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1、 1CUTTING TOOLS AND TOOLHOLDERSTo machine a workpiece successfully you must have :1. the correct kind of cutting tool or tool bit 2. the right type of toolholder3. a tool with a sharp cutting-edge4. the cutting tool set or adjusted to the correct height and position.Cutting-tool materialsTool bits u
2、sed 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 including:tool cost,size and design of tool ,metal-removal
3、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 general characteristics of the different cutting-tool m
4、aterials 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 as W steels in American Iron and Steel Institutes sy
5、stem of classification . Tools made from these materials have very sharp ,smooth cutting-edges when properly heat- 2treated. They are adequate for limited turning at a relatively low cutting speed or when old ,low-speed equipment ,such as a flat-belt lathe , is used .The main limitation of tools mad
6、e 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 steels offer great improvement in cutting efficiency over water-hardening
7、 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 temperature , the original hardness of these steels does not change . Wea
8、r 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 have greater resistance to shock than carbides or hard-cast alloys .There
9、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 to various shapes . Unground tool bits called tool-bit blanks can also be p
10、urchased . 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 3in long . High-speed steel 3tool bits are the type most used in the school machi
11、ne 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 special purpose . They reach full hardness in the as-cast condition , without heat treatment . The mu
12、st 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 tension and more brittle than high-speed steels and thus are not suitable for severe shock loads . They are k
13、nown 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 . Carbide tools are available in solid form and as inserts which are either brazed or clamped in toolholders . Clam
14、ped 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 machine tools and the proper grade of cemented carbide , it is possible to use cutting speeds 10 to 30 time fast
15、er 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
16、to enable their efficient use . 4Ceramic . With the exception of industrial diamonds , ceramic inserts are the hardest and strongest inserts available . They resist abrasive wear , chipping , and breakage . These inserts work best on very rigid machine tools and on well-supported workpieces . For most operations , cutting fluids are not needed . Diamonds . Industrial diamonds that have either circular or faceted cutting-edges are used for light finishing cuts when an extreme
