Machine Tool & Machining ME 210_2

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Machine Tool & Machining ME 210_2

  1. 1. Machine Tools and Machining ME 210 Vikrant Sharma Assistant Professor Mechanical Engineering Department FET, MITS
  2. 2. Cutting Tool Cutting tool is a device used to remove the unwanted material from the work. For carrying out the machining process, cutting tool is fundamental and essential requirement. Single point cutting tool Multi-point cutting tool Vikrant Sharma FET, MITS ME 210
  3. 3. Tool Selection Factors  Work material  Type of cut  Part geometry and size  lot size  Machinability data  Quality needed Vikrant Sharma FET, MITS ME 210
  4. 4. Elements of an Effective Tool  High hardness  Resistance to abrasion and wear  Strength to resist bulk deformation  Adequate thermal properties  Consistent tool life  Correct geometry Vikrant Sharma FET, MITS ME 210
  5. 5. Rake and Relief angle Vikrant Sharma FET, MITS ME 210
  6. 6. Variation of rake angle (positive to negative) Positive rake angle: positive rake angle have greater cutting efficiency tool penetrates more easily into work reduce cutting pressure result in fragile cutting edge limited to machining softer materials Vikrant Sharma FET, MITS ME 210
  7. 7. Negative rake angle: provide stronger cutting edge suitable for cutting high-strength alloys Vikrant Sharma FET, MITS ME 210
  8. 8. The rake angle for a tool depends on the following factors  Type of material being cut: A harder material like cast iron may be machined by smaller rake angle than that required by soft material like mid steel or aluminum.  Type of tool material: Tool material like cemented carbide permits turning at very high speed. At high speeds rake angle has little influence on cutting pressure. Under such condition the rake angle can minimum or even negative rake angle is provided to increase the tool strength.  Depth of cut: In rough turning, high depth of cut is given to remove maximum amount of material. This means that the tool has to withstand severe cutting pressure. So the rake angle should be decreased to increase the lip angle that provides the strength to the cutting edge.  Rigidity of the tool holder and machine: An improperly supported tool on old or worn out machine cannot take up high cutting pressure. So while machining under the above condition, the tool used should have larger rake angle. Vikrant Sharma FET, MITS ME 210
  9. 9. Effect of variation in relief angle Vikrant Sharma FET, MITS ME 210
  10. 10. Single Point Cutting Tool: Shank : It is the main body of the tool. Flank: Surfaces below and adjacent to the cutting edge is called flank of tool. Face: The surface on which the chip slides is called the face of the tool. Nose: It is the point where major and minor cutting edge intersect. Cutting edge: It is the edge on the face of the tool which removes the material from the work. Tool axis Shank of tool Auxiliary cutting edge Rake or Face Principal cutting edge Principal flank surface Nose Auxiliary flank surface Vikrant Sharma FET, MITS ME 210
  11. 11. A single point cutting tool may be either right or left hand cut tool depending on the direction of feed. Primary Cutting Edge Left hand cutting tool Right hand cutting tool Vikrant Sharma FET, MITS ME 210
  12. 12. Tool Terminology: Vikrant Sharma FET, MITS ME 210
  13. 13. End cutting edge angle (ECEA) Top View Nose Radius (NR) Side cutting edge angle (SCEA) Back rake angle (αb) Side rake angle (αs) Lip angle Front View Side View Side relief angle (SRA) End relief angle (ERA) Vikrant Sharma FET, MITS ME 210
  14. 14. Side Cutting Edge Angle (SCEA): Side cutting edge angle is also known as lead angle, is the angle between the side cutting edge and the side of the tool shank. Usually, the recommended value for the lead angle should range between 15° and 30°. End Cutting Edge Angle (ECEA): this is the angle between the end cutting edge and a line normal to the tool shank. The end cuttingedge angle serves to eliminate rubbing between the end cutting edge and the machined surface of the work piece. Although this angle takes values in the range of 5° to 30°, commonly recommended values are 8° to 15°. Vikrant Sharma FET, MITS ME 210
  15. 15. Side Relief Angle (SRA) : It is the angle between the portion of the side flank immediately below the side cutting edge and a line perpendicular to the base of the tool, and measured at right angle to the side flank. This angle serve to eliminate rubbing between the work piece and the side flank. The value of this angle is between 5° and 15°. End Relief Angle (ERA): It is the angle between the portion of the end flank immediately below the end cutting edge and a line perpendicular to the base of the tool, and measured at right angle to the end flank. This angle serve to eliminate rubbing between the work piece and the side flank. The value of this angle is between 5° and 15°. Vikrant Sharma FET, MITS ME 210
  16. 16. Back Rake Angle and Side Rake Angle: The back rake angle is the angle between the face of the tool and a line parallel to the base of the shank in a plane parallel to the side cutting edge. The side rake angle is the angle by which the face of the tool is inclined side ways. Both these angles determine the direction of flow of the chips onto the face of the tool. Nose Radius: Nose radius is favorable to long tool life and good surface finish. The value of nose radius range between 0.4 mm to 1.6 mm. Vikrant Sharma FET, MITS ME 210
  17. 17. Tool Designation: By designation or nomenclature of a cutting tool is meant the designation of the shape of the cutting part of the tool. It is the system of designating the principal angles of a single point cutting tool. The signature is the sequence of numbers listing the various angles, in degrees, and the size of the nose radius. There are several systems available like American Standard Association system (ASA), Orthogonal Rake System (ORS), Normal Rake System (NRS), and Maximum Rake System (MRS). The system most commonly used is American Standard Association (ASA) Vikrant Sharma FET, MITS ME 210
  18. 18. ASA System: Bake rake angle, Side rake angle, End relief angle, Side relief angle, End cutting Edge angle, Side cutting Edge angle and Nose radius. For example a tool may designated in the following sequence: 8-14-6-6-6-15-1 1. Bake rake angle is 8 2. Side rake angle is 14 3. End relief angle is 6 4. Side relief angle is 6 5. End cutting Edge angle is 6 6. Side cutting Edge angle is 15 7. Nose radius is 1 mm Vikrant Sharma FET, MITS ME 210
  19. 19. Methods of Machining: In the metal cutting operation, the tool is wedge-shaped and has a straight cutting edge. Basically, there are two methods of metal cutting, depending upon the arrangement of the cutting edge with respect to the direction of relative work-tool motion. Orthogonal cutting or two dimensional cutting. Oblique cutting or three dimensional cutting. Vikrant Sharma FET, MITS ME 210
  20. 20. Orthogonal Cutting Oblique Cutting Work Work Feed Feed Tool Tool Vikrant Sharma FET, MITS ME 210
  21. 21. Chip Thickness Ratio (Cutting Ratio): During cutting, the cutting edge of the tool is positioned a certain distance below the original work surface. This corresponds to the thickness of the chip prior to chip formation, to. As the chip is formed along the shear plane, its thickness increases to tc. The ratio of to to tc is called the chip thickness ratio (or simply the chip ratio) r Vikrant Sharma FET, MITS ME 210
  22. 22. Example: Vikrant Sharma FET, MITS ME 210
  23. 23. Forces in Metal Cutting: The friction force F is the frictional force resisting the flow of the chip along the rake face of the tool. The normal force to friction N is perpendicular to the friction force. These two components can be used to define the coefficient of friction between the tool and the chip: The friction angle is related to the coefficient of friction as Vikrant Sharma FET, MITS ME 210
  24. 24. In addition to the tool forces acting on the chip, there are two force components applied by the work piece on the chip: shear force and normal force to shear. The shear force Fs is the force that causes shear deformation to occur in the shear plane, and the normal force to shear Fn is perpendicular to the shear force. Based on the shear force, we can define the shear stress that acts along the shear plane between the work and the chip: Vikrant Sharma FET, MITS ME 210
  25. 25. None of the four force components F, N, Fs, and Fn can be directly measured in a machining operation, because the directions in which they are applied vary with different tool geometries and cutting conditions. However, it is possible for the cutting tool to be instrumented using a force measuring device called a dynamometer, so that two additional force components acting against the tool can be directly measured: cutting force and thrust force. The cutting force Fc is in the direction of cutting, the same direction as the cutting speed v, and the thrust force Ft is perpendicular to the cutting force and is associated with the chip thickness before the cut to. Vikrant Sharma FET, MITS ME 210
  26. 26. Merchant’s Analysis: Merchant established relationship between various forces acting on the chip during orthogonal metal cutting but with following assumption. Cutting velocity always remain constant. Cutting edge of tool remains sharp always during cutting. Chip does not flow sideways. Only continuous chip is produced. There is no built-up edge. Width of tool is greater than width of cut. Vikrant Sharma FET, MITS ME 210
  27. 27. Vikrant Sharma FET, MITS ME 210
  28. 28. Tool Life: Tool life is defined as the time interval for which tool works satisfactorily between two successive grindings or resharpenings of the tool. Tool life is expressed in the following ways. Time period in minutes between two successive grinding of the tool. Number of components machined between two successive grinding. Volume of metal removed between two successive grinding. In 1907 Taylor gave the following relationship between cutting speed and tool life, VTn = C Where V is cutting speed, T is tool life, C is constant and n is an exponent. n = 0.1 to 0.15 for HSS tool , 0.2 to 0.4 for carbide tool and 0.4 to 0.6 for ceramic. The tool life also depends upon the depth of cut and feed. Vikrant Sharma FET, MITS ME 210
  29. 29. Cutting Speed: Cutting speed is the distance traveled by the work surface in unit time with reference to the cutting edge of the tool. The cutting speed, v is simply referred to as speed and usually expressed in m/min. Where, D is Dia. Of work or cutter N is rev / min. of work or cutter Feed: The feed is the distance advanced by the tool into or along the workpiece each time the tool point passes a certain position in its travel over the surface. Feed f is usually expressed in mm/rev. Sometimes it is also expressed in mm/min and is called feed rate. Depth of cut : It is the distance measured perpendicularly between the machined surface and the unmachined (uncut) surface or the previously machined surface of the workpiece. The depth of cut d is expressed in mm. Vikrant Sharma FET, MITS ME 210
  30. 30. Tool Failure A properly designed and ground cutting tool is expected to perform metal cutting operation in an effective and smooth manner. If, however, it is not giving a satisfactory performance it is indicative of tool failure. Following adverse effects observed during the operation. 1. Extremely poor surface finish on the workpiece. 2. Higher consumption of power. 3. Overheating of cutting tool. 4. Work dimensions not being produced as specified. A cutting tool may fail due to one or more of the following reasons. 1. Thermal cracking and softening 2. Mechanical chipping 3. Wear Vikrant Sharma FET, MITS ME 210
  31. 31. Thermal cracking and softening A lot of heat is generated during the process of metal cutting. Due to this heat the tool tip and the area closer to cutting edge become very hot and tool material start deforming plastically at the tip and adjacent to the cutting edge. Thus the tool loses its cutting ability and is said to have failed due to softening. Factors responsible: 1. High cutting speed 2. High feed rate 3. Excessive depth of cut Carbon tool steel 2000 – 2500 High speed steel 5600 – 6000 Cemented carbide 8000-10000 Vikrant Sharma FET, MITS ME 210
  32. 32. Mechanical chipping Mechanical chipping of the nose or the cutting edge of the tool are commonly observed causes of tool failure. Reasons: 1. High cutting pressure 2. Mechanical impact 3. High vibration 4. Weak tip and cutting edge This type of failure is more common in carbide tipped and diamond tools due to the high brittleness of the tool material. Vikrant Sharma FET, MITS ME 210
  33. 33. Tool Wear: Loss of material due to rubbing of two sliding surfaces accompanying friction is called wear. In case of machining loss of cutting tool material is called tool wear. The cutting tool is subjected to, a) high localised stresses b) high temperature c) sliding of chip along the rake face d) rubbing of flank surface with freshly machined surface e) vibration and shock due to improper machining . Due to above factors the loss of material from the tool body accelerates and it loses sharp cutting edge. Vikrant Sharma FET, MITS ME 210
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  37. 37. Machinability: Machinability of a material refers to the ease with which it can be worked with a machine tool. Ease of metal removal implies:  that higher cutting speed and lower power consumption in metal cutting.  that the forces acting against the cutting tool will be relatively low.  that the chips will be broken easily.  that a good finish will result.  that the tool life will increase reducing its frequent resharpening or replacement. Ease of machining is affected by metal properties such as hardness, tensile strength, chemical composition, microstructure and strain hardening. Machine variables such as cutting speed, feed, depth of cut, tool material and its form, cutting fluid etc. also affect machinability. Vikrant Sharma FET, MITS ME 210
  38. 38. Cutting tool insert: Vikrant Sharma FET, MITS ME 210
  39. 39. Cutting Fluids The function of cutting fluids, which are often called coolants are, 1. Cool the tool and the workpiece. 2. Reduce the friction 3. Protect the work against rusting 4. Improve the surface finish 5. To prevent the formation of built-up edge 6. To wash away the chips from the cutting zone. Vikrant Sharma FET, MITS ME 210

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