B.E. First Semester Examination, December Manufacturing Processes (ME-103-E)

Transcription

1 B.E. First Semester Examination, December-2005 Manufacturing Processes (ME-103-E) Note : Attempt any five questions Q. 1. What do you understand by the following terms : (a) Simplification (b) Automation (c) Mechanization (d) Standardization (e) Interchangeability Ans. (a) Simplification : It is a process through which a limited number of grades, types and sizes of a product arc determined in order to have better control, minimise waste, simplify production and thus reduce price. By eliminating unnecessary varieties, sizes and design it leads towards interchangeable of products. (b) Automation : It involves adoption of automatic controls for the operations of different machines. This automatic control may be adopted for only a few operationos or all the operations of a machine and accordingly the machine will be known as semi-automatic or fully automatic. Also it may be adopted only for a single machine out of the whole lot or a series or group of such machines depending upon the operation required for the product. (c) Mechanization : It is a term which implies an endeavour or trend towards minimising the human efforts, to the extent. It is possible, by adopting mechanical. Means or methods for different processes. Such a trend may be in material handling, loading and unloading of components, actual operations done on the job or transportation etc. But no feedback is provided by the process, operation or machinery. (d) Standardization : It is the second step towards interchangeable manufacture increased output and higher economy. The process of standardisation involves determining the best material, manufacturing processes and allied techniques for the manufacture of a product and adhering to them very strictly so long as the better standards for all these are not known. Two types of standards prevailing in various industries : (i) Managerial standards (ii) Technical standards. (e) Interchangeability : It is a principle applied to the mass production of identical components such that any one of the components, selected at random, will suit the assembly conditions to meet the requirements ot operation. For this the components are allowed a predetermined amount of variation in their finished dimensions, the two extremities of which are called limits. All the components of which the finished dimensions lie within these limits are acceptable components. Q. 2. (a) How cast iron differs from steel? Ans. The cast iron produced consists of iron, carbon, silicon, sulphur phosphorus and manganese in varying proportions. In general use cast iron possess carbon from 3 to 4 percent. Other constituents present in

2 cast iron are silicon 1 to 3 percent, sulphur 0.02 to 0.15%, phosphorus upto 1.0% and manganese 0.5 to 1.0 percent. This metal is very brittle and has a low resistance to tension but good in compression. It is available in different forms such as grey iron, white iron. Whereas steel basically differs from cast iron in the amount of carbon content contained by it. It is not only the quantity of carbon which makes the difference but also the form in which it is present. In steel the amount of carbon present is upto 1.5%. It different form dead mild steel, mild steel, medium carbon steels, high carbon steels. Q. 2. (b) Differentiate among mild-carbon steel, medium-carbon steel, high-carbon steel and high-speed steel in terms of chemical composition, properties structure, application and limitations. Ans. Mild Carbon Steel : It is having carbon content below 0.15% used in forgings, stamping, tubes, castings. Medium Carbon Steel : It is having carbon from 0.3% to 0.8% used in drop forgings, boiler drums, marine shafts, axles, rotors. High Carbon Steel: It is having carbon from 0.8% to 1.5%, used in springs, shear blades, wood chisels, cold sets, hammers. Q. 3. (a) Make a layout of a casting shop, showing all possible tools and equipments available there. Ans. Listed of tools and equipments used in casting shop : shown at (i) Foundary cupola of required casting Nos - 7 (ii) Blower of proper capacity for cupola Nos - 6 (iii) Pit furnances For more study Nos material - 9 & 10 Log on to (iv) Oil fired fitting furnance Nos - 8

3 (v) Drying oven Nos - 5 (vi) Bench for bench moulding Nos - 2 (vii) Space for floor moulding Nos - 1 (viii) Space for care making Nos-4 (ix) Coke store Nos- 13 Q. 3. (b) Discuss the step-by-step full casting process in detail. Ans. Casting Process : (a) Pattern Making: In casting process first step is to making of pattern. A pattern may be defined as a model or replica of the object to be cast. Except for various allowances and core prints, a pattern exactly resembles the casting to be made during making of pattern design considerations are : (i) Careful selection of parting line is necessary for easy removal of the pattern from sand. (ii) As far as possible employ full core instead of joined half cores. (iii) Proper allowances should be provided. (iv) All sharp corners should be suitable provided with fillets. (b) Moulding Sand Preparation : Proper conditioning and preparation of sand is needed to get good castings sand preparation is also known as sand tempering and quality of moulding sand depends upon the manner in which it is prepared. Sand preparation means mixing the moulding sand ingredients such as sand, binder, clay, water and other additions. Sand conditioning means preparation of moulding sand, so that it becomes suitable for moulding purposes. (c) Moulding: It is a cavity in a moulding box, formed by a pattern. It is similar in shape and size so that of the actual casting plus some allowance. (d) Pouring of molten metal in a cavity. Q. 4. (a) Describe briefly the various characteristics of hot working and cold working processes. Ans. Those processes, working above the recrystallisation temperature are termed as hot working processes where those below are termed as cold working processes. Characteristics of Hot Working Processes : (i) As the material is above the recrystallisation temperature, any amount of working can be imparted since there is no strain hardening taking place. (li) At high temperature, the material would, have higher amount of ductility and therefore therse is no limit on the amount of hot working that can be done on a material even brittle materials can be hot worked. (iii) Hot working requires much less force to achieve the necessary deformation. (iv) Better mechanical properties achieved. Characteristics of Cold Working Processes : (i) Cold working increases the strength and hardness of the material due to the strain hardening which would be beneficial in some situations. There is no possibility of decarburisation of the surface. (ii) No oxide would form on the surface and consequently good surface finish is obtained. (iii) Better dimensional accuracy is achieved. (iv) It is far easier to handle cold ports and it is also economical for smaller sizes.

4 Q. 4. (b) Differentiate among Forging, Rolling and Extrusion, using principle, set-up, process, products, advantages and applications. Ans. Rolling is a process where the metal is compressed between two rotating rolls for reducing its cross-sectional area. This is one of the most widely used of all the metal working processes because of its higher productivity and low cost. Rolling would be able to produce components having constant cross-section throughout its length. Many shapes such as 1, T, L and channel sections are possible. It is also possible to produce special sections such as railway wagon wheels by rolling individual pieces. Rolling is normally a hot working process unless specifically mentioned as cold rolling. Forging is the operation where the metal is heated and then a force is applied to manipulate the metal. In such a way that the required final shape is obtained. This is the oldest of the metal working processes known to mankind since the copper age forging is generally a hot working operation though cold forging is used sometimes. Extrusion is the process of confining the metal in a closed cavity and then allowing it to flow from only one opening so that the metal will take the shape of the opening. The operation is identical to the squeezing of tooth paste but of the tooth paste tube. By the extrusion process it is possible to make components which have a constant cross-section over any length as can be had by the rolling process. The complexity of parts that can be obtained by extrusion is more than that of rolling because the die required being very simple and easier to make. Also extrusion is a single pass process unlike rolling. Q. 5. (a) Using figures, show the relative motions between work-piece and cutting tools in case of operations on lathe, shaper, planer, milling, drilling and slotter. Ans. Relative motion of

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6 of bed. Q. 5. (b) How do you specify a lathe machine? Ans. In order to specify a lathe, the following specifications should be included : (i) (a) Height of centres (b) Type of bed i.e. straight, semi-gap or gap type (c) Centre distance. (ii) (a) Swing over bed (b) Swing over cross slide (c) Swing ingap (d) Gap in front of face plate (e) Width (iii) (a) Spindle speech range (b) Spindle nose (c) Spindle bore (d) Taper in nose. (iv) (a) Metric thread pitches (b) Lead screw pitch (c) Longitudinal feeds (d) Cross feeds. (v) (a) Cross slide travel (b) Top slide travel (c) Tool section. (vi) (a) Tail stock sleeve travel (b) Taper in sleeve bore. (vii) Motor horse power and RPM. (viii) Shipping dimensions : Length x width x height x weight. Q. 6. Differentiate among gas welding, arc welding and resistance welding, using (a) principle (b) process-set-up (c) types (d) applications (e) advantages (0 tools used Ans. Gas Welding : Gas welding also called as oxy-fuel gas welding (OFW), derives the heat from the combustion of a fuel gas such as acetylene in combination with oxygen. The process is a fusion welding process wherein the joint is completely melted to obtain the fusion. The heat produced by the combustion of gas is sufficient to melt any metal and as such is universally applicable. The fuel gas generally used is acetylene because of the high temperature generated in the flame. Gas welding contains the supply units for oxygen and acetylene with associated regulators and the torch, which mixes the two gases before they are ignited. The oxygen is normally stored in strong cylinders at a pressure ranging from 13.8 MPa to 18.2 MPa. Acetylene is normally made available in the following two forms; acetylene storage cylinder and acetylene generator. The principal advantage of an oxy acetylene welding set up is the versatility of the equipment. The same equipment with a range of torches would be used for oxygen cutting as well as for brazing and braze welding. Arc Welding : In welding generation of heat by an electric arc is one of the most efficient method. Approximately 50% of the energy is liberated in the form of heat. The electric arc welding process makes use of the heat produced by the electric arc to fusion weld metallic pieces. This is one of the most widely used welding process, mainly because of the ease of use and high prduction rates that can be achieved economically. An arc is generated between two conductors of electricity, cathode and anode (DC). When they are touched to establish the flow of current and then separated by a small distance. An arc is a sustained electric discharge through the ionised gas column plasma between the two electrodes. The main requirement in an arc welding setup is the source of electric power. They are essentially of two types : (a) AC machines : (I) Transformer (ii) Motor

7 (b) DC machines : (i) Transformer with DC rectifier (ii) Motor or engine driven generator. Resistance Welding : It is a function welding process where both heat and pressure are applied on the joint but no filler metal or flux is added. The heat necessary for the melting of the joint is obtained by the heating effect of the electrical resistance of the joint and hence name resistance welding. In resistance welding (RW), a low voltage (typically 1 V) and very high current (typically 15,000 A) is passed through the joint for a very short time (typically 0.255). This high amperage heats the joint due to the contact resistance at the joint and melts it. The pressure on the joint is continuously maintained and the metal fuses together under this pressure. The heat generated in resistance welding can be expressed as H = kl 2 The maim requirement of the process is the low voltage and high current power supply. This is obtained by means of a step down transformer with a provision to have different tappings on the primary side, as required for different materials. Resistance welding are very well suited for mass production, as they give a high production rate. There are no consumables used in this process except for the electrical power and a relatively smaller electrode wear. As a result it is very economical process. It is possible to weld dissimilar metals as well as metal plates of different thicknesses. Q. 7. Differentiate among types of plant layouts in detail. Ans. Plant layout is defined as it is such a systematic and effective functional arrangement of different departments machines, equipment and services of an organisation that will facilitate the processing of the proposed or undertaken product in the most efficient and economical manner in the minimum time. Different types of layout: (a) Fixed Position Layout: In this type of layout the major part of an assembly or material remains at a fixed position and all its accessories, auxilliary material, machinery and equipment needed, tools required and the labour are brought to it to work at site. Thus the location of the major component or material is not disturbed till the product is ready for despatch. This type of layout is mostly adopted for extremely large items manufactured in very small numbers e.g. ship, aeroplanes etc. (b) Process or Functional Layout: In this type of layout similar, machines or operations are gouped together, so that all the operations of same type are performed at the same place always, for example all the lathes may be grouped together to do turning and threading etc. all drilling machines in one area to do drilling work, all buffing and polishing machines at one place to do surface finishing work and so on. This type of layout is usually preferred for the industries involved in job order type of production and manufacturing and/or maintenance activities of non-repetitive type as shown in figure. Rt

8 (c) Line Production or Product: In this type of layout all the machines are arranged in a line according to the sequence of operation i.e. each following machine or section is arranged to perform the next operation to that performed by its preceding machine or section. It is an ideal form of layout in which the handling time is minimum. (d) Combination Layout (or Group Layout): Because of the inherent advantages and disadvantages associated with both the process layout as well as product layout the best results can be obtained by combining the advantageous features of both these layouts into a single one and this is what is known as a combination layout. In this type of layout a set of machinery or equipment is grouped together in a section and so on, so that each set of machines or equipment is used to perform similar operations to produce a family of components. Q. 8. Write short notes on any three : (a) Tree structure classification of Manufacturing processes (b) Cupola furnace (c) Tool wear (d) TIG welding (e) Shearing operations Ans. (a) Tree Structure Classification of Manufacturing Process : (b) Cupola Furnace : For melting of cast iron in foundary the cupola furnace is used. It has a construction in the form of a hollow vertical cylinder made of strong mild steel plates and riverted or welded at the seams. Welded joints' are more common in modern practice. In large cupolas the lower portion is made of comparatively thicker plates so as to make it strong enough to hold the upper structure and fire brick lining. Thus the stress in the whole structure is distributed uniformly. Also such cupolas are further strengthened by providing the brick retaining rings at suitable heights. The bottom door of the shell can be in one piece hinged to a supporting leg or in two pieces, each piece hinged separately to the two opposite legs. When the cupola is in operation, the bottom door is supported by a prop so that it may not collapse due to the large weight of the charge and coke etc. it carries when we do not need the cupola for further operation, the charge feeding is stopped, air supply cut off and the prop removed. As soon as the prop is removed the door, drops down providing a clear space for the coke fire, residue of the molten metal with slag and the sand bed to fall down and thus, the fire inside ceases gradually.

9 (c) Too) Wear : It is not very difficult to realize that the success of a machining process depends on the sharpness of the tool. Even common sense tells us that the use of a blunt tool results in a large power consumption and deteriorated surface finish when a cutting tool is unable to cut consuming reasonable energy and cannot produce an acceptable finish it is considered to have failed.the failure of a cutting tool may be due to one or a combination of the following models : (i) Plastic deformation of the tool due to high temperature and large stress. (ii) Mechanical breakage of the tool due to large force and insufficient strength and toughness. (iii) Blunting of the cutting edge of the tool through a process of gradual wear. It is clear that the wearing action takes place on those surfaces along which there is a relative sliding with other surfaces. Thus, the wear takes place on the rake surface where the chip flows over the tool and on the flank surface where rubbing between the work and the tool occurs. These wears are called the crater and the flank wears. (d) TIG Welding : Tungsten Inert Gas (TIG) welding is an inert gas shielded arc welding process using non-consumable electrode. The electrodes may also contain 1 to 2% Thoria mixed along with 0.15 to 0.40% Zirconia. The pure tungsten electrodes are less expensive but will carry less current. The thoriated tungsten electrodes carry high currents are more desirable because they can strike and maintain a stable arc with relative ease. The Zirconia added tungsten electrodes are better than pure tungsten electrodes are better than pure tungsten but inferier to thoriated tungsten electrodes. The TIG welding process can be used, for the joining of a number of materials though the most common ones are aluminium magnesium and stainless steel. (e) Shearing Operation : The metal is brought to the plastic stage by pressing the sheet between two shearing blades so that fracture is initiated at the cutting points. The fractures on either side of the sheet further progressing downwards with the movement of the upper shear, Finally result in the separation of the slug from the parent strip. The metal under the upper shear is subjected to both compressive and tensile stresses. In an ideal shearing operation, the upper shear pushes the metal to a depth equal to about one third of its thickness. Because of pushing of the material into the lower shear; the area of cross-section of the metal between the cutting edge of the shears decreases and causes the initiation of the fracture. This portion of the metal which is forced into the lower shear is highly burnished and would appear as a bright band around the blank lower portion. The fractures which are initiated at both the cutting points would progress further with the movement of the upper shear and if the clearance is sufficient, would meet. Thus completing the shear action.