Microwave Oven

Microwave Oven
Background :
Microwaves are actually a segment of the electromagnetic wave spectrum, which comprises forms of energy that move through space, generated by the interaction of electric and magnetic fields. The spectrum is commonly broken into subgroups determined by the different wavelengths (or frequencies) and emission, transmission, and absorption behaviors of various types of waves. From longest to shortest wavelengths, the spectrum includes electric and radio waves, microwaves, infrared (heat) radiation, visible light, ultraviolet radiation, X-rays, gamma rays, and electromagnetic cosmic rays. Microwaves have frequencies between approximately .11 and 1.2 inches (0.3 and 30 centimeters).

Microwaves themselves are used in many different applications such as telecommunication products, radar detectors, wood curing and drying, and medical treatment of certain diseases. However, certain of their properties render them ideal for cooking, by far the most common use of microwave energy. Microwaves can pass through plastic, glass, and paper materials; metal surfaces reflect them, and foods (especially liquids) absorb them. A meal placed in a conventional oven is heated from the outside in, as it slowly absorbs the surrounding air that the oven has warmed. Microwaves, on the other hand, heat food much more quickly because they penetrate all layers simultaneously. Inside a piece of food or a container filled with liquid, the microwaves agitate molecules, thereby heating the substance.

The ability of microwave energy to cook food was discovered in the 1940s by Dr. Percy Spencer, who had conducted research on radar vacuum tubes for the military during World War II. Spencer's experiments revealed that, when confined to a metal enclosure, high-frequency radio waves penetrate and excite certain type of molecules, such as those found in food. Just powerful enough to cook the food, the microwaves are not strong enough to alter its molecular or genetic structure or to make it radioactive.

Raytheon, the company for which Dr. Spencer was conducting this research, patented the technology and soon developed microwave ovens capable of cooking large quantities of food. Because manufacturing costs rendered them too expensive for most consumers, these early ovens were used primarily by hospitals and hotels that could more easily afford the $3,000 investment they represented. By the late 1970s, however, many companies had developed microwave ovens for home use, and the cost had begun to come down. Today, microwaves are a standard household appliance, available in a broad range of designs and with a host of convenient features: rotating plates for more consistent cooking; digital timers; autoprogramming capabilities; and adjustable levels of cooking power that enable defrosting, browning, and warming, among other functions.

Design
The basic design of a microwave oven is simple, and most operate in essentially the same manner. The oven's various electronic motors, relays, and control circuits are located on the exterior casing, to which the oven cavity is bolted. A front panel allows the user to program the microwave, and the


The oven cavity and door are made using metal-forming techniques and then painted using electro-deposition, in which electric current is used to apply the paint.
The magnetron tube subassembly includes several important parts. A powerful magnet is placed around the anode to provide the magnetic field in which the microwaves will be generated, while a thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating. An antenna enclosed in a glass tube is mounted on top of the anode, and the air within the tube is pumped out to create a vacuum. Also, a blower motor used to cool the metal fins of the magnetron is attached directly to the tube.
door frame has a small window to enable the cook to view the food while it is cooking.

Near the top of the steel oven cavity is a magnetron—an electronic tube that produces high-frequency microwave oscillations—which generates the microwaves. The microwaves are funneled through a metal waveguide and into a stirrer fan, also positioned near the top of the cavity. The fan distributes the microwaves evenly within the oven. Manufacturers vary the means by which they disburse microwaves to achieve uniform cooking patterns: some use dual stirrer fans located on opposite walls to direct microwaves to the cavity, while others use entry ports at the bottom of the cavity, allowing microwaves to enter from both the top and bottom. In addition, many ovens rotate food on a turntable.

Raw Materials
The cover or outer case of the microwave oven is usually a one-piece, wrap-around metal enclosure. The oven's inside panels and doors are made of galvanized or stainless steel and are given a coating of acrylic enamel, usually light in color to offer good visibility. The cooking surface is generally made of ceramic or glass. Inside the oven, electromechanical components and controls consist of timer motors, switches, and relays. Also inside the oven are the magnetron tube, the waveguide, and the stirrer fan, all made of metal. The hardware that links the various components consists of a variety of metal and plastic parts such as gears, pulleys, belts, nuts, screws, washers, and cables.

The Manufacturing
Process
Oven cavity and door manufacture
•1 The process of manufacturing a microwave oven starts with the cavity and the door. First, the frame is formed using automatic metal-forming presses that make about 12 to 15 parts per minute. The frame is then rinsed in alkaline cleaner to get rid of any dirt or oil and further rinsed with water to get rid of the alkaline solution.
•2 Next, each part is treated with zinc phosphate, which prepares it for electro-deposition. Electro-deposition consists of immersing the parts in a paint tank at 200 volts for 2.5 minutes. The resulting coating is about 1.5 mils thick. The parts are then moved through a paint bake operation where the paint is cured at 300 degrees Fahrenheit (149 degrees Celsius) for 20 minutes.

The chassis or frame is mounted in a pallet for the main assembly operation. A pallet is a vise-like device used in conjunction with other tools.
•3 After the door has been painted, a perforated metal plate is attached to its window aperture. The plate reflects microwaves but allows light to enter the cavity (the door will not be attached to the cavity until later, when the chassis is assembled).
The magnetron tube subassembly
•4 The magnetron tube assembly consists of a cathode cylinder, a filament heater, a metal anode, and an antenna. The filament is attached to the cathode, and the cathode is enclosed in the anode cylinder; this cell will provide the electricity that will help to generate the microwaves. Metal cooling fins are welded to the anode cylinder, and a powerful magnet is placed around the anode to provide the magnetic field in which the microwaves will be generated. A metal strap holds the complete assembly together. A thermal protector is mounted directly on the magnetron to prevent damage to the tube from overheating.
•5 An antenna enclosed in a glass tube is mounted on top of the anode, and the air within the tube is pumped out to create a vacuum. The waveguide is connected to the magnetron on top of the protruding antenna, while a blower motor used to cool the metal fins of the magnetron is attached directly to the tube. Finally, a plastic fan is attached to the motor, where it will draw air from outside the oven and direct it towards the vanes. This completes the magnetron subassembly.
Main chassis assembly
•6 The chassis assembly work is performed on a pallet—a work-holding device used in conjunction with other tools—located at the station. First, the main chassis is placed on the pallet, and the cavity is screwed on to the chassis. Next, the door is attached to the cavity and chassis by means of hinges. The magnetron tube is then bolted to the side of the cavity and the main chassis.

In a completed microwave oven, the magnetron tube creates the microwaves, and the waveguide directs them to the stirrer fan. In turn, this fan points the waves into the oven cavity where they heat the food inside.
•7 The circuit that produces the voltage required to operate the magnetron tube consists of a large transformer, an oil-based capacitor, and a high voltage rectifier. All of these components are mounted directly on the chassis, close to the magnetron tube.
Stirrer fan
•8 The stirrer fan used to circulate the microwaves is mounted on top of the cavity. Some manufacturers use a pulley to drive the fan from the magnetron blower motor; others use a separate stirrer motor attached directly to the fan. Once the stirrer fan is attached, a stirrer shield is screwed on top of the fan assembly. The shield prevents dirt and grease from entering the waveguide, where they could produce arcing and damage the magnetron.
Control switches, relays, and motors
•9 The cook switch provides power to the transformer by energizing a relay and a timer. The relay is mounted close to the power transformer, while the timer is mounted on the control board. The defrost switch works like the cook switch, activating a motor and timer to operate the defrost cycle. Also mounted on the control board are a timer bell that rings when the cooking cycle is complete and a light switch that allows viewing of the cavity. A number of interlocking switches are mounted near the top and bottom of the door area. The interlocking switches are sometimes grouped together with a safety switch that monitors the other switches and provides protection if the door accidently opens during oven operation.
Front panel
•10 A front panel that allows the operator to select the various settings and features available for cooking is attached to the chassis. Behind the front panel, the control circuit board is attached. The board, which controls the various programmed operations in their proper sequence when the switches are pushed on the front panel, is connected to the various components and the front panel by means of plug-in sockets and cables.
Making and assembling the case
•11 The outer case of the microwave is made of metal and is assembled on a roll former. The case is slipped onto the preassembled microwave oven and bolted to the main chassis.
Testing and packaging the oven
•12 The power cords and dial knobs are now attached to the oven, and it is sent for automatic testing. Most manufacturers run the oven from 50-100 hours continuously as part of the testing process. After testing is complete, a palletizer robot records the model and serial data of the oven for inventory purposes, and the oven is sent for packaging. This completes the manufacturing process.
Quality Control
Extensive quality control during the manufacture of microwave ovens is essential, because microwave ovens emit radiation that can burn anyone exposed at high levels for prolonged periods. Federal regulations, applied to all ovens made after October 1971, limit the amount of radiation that can leak from an oven to 5 milliwatts of radiation per square centimeter at approximately 2 inches from the oven surface. The regulations also require all ovens to have two independent, interlocking switches to stop the production of microwaves the moment the latch is released or the door is opened.

In addition, a computer controlled scanner is used to measure emission leaks around the door, window, and back of the oven. Other scanners check the seating of the magnetron tube and antenna radiation. Each scanner operation relays data to the next-on-line operation so that any problems can be corrected.

The Future
Because of their speed and convenience, microwave ovens have become an indispensable part of modern kitchens. Many developments in the microwave market and allied industries are taking place fairly rapidly. For example, foods and utensils designed specially for microwave cooking have become a huge business. New features will also be introduced in microwaves themselves, including computerized storage of recipes that the consumer will be able to recall at the touch of a button. The display and programmability of the ovens will also be improved, and combination ovens capable of cooking with microwaves as well as by conventional methods will become a standard household product.

Where To Learn More
Books
Davidson, Homer L. Microwave Oven Repair, 2nd edition. Tab Books Inc., 1991.

Gallawa, J. Carlton. The Complete Microwave Oven Service Handbook: Operation, Maintenance. Prentice Hall, 1989.

Microwave Oven Radiation. U.S. department of Health and Human Services, 1986.

Pickett, Amold and John Ketterer. Household Equipment in Residential Design. John Wiley and Sons, 1986.

Raytheon Company. Appliance Manufacturer. Cahners Publishing, 1985.

Periodicals
Klenck, Thomas. "How It Works: Microwave Oven." Popular Mechanics. September, 1989, p. 78.

Roman, Mark. "The Little Waves That Could." Discover. November, 1989, p. 54.

— Rashid Riaz



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Laser Guided Missile

Laser Guided Missile
Background:
Missiles differ from rockets by virtue of a guidance system that steers them towards a pre-selected target. Unguided, or free-flight, rockets proved to be useful yet frequently inaccurate weapons when fired from aircraft during the World War II. This inaccuracy, often resulting in the need to fire many rockets to hit a single target, led to the search for a means to guide the rocket towards its target. The concurrent explosion of radio-wave technology (such as radar and radio detection devices) provided the first solution to this problem. Several warring nations, including the United States, Germany and Great Britain, mated existing rocket technology with new radio- or radar-based guidance systems to create the world's first guided missiles. Although these missiles were not deployed in large enough numbers to radically divert the course of the World War II, the successes that were recorded with them pointed out techniques that would change the course of future wars. Thus dawned the era of high-technology warfare, an era that would quickly demonstrate its problems as well as its promise.

The problems centered on the unreliability of the new radio-wave technologies. The missiles were not able to hone in on targets smaller than factories, bridges, or warships. Circuits often proved fickle and would not function at all under adverse weather conditions. Another flaw emerged as jamming technologies flourished in response to the success of radar. Enemy jamming stations found it increasingly easy to intercept the radio or radar transmissions from launching aircraft, thereby allowing these stations to send conflicting signals on the same frequency, jamming or "confusing" the missile. Battlefield applications for guided missiles, especially those that envisioned attacks on smaller targets, required a more reliable guidance method that was less vulnerable to jamming. Fortunately, this method became available as a result of an independent research effort into the effects of light amplification.

Dr. Theodore Maiman built the first laser (Light Amplification by Stimulated Emission of Radiation) at Hughes Research Laboratories in 1960. The military realized the potential applications for lasers almost as soon as their first beams cut through the air. Laser guided projectiles underwent their baptism of fire in the extended series of air raids that highlighted the American effort in the Vietnam War. The accuracy of these weapons earned them the well-known sobriquet of "smart weapons." But even this new generation of advanced weaponry could not bring victory to U.S. forces in this bitter and costly war. However, the combination of experience gained in Vietnam, refinements in laser technology, and similar advances in electronics and computers, led to more sophisticated and deadly laser guided missiles. They finally received widespread use in Operation Desert Storm, where their accuracy and reliability played a crucial role in the decisive defeat of Iraq's military forces. Thus, the laser guided missile has established itself as a key component in today's high-tech military technology.

Raw Materials
A laser guided missile consists of four important components, each of which contains different raw materials. These four components


The missile body is die-cast in halves: molten metal (either aluminum or steel) is poured into a metal die and cooled to form the proper shape. The two halves are then welded together.
The principal laser components—the photo detecting sensor and optical filters—are assembled in a series of operations that are separate from the rest of the missile's construction. Circuits that support the laser system are then soldered onto pre-printed boards. The circuit boards for the electronics suite are also assembled independently from the rest of the missile. If called for by the design, microchips are added to the boards at this time.
are the missile body, the guidance system (also called the laser and electronics suite), the propellant, and the warhead. The missile body is made from steel alloys or high-strength aluminum alloys that are often coated with chromium along the cavity of the body in order to protect against the excessive pressures and heat that accompany a missile launch. The guidance system contains various types of materials—some basic, others high-tech—that are designed to give maximum guidance capabilities. These materials include a photo detecting sensor and optical filters, with which the missile can interpret laser wavelengths sent from a parent aircraft. The photo detecting sensor's most important part is its sensing dome, which can be made of glass, quartz, and/or silicon. A missile's electronics suite can contain gallium-arsenide semiconductors, but some suites still rely exclusively on copper or silver wiring. Guided missiles use nitrogen-based solid propellants as their fuel source. Certain additives (such as graphite or nitroglycerine) can be included to alter the performance of the propellant. The missile's warhead can contain highly explosive nitrogen-based mixtures, fuel-air explosives (FAE), or phosphorous compounds. The warhead is typically encased in steel, but aluminum alloys are sometimes used as a substitute.

Design
Two basic types of laser guided missiles exist on the modern battlefield. The first type "reads" the laser light emitted from the launching aircraft/helicopter. The missile's electronic suite issues commands to the fins (called control surfaces) on its body in an effort to keep it on course with the laser beam. This type of missile is called a beam rider as it tends to ride the laser beam towards its target.

The second type of missile uses on-board sensors to pick up laser light reflected from the target. The aircraft/helicopter pilot selects a target, hits the target with a laser beam shot from a target designator, and then launches the missile. The missile's sensor measures the error between its flight path and the path of the reflected light. Correction messages are then passed on to the missile's control surfaces via the electronics suite, steering the missile onto its target.

Regardless of type, the missile designer must run computer simulations as the first step of the design process. These simulations assist the designer in choosing the proper laser type, body length, nozzle configurations, cavity size, warhead type, propellant mass, and control surfaces. The designer then puts together a package containing all relevant engineering calculations, including those generated by computer simulations. The electronics suite is then designed around the capabilities of the laser and control surfaces. Drawings and schematics of all components can now be completed; CAD/CAM (Computer-Aided Design/Manufacture) technology has proven helpful with this task. Electronics systems are then designed around the capabilities of the aircraft's laser and the missile's control surfaces. The following step consists of generating the necessary schematic drawings for the chosen electronics system. Another computer-assisted study of the total guided missile system constitutes the final step of the design process.

The Manufacturing
Process
Constructing the body and attaching the fins
•1 The steel or aluminum body is die cast in halves. Die casting involves pouring molten metal into a steel die of the desired shape and letting the metal harden. As it cools, the metal assumes the same shape as the die. At this time, an optional chromium coating can be applied to the interior surfaces of the halves that correspond to a completed missile's cavity. The halves are then welded together, and nozzles are added at the tail end of the body after it has been welded.
•2 Moveable fins are now added at predetermined points along the missile body. The fins can be attached to mechanical joints that are then welded to the outside of the body, or they can be inserted into recesses purposely milled into the body.
Casting the propellant
•3 The propellant must be carefully applied to the missile cavity in order to ensure a uniform coating, as any irregularities will result in an unreliable burning rate, which in turn detracts from the performance of the missile. The best means of achieving a uniform coating is to apply the propellant by using centrifugal force. This application, called casting, is done in an industrial centrifuge that is well-shielded and situated in an isolated location as a precaution against fire or explosion.
Assembling the guidance system
•4 The principal laser components—the photo detecting sensor and optical filters—are assembled in a series of operations that are separate from the rest of the missile's construction. Circuits that support the laser system are then soldered onto pre-printed boards; extra attention is given to optical materials at this time to protect them from excessive heat, as this can alter the wavelength of light that the missile will be able to detect. The assembled laser subsystem is now set aside pending final assembly. The circuit boards for the electronics suite are also assembled independently from the rest of the missile. If called for by the design, microchips are added to the boards at this time.
•5 The guidance system (laser components plus the electronics suite) can now be integrated by linking the requisite circuit boards and inserting the entire assembly into the missile body through an access panel. The missile's control surfaces are then linked with the guidance system by a series of relay wires, also entered into the missile body via access panels. The photo detecting sensor and its housing, however, are added at this point only for beam riding missiles, in which case the housing is carefully bolted to the exterior diameter of the missile near its rear, facing backward to interpret the laser signals from the parent aircraft.
Final assembly
•6 Insertion of the warhead constitutes the final assembly phase of guided missile

Current laser guided missiles work in one of two ways. The first type, a "beam rider,' reads the laser light emitted from the launching aircraft and rides the beam toward the target. The second type uses on-board sensors to pick up laser light sent by the aircraft and reflected from the target. The sensors measure the error between the missile's flight path and the path of the reflected light, and the electronics suite alters the control surfaces as necessary to guide the missile toward the target.
construction. Great care must be exercised during this process, as mistakes can lead to catastrophic accidents. Simple fastening techniques such as bolting or riveting serve to attach the warhead without risking safety hazards. For guidance systems that home-in on reflected laser light, the photo detecting sensor (in its housing) is bolted into place at the tip of the warhead. On completion of this final phase of assembly, the manufacturer has successfully constructed on of the most complicated, sophisticated, and potentially dangerous pieces of hardware in use today.
Quality Control
Each important component is subjected to rigorous quality control tests prior to assembly. First, the propellant must pass a test in which examiners ignite a sample of the propellant under conditions simulating the flight of a missile. The next test is a wind tunnel exercise involving a model of the missile body. This test evaluates the air flow around the missile during its flight. Additionally, a few missiles set aside for test purposes are fired to test flight characteristics. Further work involves putting the electronics suite through a series of tests to determine the speed and accuracy with which commands get passed along to the missile's control surfaces. Then the laser components are tested for reliability, and a test beam is fired to allow examiners to record the photo detecting sensor's ability to "read" the proper wavelength. Finally, a set number of completed guided missiles are test fired from aircraft or helicopters on ranges studded with practice targets.

Byproducts/Waste
Propellants and explosives used in warheads are toxic if introduced into water supplies. Residual amounts of these materials must be collected and taken to a designated disposal site for burning. Each state maintains its own policy pertaining to the disposal of explosives, and Federal regulations require that disposal sites be inspected periodically. Effluents (liquid byproducts) from the chromium coating process can also be hazardous. This problem is best dealt with by storing the effluents in leak-proof containers. As an additional safety precaution, all personnel involved in handling any hazardous wastes should be given protective clothing that includes breathing devices, gloves, boots and overalls.

The Future
Future laser guided missile systems will carry their own miniaturized laser on board, doing away with the need for target designator lasers on aircraft. These missiles, currently under development in several countries, are called "fire-and-forget" because a pilot can fire one of these missiles and forget about it, relying on the missile's internal laser and detecting sensor to guide it towards its target. A further development of this trend will result in missiles that can select and attack targets on their own. Once their potential has been realized, the battlefields of the world will feel the deadly venom of these "brilliant missiles" for years to come. An even more advanced concept envisions a battle rifle for infantry that also fires small, laser guided missiles. Operation Desert Storm clearly showed the need for laser guided accuracy, and, as a result, military establishments dedicated to their missions will undoubtedly invent and deploy ever more lethal versions of laser guided missiles.

Where To Learn More
Books
Bova, Ben. The Beauty of Light. John Wiley and Sons, 1988.

Hallmark, Clayton L. and Delton T. Horn. Lasers: The Light Fantastic. TAB Books, 1987.

Hecht, Jeff. Optics: Light for a New Age. Charles Scribner's Sons, 1987.

Iannini, Robert. E. Build Your Own Fiberoptic, Infrared, and Laser Space-Age Projects. TAB Books, 1987.

Laurence, Clifford L. The Laser Book: A New Technology of Light. Prentice Hall, 1986.

Von Braun, Wernher, Frederick I. Ordway III, and Dave Dooling. Space Travel: A History. Harper & Row, 1985.

Wood, Derek. Jane's World Aircraft Recognition Handbook. Jane's Information Group, 1989.

Wulforst, Harry. The Rocketmakers. Orion Books, 1990.

Periodicals
"A Dull Scalpel for the Surgical Strike on Libya." Discover. June, 1986, p. 8.

Lenorowitz, Jeffrey M. "F-1 17s Drop Laser-Guided Bombs in Destroying Most Baghdad Targets." Aviation Week and Space Technology. Feb. 4, 1991, p. 30.

Magnusson, Paul. "American Smart Bombs, Foreign Brains." Business Week. March 4, 1991, p. 18.

— Robert A. Cortese



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Automobile Windshield

Automobile Windshield

Background:
Glass is a versatile material with hundreds of applications, including windshields. Glass has a long history and was first made more than 7,000 years ago in Egypt, as early as 3,000 B.C. Glass is found in a natural state as a by-product of volcanic activity. Today, glass is manufactured from a variety of ceramic materials (main components are oxides). The main product categories are flat or float glass, container glass, cut glass, fiberglass, optical glass, and specialty glass. Automotive windshields fall into the flat glass category.

There are more than 80 companies worldwide that produce automotive glass, including windshields. Major producers in the United States include PPG, Guardian Industries Corp., and Libby-Owens Ford. According to the Department of Commerce, 25 percent of flat glass production is consumed by the automotive industry (including windows) at a total value of approximately $483 million. In Japan, 30 percent of flat glass goes to the automotive industry, valued at around $190 billion in 1989. Major Japanese flat glass manufacturers include Asahi Glass Co., Central Glass Co., and Nippon Sheet Glass Co. Little growth is expected for the flat glass industry overall in both countries. Germany has a more positive outlook, with high growth rates expected from the automotive industry.

Glass windshields first appeared around 1905 with the invention of safety glass—glass tempered (tempering is a heat treatment) to make it especially hard and resistant to shattering. This type of windshield was popular well into the middle of the century, but it was eventually replaced by windshields made of laminated glass—a multilayer unit consisting of a plastic layer surrounded by two sheets of glass. In many countries, including the U.S., auto windshields are required by law to be made of laminated glass. Laminated glass can bend slightly under impact and is less likely to shatter than normal safety glass. This quality reduces the risk of injury to the automobile's passengers.

Raw Materials
Glass is composed of numerous oxides that fuse and react together upon heating to form a glass. These include silica (SiO 2 ), sodium oxide (Na 2 O), and calcium oxide (CaO). Raw materials from which these materials are derived are sand, soda ash (Na 2 CO 3 ), and limestone (CaCO 3 ). Soda ash acts as a flux; in other words, it lowers the melting point of the batch composition. Lime is added to the batch in order to improve the hardness and chemical durability of the glass. Glass used for windshields also usually contains several other oxides: potassium oxide (K 2 O derived from potash), magnesium oxide (MgO), and aluminum oxide (AI 2 O 3 derived from feldspar).

The Manufacturing
Process
•1 The raw materials are carefully weighed in the appropriate amounts and mixed together with a small amount of water to prevent segregation of the ingredients. Cullet (broken waste glass) is also used as a raw material.
•2 Once the batch is made, it is fed to a large tank for melting using the float

The glass for automible windshields is made using the float glass process. In this method, the raw material is heated to a molten state and fed onto a bath of molten tin. The glass literally floats on top of the fin; because the fin is perfectly flat, the glass also becomes flat. From the float chamber, the glass passes on rollers through an oven (the "annealing lehr"). After exiting the lehr and cooling to room temperature, the glass is cut to the proper shape and tempered.
glass process. First, the batch is heated to a molten state, and then it is fed into a tank called the float chamber, which holds a bath of molten tin. The float chamber is very large—from about 13 feet to 26.25 feet (4 to 8 meters wide and up to almost 197 feet (60 meters) long; at its entrance, the temperature of the tin is about 1,835 degrees Fahrenheit (1,000 degrees Celsius), while at the exit the tin's temperature is slightly cooler—1,115 degrees Fahrenheit (600 degrees Celsius). In the float chamber, the glass doesn't submerge into the tin but floats on top of it, moving through the tank as though on a conveyor belt. The perfectly flat surface of the tin causes the molten glass also to become flat, while the high temperatures clean the glass of impurities. The decreased temperature at the exit of the chamber allows the glass to harden enough to move into the next chamber, a furnace.
•3 After the glass exits from the float chamber, rollers pick it up and feed it into a special furnace called a lehr. (If any solar coatings are desired, they are applied before the glass enters the lehr.) In this furnace, the glass is cooled gradually to about 395 degrees Fahrenheit (200 degrees Celsius); after the glass exits the lehr, it cools to room temperature. It is now very hard and strong and ready to be cut.
Cutting and tempering
•4 The glass is cut into the desired dimensions using a diamond scribe—a tool with sharp metal points containing diamond dust. Diamond is used because it is harder than glass. The scribe marks a cut line into the glass, which is then broken or snapped at this line. This step is usually automated and is monitored by cameras and optoelectronic measuring systems. Next, the cut piece must be bent into shape. The sheet of glass is placed into a form or mold of metal or refractory material. The glass-filled mold is then heated in a furnace to the point where the glass sags to the shape of the mold.
•5 After this shaping step, the glass must be hardened in a heating step called tempering. First, the glass is quickly heated to about 1,565 degrees Fahrenheit (850 degrees Celsius), and then it is blasted with jets of cold air. Called quenching, this process toughens the glass by putting the outer surface into compression and the inside into tension. This allows the windshield, when damaged, to break into many small pieces of glass without sharp edges. The size of the pieces can also be changed by modifying the tempering procedure so that the windshield breaks into larger pieces, allowing good vision until the wind-shield can be replaced.


A finished windshield consists of two glass layers sandwiched around a plastic interlayer. Although very thin—about .25 inch thick—such laminated glass is very strong and is less likely to shatter than normal safety glass. In the United States, windshields are required by law to be made of laminated glass.

Laminating
•6 After the glass is tempered and cleaned, it goes through a laminating process. In this process, two sheets of glass are bonded together with a layer of plastic (the plastic layer goes inside the two glass sheets). The lamination takes place in an autoclave, a special oven that uses both heat and pressure to form a single, strong unit that is resistant to tearing. The plastic interlayer is often tinted to act as an ultraviolet filter. When laminated glass is broken, the broken pieces of glass remain bound to the internal tear-resistant plastic layer, and the broken sheet remains transparent. Thus, visibility remains good. Unlike traditional safety glass, laminated glass can be further processed—cut, drilled, and edge-worked, as necessary. A typical laminated windshield is very thin: each glass layer is approximately .03 inch (.76 millimeter) thick, while the plastic interlayer is approximately .098 inch (2.5 millimeters) thick.
Assembly
•7 After laminating, the windshield is ready to be assembled with plastic moldings so it can be installed on the car. Known as glass encapsulation, this assembly process is usually done at the glass manufacturer. First, the peripheral section of the windshield is set in a predetermined position in a mold cavity. Next, molten plastic is injected into the mold; when it cools, it forms a plastic frame around the glass. The windshield assembly is then shipped to the car manufacturer, where it is installed in an automobile. The installation is done by direct glazing, a process that uses a polyurethane adhesive to bond the windshield and automobile body.
Quality Control
Process control includes testing of raw materials and monitoring such process variables as melting temperature, furnace atmosphere, and glass level. As the glass is formed, photoelectric devices are used to inspect for defects automatically. Other automatic devices have been developed to measure dimensions and radius of curvature after the windshield has been formed.

Safety glass used in windshields must meet certain specifications regarding properties such as chemical durability, impact resistance, and strength. Standards have been developed by the American Society for Testing of Materials (ASTM) for measuring these properties. Specifications have also been developed for windshield performance by SAE International, an organization of automotive engineers.

The Future
Despite the recent downturn in the automotive industry, long-term prospects are more optimistic. Motor vehicle production markets will be stronger than in recent years, raising demand for flat glass products such as windshields. Windshields are also increasing in size in order to accommodate newer aerodynamic designs, and thus the use of glass is increasing relative to the total surface area of vehicles. (In fact, some models are incorporating glass roofs as well.)

Such increase in glass area, in turn, has a negative impact on comfort systems, namely air conditioners, which must be able to adjust the higher interior temperatures to a comfortable level. To avoid having to use larger air conditioning systems, new glass compositions, coated glasses, and aftermarket films are being evaluated. These include angle-selective glazings that reject high-angle sun, and optical switching films that actively or passively change transmittance properties.

One recently developed film, a polymer multilayer solar control film, can also act as a deicing device. The coated plastic substrate simply replaces the laminated plastic film in conventional windshields. The film can be made in any color and can transmit up to 90 percent of the visible light. Another coating is a glaze that consists of silver coating used in combination with other metal oxide layers. This glaze can reject up to 60 percent of the total solar energy, reducing the infrared energy by 56 percent.

In addition, new types of laminated-glass windshields are being researched. A bi-layer windshield has been developed that only requires one outer sheet of glass, .08 to .16 of an inch (2-4 millimeters) thick, joined to a .254 of an inch (1 millimeter) sheet of polyurethane. The polyurethane sheet consists of two layers, one having high absorption properties and the other high surface resistance. Unique features of this bi-layer windshield include ultraviolet resistance, self-healing of scratches, weight savings, more complex shapes, increased safety due to retention of glass splinters, and anti-fog capability.

Recycling of windshield components may also become a standard practice. Though traditionally recycling has been difficult because of the plastic laminated films, one manufacturer has recently developed a cost-effective process to remove these layers. The recycled glass can be used in several applications, including glassphalt for road repair. Legislation may also speed up recycling practices, with the introduction of the Municipal Solid Waste and Hazardous Waste Research Act of 1992. This bill seeks to determine the obstacles to increased automotive components recycling and find ways to overcome these obstacles. This may eventually require using fewer resins during manufacturing or making sure these resins are compatible for recycling.

Where To Learn More
Books
McLellan, George W. and E. B. Shand, eds. Glass Engineering Handbook. 3rd ed., McGraw-Hill, 1984.

Pfaender, Heinz G. and Hubert Schroeder. Schott Guide To Glass. Van Nostrand Reinhold, 1983.

Scholes, Samuel R. Modern Glass Practice. CBI Publishing Company, 1975.

Periodicals
"Bill To Overcome Recycling Obstacles," Autoglass. July/August, 1992.

"Guardian Produces Largest Production Car Windshield," Autoglass, March/April, 1992.

Leventon, William. "Press and Vacuum Form Complex Windshields," Design News. November 9, 1992, p. 159.

Olosky, M. L. and M. J. Watson. "Silicon Film Adhesives: Bonding Automotive Fixtures to Glass," SAE Paper No. 931013. SAE International, 1993.

Peters, G. M. and T. W. Karwan, et al. "A Cost Effective Quality Improvement for Automotive Glass Encapsulation," SAE Paper No. 931012. SAE International, 1993.

Sheppard, L. M. "Automotive Performance Accelerate with Ceramics," Ceramic Bulletin. 1990, pp. 1012-1021.

— L. S. Millberg



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Engineering concept of the social elements: Spark Plug

Engineering concept of the social elements: Spark Plug: "Spark Plug Background: The purpose of a spark plug is to provide a place for an electric spark that is hot enough to ignite the..."

Blogger Buzz: Blogger integrates with Amazon Associates

Blogger Buzz: Blogger integrates with Amazon Associates

Spark Plug

Spark Plug

Background:
The purpose of a spark plug is to provide a place for an electric spark that is hot enough to ignite the air/fuel mixture inside the combustion chamber of an internal combustion engine. This is done by a high voltage current arcing across a gap on the spark plug



A spark plug is made of a center electrode, an insulator, a metal casing or shell, and a side electrode (also called a ground electrode). The center electrode is a thick metal wire that lies lengthwise within the plug and conducts electricity from the ignition cable hooked to one end of the plug to the electrode gap at the other end. The insulator is a ceramic casing that surrounds much of the center electrode; both the upper and lower portions of the center electrode remain exposed. The metal casing or shell is a hexagon-shaped shell with threads, which allow the spark plug to be installed into a tapped socket in the engine cylinder head. The side electrode is a short, thick wire made of nickel alloy that is connected to the metal shell and extends toward the center electrode. The tips of the side and center electrodes are about 0.020 - 0.080 inch apart from each other (depending on the type of engine), creating the gap for the spark to jump across.

The several hundred types of spark plugs available cover a variety of internal-combustion engine-driven transportation, work, and pleasure vehicles. Spark plugs are used in automobiles, trucks, buses, tractors, boats (inboard and outboard), aircraft, motorcycles, scooters, industrial and oil field engines, oil burners, power mowers and chain saws. Turbine igniters, a type of spark plug, help power the jet engines in most large commercial aircraft today while glow-plugs are used in diesel engine applications.

The heat range or rating of a spark plug refers to its thermal characteristics. It is the measure of how long it takes heat to be removed from the tip of the plug, the firing end, and transferred to the engine cylinder head. At the time of the spark, if the plug tip temperature is too cold, carbon, oil, and combustion products can cause the plug to "foul out" or fail. If the plug tip temperature is too hot, preignition occurs, the center electrode burns, and the piston may be damaged. Heat range is changed by altering the length of the insulator nose, depending on the type of engine, the load on the engine, the type of fuel, and other factors. For a "hot" plug, an insulator with a long conical nose is used; for a "cold" plug, a short-nosed insulator is used.

Spark plugs are under constant chemical, thermal, physical, and electrical attack by corrosive gases at 4,500 degrees Fahrenheit, crushing pressures of 2,000 pounds per square inch (PSI), and electrical discharges of up to 18,000 volts. This unrelenting assault under the hood of a typical automobile occurs dozens of times per second and over a million times in a day's worth of driving.

History
The spark plug evolved with the internal combustion engine, but the earliest demonstration of the use of an electric spark to ignite a fuel-air mixture was in 1777. In that year, Alessandro Volta loaded a toy pistol with a mixture of marsh gas and air, corked the muzzle, and ignited the charge with a spark from a Ley den jar.

In 1860, French engineer Jean Lenoir created what most closely resembles the spark plug


To make spark plugs, manufacturers first extrude or cold-form steel to the proper hollow shape (1). At this point, the steel forms ore called "blanks." Next, these blanks undergo further forming operations such as machining and knurling (2), and then the side electrode—with only a partial bend—is attached (3). The ceramic insulator, with a hollow bore through its center, is molded under pressure (4).
of today. He combined an insulator, electrodes, and spark gap in a single unit. As part of his patent application for the internal combustion engine that year, he devoted one sentence to describing the spark plug. He refined this spark plug in 1885.

In the early 1900s, Robert and Frank Stranahan, brothers and partners in an automobile parts importing business, set out to produce a more efficient and durable spark plug. They added gaskets between the metal shell and porcelain insulator, made manufacturing easier, and reduced the possibility of gas leakage past the gaskets. In 1909, Robert Stranahan sold the plug to one automobile manufacturer and went into the spark plug manufacturing business, cornering the market at that time.

The industry exploded as the age of the automobile opened. Eventually, variations in ignition systems, fuel, and performance requirements placed new demands on spark plugs. Although the basic design and function of the plug has changed little since its inception, a staggering variety and number of electrode and insulator materials have been tried.

Raw Materials
The electrodes in a spark plug typically consist of high-nickel alloys, while the insulator is generally made of aluminum oxide ceramic and the shell is made of steel wire.

Selection of materials for both the electrodes and the insulator have consumed much research and development time and cost. One major spark plug manufacturer claims to have tested 2,000 electrode materials and over 25,000 insulator combinations. As electrodes erode, the gap between them widens, and it takes more voltage than the ignition system can provide to fire them. High-nickel alloys have been improved and thicker electrodes have been used to reduce engine performance loss. In addition, precious and exotic metals are increasingly being used by manufacturers. Many modern plugs feature silver, gold, and platinum in the electrodes, not to mention center electrodes with copper cores. Silver has superior thermal conductivity over other electrode metals, while platinum has excellent corrosion resistance.

Insulator material also can have a dramatic effect on spark plug performance. Research continues to find a material that better reduces flashover, or electrical leakage, from the plug's terminal to the shell. The breakthrough use of Sillimanite, a material that is found in a natural state and also produced artificially, has been succeeded by the use of more heat-resistant aluminum oxide ceramics, the composition of which are manufacturers' secrets.

One major manufacturer's process for making the insulator involves wet grinding batches of ceramic pellets in ball mills, under carefully controlled conditions. Definite size and shape of the pellets produce the free-flowing substance needed to make a quality insulator. The pellets are obtained through a rigid spray-drying operation that removes the water from the ceramic mixture, until it is ready for pouring into molds.

The Manufacturing
Process
Each major element of the spark plug—the center electrode, the side electrode, the insulator, and the shell—is manufactured in a continuous in-line assembly process. Then, the side electrode is attached to the shell and the center electrode is fitted inside the insulator. Finally, the major parts are assembled into a single unit.

Shell
•1 The one-piece spark plug shells can be made in several ways. When solid steel wire is used, the steel can be cold-formed, whereby coils of steel are formed and molded at relatively low temperatures. Or, the steel can be extruded, a process in which the metal is heated and then pushed through a shaped orifice (called a die) to produce the proper hollow shape. Shells can also be made from bars of steel that are fed into automatic screw machines. These machines completely form the shell, drill the hole through it, and ream it—a process that improves the finish of the drilled hole and makes the size of the hole more exact.
•2 The formed or extruded shells—called blanks until they're molded into their final shapes—require secondary operations to be performed on them, such as machining and knurling. Knurling a shell blank involves passing it through hard, patterned rollers, which form a series of ridges on the outside of the blank. Similarly, machining-—in which machine tools cut into the exterior of the shell blank—generates shapes and contours on the outside of the shell. The shells are now in their final shape and are complete except for threads and side electrodes.
Side electrode
•3 The side electrode is made of a nickel alloy wire, which is fed from rolls into an electric welder, straightened, and welded to the shell. It is then cut to the proper length. Finally, the side electrode is given a partial bend; it is given its final bend after the rest of the plug assembly is in place.
•4 The threads are then rolled on the shells. Now complete, the shells are usually given a permanent and protective silvery finish by an electrolytic process. In this process, the shell is placed in a solution of acids, salts, or alkalis, and an electrical current is passed through the solution. The result is a thin metal coating applied evenly over the shell.
Insulator
•5 Insulators are supplied from stock storage. Ceramic material for the insulator in liquid form is first poured into rubber molds. Special presses automatically apply hydraulic pressure to produce unfired insulator blanks. The dimensions of the bore—the hollow part of the insulator—into which the center electrodes will be pressed are rigidly controlled.
•6 Special contour grinding machines give the pressed insulator blanks their final exterior shape before the insulators are fired in a tunnel kiln to temperatures in excess of 2,700 degrees Fahrenheit. The computer-controlled process produces insulators that are uniformly strong, dense, and resistive to moisture. The insulators may be fired again after identifying marks and a glaze are applied.
Center electrode
•7 The nickel alloy center electrode is first electrically welded to the basic steel terminal stud, a narrow metal wire that runs from the middle of the plug to the lower end (the opposite end from the electrode gap). The terminal stud is attached to a nut, which in turn is attached to the ignition cable that supplies the electric current to the plug.
•8 The center electrode/terminal stud assembly is sealed into the insulator and tamped under extreme pressure. Insulator assemblies are then sealed in the metal shell under 6,000 pounds pressure. After reaming to correct depth and angle, the rim or edge of the shell—called the flange —is bent or crimped to complete a gas-tight seal. Spark plug gaskets from stock are crimped over the plug body so that they won't fall off.
•9 To form the proper gap between the two electrodes, the center electrode of the now completely assembled spark plug is machine-trimmed to specifications, and the ground electrode is given a final bend.


The terminal stud and center electrode are electrically welded together and then inserted through the bore inside the insulator (5). This assembly is then sealed under extreme pressure. Finally, the center electrode is machined to its exact shape, and the side electrode is given its final bend (6).

Packaging
•10 After a final inspection, the spark plugs are placed in open cartons that have been automatically formed. The plugs are generally wrapped in plastic film, placed first in a carton, and then prepared for shipping in quantity to users.
Quality Control
Inspections and measurements are performed throughout the manufacturing and assembly operations. Both incoming parts and tooling are inspected for accuracy. New gauges are set up for use in production while other gauges are changed and calibrated.

Detailed inspections of shells from each machine are constantly made for visible flaws. The ceramic insulator contour can be checked by projecting its silhouette onto a screen at a magnification of 20 times actual size and matching the silhouette to tolerance lines. In addition, regular statistical inspections can be made on insulators coming off the production line.

During spark plug assembly, a random sampling are pressure tested to check that the center electrode is properly sealed inside the insulator. Visual inspections assure that assembly is in accordance with design specifications.

Where To Learn More
Books
Heywood, John. Internal Combustion Engine Fundamentals. McGraw-Hill, 1988.

Schwaller, Anthony. Motor Automotive Mechanics. Delmar Publishers, 1988.

Periodicals
Davis, Marlan. "Fire in the Hole: Spark-plug Design Heats up with New High-tech Materials and Design Concepts." Hot Rod. February, 1990.

"Spark Plug 'Sees' Inside Engines." Design News. October 17, 1989.

"Hot Spark Basics." Popular Mechanics. May, 1989.

— Peter Toeg

Pressure Gauge

Pressure Gauge
Background:

Many of the processes in the modern world involve the measurement and control of pressurized liquid and gas systems. This monitoring reflects certain performance criteria that must be controlled to produce the desirable results of the process and insure its safe operation. Boilers, refineries, water systems, and compressed gas systems are but a few of the many applications for pressure gauges.

The mechanical pressure indicating instrument, or gauge, consists of an elastic pressure element; a threaded connection means called the "socket"; a sector and pinion gear mechanism called the "movement"; and the protective case, dial, and viewing lens assembly. The elastic pressure element is the member that actually displaces or moves due to the influence of pressure. When properly designed, this pressure element is both highly accurate and repeatable. The pressure element is connected to the geared "movement" mechanism, which in turn rotates a pointer throughout a graduated dial. It is the pointer's position relative to the graduations that the viewer uses to determine the pressure indication.

The most common pressure gauge design was invented by French industrialist Eugene Bourdon in 1849. It utilizes a curved tube design as the pressure sensing element. A less common pressure element design is the diaphragm or disk type, which is especially sensitive at lower pressures. This article will focus on the Bourdon tube pressure gauge.

Design
In a Bourdon tube gauge, a "C" shaped, hollow spring tube is closed and sealed at one end. The opposite end is securely sealed and bonded to the socket, the threaded connection means. When the pressure medium (such as air, oil, or water) enters the tube through the socket, the pressure differential from the inside to the outside causes the tube to move. One can relate this movement to the uncoiling of a hose when pressurized with water, or the party whistle that uncoils when air is blown into it. The direction of this movement is determined by the curvature of the tubing, with the inside radius being slightly shorter than the outside radius. A specific amount of pressure causes the "C" shape to open up, or stretch, a specific distance. When the pressure is removed, the spring nature of the tube material returns the tube to its original shape and the tip to its original position relative to the socket.

Raw Materials
Pressure gauge tubes are made of many materials, but the common design factor for these materials is the suitability for spring tempering. This tempering is a form of heat treating. It causes the metal to closely retain its original shape while allowing flexing or "elasticity" under load. Nearly all metals have some degree of elasticity, but spring tempering reinforces those desirable characteristics. Beryllium copper, phosphor bronze, and various alloys of steel and stainless steel all make excellent Bourdon tubes. The type of material chosen depends upon its corrosion properties with regards to the process media (water, air, oil, etc). Steel has a limited service life due to corrosion but is adequate for oil; stainless steel alloys add cost if specific corrosion resistance is not required; and beryllium copper is usually reserved for high pressure applications. Most gauges intended


A crucial step in the manufacture of a pressure gauge is making the C-shaped bourdon tube. In this step, a metal tube is pulled through grooved rollers on an automatic rolling machine. One roller grasps the tubing end and forms the inside radius, while the other provides outside pressure to maintain uniform contact with the tubing. The same roller that grabs and bends the tubing also contains a saw blode. As the roller continues turning after creating the bend, the saw blade on it cuts the tubing to the proper length.
for general use of air, light oil, or water utilize phosphor bronze. The pressure range of the tubes is determined by the tubing wall thickness and the radius of the curvature. Instrument designers must use precise design and material selection, because exceeding the elastic limit will destroy the tube and accuracy will be lost.

The socket is usually made of brass, steel, or stainless steel. Lightweight gauges sometimes use aluminum, but this material has limited pressure service and is difficult to join to the Bourdon tube by soldering or brazing. Extrusions and rolled bar stock shapes are most commonly used.

The movement mechanism is made of glass filled polycarbonate, brass, nickel silver, or stainless steel. Whichever material is used, it must be stable and allow for a friction-free assembly. Brass and combinations of brass and polycarbonate are most popular.

To protect the Bourdon tube and movement, the assembly is enclosed within a case and viewing lens. A dial and pointer, which are used to provide the viewer with the pressure indication, are made from nearly all basic metals, glass, and plastics. Aluminum, brass, and steel as well as polycarbonate and polypropylene make excellent gauge cases and dials. Most lenses are made of polycarbonate or acrylic, which are in favor over glass for obvious safety reasons. For severe service applications, the case is sealed and filled with glycerine or silicone fluid. This fluid cushions the tube and movement against damage from impact and vibration.



After the Bourdon tube is made, its closed end is attached to the socket by soldering, brazing, or welding. The free end of the Bourdon tube is precisely located during this assembly operation, and then sealed, usually by the some means used to join the tube to the socket.
Once the Bourdon tube and socket assembly is secure, the tip of the unsupported end of the 'C ' is attached to an endpiece. This endpiece contains a small hole that connects the tip to the geared movement mechanism. The other components—the movement, pointer, and dial—are then assembled onto the socket as a group.

The Manufacturing
Process
Making the Bourdon tube
•1 The Bourdon tube is the most important part of the instrument. The tube may be made from solid bar stock by drilling the length to the desired inside diameter and turning the outside diameter on a lathe to achieve the appropriate wall thickness. However, most general purpose gauges utilize preformed tubing purchased from a metals supplier. The gauge builder specifies the desired wall thickness, material, configuration, and diameter. The supplier provides the material in 10- to 12-foot (3- to 3.65-meter) lengths, ready for production.
•2 Most manufacturers have closely guarded proprietary rolling methods for rolling the tubing into the "C" shape. The "C" shape of the tube is generally formed in an automatic rolling machine. This machine contains two precision, powered rollers, through which the tubing passes. One roller grasps the tubing end and forms the inside radius, while the other provides outside pressure to maintain uniform contact with the tubing. Each roller contains a groove that fits around the outside of the tubing; these grooves allow the tubing to maintain its circular shape rather than being flattened. In the rolling process, a steel mandrel—a bar that guides the tubing into the rollers and helps it keep its shape—is first inserted though the free end of the tubing and positioned just before the rollers. This lubricated mandrel is of the desired interior shape of the oval. The tubing then passes over the mandrel and between the rollers. One roller contains a clip that grabs the tubing; as the roller turns, it pulls the tubing and bends it into the "C" shape.
•3 The same roller that grabs and bends the tubing also contains a saw blade. As the roller continues turning after creating the bend, the saw blade on it cuts the tubing to the proper length. The tubing is then heat treated in ovens.
Other components
•4 The socket is basically a block of metal that serves as a connector to the source of the pressure medium; a mount for the case, dial, and movement; and as an attachment slot for the Bourdon tube. One end of the socket is threaded, which allows it to be screwed into the pressure-providing apparatus. The socket may be cast, forged, extruded, or machined from bar stock. Most sockets are made on automated machining centers that turn, drill, mill, and thread all in one cycle. General machining practices apply to most socket manufacture.
•5 Movements are geared mechanisms that contain a pinion (a rotating shaft), sector, support plates, hairspring, and spacer columns. The mechanism converts the somewhat linear displacement of the Bourdon tip into rotary movement, as well as providing a means for calibration adjustment. The pointer is fastened to the rotating shaft, or pinion, and sweeps across the graduated dial indicating the pressure amount. Most movements are supplied to the gauge builder ready to use. Many types of manufacturing processes are used to produce the movement components, and the workmanship of the mechanism closely resembles a clockwork when completed.
•6 The case, dial, and pointer may be sheet metal stampings, plastic moldings, or castings. Stampings and moldings require little further processing, but castings will require some machining—trimming off excess material, for instance—to meet the final requirements. These components are painted as required, and the dials are printed with the appropriate artwork. Common printing practice, utilizing both offset and direct methods, is used. The lens most commonly is a plastic part made by injection molding, whereby the plastic is heated into a molten state and then poured into a mold of the desired shape. The attachment feature that secures and seals the lens to the case is designed into the mold. Glass lenses are still used, but must be retained by a ring of some type. Glass has fallen out of favor because of the safety problems of breakage.
Final assembly
•7 After the Bourdon tube is made, its closed end is attached to the socket by soldering, brazing, or welding. The free end of the Bourdon tube is precisely located during this assembly operation, and then sealed, usually by the same means used to join the tube to the socket. Once the Bourdon tube and socket assembly is secure, the tip of the unsupported end of the "C" is attached to an endpiece. This endpiece contains a small hole that connects the tip to the geared movement mechanism. The Bourdon tip doesn't move a great distance within its pressure range, typically .125 to .25 inch (.31 to .63 centimeter). Understandably, the greater the pressure, the farther the tip moves. The other components—the movement, pointer, and dial—are then assembled onto the socket as a group.
Calibration
Calibration occurs just before the final assembly of the gauge to the protective case and lens. The assembly consisting of the socket, tube, and movement is connected to a pressure source with a known "master" gauge. A "master" gauge is simply a high accuracy gauge of known calibration. Adjustments are made in the assembly until the new gauge reflects the same pressure readings as the master. Accuracy requirements of 2 percent difference are common, but some may be 1 percent, .5 percent, or even .25 percent. Selection of the accuracy range is solely dependant upon how important the information desired is in relationship to the control and safety of the process. Most manufacturers use a graduated dial featuring a 270 degree sweep from zero to full range. These dials can be from less than I inch (2.5 centimeters) to 3 feet (.9 meter) in diameter, with the largest typically used for extreme accuracy. By increasing the dial diameter, the circumference around the graduation line is made longer, allowing for many finely divided markings. These large gauges are usually very fragile and used for master purposes only. Masters themselves are inspected for accuracy periodically using dead weight testers, a very accurate hydraulic apparatus that is traceable to the National Bureau of Standards in the United States.

It is interesting to note that when the gauge manufacturing business was in its infancy, the theoretical design of the pressure element was still developing. The Bourdon tube was made with very general design parameters, because each tube was pressure tested to determine what range of service it was suitable for. One did not know exactly what pressure range was going to result from the rolling and heat treating process, so these instruments were sorted at calibration for specific application. Today, with the development of computer modeling and many decades of experience, modern Bourdon tubes are precisely rolled to specific dimensions that require little, if any, calibration. Modern calibration can be performed by computers using electronically controlled mechanical adjusters to adjust the components. This unfortunately eliminates the image of the master craftsman sitting at the calibration bench, finely tuning a delicate, watch-like movement to extreme precision. Some instrument repair shops still perform this unique work, and these beautiful pressure gauges stand as equals to the clocks and timepieces created by master craftsmen years ago.

Applications and Future
Once the calibrated gauge is assembled and packaged, it is distributed to equipment manufacturers, service companies, and testing laboratories for use in many different applications. These varied applications account for the wide range in design of the case and lens enclosure. The socket may enter the case from the back, top, bottom or side. Some dials are illuminated by the luminescent inks used to print the graduations or by tiny lamps connected to an outside electrical source. Gauges intended for high pressure service usually are of "dead front" safety design, a case design feature that places a substantial thickness of case material between the Bourdon tube and the dial. This barrier protects the instrument viewer from gauge fragments should the Bourdon tube rupture due to excess pressure. The internal case design directs these high velocity pieces out the back of the gauge, away from the viewer. Many applications involve mounting the gauge directly to the running machinery, resulting in the need for liquid filling. Unfilled gauges quickly succumb to the destructive effects of vibration. Special mounting flanges are secured to the cases to allow for panel and surface mounting independent of the pressure plumbing. Case and lens materials are chosen to cope with a variety of abusive or contaminated environments, and are sealed by various means to keep moisture and contaminants out of the movement mechanism.

The use of pressure gauges in the future appears to be dependant on the quickly growing electronic sensor industry. These sensors are electronic components that provide an electrical signal and have essentially no moving parts. Many gauges today already have these sensors mounted within the case to send information to process control computers and controllers. These sensors are intrinsically safe, allowing their use in flammable or explosive environments. The whole process control issue has grown in recent years as a result of the need to prevent accidental releases of the process media, many of which are harmful to the environment. As environmental concerns grow, this interface will be in demand and the mechanical gauge may fall out of favor. However, the mechanical gauge does not require the electrical power source or the computer equipment needed by the electronic sensor. That makes the gauge cost effective for most general uses, and it is in this area that industry expects to continue to thrive.

Where To Learn More
Books
Kardos, Geza, ed. Bourdon Tubes and Bourdon Tube Gauges: An Annotated Bibliography. Books on Demand, 1989.

Pressure Gauge Handbook. M. Dekker, 1985.

Periodicals
Arslanian, Russ. "How to Select a Pressure Calibration Device." InTech. June, 1989, pp. 84-85.

Garrett, D. Dewayne and M. C. Banta. "A Suggested Improvement for the Fabrication of Low-Cost Manometers." Journal of Chemical Education. June, 1990, p. 523.

Jimenez-Dominguez, H., F. Figueroa-Lara, and S. Galindo. "Bourdon Gauge Absolute Manometer." Review of Scientific Instruments. March, 1986, p. 499.

— Douglas E. Betts



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Mirror

Mirror
Background:

From the earliest recorded history, humans have been fascinated by reflections. Narcissus was supposedly bewitched by his own reflection in a pool of water, and magic powers are ascribed to mirrors in fairy tales. Mirrors have advanced from reflective pools and polished metal surfaces to clear glass handheld and bathroom mirrors. They have been used in interior decoration since the 17th century, and reflective surfaces on cars and in hotel lobbies are still popular in modern design. Mirrors are used for practical purposes as well: examining our appearance, examining what is behind us on the road, building skyscrapers, and making scientific research instruments, such as microscopes and lasers.

The nature of modernn mirrors is not fundamentally different from a pool of water. When light strikes any surface, some of it will be reflected. Mirrors are simply smooth surfaces with shiny, dark backgrounds that reflect very well. Water reflects well, glass reflects poorly, and polished metal reflects extremely well. The degree of reflectivity—how much light bounces off of a surface—and the diffusivity of a surface—what direction light bounces off of a surface—may be altered. These alterations are merely refinements, however. In general, all reflective surfaces, and hence, all mirrors, are really the same in character.

Man-made mirrors have been in existence since ancient times. The first mirrors were often sheets of polished metal and were used almost exclusively by the ruling classes. Appearance often reflected, and in some cases determined, position and power in society, so the demand for looking glasses was high, as was the demand for the improvement of mirror-making techniques. Silvering—the process of coating the back of a glass sheet with melted silver—became the most popular method for making mirrors in the 1600s. The glass used in these early mirrors was often warped, creating a ripple in the image. In some severe cases, the images these mirrors reflected were similar to those we'd see in a fun-house mirror today. Modern glassmaking and metallurgical techniques make it easy to produce sheets of glass that are very flat and uniformly coated on the back, improving image clarity tremendously. Still, the quality of a mirror depends on the time and materials expended to make it. A handheld purse mirror may reflect a distorted image, while a good bathroom mirror will probably have no noticeable distortions. Scientific mirrors are designed with virtually no imperfections or distorting qualities whatsoever.

Materials technology drastically affects the quality of a mirror. Light reflects best from surfaces that are non-diffusive, that is, smooth and opaque, rather than transparent. Any flaw in this arrangement will detract from the effectiveness of the mirror. Innovations in mirror making have been directed towards flattening the glass used and applying metal coatings of uniform thickness, because light traveling through different thicknesses of glass over different parts of a mirror results in a distorted image. It is due to these irregularities that some mirrors make you look thinner and some fatter than normal. If the metal backing on a mirror is scratched or thin in spots, the brightness of the reflection will also be uneven. If the coating is very thin, it may be possible to see through the mirror. This is how one-way mirrors are made. Non-opaque coating is layered over the thin, metal backing and only one side of the mirror (the reflecting side) is lit. This allows a viewer on the other side, in a darkened room, to see through.

Raw Materials
Glass, the main component of mirrors, is a poor reflector. It reflects only about 4 percent of the light which strikes it. It does, however, possess the property of uniformity, particularly when polished. This means that the glass contains very few pits after polishing and will form an effective base for a reflective layer of metal. When the metal layer is deposited, the surface is very even, with no bumps or wells. Glass is also considered a good material for mirrors because it can be molded into various shapes for specialty mirrors. Glass sheets are made from silica, which can be mined or refined from sand. Glass made from natural crystals of silica is known as fused quartz. There are also synthetic glasses, which are referred to as synthetic fused silica. The silica, or quartz, is melted to high temperatures, and poured or rolled out into sheets.

A few other types of glass are used for high-quality scientific grade mirrors. These usually contain some other chemical component to strengthen the glass or make it resistant to certain environmental extremes. Pyrex, for example, is a borosilicate glass—a glass composed of silica and boron—that is used when mirrors must withstand high temperatures.

In some cases, a plastic substrate will do as well as a glass one. In particular, mirrors on children's toys are often made this way, so they don't break as easily. Plastic polymers are manufactured from petroleum and other organic chemicals. They can be injection molded into any desired shape, including flat sheets and circles, and can be opaque or transparent as the design requires.

These base materials must be coated to make a mirror. Metallic coatings are the most common. A variety of metals, such as silver, gold, and chrome, are appropriate for this application. Silver was the most popular mirror backing one hundred years ago, leading to the coinage of the term "silvering." Old silver-backed mirrors often have dark lines behind the glass, however, because the material was coated very thinly and unevenly, causing it to flake off, scratch or tarnish. More recently, before 1940, mirror manufacturers used mercury because it spread evenly over the surface of the glass and did not tarnish. This practice was also eventually abandoned, for it posed the problem of sealing in the toxic liquid. Today, aluminum is the most commonly used metallic coating for mirrors.

Scientific grade mirrors are sometimes coated with other materials, like silicon oxides and silicon nitrides, in up to hundreds of layers of, each a 10,000th of an inch thick. These types of coatings, referred to as dielectric coatings, are used both by themselves as reflectors, and as protective finishes on metallic coatings. They are more scratch resistant than metal. Scientific mirrors also use silver coatings, and sometimes gold coatings as well, to reflect light of a particular color of light more or less well.

Design
Surface regularity is probably the most important design characteristic of mirrors. Mirrors for household use must meet roughly the same specifications as window panes and picture frame glass. The glass sheets used must be reasonably flat and durable. The designer need only specify the thickness required; for example, thicker mirrors are more durable, but they are also heavier. Scientific mirrors usually have specially designed surfaces. These surfaces must be uniformly smooth within several lOOOths of an inch, and can be designed with a specific curvature, just like eyeglass lenses. The design principle for these mirrors is the same as that of eyewear: a mirror may be intended to focus light as well as reflect it.

The mirror design will also specify the type of coating to be used. Coating material is chosen based on required durability and reflectivity and, depending on the intended purpose of the mirror, it may be applied on the front or back surface of the mirror. Any subsequent layers of protective coatings must also be specified at this stage. For most common mirrors, the reflective coating will be applied on the back surface of the glass because it is less likely to be harmed there. The back side is then frequently mounted in a


The initial step in mirror manufacture involves cutting and shaping the glass blanks. Cutting is usually done with a saw with diamond dust embedded in the tips. Next, the blanks are put in optical grinding machines, which use abrasive liquid plus a grinding plate to produce a very even, smooth finish on the blanks. The reflective material is then applied in an evaporator, which heats the metal coating until it evaporates onto the surface of the blanks.
plastic or metal frame so as to entirely seal the coating from the air and sharp objects.

For scientific use, the color, or wavelength of light, which the mirror will reflect must be considered. For standard visible light or ultraviolet light mirrors, aluminum coatings are common. If the mirror is to be used with infrared light, a silver or gold coating is best. Dielectric coatings are also good in the infrared range. Ultimately, however, the choice of coating will depend on durability as well as wavelength range, and some reflectivity may be sacrificed for resilience. A dielectric coating, for example, is much more scratch resistant than a metallic coating and, despite the additional cost, these coatings are often added on top of metal to protect it. Coatings on scientific grade mirrors are usually applied on the front surface of the glass, because light which travels through glass will always distort to a small degree. This is undesirable in most scientific applications.

The Manufacturing
Process
Cutting and shaping the glass
•1 The first step in manufacturing any mirror is cutting the outline of the glass "blank" to suit the application. If the mirror is for an automobile, for example, the glass will be cut out to fit in the mirror mount on the car. Although some mirror manufacturers cut their own glass, others receive glass that has already been cut into blanks. Regardless of who cuts the glass, very hard, finely pointed blades are used to do the cutting. Diamond scribes or saws—sharp metal points or saws with diamond dust embedded in them—are often used because the diamond will wear down the glass before the glass wears down the diamond. The cutting method used depends entirely on the final shape the mirror will take. In one method, the blades or scribes may be used to cut partway through the glass; pressure can then be used to break the glass along the score line. In another method, a machine uses a diamond saw to cut all the way through the glass by drawing the blade back and forth or up and down multiple times, like an automated bandsaw. Cutting is usually done before the metal coating is applied, because the coating may flake off the glass as a result of the cut. An alternative to cutting the glass to form blanks is to mold the glass in its molten state.
•2 Blanks are then placed in optical grinding machines. These machines consist of large base plates full of depressions that hold the blanks. The blank-filled base is placed against another metal plate with the desired surface shape: flat, convex, or concave. A grinding compound—a gritty liquid—is spread over the glass blanks as they are rubbed or rolled against the curved surface. The action is similar to grinding spices with a mortar and pestle. The grit in the compound gradually wears away the glass surface until it assumes the same shape as the grinding plate. Finer and finer grits are used until the surface is very smooth and even.
Hand grinding techniques exist as well, but they are extremely time-consuming and difficult to control. They are only used in cases where mechanical grinding would be impossible, as is the case with very large or unusually shaped surfaces. A commercial optical grinder can accommodate 50 to 200 blanks, which are all polished simultaneously. This is much more efficient than hand grinding. Even specialty optics can be made mechanically in adjustable equipment.

Applying the reflective material
•3 When the glass surfaces are shaped appropriately and polished to a smooth finish, they are coated with whatever reflective material the designer has chosen. Regardless of the coating material, it is applied in an apparatus called an evaporator. The evaporator is a large vacuum chamber with an upper plate for supporting the blank mirrors, and a lower crucible for melting the coating metal. It is so called because metal is heated in the crucible to the point that it evaporates into the vacuum, depositing a coating on the surface of the glass much like hot breath will steam a cold window. Blanks are centered over holes in the upper plate that allow the metal vapor to reach the surface of the glass. Metals can be heated to several hundreds or thousands of degrees (depending on the boiling point of the metal), before they vaporize. The temperature and timing for this procedure are controlled very precisely to achieve exactly the right thickness of metal. This method of coating creates very uniform and highly reflective surfaces.
•4 The shape of the holes in the upper plate will be transferred to the glass in metal, like paint through a stencil. This effect is often used to intentionally pattern the mirror. Metal stencils, or masks, can be applied to the surface of the glass to create one or more patterns.
•5 Dielectric coatings—either as reflective layers or as protective layers over metal ones—are applied in much the same way, except that gases are used instead of metal chunks. Silicon oxides and silicon nitrides are typically used as dielectric coatings. When these gases combine in extreme heat, they react to form a solid substance. This reaction product forms a coating just like metal does.
•6 Several evaporation steps may be combined to make a multiple-layer coating. Clear dielectric materials may be evaporated on top of metal or other dielectrics to change the reflective or mechanical properties of a surface. Mirrors with silvering on the back of the glass, for instance, often have an opaque dielectric layer applied to improve the reflectivity and keep the metal from scratching. One-way mirrors are the exception to this procedure, in which case great care must be taken not to damage the thin metal coating.
•7 Finally, when the proper coatings have been applied, the finished mirror is mounted in a base or packed carefully in a shock resistant package for shipping.
Quality Control
How good does a mirror have to be? Is it sufficient to have 80 percent of the light bounce off? Does all 80 percent have to bounce in exactly the same direction? The answer is dependent on the application. A purse mirror might only be 80 or 90 percent reflective, and might have some slight irregularity in the thickness of the glass (like ripples on the surface of a pond). The image would be slightly distorted in this case, but the distortion would be barely visible to the naked eye. If, however, a mirror is to be used for a scientific application, for example in a telescope, the shape of the surface and the reflectivity of the coating must be known to a very specific degree, to insure the reflected light goes exactly where the telescope designer wants it, and at the right intensity. The tolerances on the mirror will affect the cost and ease with which it can be manufactured.

Batch mirror uniformity is the first and fore-most job of quality assurance. Mirrors on the edge of a grinding plate or evaporator chamber may not have the same surface or coating as those in the center of the apparatus. If there is a wide range of metal thicknesses or surface flatnesses in a single batch of mirrors, the process must be adjusted to improve uniformity.

Several methods are employed to test the integrity of a mirror. The surface quality is examined first visually for scratches, unevenness, pits, or ripples. This can be done with the unaided eye, with a microscope, or with an infrared photographic process designed to show differences in metal thicknesses.

For more stringent surface control, a profile of the mirror can be measured by running a stylus along the surface. The position of the stylus is recorded as it is dragged across the mirror. This is similar to the way a record player works. Like the record player, the drawback to a mechanical stylus is that it can damage the surface it is detecting. Mirror manufacturers have come to the same solution as the recording industry: use a laser. The laser can be used for non-destructive testing in the same way a compact disc player reads the music from a disc without altering its surface.

In addition to these mechanical tests, mirrors may be exposed to a variety of environmental conditions. Car mirrors, for example, are taken through extremes of cold and heat to


A typical mirror can include a metal reflective layer and one or more dielectric coatings—as protective layers over the metal one. Dielectric coatings are applied in much the same way as metal layers, except that gases such as silicon oxides and silicon nitrides are used instead of metal chunks.
insure that they will withstand weather conditions, while bathroom mirrors are tested for water resistance.

The Future
As glassmaking techniques improve, mirrors find a more elaborate place in art and architecture. Stronger, lighter glasses are more attractive to designers. Some one-way mirror manufacturing techniques allow windows to be manufactured that are mirrored on the outside. This creates a distinctive appearance on a building and also makes the building's air conditioning system more efficient by deflecting heat during the summer. This type of mirror is now commonly seen on office buildings.

Mirrors will also continue to be used in sophisticated optical applications, from microscopes and telescopes to laser-based reading systems such as compact disc players and bar code scanners.

Where To Learn More
Books
Hecht, Eugene. Optics. Addison-Wesley Publishing Co., 1974.

Korsch, Dietrich. Reflective Optics. Academic Press, 1991.

Londono, ed. Recent Trends in Optical Systems Design: Computer Lens Design. SPIE-International Society for Optical Engineering, 1987.

Periodicals
Derra, Skip. "Spin Casting Method Makes the Grade for Telescopic Mirrors." Research & Development. August, 1989, p. 24.

Folger, Tim and Roger Ressmeyer. "The Big Eye." Discover. November, 1991, p. 40.

Hogan, Brian J. "Astronomy Gets a Sharper Vision." Design News. August 26, 1991, p. 110.

"Custom Optics." Laser Focus World. December, 1992.

Nash, J. Madeline. "Shoot for the Stars." Time. April 27, 1992, p. 56.

Walker, Jearl. "Wonders with the Retroreflector, a Mirror That Removes Distortion from a Light Beam." Scientific American. April, 1986, p. 118.

— Leslie G. Melcer



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