Manufacturing Operations

Manufacturing Operations

CONTENTS

     I.        Manufacturing Industries and Products

   II.        Manufacturing Operations

a.   Processing and Assembly Operations

b.   Other Factory Operations

  III.        Product/Production Relationships

a.   Production Quantity and Product Variety

b.   Product and Part Complexity

c.   Limitations and Capabilities of a Manufacturing Plant

 IV.        Production Concepts and Mathematical Models

a.   Production Rate

b.   Plant Capacity

c.   Utilization and Availability (Reliability)

d.   Manufacturing Lead Time

e.   WorkinProcess

  V.        Costs of Manufacturing Operations

a.   Fixed and Variable Costs

b.   Direct Labor, Material, and Overhead

c.   Cost of Equipment Usage

Manufacturing can be defined as the application of physical and chemical processes to alter the geometry, properties, and/or appearance of a given starting material to make parts

or products; manufacturing also includes the joining of multiple parts to make assembled products. The processes that accomplish manufacturing involve a combination of machinery, tools, power, and manual labor, as depicted in Figure 2.1(a). Manufacturing is almost always carried out as a sequence of operations. Each successive operation brings the material closer to the desired final state.

From an economic viewpoint, manufacturing is the transformation of materials into items of greater value by means of one or more processing and/or assembly operations, as depicted in Figure 2.1(b). The key point is that manufacturing adds value to the material by changing its shape or properties or by combining it with other materials that have been similarly altered. The material has been made more valuable through the manufacturing operations performed on it.When iron ore is converted into steel, value is added.When sand is transformed into glass, value is added. When petroleum is refined into plastic, value is added. And when plastic is molded into the complex geometry of a patio chair, it is made even more valuable.

In this chapter, we provide a survey of manufacturing operations. We begin by examining the industries that are engaged in manufacturing and the types of products they produce.We then discuss fabrication and assembly processes used in manufacturing as well as the activities that support the processes, such as material handling and inspection. The chapter concludes with descriptions of several mathematical models of manufacturing operations. These models help to define certain issues and parameters that are important in manufacturing and to provide a quantitative perspective on manufacturing operations.

We might observe here that the manufacturing operations, the processes in particular, emphasize the preceding technological definition of manufacturing, while the production systems discussed in Chapter 1 stress the economic definition. Our emphasis in this book is on the systems. The history of manufacturing includes both the development of manufacturing processes, some of which date back thousands of years, and the evolution of the production systems required to apply and exploit these processes (Historical Note 2.1).

Historical Note 2.1        History of manufacturing

The history of manufacturing includes two related topics: (1) man’s discovery and invention of materials and processes to make things and (2) the development of systems of production. The materials and processes predate the systems by several millennia. Systems of production refer to the ways of organizing people and equipment so that production can be performed more efficiently. Some of the basic processes date as far back as the Neolithic period (circa 8000–3000 B.C.), when operations such as the following were developed: woodworking, forming, and firing of clay pottery, grinding and polishing of stone, spinning and weaving of textiles, and dyeing of cloth. Metallurgy and metalworking also began during the Neolithic, in Mesopotamia and other areas around the Mediterranean. It either spread to, or developed independently in, regions of Europe and Asia. Gold was found by early man in relatively pure form in nature; it could be hammered into shape. Copper was probably the first metal to be extracted from ores, thus requiring smelting as a processing technique. Copper could not be readily hammered because it strainhardened; instead, it was shaped by casting. Other metals used during this period were silver and tin. It was discovered that copper alloyed with tin produced a more workable metal than copper alone (casting and hammering could both be used). This heralded the important period known as the Bronze Age (circa 3500–1500 B.C.).

Iron was also first smelted during the Bronze Age. Meteorites may have been one source of the metal, but iron ore was also mined. The temperatures required to reduce iron ore to metal are significantly higher than for copper, which made furnace operations more difficult. Other processing methods were also more difficult for the same reason. Early blacksmiths learned that when certain irons (those containing small amounts of carbon) were sufficiently heated and then quenched, they became very hard. This permitted the grinding of very sharp cutting edges on knives and weapons, but it also made the metal brittle.Toughness could be increased by reheating at a lower temperature, a process known as tempering. What we have described is, of course, the heat treatment of steel. The superior properties of steel caused it to succeed bronze in many applications (weaponry, agriculture, and mechanical devices). The period of its use has subsequently been named the Iron Age (starting around 1000 B.C.). It was not until much later, well into the nineteenth century, that the demand for steel grew significantly and more modern steelmaking techniques were developed.

The early fabrication of implements and weapons was accomplished more as crafts and trades than by manufacturing as we know it today. The ancient Romans had what might be called factories to produce weapons, scrolls, pottery, glassware, and other products of the time, but the procedures were largely based on handicraft. It was not until the Industrial Revolution (circa 1760–1830) that major changes began to affect the systems for making things. This period marked the beginning of the change from an economy based on agriculture and handicraft to one based on industry and manufacturing. The change began in England, where a series of important machines were invented, and steam power began to replace water, wind, and animal power. Initially, these advances gave British industry significant advantages over other nations, but eventually the revolution spread to other European countries and to the United States.The Industrial Revolution contributed to the development of manufacturing in the following ways:

(1) Watt’s steam engine, a new powergenerating technology; (2) development of machine tools, starting with John Wilkinson’s boring machine around 1775, which was used to bore the cylinder on Watt’s steam engine; (3) invention of the spinning jenny, power loom, and other machinery for the textile industry, which permitted significant increases in productivity; and (4) the factory system, a new way of organizing large numbers of production workers based on the division of labor.

Wilkinson’s boring machine is generally recognized as the beginning of machine tool technology. It was powered by water wheel. During the period 1775–1850, other machine tools were developed for most of the conventional machining processes, such as boring, turning, drilling, milling, shaping, and planing. As steam power became more prevalent, it gradually became the preferred power source for most of these machine tools. It is of interest to note that many of the individual processes predate the machine tools by centuries; for example, drilling and sawing (of wood) date from ancient times and turning (of wood) from around the time of Christ.

Assembly methods were used in ancient cultures to make ships, weapons, tools, farm implements, machinery, chariots and carts, furniture, and garments. The processes included binding with twine and rope, riveting and nailing, and soldering. By around the time of Christ, forge welding and adhesive bonding had been developed. Widespread use of screws, bolts, and nuts—so common in today’s assembly—required the development of machine tools, in particular, Maudsley’s screw cutting lathe (1800), which could accurately form the helical threads. It was not until around 1900 that fusion welding processes started to be developed as assembly techniques.

While England was leading the Industrial Revolution, an important concept related to assembly technology was being introduced in the United States: interchangeable parts manufacture. Much credit for this concept is given to Eli Whitney (1765–1825), although its importance had been recognized by others [2]. In 1797, Whitney negotiated a contract to produce 10,000 muskets for the U.S. government. The traditional way of making guns at the time was to custom–fabricate each part for a particular gun and then hand–fit the parts together by filing. Each musket was therefore unique, and the time to make it was considerable. Whitney believed that the components could be made accurately enough to permit parts assembly without fitting. After several years of development in his Connecticut factory, he traveled to Washington in 1801 to demonstrate the principle. Before government officials, including Thomas Jefferson, he laid out components for 10 muskets and proceeded to select parts randomly to assemble the guns. No special filing or fitting was required, and all of the guns worked perfectly. The secret behind his achievement was the collection of special machines, fixtures, and gages that he had developed in his factory. Interchangeable parts manufacture required many years of development and refinement before becoming a practical reality, but it revolutionized methods of manufacturing. It is a prerequisite for mass production of assembled products. Because its origins were in the United States, interchangeable parts production came to be known as the American System of manufacture.

The mid and late1800s witnessed the expansion of railroads, steam–powered ships, and other machines that created a growing need for iron and steel. New methods for producing steel were developed to meet this demand.Also during this period, several consumer products were developed, including the sewing machine, bicycle, and automobile. To meet the mass demand for these products, more efficient production methods were required. Some historians identify developments during this period as the Second Industrial Revolution, characterized in terms of its effects on production systems by the following: (1) mass production, (2) assembly lines,

(3) scientific management movement, and (4) electrification of factories.

Mass production was primarily an American phenomenon. Its motivation was the mass market that existed in the United States. Population in the United States in 1900 was 76 million and growing. By 1920 it exceeded 106 million. Such a large population, larger than any western European country, created a demand for large numbers of products. Mass production provided those products. Certainly one of the important technologies of mass production was the assembly line, introduced by Henry Ford (1863–1947) in 1913 at his Highland Park plant (Historical Note 17.1). The assembly line made mass production of complex consumer products possible. Use of assembly line methods permitted Ford to sell a Model T automobile for less than $500 in 1916, thus making ownership of cars feasible for a large segment of the American population.

The scientific management movement started in the late 1800s in the United States in response to the need to plan and control the activities of growing numbers of production workers. The movement was led by Frederick W. Taylor (1856–1915), Frank Gilbreath (1868–1924) and his wife Lilian (1878–1972), and others. Scientific management included: (1) motion study, aimed at finding the best method to perform a given task; (2) time study, to establish work standards for a job; (3) extensive use of standards in industry; (4) the piece rate system and similar labor incentive plans; and (5) use of data collection, record keeping, and cost accounting in factory operations.

In 1881, electrification began with the first electric power generating station being built in New York City, and soon electric motors were being used as the power source to operate factory machinery. This was a far more convenient power delivery system than the steam engine, which required overhead belts to distribute power to the machines. By 1920, electricity had overtaken steam as the principal power source in U.S. factories. Electrification also motivated many new inventions that have affected manufacturing operations and production systems.The twentieth century has been a time of more technological advances than in all other centuries combined. Many of these developments have resulted in the automation of manufacturing. Historical notes on some of these advances in automation are covered in this book.