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. See Drill.

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.

F

Grinders

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.

G

Saws

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.

H

Cutting Tools and Fluids

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.

I

Presses

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.

IV

UNCONVENTIONAL MACHINE TOOLS

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.

A

Plasma Arc

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.

B

Laser

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.

C

Electrodischarge

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.

D

Electrochemical

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.

E

Ultrasonic

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.

F

Electron Beam

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.

Flame Retardant

I

INTRODUCTION

Flame Retardant, material added or applied to a product to increase the resistance of that product to fire. Flame retardants, also called fire retardants, are less flammable than the materials they protect, burn slowly, and do not propagate fire. Some flame retardants prevent the spread of flame; others burn and thereby create a layer of char that inhibits further combustion.

Flame retardants are generally added to wood, paper, plastics, textiles, and composites to meet governmental regulations for buildings, aircraft, automobiles, and ships. Flame retardants can be incorporated into a material either as a reactive component or as an additive component. Reactive-type flame retardants are preferable because they produce stable and more uniform products. Such flame retardants are incorporated into the polymer structure of some plastics. Additive-type flame retardants, on the other hand, are more versatile and economical. They can be applied as a coating to wood, woven fabrics, and composites, or as dispersed additives in bulk materials such as plastics and fibers.

The chemicals in a flame retardant determine how it works. Most flame retardants contain elements from any of three groups in the periodic table of elements: group IIIa (including boron and aluminum); group Va (including nitrogen, phosphorus, arsenic, and antimony); and group VIIa (including fluorine, chlorine, and bromine). Elements of different groups that are combined in a single flame retardant may work more effectively together than they would separately.

II

GROUP IIIA FLAME RETARDANTS

Flame retardants that contain boron or aluminum increase the amount of char, or burnt material, formed in the early stage of a fire. The char forms a protective layer that prevents oxygen from reaching the inner layers of the material and thus sustaining the fire (see Combustion). Chemicals commonly used for this purpose include borax, boric acid, and hydrated aluminum oxide.

III

GROUP VA FLAME RETARDANTS

Phosphorus can function as a flame retardant in both its solid phase and its liquid phase. Phosphorus-containing compounds such as phosphoric acid work by forming a surface layer of protective char. Nitrogen is used mainly in combination with phosphorus; such combinations have proved effective in cellulose, polyester, and polyurethane products. Arsenic, because of its toxicity, is now rarely used in flame retardants. Antimony by itself is ineffective as a flame retardant and is used only in combination with halogens, especially bromine and chlorine.

IV

GROUP VIIA FLAME RETARDANTS

Bromine works as a flame retardant in its gaseous phase. Bromine-containing compounds are incorporated into flammable materials. When these materials are exposed to flame, the bromine dissociates from the material and forms a heavy gas. This dissociation disperses heat, and the bromine gas forms an insulating layer around the material. The layer prevents flames from spreading by inhibiting access to oxygen and by slowing the transfer of heat. Chlorine works in a similar manner in both its liquid and gaseous phases. The most important fluorine-containing flame retardants are the chlorofluorocarbons, which are used as blowing agents in polyurethane and polystyrene foams. The use of bromine and chlorine in fire retardants is somewhat restricted, however, because a high concentration of these elements can diminish the flexibility, mechanical properties, and durability of materials.

Welding

I

INTRODUCTION

Welding, in engineering, any process in which two or more pieces of metal are joined together by the application of heat, pressure, or a combination of both. Most of the processes may be grouped into two main categories: pressure welding, in which the weld is achieved by pressure; and heat welding, in which the weld is achieved by heat. Heat welding is the most common welding process used today. Brazing and soldering (see Solder) are other means of joining metals.

With the development of new techniques during the first half of the 20th century, welding replaced bolting and riveting in the construction of many types of structures, including bridges, buildings, and ships. It is also a basic process in the automotive and aircraft industries and in the manufacture of machinery. Along with soldering and brazing, it is essential in the production of virtually every manufactured product involving metals.

The welding process best suited to joining two pieces of metal depends on the physical properties of the metals, the specific use to which they are applied, and the production facilities available. Welding processes are generally classified according to the sources of heat and pressure used.

The original pressure process was forge welding. Forge welding was practiced for centuries by blacksmiths and other artisans. The metals are brought to a suitable temperature in a furnace, and the weld is achieved by hammering or other mechanical pressure. Forge welding is used rarely in modern manufacturing.

The welding processes most commonly employed today include gas welding, arc welding, and resistance welding. Other joining processes include thermite welding, laser welding, and electron-beam welding.

II

GAS WELDING

Gas welding is a nonpressure process using heat from a gas flame. The flame is applied directly to the metal edges to be joined and simultaneously to a filler metal in wire or rod form, called the welding rod, which is melted to the joint. Gas welding has the advantage of involving equipment that is portable and does not require an electric power source. The surfaces to be welded and the welding rod are coated with flux, a fusible material that shields the material from air, which would result in a defective weld.

III

ARC WELDING

Arc-welding processes, which have become the most important welding processes, particularly for joining steels, require a continuous supply of either direct or alternating electrical current. This current is used to create an electric arc, which generates enough heat to melt metal and create a weld (see Electric Arc).

Arc welding has several advantages over other welding methods. Arc welding is faster because of its high heat concentration, which also tends to reduce distortion in the weld. Also, in certain methods of arc welding, fluxes are not necessary. The most widely used arc-welding processes are shielded metal arc, gas-tungsten arc, gas-metal arc, and submerged arc.

A

Shielded Metal Arc

In shielded metal-arc welding, a metallic electrode, which conducts electricity, is coated with flux and connected to a source of electric current. The metal to be welded is connected to the other end of the same source of current. By touching the tip of the electrode to the metal and then drawing it away, an electric arc is formed. The intense heat of the arc melts both parts to be welded and the point of the metal electrode, which supplies filler metal for the weld. This process, developed in the early 20th century, is used primarily for welding steels.

B

Gas-Tungsten Arc

In gas-tungsten arc welding, a tungsten electrode is used in place of the metal electrode used in shielded metal-arc welding. A chemically inert gas, such as argon or helium, is used to shield the metal from oxidation. The heat from the arc formed between the electrode and the metal melts the edges of the metal. Metal for the weld may be added by placing a bare wire in the arc or the point of the weld. This process can be used with nearly all metals and produces a high-quality weld. However, the rate of welding is considerably slower than in other processes.

C

Gas-Metal Arc

In gas-metal welding, a bare electrode is shielded from the air by surrounding it with argon or carbon dioxide gas or by coating the electrode with flux. The electrode is fed into the electric arc, and melts off in droplets to enter the liquid metal that forms the weld. Most common metals can be joined by this process.

D

Submerged Arc

Submerged-arc welding is similar to gas-metal arc welding, but in this process no gas is used to shield the weld. Instead, the arc and tip of the wire are submerged beneath a layer of granular, fusible material formulated to produce a proper weld. This process is very efficient but is generally only used with steels.

IV

RESISTANCE AND THERMITE WELDING

In resistance welding, heat is obtained from the resistance of metal to the flow of an electric current. Electrodes are clamped on each side of the parts to be welded, the parts are subjected to great pressure, and a heavy current is applied briefly. The point where the two metals meet creates resistance to the flow of current. This resistance causes heat, which melts the metals and creates the weld. Resistance welding is extensively employed in many fields of sheet metal or wire manufacturing and is particularly adaptable to repetitive welds made by automatic or semiautomatic machines.

In thermite welding, heat is generated by the chemical reaction that results when a mixture of aluminum powder and iron oxide, known as thermite, is ignited. The aluminum unites with the oxygen and generates heat, releasing liquid steel from the iron. The liquid steel serves as filler metal for the weld. Thermite welding is employed chiefly in welding breaks or seams in heavy iron and steel sections. It is also used in the welding of rail for railroad tracks.

V

NEW PROCESSES

The use of electron beams and lasers for welding has grown during the second half of the 20th century. These methods produce high-quality welded products at a rapid rate. Laser welding and electron-beam welding have valuable applications in the automotive and aerospace industries.

Brazing

Brazing, a method of joining two metal surfaces by using nonferrous filler metal heated to above 430° C (800° F), but below the melting point of the metals to be joined. The kinds of filler metal used include brass, bronze, or a silveralloy; the filler metal distributes itself between the surfaces to be bonded by capillary action. Brazing is different from welding; in welding, partial melting of the surfaces may occur, and the filler metal is not distributed by capillarity. Brazing differs from ordinary soldering only in the temperature of the operation; ordinary, or soft, solder melts at temperatures below 430° C, but brazing alloys, sometimes called hard solder, melt above that temperature.

In general, brazing requires careful cleaning of the surfaces to be joined and the use of flux, such as borax, to reduce any oxide film on the surfaces. In mass production, furnaces are often used to heat the parts to be brazed, or the parts are brazed by dipping in baths of molten filler alloys. For single, nonrepetitive operations, the joint is usually heated with a gas, oxyacetylene, or oxyhydrogen torch.

Electric Arc

Electric Arc, type of continuous electric discharge, giving intense light and heat, formed between two electrodes in a gas at low pressure or in open air. It was first discovered and demonstrated by the British chemist Sir Humphry Davy in 1800.

To start an arc, the ends of two pencil-like electrodes, usually made of carbon, are brought into contact and a large current (about 10 amp) is passed through them. This current causes intense heating at the point of contact, and if the electrodes are then separated, a flamelike arc is formed between them. The discharge is carried largely by electrons traveling from the negative to the positive electrode, but also in part by positive ions traveling in the opposite direction. The impact of the ions produces great heat in the electrodes, but the positive electrode is hotter, because the electrons impinging on it have greater total energy. In an arc in air at normal pressure, the positive electrode reaches a temperature of 3500° C (6332° F).

The intense heat of the electric arc is often utilized in special furnaces to melt refractory materials. Temperatures of about 2800° C (5072° F) can easily be obtained with such a furnace. Arcs are also used as a high-intensity light source. Arc lights have the advantage of being concentrated sources of light, because some 85 percent of the light intensity comes from a small area of the tip of the positive carbon electrode. Such lamps were formerly much used for street lighting, but are now chiefly employed in motion picture projectors. Mercury-vapor lamps and sodium-vapor lamps are enclosed arc lamps in which the arc is maintained in an atmosphere of mercury or sodium vapor at reduced pressure.

Forging

I

INTRODUCTION

Forging, process of shaping iron and other malleable metals by hammering or pressing them after making them plastic by application of heat. Forging techniques are useful in the working of metal because the metal can be given the desired form, and the process improves the structure of the metal, particularly by refining the grain size of the metal. Forged metal is stronger and more ductile than cast metal and exhibits greater resistance to fatigue and impact.

II

HAND FORGING

Sometimes called smithing, or blacksmithing, hand forging is the simplest form of forging and it is one of the methods by which metal was first worked. The metal to be forged is first heated to red heat in the fire of a forge, and then is beaten into shape on a metal anvil with sledges or hammers. The forge consists of an open hearth, made of some durable, refractory substance such as firebrick, which is provided with a number of air openings, or tuyeres, through which air is forced by a bellows or blower fan. Charcoal, coke, and coal are used as fuels in the forge. Hammers and other tools are employed by the blacksmith in the various forging operations.

In general, six basic types of forging exist: upsetting, or decreasing the length and increasing the diameter of the metal; swaging, decreasing the diameter of the metal; bending; welding, joining two pieces of metal together by semifusion; punching, the forming of small openings in the metal; and cutting out, the forming of large holes in the metal.

A piece of metal, called the work, is upset when it is struck along the longest dimension (for example, the end of a rod or bar), which shortens and thickens it. Swaging is accomplished by hammering the metal stock while it is held on the anvil within any one of various concave tools called swages. Bending is accomplished either by hammering the work around a form or by leveraging it against a supporting fulcrum. In forge welding of iron, a flux such as borax is first applied to the heated metal to remove any oxides from the surfaces of the two pieces, and the pieces are then joined by hammering them together at high temperature; a welded joint of this kind, when properly made, is entirely homogeneous and is as strong, that is, uniform, as the parent metal. To punch small holes, the work is supported on a ring-shaped piece of metal atop the anvil, and a punch of the proper shape is driven through the work by hammer blows. Larger holes are cut out, and portions of the work are cut off with heavy, sharp chisels similar to cold chisels used to cut cold metal. Combinations of several of these operations can produce forgings of a wide variety of shapes.

III

MACHINE FORGING

The chief difference between hand forging and machine forging is that in the latter technique various types of machine-powered hammers or presses are used instead of hand sledges. These machines enable the operator to strike heavy blows with great rapidity and thus to produce forgings of large size and high quality as swiftly as required by modern production-line methods. Another advantage of machine forging is that the heavier the blows struck during forging, the greater the improvement in the quality of metallic structure. Fine-grain size in the forging, which is particularly desirable for maximum impact resistance, is obtained by working the entire piece. With large, hand-forged metal, only the surface is deformed, whereas the machine hammer or press will deform the metal throughout the entire piece.

A special type of machine forging is drop forging, also called impact-die forging. Drop forging consists of placing soft, hot metal between two shaping dies (see Die). The upper one of these dies is hammered, or dropped, on the lower die, forcing the heated metal into the shaped die cavities, as in coin-making operations.

For reducing part of a piece of metal stock to a predetermined size, forging rolls are sometimes employed. These consist of a pair of grooved, cam-shaped rollers through which the metal is passed. The rollers touch each other and work on the metal during only part of each rotation and therefore reduce only part of the stock that is fed to them.

Machine-forging operations are frequently accomplished by use of a series of dies mounted on the same press or hammer. The dies are arranged in sequence so as to form the finished forging in a series of steps. After the piece has been partially formed by one stroke, it is moved to the next die for further shaping on the next stroke.

Fatigue (materials)

Fatigue (materials), in metals, progressive deterioration, that ultimately results in the breaking of the metal. Fatigue is caused by repeated application of stress to the metal, and the deformation of a material or object as a result of the stress is known as creep. The fatigue strength of a typical steel alloy is about 50 percent of the ultimate strength and 75 percent of the elastic strength but may be considerably lower, particularly for the strongest heat-treated steels. If the elastic strength of a steel beam is about 45,000 kg (about 100,000 lb), it could withstand a continuous stress of about 41,000 kg (about 90,000 lb) for centuries, with no measurable yielding. A stress of about 36,000 kg (about 80,000 lb) alternately applied and withdrawn, however, would probably cause fatigue failure after a few million applications. Fatigue is not important in civil engineering structures, in which stress is generally continuous, but in an engine turning at 3000 rpm, any stress to which an engine part is subjected will often be applied millions of times within a few hours of operation. Fatigue failures account for an overwhelming majority of all structural failures in cyclic devices such as engines, and design engineers must consider fatigue strength, rather than elastic strength or ultimate strength, in their calculations.

The problem of metal fatigue has gained great importance in the field of air transport since the end of World War II. The increased stresses of high-speed flight with heavy loads at high altitudes have posed difficult problems for structural engineers, especially in wing and engine design. The exact structural changes that occur as a result of fatigue are not known. The failure usually starts at a point of stress concentration and proceeds along the intercrystalline planes of the metal. The break often shows a characteristic coarsely crystalline structure except where the surfaces are worn smooth by rubbing against one another after the break has started. The term fatigue is not an entirely appropriate one, because no amount of rest between stress applications has any measurable effect upon the ultimate failure. See Materials Science and Technology; Metallography; Metals; Tensile Strength.

Flame

Flame, glowing body of mixed gases undergoing the process of combustion. Flames generally consist of a mixture of oxygen (or air) and another gas, usually such combustible substances as hydrogen, carbon monoxide, or hydrocarbon.

A typical flame is that of a burning candle. When the candle is lighted, the heat of the match melts the wax, which is carried up the wick and then vaporized by the heat. The vaporized wax is then broken down by the heat and, finally, combines with the oxygen of the surrounding air, producing a flame and generating heat and light. The candle flame consists of three zones that are easily distinguished. The innermost zone, a nonluminous cone, is composed of a gas-air mixture at a comparatively low temperature. In the second, or luminous, cone, hydrogen and carbon monoxide are produced by decomposition and begin to react with oxygen to form water and carbon dioxide, respectively. In this cone the temperature of the flame—about 590° to 680° C (about 1090° to 1250° F)—is great enough to dissociate the gases in the flame and produce free particles of carbon, which are heated to incandescence and then consumed. The incandescent carbon produces the characteristic yellow light of this portion of the flame. Outside the luminous cone is a third, invisible cone in which the remaining carbon monoxide and hydrogen are finally consumed.

If a cold object is introduced into the outer portions of a flame, the temperature of that part of the flame will be lowered below the point of combustion, and unburned carbon and carbon monoxide will be given off. Thus, if a porcelain dish is passed through a candle flame, it will receive a deposit of carbon in the form of soot. Operation of any kind of flame-producing stove in a room that is unventilated is dangerous because of the production of carbon monoxide, which is poisonous.

All combustible substances require a definite proportion of oxygen for complete burning. (A flame can be sustained in an atmosphere of pure chlorine, although combustion is not complete.) In the burning of a candle, or of solids such as wood or coal, this oxygen is supplied by the surrounding atmosphere. In blowpipes and various types of gas burners, air or pure oxygen is mixed with the gas at the base of the burner so that the carbon is consumed almost instantaneously at the mouth of the burner. For this reason such flames are nonluminous. They also occupy a smaller volume and are proportionately hotter than a simple candle flame. The hottest portion of the flame of a Bunsen burner has a temperature of about 1600° C (about 2910° F). The hottest portion of the oxygen-acetylene flames used for welding metals reaches 3500° C (6330° F); such flames have a bluish-green cone in place of the luminous cone. If the oxygen supply is reduced, such flames have four cones: nonluminous, bluish-green, luminous, and invisible.

The blue-green cone of any flame is often called the reducing cone, because it is insufficiently supplied with oxygen and will take up oxygen from substances placed within it. Similarly, the outermost cone, which has an excess of oxygen, is called the oxidizing cone. Intensive studies of the molecular processes taking place in various regions of flames are now possible through the techniques of laser spectroscopy.