1、 中文 6500字，4200英文单词，18500英文字符 1 CUTTING TOOLS AND TOOLHOLDERS To machine a workpiece successfully you must have : 1. the correct kind of cutting tool or tool bit 2. the right type of toolholder 3. a tool with a sharp cutting-edge 4. the cutting tool set or adjusted to the correct hei
2、ght and position. Cutting-tool materials Tool bits used on the lathe are made form one of six basic materials: water-hardening steels,high-speed steels,hard-cast,nonferrous alloys,sintered (cemented) carbides,ceramics,and diamonds. The selection of the material used depends upon many factors includi
3、ng:tool cost,size and design of tool ,metal-removal rate ,length of run ,finish and tolerance of part,and condition and capability of the machine tool . Because of these factors ,material selection is more often based on general experience than on precise evaluation . There are, however ,certain gen
4、eral characteristics of the different cutting-tool materials you should understand. Water-hardening Steels.These include the high-carbon tool steels (either plain carbon or those with minor additions of chromium , vanadium,or tungsten) .The different grades of water-hardening tool steels are classed
5、 as W steels in American Iron and Steel Institutes system of classification . Tools made from these materials have very sharp ,smooth cutting-edges when properly 2 heat-treated. They are adequate for limited turning at a relatively low cutting speed or when old ,low-speed equipment ,such as a
6、 flat-belt lathe , is used . The main limitation of tools made form water-hardening steels is that they soften if the cutting-edge temperature exceeds approximately 300-400F during sharpening or cutting .A second disadvantage is low resistance to edge wear . High-speed Steels. High-speed steel
7、s offer great improvement in cutting efficiency over water-hardening tool steels .Tools made from high-speed steels retain enough hardness to machine at rapid rates even when the tool temperature reaches 1050F . They can be used even though they become dull red with heat . Upon cooling to room tempe
8、rature , the original hardness of these steels does not change . Wear resistance of high-speed steels is much better than that of the carbon or alloy steels . This is due to the high carbide content ,especially in the higher-alloy types of high-speed steel . Fully hardened , high-speed steels
9、have greater resistance to shock than carbides or hard-cast alloys . There are two main types of high-speed steels designated in the American Iron and Steels Institute system , M steels (molybdenum base and T steels tungsten base . Tool bits made from these materials can be purchased already ground
10、to various shapes . Unground tool bits called tool-bit blanks can also be purchased . These tool-bit blanks are made in standard size to fit the commonly used lathes . The common sizes are 3/16in square by 1 in long ,1/4in square by 2in long ,5/16in square by 2-1/2in long ,and 3/8in square by
11、3in long . High-speed 3 steel tool bits are the type most used in the school machine shop . Hard-cast Alloy . These materials do not contain sufficient iron to be classed as steels . Rather , they are mainly alloys of cobalt , chromium , and tungsten with other elements added for
12、special purpose . They reach full hardness in the as-cast condition , without heat treatment . The must be ground to size after casting . In terms of resistance to heat , wear ,shock ,and initial cost , cast alloys rank between high-speed steels and carbides . Hard-cast alloys are weaker in te
13、nsion and more brittle than high-speed steels and thus are not suitable for severe shock loads . They are known by such commercial names as stellite , Rex alloy ,and tantung . Sintered Carbides . For efficient and high-speed machining ,best results can be obtained with sintered carbide tools . Carbi
14、de tools are available in solid form and as inserts which are either brazed or clamped in toolholders . Clamped inserts are usually round , square , or triangular in shape and have all edge is always available . These inserts can be rotated so that a sharp edge is always available . With modern mach
15、ine tools and the proper grade of cemented carbide , it is possible to use cutting speeds 10 to 30 time faster than those feasible with high-speed steels . Carbides are suitable for most machining operations such as single-point turning , drilling ,milling , thread cutting, and reaming . Carbides should be used only when they can be supported rigidly and when the machine tool has adequate power and speed to enable their efficient use .
