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MANUFACTURING
ENGINEERING AND TECHNOLOGY HOMEPAGE
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A variety of cutting-tool materials have been developed over the past century for specific applications in machining operation. These material have a wide range of mechanical and physical properties, such as hot hardness, toughness, chemical stability and resistance to chipping and wear. Various coating have been developed, resulting in major improvement in tool life. The selection of appropriate tool material not only on the material to be machined, but also on process parameters and the characteristics of the machine tools. Cutting fluid are an important factor in machining operations. Generally, slower operations with high tool pressures require a fluid with good lubricating characteristics. In high-speed operations with significant temperature rise, fluids with cooling capacity are preferred. Selection should be include consideration of various adverse effects of cutting fluids on products, machinery, personnel, and the environment.
Machining Processes for Producing Round Shape. Cutting processes that produce external and internal circular profile are turning, boring and drilling. Reaming, tapping and die threading are processes for finishing workpieces. Chip formation in all these processes is essentially the same. However, because of the three-dimensional nature of the cut, chip movement and its control are important considerations since otherwise they interfere with the cutting operation. Chip removal can be significant problem especially in drilling and tapping and can be lead to tool breakage. Each process should be studied in order to understand the interrelationships of design parameters, such as dimensional accuracy, surface finish and integrity, and process parameters such as speed, feed, depth of cut, tool material and shape, and cutting fluids. Design guidelines should be followed carefully to take full advantage of the capabilities of each process. Part to be machined may have been produced by casting forging, extrusion, powder metallurgy, and so on. The closer to the blank to be machined to the final shape desired, the fewer the number and extent of machining processes required. Such net-shape manufacturing is of major significance in minimizing costs.
Machining Processes for Producing Various Shapes. Some of the most versatile machining processes are milling, planing, shaping, broaching and sawing. Milling is the one of the most useful processes because of its capability to produce a variety of shapes from workpieces. Although there are similarities with processes such as turning, drilling, and boring, most of these processes utilize multitooth tools and cutters at various axes with respect to the workpieces. The machine tools employed have various features, attachments and considerable flexibility in operation. Gear manufacturing by various forming and shaping processes. It also produced by machining, either by form cutting or generating, with the latter producing gears with better surface finish and greater dimensional accuracy. The surface finish and accuracy of tooth profile are further improved by gear finishing processes such as shaving, burnishing and grinding.
Machining and Turning Centers, Machine-Tool Structures, and Machining Economics.
Because of their versatility and capability of performing a variety of cutting operations on small and large workpieces, machining centers have become one of the most important developments in machine tools. Various type of machining and turning centers are available, and their selection depends on factors such as part complexity, the number and type of cutting involve, dimensional accuracy, and production rate required. Vibration and chatter in machining are important considerations for workpiece dimensional accuracy and surface finish, as well as tool life. Stiffness and damping capacity of machine tools are important factors in controlling vibration and chatter. New materials are being developed used for constructing machine-tool structures. The economic of machining processes depend on nonproductive, machining, tool-change, and tool costs. Optimum cutting speeds can be determined mathematically for minimum machining time per piece and minimum cost per piece, respectively.
Abrasive Machining and Finishing Operations Grinding and various abrasive-removal processes are capable of producing the finest accuracy and surface finish in manufactured products. The majority of abrasive processes are basically finishing operations that are usually performed on machined or cold-worked parts. However, abrasives are also used for large-scale material-removal processes, such as creep-feed grinding and snagging in foundries. A variety of abrasive processes and machinery are available for surface, external, and internal grinding. The selection of abrasives and process variable in these operations must be controlled in order to obtain the desired surface and dimensional accuracy. Otherwise, damage to surfaces such as burning, heat checking, and harmful residual stresses may develop. Several finishing operations are available for debarring. Because contribute significantly to product cost, proper selection and implementation of finishing operations are important.
Nontraditional Machining Processes Machining processes involve not only single-point or multipoint tools, but also other method using chemical, electrical, and high-energy-beam sources of energy. The mechanical properties of the workpiece material are not significant because this processes rely on mechanism that do not involve the strength, hardness, ductility, or toughness of the material. Rather, they involve physical, chemical, and electrical properties. Chemical and electrical methods of machining are particularly suitable for hard materials and complex shapes. They do not produce forces (hence can be used for slender and flexible workpiece), significant temperatures, or residual stresses. However, the effect of these processes on surface integrity must be understood, as they can damage surfaces considerably, thus reducing fatigue life.
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