Mechanical Working Notes PDF

Manufacturing Technology 4. Mechanical working of metals Introduction Mechanical working processes are used to achieve optimum mechanical properties in the metal. Metal working reduces any internal voids...

Manufacturing Technology 4. Mechanical working of metals Introduction Mechanical working processes are used to achieve optimum mechanical properties in the metal. Metal working reduces any internal voids or cavities present and thus make the metal dense. The impurities present in the metal also get elongated with the grains and in the process they get broken and dispersed throughout the metal. This decreases the harmful effect of the impurities and improves the mechanical strength. Hot working and cold working The metal working processes are traditionally divided into hot working and cold working processes. The division is on the basis of the amount of heating applied to the metal before applying the mechanical force. These processes, working above the recrystallisation temperature, are termed as hot working processes whereas those below are termed as cold working processes. Hot Working Properties 1. Hot working is done at a temperature above recrystallization but below its melting point. It can therefore be regarded as a simultaneous process of deformation and recovery. 2. Hardening due to plastic deformation is completely eliminated by recovery and recrystallization. 3. Improvement of mechanical properties such as elongation, reduction of area and impact values. 4. No effect on ultimate tensile strength, yield point, fatigue strength and hardness. 5. Poor surface finish due to oxidation and scaling. 6. Refinement of crystals occurs. 7. Due to hot working cracks and blowholes are welded up. 8. No internal or residual stress developed. 9. Force required for deformation is less. 10. Light equipment is used in hot working. 11. Difficult to handle a hot worked metal. 12. Hot working processes are—hot forging, hot rolling, hot spinning, hot extrusion, hot drawing, and hot piercing, pipe welding. Advantages of Hot Working 1. Porosity in the metal is largely eliminated. Most ingots contain many small blow holes. These are pressed together and eliminated. 2. Impurities in the form of inclusions are broken up and distributed throughout the metal. Department of Mechanical & Manufacturing Engineering, MIT, Manipal 1  Manufacturing Technology 3. Coarse or columnar grains are refined. Since this hot work is in the recrystalline temperature range, it should be continued until the low limit is reached to provide a tine grain structure. 4. Physical properties are generally improved owing principally to grain refinement. Ductility and resistance to impact are improved, strength is increased, and greater homogeneity is developed in the metal. The greatest strength of rolled steel exists in the direction of metal flow. 5. The amount of energy necessary to change the shape of steel in the plastic state is far less than that required when the steel is cold. Disadvantages/Limitations of Hot Working 1. Because of the high temperature of the metal, there is rapid oxidation or scaling of the surface with accompanying poor surface finish. 2. Difficult to achieve close tolerances due to scaling. 3. Some metals cannot be hot worked because of their brittleness at high temperatures. 4. Hot working equipment and maintenance costs are high Cold Working Properties l. Cold working is done at temperature below recrystallization temperature. So, no appreciable recovery can take place during deformation. 2. Hardening is not eliminated since working is done below recrystallization temperature. 3. Decreases elongation, reduction of area etc. 4. Increase in ultimate tensile strength, yield point and hardness. 5. Good surface finish is obtained. 6. Crystallization does not occur. Grains are only elongated. 7. Possibility of crack formation and propagation is great. 8. Internal and residual stresses are developed in the metal. 9. Force required for deformation is high. 10. Heavy and powerful equipment is used for cold working. 11. Easier to handle cold parts. 12. Cold working processes are—cold rolling, cold extrusion, press work (drawing, squeezing, bending, and shearing). Advantages of Cold Working 1. Cold working increases the strength and hardness of the material due to the strain hardening which would be beneficial in some situations. Further, there is no possibility of decarburization of the surface. 2. Since the working is done in cold state, hence no oxide formation on the surface and consequently, good surface finish is obtained. 3. Greater dimensional accuracy is achieved. Department of Mechanical & Manufacturing Engineering, MIT, Manipal 2  Manufacturing Technology 4. Easier to handle cold parts and also economical for small sizes. 5. Better mechanical properties are achieved. Disadvantages/Limitations of Cold Working 1. Only small sized components can be easily worked as greater forces are required for large sections. Due to large deforming forces, heavy and expensive capital equipment is required. 2. The grain structure is not refined and residual stresses have harmful effects on certain properties of metals. 3. Many of the metals have less ductility e.g., carbon steel and certain alloy steels, cannot be cold worked at room temperature. It is therefore, limited to ductile metals and the range of shapes produced is not as wide as can be obtained by machining. 4. Tooling costs are high and as such it is used when large quantities of similar components are required. Sheet Metal Forming: Shearing: Shearing is a process which involves cutting sheet metal by subjecting it to shear stresses, usually between a punch and a die Figure 4.1: Schematic illustration of shearing process with die and punch The major variables in the shearing process are the punch force (F), the speed of the punch, lubrication, the edge condition of the sheet, the punch and die materials, the comer radii of the punch and die, and the clearance between the punch and die. Fig. 4.1 shows the overall features of a typical sheared edge for the two sheared surfaces i.e., the sheet and the slug. The clearance ‘c’ is the major factor that determines the shape and quality of the sheared edge. Usually shearing starts with the formation of cracks on both the top and bottom edges of the workpiece (A and B in Fig. 4.1). These cracks eventually meet and complete separation takes place Shearing Operations: Department of Mechanical & Manufacturing Engineering, MIT, Manipal 3  Manufacturing Technology l. Blanking and punching: It is a kind of shearing operation, carried out along a closed contour. The desirable part in this operation is the metal inside the sheared contour, called blank. Example is making circular blanks out of sheets for subsequent deep drawing of cups. Punching is the operation in which the desired part is the sheet left out after making a punch hole or contour shearing. Die-punch clearance is very critical for blanking and punching, as it governs the kind of finish obtained on the final part. Figure 4.2: Blanking/Punching 2. Lancing Lancing - Creating a partial cut in the sheet, so that no material is removed. The material is left attached to be bent and form a shape, such as a tab, vent, or louver. Lancing is a piercing operation in which the workpiece is sheared and bent with one strike of the die. A key part of this process is that there is not reduction of material, only a modification in its geometry. This operation is used to make tabs, vents, and louvers.The cut made in lancing is not a closed cut, like in perforation even though a similar machine is used, but a side is left connected to be bent sharply or in more of a rounded manner. Lancing can be used to make partial contours and free up material for other operations further down the production line. Along with these reasons lancing is also used to make tabs (where the material is bent at a 90 degree angle to the material), vents (where the bend is around 45 degrees), and louvers (where the piece is rounded or cupped).It also help to cut or slight shear of sheet on cylindrical shape.Normally lancing is done on a mechanical press, lancing requires the use of punches and dies to be used. The different punches and dies determine the shape and angle (or curvature) of the newly made section of the material. The dies and punches are needed to be made of tool steel to withstand the repetitious nature of the proced Department of Mechanical & Manufacturing Engineering, MIT, Manipal 4  Manufacturing Technology 3. Slitting: It is also a shearing process, but rather than making cuts at the end of a workpiece like shearing, slitting is used to cut a wide coil of metal into a number of narrower coils as the main coil is moved through the slitter. During the slitting process, the metal coil passes lengthwise through the slitter’s circular blades. Fig. 4.4 provides a two-dimensional look at a typical coil slitting process. Note, how the metal workpiece is drawn past the upper and lower slitting blades, leaving two coils of the same length as the original wide coil. Figure 4.4: Slitting 4. Notching: Figure 4.5: Notching It is a shearing process during which a metal scrap piece is removed from the outside edge of a metal workpiece. Notching is typically a manually operated, low-production process. Department of Mechanical & Manufacturing Engineering, MIT, Manipal 5  Manufacturing Technology During a notching operation, the metal workpiece has an outside edge removed by the use of multiple shear blades that are set at right angles to each other. Fig. 4.5 provides a two- dimensional look at a typical notching process. Note, how the tool removes the notched part. As mentioned above, the notching process removes material from only the outside edges of the workpiece. Fig. 4.6 shows how a metal workpiece may look after the notching process is employed and has removed material from the outside edges. Figure 4.6: Notched shapes 5. Perforating: It is the punching of many holes, usually identical and arranged in a regular pattern, in a sheet, workplace blank, or previously formed part. The holes are usually round, but may be of any shape. The operation is also called multiple punching. (Fig. 4.7). Figure 4.7: Perforating 6. Nibbling : It is the process of cutting out a contour or other shape by punching a series of overlapping round or square holes along the edge of the part (Fig. 4.8). Department of Mechanical & Manufacturing Engineering, MIT, Manipal 6  Manufacturing Technology Figure 4.8: Nibbling 7. Shaving:It is a finishing or sizing process and very little material is removed from the edges. Usually, the edge has been press worked previously. Shaving is intended to produce tight tolerance holes or blanks. 8. Slotting: It is the cutting of elongated holes or slots. 9. Trimming: It is the cutting away of excess material in a flange or flash from a piece. Figure 4.9: Trimming 10. Lancing: It is a cutting operation in which a hole is partially cut and then one side is bent down to form a sort of tab. Department of Mechanical & Manufacturing Engineering, MIT, Manipal 7  Manufacturing Technology Figure 4.10: Lancing Embossing Embossing is a process that produces relatively shallow indentations or raised portions with no change in metal thickness, and is done on relatively thin sections which is carried out by means of a punch and a die (Fig 4.11) The operation is a combination of bending and stretching operations, and there is no lateral flow of metal as in coining. This process is used in forming projections as in the case of 'projection welding' Figure 4.11: Embossing Coining Coining is a cold-forging operation in which deformation takes place mainly by compression. This Process is used for the manufacture of coins, badges, medals, keys, metal plaques, tokens, etc. The process is carried out in a closed die and there is no provision for the extrusion of excess metal (Fig 4.12). Care must be taken to see that the gauge of the blanks is not excessive, otherwise which may damage the punch and die. Department of Mechanical & Manufacturing Engineering, MIT, Manipal 8  Manufacturing Technology Figure 4.12: Coining The coining pressure depends mainly on the malleability of the metal and the extent to which it should flow. Because of the metal flow in cold conditions, very high pressures are required (around 1500 N/mm2) For this purpose usually heavy duty hydraulic presses are used. Bending In bending, a straight length of metal is worked into a curved length. The common bending operations are the formation of channels. drums, tanks, etc., using plam sheets. Fig 4.14 illustrates the terms used in bending Figure 4.14: Terms in bending Department of Mechanical & Manufacturing Engineering, MIT, Manipal 9  Manufacturing Technology Bend radius, R: It is defined as the radius of curvature on the concave surface, i.e..the inside surface of the bend. Neutral Axis, NA: When a maternal is bent. the layers at the outer side of the bend are stretched and lengthened, while those on the inside are compressed 8nd shortened At some intermediate layer there will be neither lengthening nor shortening and the length of this layer will remain unaltered upon bending. This layer is called the "neutral axis". Bend allowance, B: when metal is bent, its final length is increased over its original length because the metal thickness is reduced. As the bend radius becomes smaller, the decrease in thickness increases. The developed length of the centre line of the bent section is called as 'Bend Allowance'. Minimum bend radius: For a given bending operation, the bend radius cannot be below certain value, at which the metal will crack on the outer tensile surface. This minimum value of radius, below which the metal may crack is called 'the minimum bend radius'. Spring back in Bending It is the common forming difficulty encountered in all bending operations. Spring back is in dimensional change of the formed part after the pressure of the forming tool has been released. It is the effort of metal that is not stressed beyond its elastic limit to reshape itself to its original position. If the metal nearest to the neutral axis has been stressed below the elastic limit the metal will try to return to its preformed shape when the forming forces are removed. A metal that has been stressed beyond its elastic limit will resist returning to its original shape, but a certain amount of spring back will occur in a formed part, the forces causing the spring back are just reverse of the stresses placed on the metal during forming. Harder and thicker metals exhibit higher degree of springback. Bulging: Department of Mechanical & Manufacturing Engineering, MIT, Manipal 10  Manufacturing Technology In bulging applications, urethane offers much longer service life than rubber, and is very cost effective compared to expanding/shrinking steel punches. Displacement of urethane punch under compression forces metal outward into desired shape. Amount of bulge is determined by depth of stroke. When ram is raised (after stroke), urethane resumes its normal shape and bulged part can be easily removed. Curling: Curling sheet metal is the process of adding a hollow, circular roll to the edge of the sheet. The curled edge provides strength to the edge and makes it safe for handling. Curling is different than a tear drop hem because in a curl the edge finishes inside itself, where a hem leaves the initial edge exposed. Sheet Metal Hems are formed using very different methods, though produces features with similar uses and functionality. Curls are most often used to remove a sharp untreated edge and make it safe for handling. Roll forming Bending the sheet metal using rollers is called as "roll-forming". In roll forming the strip is progressively bent into complex shapes by passing it through a series of driven rolls. The thickness of the sheet is not appreciably reduced. Roll forming can be subdivided into two types: a) Cold-roll forming b) Roll bending a) Cold-roll forming In cold-roll forming (Fig 4.15 a) a flat metal strip is fed lengthwise between one or more pairs of forming rolls. These pairs of rolls are mounted in stands in a straight line like a continuous rolling mill. Only straight sections are formed by cold-roll forming. Cold-roll forming is an economical method for forming straight sections in long lengths and in large quantities. Department of Mechanical & Manufacturing Engineering, MIT, Manipal 11  Manufacturing Technology b) Roll bending Roll bending (Fig 4.15 b and c) uses three rolls to form the metal sheets and plates. To start with, the work piece is bent by some other method so that it is easily gripped by rolls for further forming. Even cylindrical shapes can be produced by this method. One or two rolls are adjustable, so that bending can be achieved to the required degree. The process is very suitable for longer work pieces. Figure 4.15: Roll forming Department of Mechanical & Manufacturing Engineering, MIT, Manipal 12