机电专业英语 教学课件 作者 杨寿智 刘水平 机电专业英语.Lesson 2.pptVIP

Lesson 2 TOOL BIT MATERIALS Text The materials used for tool bits must possess certain qualities to be effective. They must possess hardness, strength, and toughness, and they must be heat resistant. There are conditions associated with the material being cut that affect the selection of the tool materials: the characteristics and condition of the workpiece material, the amount of stock to be removed, the required provision of the finished workpiece, the surface finish, the rigidity of the workpiece and toolholder, the condition of the machine tool, the heat generated by a particular operation, and so on. All of these items must be considered when selecting a tool material. Cutting tools, to operate efficiently, must have properties such as hot hardness, toughness, resistance to cratering, wear resistance, and resistance to shock. It should be noted that some hardened materials will retain their hardness after being subjected to high temperatures during the cutting operation and cooled to room temperature. Such tool materials are ceramics, carbides, cast materials, and high-speed steel(HSS) up to 1000℉. Carbon steel loses its hardness almost immediately upon being cooled from elevated to room temperature. High-carbon steel: Tools made from high-carbon steels, 0.80 to 1.3%, are used for small-quantity production of wood parts or machining soft materials such as free-cutting steels and brass. It is important that the operational temperatures be kept below 400℉ because the material loses its hardness above this temperature. For this reason coolants should be used freely. Sometimes vanadium and chromium are added to give better hardening ability. However, any temperature increase will affect the hardness and effectiveness of the tool. High-speed steel: These tool materials are of several varieties. The most common type is the 18-4-1 high-speed steel. This tool material has approximately 18% tungsten, 4% chromium, and 1% vanadium as its alloying elements. Another type h