There are certain basic activities that must be carried out in a factory to convert raw materials into finished products. Limiting our scope to a plant engaged in making discrete products, the factory activities are: (1) processing and assembly operations, (2) material handling, (3) inspection and test, and (4) coordination and control.
The first three activities are the physical activities that “touch” the product as it is being made. Processing and assembly operations alter the geometry, properties, and/or appearance of the work unit. They add value to the product. The product must be moved from one operation to the next in the manufacturing sequence, and it must be inspected and/or tested to insure high quality. It is sometimes argued that these material handling and inspection activities not add value to the product. However, our viewpoint is that value is added through the totality of manufacturing operations performed on the product. Unnecessary operations, whether they are processing, assembly, material handling, or inspection, must be eliminated from the sequence of steps performed to complete a given product.
1 Processing and Assembly Operations
Manufacturing processes can be divided into two basic types: (1) processing operations and (2) assembly operations. A processing operation transforms a work material from one state of completion to a more advanced state that is closer to the final desired part or product. It adds value by changing the geometry, properties, or appearance of the starting material. In general, processing operations are performed on discrete workparts, but some processing operations are also applicable to assembled items, for example, painting a welded sheet metal car body. An assembly operation joins two or more components to create a new entity, which is called an assembly, subassembly, or some other term that refers to the specific joining process.
Processing Operations. A processing operation uses energy to alter a workpart’s shape, physical properties, or appearance to add value to the material. The forms of energy include mechanical, thermal, electrical, and chemical. The energy is applied in a controlled way by means of machinery and tooling. Human energy may also be required, but human workers are generally employed to control the machines, to oversee the operations, and to load and unload parts before and after each cycle of operation. A general model of a processing operation is illustrated in Figure 2.1(a). Material is fed into the process, energy is applied by the machinery and tooling to transform the material, and the completed workpart exits the process. As shown in our model, most production operations produce waste or scrap, either as a natural byproduct of the process (e.g., removing material as in machining) or in the form of occasional defective pieces. An important objective in manufacturing is to reduce waste in either of these forms.
More than one processing operation are usually required to transform the starting material into final form. The operations are performed in the particular sequence to achieve the geometry and/or condition defined by the design specification.
Three categories of processing operations are distinguished: (1) shaping operations,
(2) property–enhancing operations, and (3) surface processing operations. Shaping operations apply mechanical force or heat or other forms and combinations of energy to effect a change in geometry of the work material.There are various ways to classify these processes. The classification used here is based on the state of the starting material, by which we have four categories:
1. Solidification processes. The important processes in this category are casting (for metals) and molding (for plastics and glasses), in which the starting material is a heated liquid or semifluid, in which state it can be poured or otherwise forced to flow into a mold cavity where it cools and solidifies, taking a solid shape that is the same as the cavity.
2. Particulate processing. The starting material is a powder. The common technique involves pressing the powders in a die cavity under high pressure to cause the powders to take the shape of the cavity. However, the compacted workpart lacks sufficient strength for any useful application. To increase strength, the part is then sintered— heated to a temperature below the melting point, which causes the individual particles to bond together. Both metals (powder metallurgy) and ceramics can be formed by particulate processing.
Deformation processes. In most cases, the starting material is a ductile metal that is shaped by applying stresses that exceed the metal’s yield strength. To increase ductility, the metal is often heated prior to forming. Deformation processes include forg ing, extrusion, and rolling.Also included in this category are sheet metal processes such as drawing, forming, and bending.
4. Material removal processes. The starting material is solid (commonly a metal, ductile or brittle), from which excess material is removed from the starting workpiece so that the resulting part has the desired geometry. Most important in this category are machining operations such as turning, drilling, and milling, accomplished using cutting tools that are harder and stronger than the work metal. Grinding is another common process in this category, in which an abrasive grinding wheel is used to remove material. Other material removal processes are known as nontraditional processes because they do not use traditional cutting and grinding tools. Instead, they are based on lasers, electron beams, chemical erosion, electric discharge, or electrochemical energy.
Property–enhancing operations are designed to improve mechanical or physical properties of the work material. The most important property–enhancing operations involve heat treatments, which include various temperatureinduced strengthening and/or toughening processes for metals and glasses. Sintering of powdered metals and ceramics, mentioned previously, is also a heat treatment, which strengthens a pressed powder workpart. Propertyenhancing operations do not alter part shape, except unintentionally in some cases, for example, warping of a metal part during heat treatment or shrinkage of a ceramic part during sintering.
Surface processing operations include: (1) cleaning, (2) surface treatments, and (3) coating and thin film deposition processes. Cleaning includes both chemical and mechanical processes to remove dirt, oil, and other contaminants from the surface. Surface treatments include mechanical working, such as shot peening and sand blasting, and physical processes, like diffusion and ion implantation. Coating and thin film deposition processes apply a coating of material to the exterior surface of the workpart. Common coating processes include electroplating, anodizing of aluminum, and organic coating (call it painting). Thin film deposition processes include physical vapor deposition and chemical vapor deposition to form extremely thin coatings of various substances. Several surface processing operations have been adapted to fabricate semiconductor materials (most commonly silicon) into integrated circuits for microelectronics. These processes include chemical vapor deposition, physical vapor deposition, and oxidation. They are applied to very localized areas on the surface of a thin wafer of silicon (or other semiconductor material) to create the microscopic circuit.
3. Assembly Operations. The second basic type of manufacturing operation is assembly, in which two or more separate parts are joined to form a new entity. Components of the new entity are connected together either permanently or semipermanently. Permanent joining processes include welding, brazing, soldering, and adhesive bonding. They combine parts by forming a joint that cannot be easily disconnected. Mechanical assembly methods are available to fasten two (or more) parts together in a joint that can be conveniently disassembled. The use of threaded fasteners (e.g., screws, bolts, nuts) are important traditional methods in this category. Other mechanical assembly techniques that form a permanent connection include rivets, press fitting, and expansion fits. Special assembly methods are used in electronics. Some of the methods are identical to or adaptations of the above techniques. For example, soldering is widely used in electronics assembly. Electronics assembly is concerned primarily with the assembly of components (e.g., integrated circuit packages) to printed circuit boards to produce the complex circuits used in so many of today’s products.
2 Other Factory Operations
Other activities that must be performed in the factory include material handling and storage, inspection and testing, and coordination and control.
Material Handling and Storage. A means of moving and storing materials between processing and/or assembly operations is usually required. In most manufacturing plants, materials spend more time being moved and stored than being processed. In some cases, the majority of the labor cost in the factory is consumed in handling, moving, and storing materials. It is important that this function be carried out as efficiently as possible. In Part II of our book, we consider the material handling and storage technologies that are used in factory operations.
Eugene Merchant, an advocate and spokesman for the machine tool industry for many years, observed that materials in a typical metal machining batch factory or job shop spend more time waiting or being moved than in processing . His observation is illustrated in Figure 2.3. About 95% of a part’s time is spent either moving or waiting (temporary storage). Only 5% of its time is spent on the machine tool. Of this 5%, less than 30% of the time on the machine (1.5% of the total time of the part) is time during which actual cutting is taking place. The remaining 70% (3.5% of the total) is required for loading and unloading, part handling and positioning, tool positioning, gaging, and other elements of nonprocessing time. These time proportions provide evidence of the significance of material handling and storage in a typical factory.
Inspection and Test. Inspection and test are quality control activities. The purpose of inspection is to determine whether the manufactured product meets the established design standards and specifications. For example, inspection examines whether the actual dimensions of a mechanical part are within the tolerances indicated on the engineering drawing for the part. Testing is generally concerned with the functional specifications of the final product rather than with the individual parts that go into the product. For example, final testing of the product ensures that it functions and operates in the manner specified by the product designer. In Part IV of this text, we examine the inspection and testing function.
Coordination and Control. Coordination and control in manufacturing includes both the regulation of individual processing and assembly operations as well as the management of plant level activities. Control at the process level involves the achievement of certain performance objectives by properly manipulating the inputs and other parameters of the process. Control at the process level is discussed in Part I of the book.
Control at the plant level includes effective use of labor, maintenance of the equipment, moving materials in the factory, controlling inventory, shipping products of good quality on schedule, and keeping plant operating costs at a minimum possible level..The manufacturing control function at the plant level represents the major point of intersection between the physical operations in the factory and the information processing activities that occur in production. We discuss many of these plant and enterprise level control functions in Parts IV and V.