Mody Institute of Technology and Science, LakshmangarhFaculty of Engineering and Technology

II-Mid Term Examination, Spring Semester 2011-2012B.Tech. (CSE, IT, ME, MBA-ECE & CSE Branch I Year)

Course Name: Basic Manufacturing Processes Weight: 50 %Course Code: ME 107 Max. Marks: 100

Time: 3 hrNote: Question one is compulsory and attempts any Five from other questions.

SolutionQ.1 (A) (i) (b) (ii) (d) (iii) (c) (iv) (b) (v) (c) (vi) (b) (vii) (d) (viii) (a) (ix) (b) (x) (c)Q.1 (B) (i) Cast iron is iron or a ferrous alloy which has been heated until it liquifies, and is then poured into a mould to solidify. It is usually made from pig iron. The alloy constituents affect its colour when fractured: white cast iron has carbide impurities which allow cracks to pass straight through. Grey cast iron, or grey iron, has graphitic flakes which deflect a passing crack and initiate countless new cracks as the material breaks. (ii) A pattern is a model or the replica of the object (to be casted). It is embedded in molding sand and suitable ramming of molding sand around the pattern is made. The pattern is then withdrawn for generating cavity (known as mold) in molding sand. Thus it is a mould forming tool. Pattern can be said as a model or the replica of the object to be cast except for the various al1owances a pattern exactly resembles the casting to be made. It may be defined as a model or form around which sand is packed to give rise to a cavity known as mold cavityin which when molten metal is poured, the result is the cast object. When this mould/cavityis filled with molten metal, molten metal solidifies and produces a casting (product). So thepattern is the replica of the casting. A pattern prepares a mold cavity for the purpose of making a casting. It may also possess projections known as core prints for producing extra recess in the mould for placement of core to produce hol1owness in casting. (iii) A single point cutting tool is held in the tool holder, which is mounted on the ram. The workpiece is rigidly held in a vice or clamped directly on the table. The table may be supported at the outer end. The ram reciprocates and thus cutting tool held in tool holder moves forward and backward over the workpiece. In a standard shaper, cutting of material takes place during the forward stroke of the ram. The backward stroke remains idle and no cutting takes place during this stroke. The feed is given to the workpiece and depth of cut is adjusted by moving the tool downward towards the workpiece. The time taken during the idle stroke is less as compared to forward cutting stroke and this is obtained by quick return mechanism. The cutting action and functioning of clapper box is shown in Fig during forward and return stroke.

(iv) Gas welding, sometimes referred to by its subtypes metal inert gas (MIG) welding or metal active gas (MAG) welding, is a welding process in which an electric arc is formed between a consumable wire electrode and the workpiece metal(s), which heats the workpiece metal(s), causing them to melt, and join. Along with the wire electrode, a shielding gas is fed through the welding gun, which shields the process from contaminants in the air. The process can be semi-automatic or automatic. A constant voltage, direct current power source is most commonly used with GMAW, but constant current systems, as well as alternating current, can be used. There are four primary methods of metal transfer in GMAW, called globular, short-circuiting, spray, and pulsed-spray, each of which has distinct properties and corresponding advantages and limitations.

(v) Recrystallization is a process by which deformed grains are replaced by a new set of unreformed grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility. Thus, the process may be introduced as a deliberate step in metals processing or may be an undesirable by product of another processing step. The most important industrial uses are the softening of metals previously hardened by cold work, which have lost their ductility, and the control of the grain structure in the final product.

Q.2 (a) Manufacturing is the use of machines, tools and labour to produce goods for use or sale. The term may refer to a range of human activity, from handicraft to high tech, but is most commonly applied to industrial production, in which raw materials are transformed into finished goods on a large scale. Such finished goods may be used for manufacturing other, more complex products, such as aircraft, household appliances or automobiles, or sold to wholesalers, who in turn sell them to retailers, who then sell them to end users – the "consumers".

(b) MALLEABILITY AND DUCTILITY

Both these properties relate to the plasticity of the material. Malleability refers to the ability of plastic deformation under compressive loads, while ductility refers to plastic deformation under tensile loads.A malleable material can be beaten into thin sheets and even thinner foils. A ductile material can be drawn into wires.A measure of ductility is “percentage elongation”. Before the tensile test begins two punchmarks are made on the stem of the tensile test piece. Distance between these marks is noted and isknown as gauge length (l0). After the tensile test piece fractures in two pieces, the two pieces are retrieved and placed together as close to each other as possible. Now the distance between the two punch marks is measured and noted again. Let this distance be l1. The % elongation is calculated as High values of percentage elongation indicate that material is very ductile. Low values indicate that material is brittle and has low ductility. For mild steel, the percentage elongation usually is 20% or more.

BRITTLENESSBrittleness can be thought of as opposite of ductility. It is a property which is possessed in great measure by glass and other ceramics. A piece of glass, if dropped on a hard surface shatters and is broken in many pieces. The real cause of brittleness is inability of the material to withstand shock loads. Of course, glass is an extreme case of brittle material.

STIFFNESS AND RESILIENCEA material with high value of modulus of elasticity is said to be stiff and a material with low value of modulus of elasticity is said to be resilient. Consider a material undergoing tensile stress within the elastic range. If the material possesses a high value of Young’s modulus (which is the modulus of elasticity corresponding to tensile stress), the material will not stretch much. It will behave as a “stiff ” material. In this case, the slope of the line OA (Fig. 1.1) will be more. Resilience is a property which is totally opposite to stiffness. A beam made

of stiff material will deflect to a lesser extent as compared to another made of resilient material under identical loading condition.

TOUGHNESS AND IMPACT STRENGTHToughness and impact strength are allied or similar properties (although these are some differences as mentioned later). They represent the ability of the material to absorb energy before actual failure/fracture occurs. Refer to Fig. 1.1. If the scale of y-axis is changed and if force is plotted on this axis and, if actual elongation is plotted on x-axis instead of strain, we shall obtain a force-elongation curve instead of stress-strain curve. The shape of curve will remain the same; only scales of x and y axes will change. Now the area under this curve will represent energy required to fracture the material. Higher Properties of Materials the energy, higher is the toughness of material. Toughness comes from a combination of strength and percentage elongation. Since this property enables a material to withstand both elastic and plastic strains, it is considered very important.Higher impact strength goes with higher toughness. In actual impact testing, loads used aredynamic loads and the load is directed to the specimen through a sharp notch. Two tests have been standardised to measure the impact strength of a material (as also its toughness).

HARDNESSHardness is a very important property of materials. Hardness indicates wear-resistance and resistance against abrasion or scratching. A hard material also offers resistance to penetration by another body. In the olden days, a scale of hardness was established and diamond, which is the hardest known material was put on top of this scale. Glass and other materials were put lower down on this scale. The criterion used was a simple scratch test. If a material could scratch another material, then the former was considered harder than the latter material and was placed higher in the scale of hardness.

(c) Patterns are replicas of the casting required. It is similar in shape and size to the final product, but not exactly. Usually, the mould is prepared in wet sand, to which some binder is added to hold sand particles together. The pattern is then withdrawn from inside the sand mould in such a manner that the impression/cavity made in the mould is not damaged or broken in anyway. Finally molten metal is poured into this cavity and allowed to solidify and cool down to room temperature. Patterns are usually made of good quality wood. Wood is easy to work, acquires good smooth surface and properly seasoned wood retains its size. It is also relatively cheap and abundant. However, if a very large number of castings are required, metal patterns may be used. Usually, they are made of aluminium-magnesium alloys.

Q.3 (a) Pattern may be made from wood or metal and its colour may not be same as that of the casting. The material of the pattern is not necessarily same as that of the casting. Pattern carries an additional allowance to compensate for metal shrinkage. It carries additional allowance for machining. It carries the necessary draft to enable its easy removal from the sand mass. It carries distortions allowance also. Due to distortion allowance, the shape of casting is opposite to pattern. Pattern may carry additional projections, called core prints to produce seats or extra recess in mold for setting or adjustment or location for cores in mold cavity. It may be in pieces (more than one piece) whereas casting is in one piece. Sharp changes are not provided on the patterns. These are provided on the casting with the help of machining. Surface finish may not be same as that of casting.

1. Shrinkage AllowanceIn practice it is found that all common cast metals shrink a significant amount when they are cooled from the molten state. The total contraction in volume is divided into the following parts:1. Liquid contraction, i.e. the contraction during the period in which the temperature of the liquid metal or alloy falls from the pouring temperature to the liquidus temperature.2. Contraction on cooling from the liquidus to the solidus temperature, i.e. solidifying contraction.

3. Contraction that results thereafter until the temperature reaches the room temperature. This is known as solid contraction.

2. Machining AllowanceIt is a positive allowance given to compensate for the amount of material that is lost in machining or finishing the casting. If this allowance is not given, the casting will become undersize after machining. The amount of this allowance depends on the size of casting, methods of machining and the degree of finish. In general, however, the value varies from 3 mm. to 18 mm.

3. Draft or Taper AllowanceTaper allowance (Fig. 10.11) is also a positive allowance and is given on all the vertical surfaces of pattern so that its withdrawal becomes easier. The normal amount of taper on the external surfaces varies from 10 mm to 20 mm/mt. On interior holes and recesses which are smaller in size, the taper should be around 60 mm/mt. These values are greatly affected by the size of the pattern and the molding method. In machine molding its, value varies from 10 mm to 50 mm/mt.

4. Rapping or Shake AllowanceBefore withdrawing the pattern it is rapped and thereby the size of the mould cavity increases. Actually by rapping, the external sections move outwards increasing the size and internal sections move inwards decreasing the size. This movement may be insignificant in the case of small and medium size castings, but it is significant in the case of large castings. This allowance is kept negative and hence the pattern is made slightly smaller in dimensions 0.5-1.0 mm.

5. Distortion AllowanceThis allowance is applied to the castings which have the tendency to distort during cooling due to thermal stresses developed. For example a casting in the form of U shape will contract at the closed end on cooling, while the open end will remain fixed in position. Therefore, to avoid the distortion, the legs of U pattern must converge slightly so that the sides will remain parallel after cooling.

Q.3 (b) Injection molding (British English: moulding) is a manufacturing process for producing parts from both thermoplastic and thermosetting plastic materials. Material is fed into a heated barrel, mixed, and forced into a mold cavity where it cools and hardens to the configuration of the cavity.In this process, thermoplastic materials soften when heated and re-harden when cooled. No chemical change takes place during heating and cooling. Fig. 13.11 illustrates the injection molding process. The process involves granular molding material is loaded into a hopper from where it is metered out in a heating cylinder by a feeding device. The exact amount of material is delivered to the cylinder which is required to fill the mold completely. The injection ram pushes the material into a heating cylinder and doing so pushing bushes a small amount of heated material out of other end of cylinder through the nozzle and screw bushing and into cavities of the closed mold. The metal cooled in rigid state in the mold. Then mold is opened and piece is ejected out material heating temperature is usually between 180°- 280°C. Mold is cooled in order to cool the mold articles. Automatic devices are commercially available to maintain mold temperature at required level. Injection molding is generally limited to forming thermoplastic materials, but equipment is available for converting the machines for molding thermosetting plastics and compounds of rubber.

Q.4 (a) A live center or revolving center is constructed so that the 60° center runs in its own bearings and is used at the non driven or tailstock end of a machine.[2] It allows higher turning speeds without the need for separate lubrication, and also greater clamping pressures. CNC lathes use this type of center almost exclusively and they may be used for general machining operations as well. Spring loaded live centers are designed to compensate for center variations, without damage to the work piece or center tip. This assures the operator of uniform constant tension while machining. Some live centers also have interchangeable shafts. This is valuable when situations require a design other than a 60° male tip.A dead center (one that does not turn freely, i.e., dead) may be used to support the workpiece at either the fixed or rotating end of the machine. When used in the fixed position, a dead center produces friction between the workpiece and center, due to the rotation of the workpiece.

(b) A simple lathe comprises of a bed made of grey cast iron on which headstock, tailstock, carriage and other components of lathe are mounted. The different parts of engine lathe or central lathe. The major parts of lathe machine are given as under:1. Bed2. Head stock3. Tailstock4. Carriage5. Feed mechanism6. Thread cutting mechanism

1. BedThe bed of a lathe machine is the base on which all other parts of lathe are mounted. It is massive and rigid single piece casting made to support other active parts of lathe. On left end of the bed, headstock of lathe machine is located while on right side tailstock is located. The carriage of the machine rests over the bed and slides on it. On the top of the bed there are two sets of guideways-innerways and outerways. The innerways provide sliding surfaces for the tailstock and the outerways for the carriage. The guideways of the lathe bed may be flat and inverted V shape. Generally cast iron alloyed with nickel and chromium material is used for manufacturing of the lathe bed.

2. Head StockThe main function of headstock is to transmit power to the different parts of a lathe. It comprises of the headstock casting to accommodate all the parts within it including gear train arrangement. The main spindle is adjusted in it, which possesses live centre to which the work can be attached. It supports the work and revolves with the work, fitted into the main spindle of the headstock. The cone pulley is also attached with this arrangement, which is used to get various spindle speed through electric motor. The back gear arrangement is used for obtaining a wide range of slower speeds. Some gears called change wheels are used to produce different velocity ratio required for thread cutting.

3. Tail Stock the tail stock of central lathe, which is commonly used for the objective of primarily giving an outer bearing and support the circular job being turned on centers. Tail stock can be easily set or adjusted for alignment or non-alignment with respect to the spindle centre and carries a centre called dead centre for supporting one end of the work. Both live and dead centers have 60° conical points to fit centre holes in the circular job, the other end tapering to allow for good fitting into the spindles. The dead centre can be mounted in ball bearing so that it rotates with the job avoiding friction of the job with dead centre as it important to hold heavy jobs.

4. CarriageCarriage is mounted on the outer guide ways of lathe bed and it can move in a direction parallel to the spindle axis. It comprises of important parts such as apron, cross-slide, saddle, compound rest, and tool post. The lower part of the carriage is termed the apron in which there are gears to constitute apron mechanism for adjusting the direction of the feed using clutch mechanism and the split half nut for automatic feed. The cross-slide is basically mounted on the carriage, which generally travels at right angles to the spindle axis. On the cross-slide, a saddle is mounted in which the compound rest is adjusted which can rotate and fix to any desired angle. The compound rest slide is actuated by a screw, which rotates in a nut fixed to the saddle. The tool post is an important part of carriage, which fits in a tee-slot in the compound rest and holds the tool holder in place by the tool post screw. The tool post of centre lathe.

Q.4 (C)

BaseIt is rigid and heavy cast iron body to resist vibration and takes up high compressive load. It supports all other parts of the machine, which are mounted over it. The base may be rigidly bolted to the floor of the shop or on the bench according to the size of the machine.

ColumnThe column is a box shaped casting mounted upon the base. It houses the ram-driving mechanism. Two accurately machined guide ways are provided on the top of the column on which the ram reciprocates.

Cross railCross rail of shaper has two parallel guide ways on its top in the vertical plane that is perpendicular to the rai1 axis. It is mounted on the front vertical guide ways of the column. It consists mechanism for raising and lowering the table to accommodate different sizes of jobs by rotating an elevating screw which causes the cross rail to slide up and down on the vertical face of the column. A horizontal cross feed screw is fitted within the cross rail and parallel to the top guide ways of the cross rail. This screw actuates the table to move in a crosswise direction.

SaddleThe saddle is located on the cross rail and holds the table on its top. Crosswise movement of the saddle by rotation the cross feed screw by hand or power causes the table to move sideways.

TableThe table is a box like casting having T -slots both on the top and sides for clamping thework. It is bolted to the saddle and receives crosswise and vertical movements from the saddle and cross rail.

RamIt is the reciprocating part of the shaper, which reciprocates on the guideways provided above the column. Ram is connected to the reciprocating mechanism contained within the column.

Tool headThe tool head of a shaper performs the following functions-(1) It holds the tool rigidly,(2) It provides vertical and angular feed movement of the tool, and(3) It allows the tool to have an automatic relief during its return stroke.The various parts of tool head of shaper are apron clamping bolt, clapper box, tool post, down feed, screw micrometer dial, down feed screw, vertical slide, apron washer, apron swivel pin, and swivel base. By rotating the down feed screw handle, the vertical slide carrying the tool gives down feed or angular feed movement while machining vertical or angular surface. The amount of feed or depth of cut may be adjusted by a micrometer dial on the top of the down feed screw. Apron consisting of clapper box, clapper block and tool post is clamped upon the vertical slide by a screw. The two vertical walls on the apron called clapper box houses the clapper block, which is connected to it by means of a hinge pin. The tool post is mounted upon the clapper block. On the forward cutting stroke the clapper block fits securely to the clapper box to make a rigid tool support. On the return stroke a slight frictional drag of the tool on the work lifts the block out of the clapper box a sufficient amount preventing the tool cutting edge from dragging and consequent wear. The work surface is also prevented from any damage due to dragging.

Q.5 (a) Difference between Shaper and Planer

(b) DrillingThis is the operation of making a circular hole by removing a volume of metal from the job by a rotating cutting tool called drill as shown in Fig. Drilling removes solid metal from the job to produce a circular hole. Before drilling, the hole is located by drawing two lines at right angle and a center punch is used to make an indentation for the drill point at the center to help the drill in getting started. A suitable drill is held in the drill machine and the drill machine is adjusted to operate at the correct cutting speed. The drill machine is started and the drill starts rotating. Cutting fluid is made to flow liberally and the cut is started. The rotating drill is made to feed into the job. The hole, depending upon its length, may be drilled in one or more steps. After the drilling operation is complete, the drill is removed from the hole and the power is turned off.

ReamingThis is the operation of sizing and finishing a hole already made by a drill. Reaming is performed by meansof a cutting tool called reamer as shown in Fig. Reaming operation serves to make the hole smooth, straight and accurate in diameter. Reaming operation is performed by means of a multitooth tool called reamer. Reamer possesses several cutting edges on outer periphery and may be classified as solid reamer and adjustable reamer.

BoringThe boring operation where enlarging a hole by means of adjustable cutting tools with only one cutting edge is accomplished. A boring tool is employed for this purpose.

Q.5 (c) CUTTING SPEEDCutting speed for lathe work may be defined as the rate in meters per minute at which the surface of the job moves past the cutting tool. Machining at a correct cutting speed is highly important for good tool life and efficient cutting. Too slow cutting speeds reduce productivity and increase manufacturing costs whereas too high cutting speeds result in overheating of the tool and premature failure of the cutting edge of the tool. The following factors affect the cutting speed:(i) Kind of material being cut,(ii) Cutting tool material,(iii) Shape of cutting tool,(iv) Rigidity of machine tool and the job piece and(v) Type of cutting fluid being used.Calculation of cutting speed Cs, in meters per minuteCs = ((22/7) × D × N)) /1000WhereD is diameter of job in mm.N is in RPM

FEEDFeed is defined as the distance that a tool advances into the work during one revolution of the headstock spindle. It is usually given as a linear movement per revolution of the spindle or job. During turning a job on the center lathe, the saddle and the tool post move along the bed of the lathe for a particular feed for cutting along the length of the rotating job.

Depth of cut:Depth of chip taken by cutting tool and one-half total amount removed from workpiece in one cut.

Q.6 (a) Push Type ShaperThis is the most general type of shaper used in common practice, in which the metal is removed when the ram moves away from the column, i.e. pushes the work. (b)

1. Plain milling or slab milling the plain and slab milling operation. It is a method of producing a plain, flat, horizontal surface parallel to the axis of rotation of the cutter. 2. Face milling

Drilling OperationReaming Operation Boring Operation

the face milling operation. It is a method of producing a flat surface at right angles to the axis of the cutter.3. Side milling the side milling operation. It is the operation of production of a flat vertical surface on the side of a work-piece by using a side milling cutter.

4. Angular milling angular milling operation. It is a method of producing a flat surface making an angle to the axis of the cutter.

5. Gang-milling the gang milling operation. It is a method of milling by means of two or more cutters simultaneously having same or different diameters mounted on the arbour of the milling machine.

6. Form millingthe form milling operation. It is, a method of producing a surface having an irregular outline.

7. End millingEnd milling operation. It is a method of milling slots, flat surfaces, and profiles by end mills.

Q.6 (c)

BaseIt is a foundation member for all the other parts, which rest upon it. It carries the column at its one end. In some machines, the base is hollow and serves as a reservoir for cutting fluid.ColumnThe column is the main supporting member mounted vertically on the base. It is box shaped, heavily ribbed inside and houses all the driving mechanism for the spindle and table feed. The front vertical face of the column is accurately machined and is provided with dovetail guideway for supporting the knee.KneeThe knee is a rigid grey iron casting which slides up and down on the vertical ways of the column face. An elevating screw mounted on the base is used to adjust the height of the knee and it also supports the knee. The knee houses the feed mechanism of the table, and different controls to operate it.SaddleThe saddle is placed on the top of the knee and it slides on guideways set exactly at 90° to the column face. The top of the saddle provides guide-ways for the table.TableThe table rests on ways on the saddle and travels longitudinally. A lead screw under the table engages a nut on the saddle to move the table horizontally by hand or power. In universal machines, the table may also be swiveled horizontally. For this purpose the table is mounted on a circular base. The top of the table is accurately finished and T -slots are provided for clamping the work and other fixtures on itOverhanging armIt is mounted on the top of the column, which extends beyond the column face and serves as a bearing support for the other end of the arbor.Front braceIt is an extra support, which is fitted between the knee and the over-arm to ensure further rigidity to the arbor and the knee.SpindleIt is situated in the upper part of the column and receives power from the motor through belts, gears. and clutches and transmit it to the arbor.ArborIt is like an extension of the machine spindle on which milling cutters are securely mounted and rotated. The arbors are made with taper shanks for proper alignment with the machine spindles having taper holes at their nose. The draw bolt is used for managing for locking the arbor with the spindle and the whole assembly. The arbor assembly consists of the following components.

Q.7 (a) Welding is a process for joining two similar or dissimilar metals by fusion. It joins different metals/alloys, with or without the application of pressure and with or without the use of filler metal. The fusion of metal takes place by means of heat. The heat may be generated either from combustion of gases, electric arc, electric resistance or by chemical reaction. During some type of welding processes, pressure may also be employed, but this is not an essential requirement for all welding processes. Welding provides a permanent joint but it normally affects the metallurgy of the components.

(b) Welding ElectrodesAn electrode is a piece of wire or a rod of a metal or alloy, with or without coatings. An arc is set up between electrode and workpiece. Welding electrodes are classified into following types-(1) Consumable Electrodes(a) Bare Electrodes(b) Coated Electrodes(2) Non-consumable Electrodes(a) Carbon or Graphite Electrodes(b) Tungsten Electrodes

Consumable electrode is made of different metals and their alloys. The end of this electrode starts melting when arc is struck between the electrode and workpiece. Thus consumable electrode itself acts as a filler metal. Bare electrodes consist of a metal or alloy wire without any flux coating on them. Coated electrodes have flux coating which starts melting as soon as an electric arc is struck. This coating on melting performs many functionslike prevention of joint from atmospheric contamination, arc stabilizers etc. Non-consumable electrodes are made up of high melting point materials like carbon, pure tungsten or alloy tungsten etc. These electrodes do not melt away during welding. But practically, the electrode length goes on decreasing with the passage of time, because of oxidation and vaporization of the electrode material during welding. The materials of nonconsumable electrodes are usually copper coated carbon or graphite, pure tungsten, thoriated or zirconiated tungsten.

(C) SOLDERINGSoldering is a method of joining similar or dissimilar metals by heating them to a suitable temperature and by means of a filler metal, called solder, having liquidus temperuatre notexceeding 450°C and below the solidus of the base material. Though soldering obtains a good joint between the two plates, the strength of the joint is limited by the strength of the filler metal used. Solders are essentially alloys of lead and tin. To improve the mechanical properties and temperature resistance, solders are added to other alloying elements such as zinc, cadmium and silver in various proportions. Soldering is normally used for obtaining a neat leak proof joint or a low resistance electrical joint. The soldered joints are not suitable for high temperature service because of the low melting temperatures of the filler metals used. The soldering joints also need to be cleaned meticulously to provide chemically clean surfaces to obtain a proper bond. Solvent cleaning, acid pickling and even mechanical cleaning are applied before soldering. To remove the oxides from the joint surfaces and to prevent the filler metal from oxidizing, fluxes are generally used in soldering. Rosin and rosin plus alcohol based fluxes are least active type and are generally used for electrical soldering work. Because of the content of acids, these are corrosive at soldering temperature. They can be easily cleaned after the soldering. The organic fluxes such as zinc chloride and ammonium chloride are quick acting and produce efficient joints. But because of their corrosive nature the joint should be thoroughly cleaned of the entire flux residue from the joint.

BRAZINGLike soldering, brazing is a process of joining metals without melting the base metal. Filler material used for brazing has liquidus temperature above 450°C and below the solidus temperature of the base metal. The filler metal is drawn into the joint by means of capillary action (entering of fluid into tightly fitted surfaces). Brazing is a much widely used joining process in various industries because of its many advantages. Due to the higher melting point of the filler material, the joint strength is more than in soldering. Almost all metals can be joined by brazing except aluminium and magnesium which cannot easily be joined by brazing. Dissimilar metals, such as stainless steel to cast iron can be joined by brazing. Because of the lower temperatures used there is less distortion in brazed joints. Also, in many cases the original heat treatment of the plates being joined is not affected by the brazing heat. The joint can be quickly finished without much skill. Because of the simplicity of the process it is often an economical joining method with reasonable joint strength. The brazed joints are reasonably stronger, depending on the strength of the filler metal used. But the brazed joint is generally not useful for high temperature service because of the low melting temperature of the filler metal. The color of the filler metal in the brazed joint also, may not match with that of the base metal. Because the filler metal reaches the joint by capillary action, it is essential that the joint is designed properly. The clearance between the two parts to be joined should be critically controlled. During brazing, the base metal of the two pieces to be joined is not melted. An important requirement is that the filler metal must wet the base metal surfaces to which it is applied. The diffusion or alloying of the filler metal with the base metal place even though the base metal does not reach its solidus temperature. The surfaces to be joined must be chemically clean before brazing.

Q.8 (a) Comparison of Hot Working with Cold Working

(b)

(i) Forward or direct extrusion process: In this process, the material to be extruded is in the form of a block. It is heated to requisite temperature and then it is transferred inside a chamber as shown in fig. In the front portion of the chamber, a die with an opening in the shape of the cross-section of the extruded product, is fitted. The block of material is pressed from behind by means of a ram and a follower pad. Since the chamber is closed on all sides, the heated material is forced to squeeze through the die-opening in the form of a long strip of the required cross-section. The process looks simple but the friction between the material and the chamber walls must be overcome by suitable lubrication. When extruding steel products, the high temperautre to which the steel has to be heated makes it difficult to find a suitable lubricant. The problem is solved by using molten glass as a lubricant. When lower temperatures are used, a mixture of oil and graphite is used as a lubricant.At the end of the extrusion process, a small piece of metal is left behind in the chamber which cannot be extruded. This piece is called butt—end scrap and is thrown away. To manufacture a tubular rod, a mandrel of diameter equal to that of tube—bore is attached to the ram. This mandrel passes centrally through the die when the material is extruded. The outside diameter of the tube produced will be determined by the hole in the die and the bore of tube will be equal to mandrel diameter. The extrusion process will then called “tubular extrusion”.

(ii) Backward or indirect extrusion: This process is depicted in Fig. 4.3. As shown, the block of heated metal is inserted into the container/chamber. It is confined on all sides by the container walls except in front; where a ram with the die presses upon the material. As the ram presses backwards, the material has to flow forwards through the opening in the die. The ram is made hollow so that the bar of extruded metal may pass through it unhindered. This process is called backward extrusion process as the flow of material is in a direction opposite to the movement of the ram. In the forward extrusion process the flow of material and ram movement were both in the same direction. The following table compares the forwards (direct) and backwards (indirect extrusion process):

(c) Rolling: In this process, metals and alloys are plastically deformed into semifinished or finished products by being pressed between two rolls which are rotating. The metal is initially pushed into the space between two rolls, thereafter once the roll takes a ‘‘bite’’ into the edge of the material, the material gets pulled in by the friction between the surfaces of the rolls and the material. The material is subjected to high compressive force as it is squeezed (and pulled along) by the rolls. This is a process to deal with material in bulk in which the cross-section of material is reduced and its length increased. The final cross-section is determined by the impression cut in the roll surface through which the material passes and into which it is compressed.Rolling is done both hot and cold. In a rolling mill attached to a steel plant, the starting point is a cast ingot of steel which is broken down progressively into blooms, billets and slabs. The slabs are further hot rolled into plate, sheet, rod, bar, rails and other structural shapes like angles, channels etc. Conversion of steel into such commercially important sections is usually done in another rolling mill called merchant mill.

Rolling is a very convenient and economical way of producing commercially important sections. In the case of steel, about three-fourth’s of all steel produced in the country is ultimately sold as a rolled product and remaining is used as forgings, extruded products and in cast form. This shows the importance of rolling process.

Forging: In forging, metal and alloys are deformed to the specified shapes by application of repeated blows from a hammer. It is usually done hot; although sometimes cold forging is also done. The raw material is usually a piece of a round or square cross-section slightly larger in volume than the volume of the finished component. Depending on the end use of the component, the forged part may be used as such or (more frequently) it has to be machined to correct size to close tolerances. The initial volume of material taken must, therefore, allow for loss due to scaling and the machining allowance.

(C)(i) Blooms: It is the first product obtained from the breakdown of Ingots. A bloom has a crosssection ranging in size from 150 mm square to 250 mm square or sometimes 250 × 300 mm rectangle.(ii) Billet: A billet is the next product rolled from a bloom. Billets vary from 50 mm square to125 mm square.