, and wear resistance at elevated temperatures. These requirements are met in varying degrees by such cutting-tool materials as carbon steels (steel containing 1 to 1.2 percent carbon), high-speed steels (iron alloys containing tungsten, chromium, vanadium, and carbon), tungsten carbide, and diamonds and by such recently developed materials as ceramic, carbide ceramic, and aluminum oxide.

In many cutting operations fluids are used to cool and lubricate. Cooling increases tool life and helps to stabilize the size of the finished part. Lubrication reduces friction, thus decreasing the heat generated and the power required for a given cut. Cutting fluids include water-based solutions, chemically inactive oils, and synthetic fluids.

Presses shape workpieces without cutting away material, that is, without making chips. A press consists of a frame supporting a stationary bed, a ram, a power source, and a mechanism that moves the ram in line with or at right angles to the bed. Presses are equipped with dies (see Die) and punches designed for such operations as forming, punching, and shearing. Presses are capable of rapid production because the operation time is that needed for only one stroke of the ram.

Unconventional machine tools include plasma-arc, laser-beam, electrodischarge, electrochemical, ultrasonic, and electron-beam machines. These machine tools were developed primarily to shape the ultrahard alloys used in heavy industry and in aerospace applications and to shape and etch the ultrathin materials used in such electronic devices as microprocessors.

Plasma-arc machining (PAM) employs a high-velocity jet of high-temperature gas (see Plasma) to melt and displace material in its path. The materials cut by PAM are generally those that are difficult to cut by any other means, such as stainless steels and aluminum alloys.

Laser-beam machining (LBM) is accomplished by precisely manipulating a beam of coherent light (see Laser) to vaporize unwanted material. LBM is particularly suited to making accurately placed holes. The LBM process can make holes in refractory metals and ceramics and in very thin materials without warping the workpiece. Extremely fine wires can also be welded using LBM equipment.

Electrodischarge machining (EDM), also known as spark erosion, employs electrical energy to remove metal from the workpiece without touching it. A pulsating high- frequency electric current is applied between the tool point and the workpiece, causing sparks to jump the gap and vaporize small areas of the workpiece. Because no cutting forces are involved, light, delicate operations can be performed on thin workpieces. EDM can produce shapes unobtainable by any conventional machining process.

Electrochemical machining (ECM) also uses electrical energy to remove material. An electrolytic cell is created in an electrolyte medium, with the tool as the cathode and the workpiece as the anode. A high-amperage, low-voltage current is used to dissolve the metal and to remove it from the workpiece, which must be electrically conductive. A wide variety of operations can be performed by ECM; these operations include etching, marking, hole making, and milling.

Ultrasonic machining (USM) employs high-frequency, low-amplitude vibrations to create holes and other cavities. A relatively soft tool is shaped as desired and vibrated against the workpiece while a mixture of fine abrasive and water flows between them. The friction of the abrasive particles gradually cuts the workpiece. Materials such as hardened steel, carbides, rubies, quartz, diamonds, and glass can easily be machined by USM.

In electron-beam machining (EBM), electrons are accelerated to a velocity nearly three-fourths that of light. The process is performed in a vacuum chamber to reduce the scattering of electrons by gas molecules in the atmosphere. The stream of electrons is directed against a precisely limited area of the workpiece; on impact, the kinetic energy of the electrons is converted into thermal energy that melts and vaporizes the material to be removed, forming holes or cuts. EBM equipment is commonly used by the electronics industry to aid in the etching of circuits in microprocessors. See Microprocessor.