05-06-2013, 02:12 PM
Hard Turning
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INTRODUCTION
As the hardness of the workpiece increases, its machinability decreases accordingly. And tool wear and fracture, as well as surface finish and integrity, can become a significant problem. There are several other mechanical processes and non-mechanical methods of removing material economically from hard or hardened metals. However, it is still possible to apply traditional cutting processes to hard metals and alloys by selecting an appropriate tool material and machine tools with high stiffness and high-speed spindles.
One common example is finish machining of heat-treated steel machine and automotive components using polycrystalline cubic boron nitride (PCBN) cutting tools. This process produces machined parts with good dimensional accuracy, surface finish and surface integrity. It can compete successfully with grinding the same components, from both technical and economic aspects. According to some calculations grinding is more than ten times more costly than hard turning.
Advanced cutting tool materials, such as polycrystalline cubic boron nitride (PCBN) and ceramics (discussed in Chapter 1, Cutting Tool Materials), have made the turning of hardened steel a cost effective alternative to grinding. Many machine shops have retired their cylindrical grinders in favor of less expensive and more versatile CNC lathes.
Hard turning requires much less energy than grinding, thermal and other damage to the workpiece is less likely to occur, cutting fluids may not be necessary and the machine tools are less expensive. In addition, finishing the part while still chucked in the lathe eliminates the need for material handling and setting the part in the grinder. However, work holding devices for large and slender workpieces for hard turning can present problems, since the cutting forces are higher than in grinding.
Furthermore, tool wear and its control can be a significant problem as compared to the automatic dressing of grinding wheels. It is thus evident that the competitive position of hard turning versus grinding must be elevated individually for each application and in terms of product surface integrity, quality, and overall economics