Lecture 21_MECHANICS OF METAL REMOVAL IN ORTHOGONAL CUTTING (1).pdf

VIDEO DISCLAIMER The information contained in the multimedia content “Mechanics Of Metal Removal In Orthogonal Cutting” posted by Thapar Institute of Engineering & Technology is purely for education (class teaching) and informational purpose only and not for any commercial use....

VIDEO DISCLAIMER The information contained in the multimedia content “Mechanics Of Metal Removal In Orthogonal Cutting” posted by Thapar Institute of Engineering & Technology is purely for education (class teaching) and informational purpose only and not for any commercial use. UME 505: MANUFACTURING TECHNOLOGY  MECHANICS OF METAL REMOVAL IN ORTHOGONAL CUTTING UME 505: MANUFACTURING TECHNOLOGY Introduction Of all the manufacturing processes available, metal removable is perhaps the most expensive one. The reason being that from the raw material, quite a substantial amount of material is removed in the form of chips in order to achieve the final shape required. Also, lot of energy is expended in the process of material removal. So the choice of material removal as an option for manufacturing should be considered when no other manufacturing process suits the purpose. The material removal processes are a family of shaping operations in which excess material is removed from a starting work part so that what remains is the desired final geometry. The most important branch of the family is conventional machining, in which a sharp cutting tool is used to mechanically cut the material to achieve the desired geometry. Another group of material removal processes is the abrasive processes, which mechanically remove material by the action of hard, abrasive particles. There are the non-traditional processes, which use various energy forms other than a sharp cutting tool or abrasive particles to remove material. UME 505: MANUFACTURING TECHNOLOGY Classification of Material Removal Processes Material removal processes Conventional Abrasive Non-traditional machining processes machining Turning and Grinding Mechanical energy related operations operations processes Other abrasive Electrochemical Drilling and related processes machining operations Thermal energy Milling processes Other machining Chemical operations machining UME 505: MANUFACTURING TECHNOLOGY  Machining is a manufacturing process in which a sharp cutting tool is used to cut away material to leave the desired part shape. The predominant cutting action in machining involves shear deformation of the work material to form a chip; as the chip is removed, a new surface is exposed. Machining is most frequently applied to shape metals. Machining is one of the most important manufacturing processes. The Industrial Revolution and the growth of the manufacturing-based economies of the world can be traced largely to the development of the various machining operations UME 505: MANUFACTURING TECHNOLOGY Machining is important commercially and technologically for several reasons: ✓ Variety of work materials: Machining can be applied to a wide variety of work materials. Virtually all solid metals can be machined. Plastics and plastic composites can also be cut by machining. ✓ Variety of part shapes and geometric features: Machining can be used to create any regular geometries, such as flat planes, round holes, and cylinders. By introducing variations in tool shapes and tool paths, irregular geometries can be created, such as screw threads and T-slots. By combining several machining operations in sequence, shapes of almost unlimited complexity and variety can be produced. ✓ Dimensional accuracy: Machining can produce dimensions to very close tolerances. Some machining processes can achieve tolerances of 0.025 mm, much more accurate than most other processes. ✓ Good surface finishes: Machining is capable of creating very smooth surface finishes. Roughness values less than 0.4 microns can be achieved in conventional machining operations. Some abrasive processes can achieve even better finishes. UME 505: MANUFACTURING TECHNOLOGY Variables of a Machining Process Input (independent) variables Output (dependent) variables Workpiece material, like composition and Cutting force and power influences metallurgical features deflection and chattering, both affect Starting geometry of the workpiece, part size and accuracy. including preceding processes Geometry of finished product, thus Selection of process, which may be obtaining a machined surface of desired conventional or nonconventional shape, tolerance, and mechanical processes properties. Tool material Surface finish Machining parameters Tool failure due to the increased power Work-holding devices ranging from vises consumption. to specially designed jigs and fixtures Economy of the machining process Cutting fluids Ecological aspects and health hazards UME 505: MANUFACTURING TECHNOLOGY Methods of Metal Cutting There are two basic methods of metal cutting based on cutting edge and direction of relative motion between tool and work: Orthogonal Cutting Process: In orthogonal cutting process, the cutting edge is perpendicular (90 degree) to the direction of feed. The chip flows in a direction normal to cutting edge of the tool. A perfectly sharp tool will cut the metal on rack surface. Examples are Lathe cut-off operation, Straight milling, etc Oblique Cutting Process: In oblique cutting process, the cutting edge is inclined at an acute angle (less than 90 degree) to the direction of feed. The chip flows sideway in a long curl. The chip flows in a direction at an angle with normal to the cutting edge of the tool. Examples are Drilling, Slab milling, End milling, etc UME 505: MANUFACTURING TECHNOLOGY Difference between Orthogonal and Oblique Cutting Orthogonal Cutting Oblique Cutting In orthogonal cutting, the cutting edge of In oblique cutting, the cutting edge of the the tool makes right angle to the direction of tool is inclined to the direction of feed feed motion. motion. In orthogonal cutting, there are only two In oblique cutting, three components of components of force, cutting force and force are considered, that is thrust force, thrust force, acting mutually perpendicular. radial force and cutting force. Lesser cutting life Higher cutting life High heat concentration at the cutting Less heat concentration in cutting region. region. The cutting edge is larger than the cutting The cutting edge may or may not be width. larger than cutting width. The chips flow in the direction normal to The chips flow along the sideways. the cutting edge. The surface finish obtained is very poor. The surface finish obtained is fairly good. The shear force that act per unit area is The shear force per unit is low; a factor high, a factor which increases the heat which decreases heat developed per unit developed per unit area. area hence increasing tool life. UME 505: MANUFACTURING TECHNOLOGY Common Cutting Processes Turning, in which the workpiece is rotated and a cutting tool removes a layer of material as the tool moves along its length. Cutting off, in which the tool moves radially inward, and separates the piece on the right from the blank. Face milling is a machining process in which the milling cutting is placed perpendicular to the workpiece. When engaged, the top of the milling cutting grinds away at the top of the workpiece to remove some of its material. UME 505: MANUFACTURING TECHNOLOGY  Drilling is performed with a rotating cylindrical tool that has two cutting edges on its working end. The rotating drill feeds into the stationary work piece to form a hole whose diameter is equal to the drill diameter Slab milling, in which a rotating cutting tool removes a layer of material from the surface of the workpiece. End milling, in which a rotating cutter travels along a certain depth in the workpiece and produces a cavity. UME 505: MANUFACTURING TECHNOLOGY Mechanics of Orthogonal Metal Cutting There are two theories to analyse the metal removal process. The first theory is that the deformation zone is very thin and planar. The other theory is that the actual deformation zone is a thick one with a fan shape. Thick shear Thin shear zone model plane model Though the first model is convenient from the stand point of analysis, physically it is impossible to create. The stress gradient across the shear plane has to be very large to be practical. In the second model, by marking the shear zone over a region, the transitions in velocities and the shear stresses could be realistically accounted for. The angle made by the shear plane with the cutting speed vector, φ is a very important parameter in metal cutting. UME 505: MANUFACTURING TECHNOLOGY  Higher the shear angle, better is the cutting performance. From a view of the thin shear plane model, it can be observed that higher rake angles give rise to higher shear angles. The current analysis is based on Merchant’s thin shear plane model considering the minimum energy principle. This model would be applicable at very high cutting speeds, which are generally practised in production. IMPRTANT ASSUMPTIONS: ✓ The tool is perfectly sharp, and there is no contact along the clearance face. ✓ The shear surface is a plane extending upward from the cutting edge. ✓ The cutting edge is a straight line extending perpendicular to the direction of motion and generates a plane surface as the work moves past to it. ✓ The chip does not flow to either side. ✓ The depth of cut is constant. ✓ The width of tool is greater than that of the workpiece. ✓ The work moves relative to the workpiece with uniform velocity. ✓ A continuous chip is produced. ✓ The shear and normal stresses along the shear plane and the tool are uniform. UME 505: MANUFACTURING TECHNOLOGY Determination of Shear Plane Angle During orthogonal cutting, the workpiece material undergoes instantaneous deformation along the shear plane. The angle that this plane creates with the cutting speed vector, in a plane normal to the machined surface, is called the shear angle (φ). Let t, l, and b denote the thickness, length, and width of the uncut chip, respectively. The corresponding dimensions of the cut chip are tc, lc, and bc. UME 505: MANUFACTURING TECHNOLOGY  During machining, it is assumed that the change of density is negligible and, consequently, the volume of the uncut chip is equal to that of the cut chip: Assuming a negligible change in chip width during orthogonal cutting, we assume b = bc. Hence eq. 1 changes to The eq.2 can be rewritten as where rc is the chip thickness ratio. The eq.3 can be rewritten in terms of chip velocity Vc and cutting velocity V as UME 505: MANUFACTURING TECHNOLOGY  From the geometry of the figure, we can write the length of shear plane AC accordingly where α is the normal rake angle. On rearranging eq.5, we get From eq. 4, we know that t/tc = rc. Rewriting eq.6, On solving eq.7 for φ, we get UME 505: MANUFACTURING TECHNOLOGY References Rao, P.N., Manufacturing Technology Volume 1, McGraw Hill Education (India) Private Ltd. Groover, M.P., Principles of Modern Manufacturing, John Wiley and Sons (2011). Juneja, B.L., Sekhon, G.S., Seth, N., Fundamentals of Metal Cutting and Machine Tools, New Age International Publishers (2017). El-Hofy, H.A.G., Fundamentals of Machining Processes: Conventional and Nonconventional Processes, CRC Press, (2014) Kalpakjian, S. and Schmid, S.R., Manufacturing Engineering and Technology, 4th ed., Pearson Education (2001). UME 505: MANUFACTURING TECHNOLOGY THANK YOU UME 505: MANUFACTURING TECHNOLOGY