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0CUTTING 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 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 including: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 general 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 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 heat-1treated. 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 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 steels 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 temperature , 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 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 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 2,1/4in square by 2in long ,5/16in square by 2-1/2in long ,and 3/8in square by 3in long . High-speed 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 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 tension 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 . Carbide 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 machine 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 3used only when they can be supported rigidly and when the machine tool has adequate power and speed to enable their efficient use . Ceramic . 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 extremely high-quality surface finish required . Although a very smooth finish can be achieved using other cutting-tool materials , diamond turning can provide even smoother finishes with very small tolerances .TOOLHOLDERSThe toolholder holds the cutting tool rigid during cutting operations . Four types of toolholders are in general use . 1 . The tool post with standard toolholders . The tool too post is comprised of the post , screw , washer , collar , and rocker . The washer fits the top slidee piece slot . The collar and the rocker elevate or lower the point of too . The screw clamps the toolholder in place . The standard toolholder for high-speed steel cutter bits comes in three common shapes : straight , right-hand offset or shank , and left-hand offset or shank . You can identify right-hand and left-hand offset holders by holding 4the setscrew end in your hand . If the shank bends to the right , it is a left-hand offset holder .The straight toolholder is fused for most work . The left-hand toolholder is used when you need to cut close to the chuck or lathe dog . The right-hand holder is used when feeding toward the tailstock of the lathe . The cutting-tool bit is held in each of these toolholders at an angle of 14 to 16.5 degrees . This is called the toolholder angle . Carbide toolholders also come in three styles and are similar in appearance to those mentioned above . The hole for the cutter bit , however , is parallel to the bottom edge of the holder . 2. The open-side or heavy-duty tool block holds one tool at a time and consists of a T-slot clamp , a C-shaped block , and two or more tool clamping screws . Because this unit is very rigid , it is especially useful for heavy cuts . A tool bit can be mounted directly in the tool block or some type of carbide tooolholder can be used . 3. The turret tool block or four-way toolholder consists of a swiveling block in which the tools are clamped Common turret block hold four tools . Each can be quickly swiveled into cutting position and clamped in place . Some turret blocks have eight tool stations . Frequently ,an open-side-type tool block is also mounted on the rear of the cross slide to add one additional cutting operation . With this arrangement , anywhere form five to nine different kinds of tools can be mounted and operated in sequence for turning , 5forming ,facing ,knurling , and cutting off duplicate parts . 4. The quick-change-type tool system holds only one tool at a time , but three different sides can be used to position the tool . It consists of a quick-change tool post with a clamping lever and a series of toolholders for turning , facing , boring , cutting off , threading , knurling , and thread cutting .Single-point Cutting Tools Tool Parts . Before you can grind a tool bit , you must become acquainted with some of the terms used to describe the various parts of the cutting tool . 1 . The face is the top of the tool bit . It is the surface on which a part of the chip attaches as it is cut away form the workpiece .2. The cutting-edge is that part of the cutter bit which actually does the cutting .3. The nose is the corner or are formed by the side and end cutting-edge . 4. The flank is the surface below the cutting-edge . 5. The point is the part of the tool bit which is shaped to form the cutting-edge and face . Tool Angles .The following are important tool angles used for all single-point cutting tools .1. The side relief angle is the angle between the ground surface and the vertical side of the tool bit before it is ground . This angle was formerly called side clearance , and many machinists still use this term .The side relief angle provides clearance between the cut surface of the work and the flank of tool . Tool wear reduces the effective side clearance angle .If the 6angle is too small, the cutter will rub and heat . If the angle is too large , the cutting-edge will be weak and the tool will have a tendency to dig into the workpiece.2. The end relief angle is the angle formed between the end of the cutting-edge and a vertical line. It was formerly called front clearance. The end relief angle provides clearance between the finished surface of the work and the tool. If this angle is too small, the tool will rub on the finished surface and produce a poor finish. Wear tends to reduce this angle. If the angle is too large , the tool may dig into the work , chatter, and fail through chipping . An angle of 8 to 15 is recommened for steel tools and 6 to 8 for carbide tools .If the tool is set above the center of rotation , the effective clearance angle is reduce . This must be considered in choosing the proper angle . 3. The end cutting-edge angle provides clearance between the cutter and the finished surface of the work .If this angle is too small, it may cause chatter. A small angle of about 6 is desirable on light finishing cuts, however, in order to produce a smooth finish . 4. The side cutting-edge angle turns the chip away form the finished surface .Recommended angles are between 5 and 15 .5. The nose radius removes the fragile corner of the tool , prolongs tool life , and improves finish . The radius may be large for maximum-strength rough-cutting tools and may be reduced for light feeds . The larger the 7nose radius ,the better the finish as long as chatter dose not occur .Recommended nose radii are 0.010 to 0.030 in . or more for finishing cuts , and 1/30 to 1/2 in for roughing cuts . 6. To help shape the cutting-edge and face , it is necessary to grind rake angles on a tool bit .Rake is an inclination form the vertical or horizontal .The two rake angles are back rake and side rake . Back rake , in a single-point feeding to the side , turns the chip away form the finished work and gives the tool a slicing action .A zero back rack tends to make a spiral chip , and a back rake angle greater than zero tends to stretch the spiral chip out into a helix . A back-rake angle of form 5 to 15 is used to keep chips form scratching the workpiece . High-speed steel tool bits are always ground with a positive rake .However , cemented carbide tools may have either a positive or negative rake .Negative rake increases the shear angle at which the chip is formed , providing for a good chip and a good surface finish .Negative rake tools are generally used on a heavy-duty lathe that is operated at high speed with a heavy feed . Side rake refers to the angle between the face of the tool and a line that would represent the top of the unground tool bit as viewed form the end . Side rake controls the type of chip produced during machining as well as the direction in which the chip will travel .A tool with a small side-rake angle will produce shorter chips than one with a large rake angle .7 .The nose angle is the angle between the side-cutting edge and the end-8cutting edge .Classes of Single-point Tools Different shapes of tool bits are needed to do certain machining operations . Most tool bits are ground to cut in one direction only .The two common types are referred to as right cut and left cut .These were formerly called right-hand and left-hand tool bits .A right-cut single-point tool is one that , when viewed form the point end with the face up , has its cutting-edge on the right side .When the tool bits is placed in the lathe , the cutting-edge is on the left side .This tool bit cuts form the right to the left , or form the tailstock end toward the headstock of the lathe .A left-cut tool bit has the cutting-edge on the left when viewed form the point end with the face up . This tool bit is ground to cut form the left to the right or toward the tailstock of the lathe .Commonly used types of tool bits include the following:1. A roughing tool is a tool bit designed to take heavy roughing cuts to reduce the diameter of a workpiece to approximate size . Because finish is not important when roughing work , such a tool bit may be ground to almost a sharp point . However , the point is usually rounded very slightly to prevent its breaking down . 2. A finishing tool is one that has a keenly ground cutting-edge which may be honed with an oilstone to produce a very smooth finish ,A finish tool generally has a larger rounded nose than a roughing tool .93. A roundnose cutting tool is a general-purpose tool used for many types of work .When ground flat on the top , it can be used for both right and left cuts and for turning brass . It may also be used to form a radius at the corner of a shoulder . Roundnose tool bits are used as finishing tools . 4. The square-nose tool cuts on the end only . It is used for chamfering and for roughing cuts to square a shoulder . 5. A cutoff or parting tool cuts on the end only and is used for cutting off stock or workpieces held in a chuck .6. Side-facing tools or side tools , as they are often called , are used for finishing the ends of work square and smooth .A right-cut side-facing tool is always used to finish the end of a shaft . A left-cut side-facing tool may be used to finish the left side of a shoulder .Chip BreakersTo eliminate the problem of a continuous chip , a chip breaker is often ground on a high-speed steel tool bit .Chip breakers can either be ground on an off-hand grinding wheel , or in the case of the grooved chip breaker , it can be done with a thin grinding wheel on a surface grinder .A separate chip breaker is often used with insert-type carbide tools .Cutting ActionIn lathe turning , there are three basic cutting forces , the longitudinal force of the workpiece against the side of the tool , the radial force of the workpiece against the front of tool , and the tangential force of the workpiece against the 10top of the tool . The tangential cutting force is by far the greatest and has the most influence on the cutting action . This tremendous force is exerted against the cutting-edge . Pressures as great as a quarter-million pounds per square foot have been measured on large metal-cutting lathes . If the tool is shaped incorrectly or set at an incorrect angle ,it will dull rapidly .Here is what actually happens when cutting on the lathe ,As the workpiece meets the cutting-edge of the tool bit , chips or a continuous ribbon of metal are wedged away from the material being machined ,On soft , ductile material , this wedging is continuous. On harder materials , the wedging force causes the metal to compress . Compression continues until a shearing point is reached , and the compressed metal is separated from the workpiece . This is repeated throughout the cutting operation .The shape is much more important than the actual sharpness of a cutting-edge . Lack of clearance permits the tool to drag on the workpiece , greatly increasing the pressure on the cutting-edge and interfering withtool performance . Too-large clearance angles make the tool weak and do not edge breaks off or wears out rapidly . Grinding a High-speed Steel Tool BitA tool bit is ground:1.To provide a sharp cutting-edge 2. To obtain the correct or best shape for a particular operation 3. To provide clearance away form the end of the tool bit 114. To provide clearance away form the side of the tool bit 5. To provide good chip movement over the face of the tool bit and away the cutting-edge Here is the correct procedure for grinding a right-cut roundnose tool bit :1. Check the grinding wheels to make certain the faces of the wheels are dressed properly. It is difficult to grind good cutting tools on wheels that are uneven or have grooves in them. Use a bench or floor grinder that is used only for grinding cutting tools. Such a grinder should have a coarse-grit aluminum-oxide wheel on one side and a fine-grit wheel on the other side. Use the coarse-grit wheel for finishing the tool bit to shape. Then use the fine-grit wheel for finishing the tool bit.2. The tool grinder should be equipped with tool rests. Rest your hands on them to control the movement of the tool bit. Hold the tool bit firmly to keep it form bouncing around on the wheel. Do not grip it so tightly, however, that you have difficulty move it.3. Grind the side-relief angle to form the side-cutting-edge angle by holding the tool bit against the wheel, as shown in Fig 18. This angle should be about 6 for mild steel. Hold the tool bit against the wheel, and tilt the bottom inward to get this angle. As you grind, move the tool bit back and forth across the face of the wheel without changing its position. This helps to grind the tool faster and prevents grooving the wheel. As the tool bit heats up, dip it in water to cool it. A beginner often makes the mistake of 12moving the tool bit up and down. This forms many different angles on the flank, and it becomes very irregular. To avoid this, hold the tool bit in affixed position on the wheel. Move it back and forth, but never up or down.4. Grind the side-relief angle on the opposite side to form the shape of the tool bit, as shown in. 5. To grind the
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