Workshop Practice PDF

Workshop Practice C.S. Baladhiya J.B. Raol  WORKSHOP PRACTICE Writer C.S. Baladhiya & J.B. Raol Department of Dairy Engineering AAU, Anand  Index SN Lesson...

Workshop Practice C.S. Baladhiya J.B. Raol  WORKSHOP PRACTICE Writer C.S. Baladhiya & J.B. Raol Department of Dairy Engineering AAU, Anand  Index SN Lesson Page No Module 1 Introduction to workshop practice Lesson 1 Introduction to workshop practice, safety, care and precaution in 4-8 workshop Module 2 Bench work tools and processes Lesson 2 The bench work tools and its uses 9-17 Lesson 3 The bench work tools, its uses and processes 18-24 Module 3 Smithy and forgoing tools and operation Lesson 4 Smithy and forging tools and equipment 25-32 Lesson 5 Smithy and forging operation 33-37 Module 4 Heat treatment process Lesson 6 Heat treatment processes: Hardening, tempering, annealing and 38-44 normalizing Lesson 7 Metal Cutting 45-53 Module 5 Welding Lesson 8 Electric arc welding 54-62 Lesson 9 Gas Welding 63-68 Module 6 Lathe machine, drilling machine, etc Lesson 10 Introduction to lathe machine 69-76 Lesson 11 Introduction to drilling machine 77-82 Lesson 12 Introduction to milling and grinding machine 83-88 Lesson 13 Introduction to shaper and planer machine, CNC machines 89-92 Module 7 Carpentry Lesson 14 Wood working tools and their works 93-100 Lesson 15 Carpentry and Pattern Making, Mould Material and Their Application 101-107 Module 8 Jigs and fixtures Lesson 16 Use of jigs and fixtures in production 108-113 Reference 114 Workshop Practice Module 1. Introduction to workshop practice Lesson-1 Introduction to workshop practice, safety, care and precaution in workshop 1.0. Introduction Workshop practice is a very vast one and it is very difficult for anyone to claim a mastery over it. It provides the basic working knowledge of the production and properties of different materials used in the industry. It also explains the use of different tools, equipments, machinery and techniques of manufacturing, which ultimately facilitate shaping of these materials into various usable forms. In general, various mechanical workshops know by long training how to use workshop tools, machine tools and equipment. Trained and competent persons should be admitted to this type of mechanical works and permitted to operate equipment. Processes: 1. Primary shaping processes 2. Machining processes 3. Joining processes 4. Surface finishing processes 5. Processes effecting change in properties. 1.1.1. Primary shaping processes Some of these finish the product to its usable form whereas others do not and it requires further working to finish the component to the desired shape and size. Wire drawing lead to the directly usable articles, which do not need further processing before use. 4 www.AgriMoon.Com Workshop Practice Casting,forging, bending, rolling, drawing, power metal forging, etc 1.1.2.Machining processes Large number of components need further processing after primary processes known as secondary operation to obtain desired shape and dimensional accuracy. These operations require the use of one or more machine tools, various types of cutting tools and cutters, job holding devices, marking and measuring instruments, testing devices and gauges etc. Common machining operations are: Turning,Threading. Drilling, Boring, Planning, Shaping, Sawing, Milling, Grinding,Slotting, etc. 1.1.3. Joining processes These processes are used for joining metal parts and in general fabrication work. Such requirement usually occur when larger lengths of standard sections are required or several pieces are to be joined together to fabricate a desired structure. Common processes are Welding, Soldering, Brazing, Riveting, Screwing, Pressing, etc. 1.1.4.Surface Finishing Processes These processes should not be misunderstood as metal removing processes in any case as they are primarily intended to provide a good surface finish or a decorative and/or protective coating on to the metal surface, although a very negligible amount of metal removal or addition may take place. Thus, any appreciable variation in dimensions will not be effected by these processes. The common processes employed for obtaining desired surface finish are the following: 1. Buffing 2. Polishing 3. Lapping 4. Belt grinding 5. Metal spraying 6. Painting 1.1.5. Processes Effecting Change in Properties These processes are employed to impart certain specific properties to the metal parts so as to make them suitable for particular operations. Most physical properties like hardening, softening and grain refinement etc., call for particular heat treatment. Heat treatments not only effect the physical properties, but in most cases also make a marked change in the internal structure of the metal. So is the case with cold and hot working of metals. 1. Heat treatment 2. Cold working 3. Hot working Workshop safety The safety in Workshops has been written not only to provide appropriate safety procedures but also to assist trained workshop personnel with the provision of a reference document outlining the general 5 www.AgriMoon.Com Workshop Practice principles of safe working practices relevant to the mechanical engineering aspects. It relates to specific are as where definite safety measures are required for workshop operations Factories Act and Accident Various acts relating to accidents are spelt out in workmen’s compensation Act-1923, The factories act- 1948 and Fatal Accidents Act-1855. These acts describe the regulations for fencing and guarding the dangerous machinery, items and employer’s liabilities. 1.1.6. Concept of Accident It is very difficult to give a definition of the word‘Accident’. However, a generally accepted conception that an accident is a mishap, a disaster that results in some sort of injury, to men, machines or tools and equipments and in general loss to the organization. The said injury or loss may be of minor or major nature and the accident is termed as non-reportable or reportable. For example, a small cut on the body will be reportable accident in a training workshop. It can be treated by first aid and does not involve any appreciable loss of time, and will not be considered a reportable accident in a production unit. 1.1.7. Causes of Accidents The 98% accidents could be easily avoided provided due precautions are taken well in time. A very familiar slogan goes on to say that accidents do not just happen but are caused due to the failure of one element or the other, and the most unfortunate factor is that the human element is the most pronounced of all which fail. The common causes which lead to accidents are the following: 1. Unsafe working position. 2. Improper or defective tools or their improper use. 3. Improper acts- which result in violation of safety rules and non-observance of safety precautions. 1.1.8. Common Sources of Accidents The large number of machines in use and an even larger number of parts. This can be regarded as sources of danger and require guarding for protection against accidents. Some common sources of accident are listed below : Projecting nips between sets of revolving parts, viz., gears, rolls and friction wheels, etc. 1. Projecting fasteners on revolving parts. 2. Revolving cutting tools, circular saw blades. 3. Revolving drums, crushers, spiked cylinder and armed mixers, etc. 4. Revolving shafts, spindles, bars and tools like drills,reamers, boring bars and chucks, etc. 6 www.AgriMoon.Com Workshop Practice 5. Projecting sharp edges or nips of belt and chain drives viz., belt, pulleys, chains, sprockets and belt fasteners. 6. Reciprocating tools and dies of power presses, drop hammers, and revolving presses, etc. 7. Grinding wheels and stones. 8. Reciprocating knives and saw blades such as cutting and trimming machines and power hack-saws, etc. 9. Revolving drums and cylinders without casing, such as concrete and other mixers. 10. Intermittent feed mechanisms. 11. Projecting nips between various links and mechanisms, like cranks connecting rods, piston rods, rotating wheels and discs, etc. 1.2. Common Methods of Protection The common methods of protection against accidents are the following: 1. Safety by position. 2. Safety by construction. 3. Safety by using interlock guards. 4. Safety by using fixed guards. 5. Safety by using automatic guards. 6. Safety by using distance guards. 1.2.1. Safety by construction When a new machine is designed, it should be ensured that all its dangerous parts are either enclosed in suitable housings or provided with suitable safety guards. For example, the belt drive and motor in a lathe or milling machine are enclosed, the back gears in a lathe are either enclosed or provided with cast iron guards or covers. Lubricating points are provided on the outer surfaces so that the interior parts are not required to be opened every time. 1.2.2. Safety by Position The machine design is in such a way that the dangerous parts are located such that they are always beyond the reach of the operator. The dangerous parts of all the machines should invariably be guarded and undertaking should be made to make them enclosed in the body or housing of the machines. 1.2.3. Safety by using interlock guards 7 www.AgriMoon.Com Workshop Practice It is a very efficient and sound method of guarding in that the guard cannot be removed and dangerous parts exposed until and unless the machine is totally stopped. Similarly, the machine cannot be started to work unless the guard returns in position and protects the dangerous parts. An interlocking guard may be mechanical,electrical or some sort of a combination of these. It is essential that it should: 1. Prevent the starting and operation of the machine in case the interlocking device fails. 2. Always acquire its position to guard the dangerous part before the machine can be started. 3. Remain closed in position until the dangerous part is completely at rest. 1.2.4. Safety by using fixed guards These guards either for man integral part of the machine or are tightly secured to them. They should be made to have rigid construction and should be so placed that any access to the dangerous parts of the machine is totally prevented in the running condition of the machines. Steel sheets can be advantageously used and they facilitate an easy fabrication of guards and are lighter in weight. In some cases the fixed guards are made adjustable in order to accommodate different kinds of works or sets of tools. In some cases the fixed guards are provided at a distance from the danger point. 1.2.5. Safety by using distance guards The principle of a distance guards is that a fencing, enough high, is made of bars,at a suitable distance from the machine such that even if the operative, by chance, extends his hands over it, his fingers, clothes or any part of the body does not reach within the area of dangerous parts. An additional measure of safety, some sort of tripping device is also usually incorporated to stop the machine quickly in case of an accident. 1.2.6. Safety by using automatic guards The principle of an automatic guard is that its operation is actuated by some moving part of the machine. It may linked that the part will automatically bring the guard in protecting position before the operation of the machine starts. The design of the guard is such that it automatically forces the operative away from the dangerous area of work before the operation starts and does not permit his access to the area again until and unless the machine stops. It may be noted that due to enough time being required for their operation, this type of guards are not suitable for quick-acting and fast-running machines. Their use is largely favoured for heavy and slow acting machines like heavy power presses. 8 www.AgriMoon.Com Workshop Practice Module 2. Bench work tools and processes Lesson-2 The Bench Work Tools and its Uses Introduction Bench work has its own essential position in all engineering works. In the mechanized workshops, where most of the work is carried out on an automatic machine, while bench work has its own importance. The jobs can be finished to a fairly good degree of accuracy through machining operation; they often require the hand operations to be done on them to finish to the desired accuracy. A fitter’s work is unavoidable when different parts are to be assembled in position after they have been finished. Alignment of machine parts, bearings, engine slide valves and similar other works call for a fitter’s work. Reconditioning and refitting of machines and machine parts cannot be done without a skilled fitter. All the above types of works require the use of a large number of hand tools and a fitter must have good working knowledge of all these tools and instruments. 2.1. Filter’s vices Vices are the most suitable and widely used tools for gripping different jobs in position during various operations carried out in a fitting shop. There are a fairly good number of different types of vices such as parallel jaw vice, machine vice, hand vice and pipe vice. From these, the parallel jaw vice is the most commonly used in general fitting work. These vices are available in different trade sizes and the selection of a suitable size will depend upon the maximum size of the work. The width of the jaws determines the size of the vice. In fixing it on the fitter’s bench it is held with the help of bolts passing through the planks of the bench. The bolts are tightened by means of nuts and the vice is held firmly on the bench. The jaws of the vice are usually kept overhanging the edge of the bench. Bench vice It is the most commonly used vice sometimes also known as parallel jaw vice. It essentially consists of a cast steel body, a movable jaw, a fixed jaw, both made of cast steel, a handle, a square threaded screw and a nut all made of mild steel. A separate cast steel plates known as jaw plates with teeth are fixed to the jaws by means of set screws and they can be replaced when worn. The movement of the vice is caused by the screw which passes through the nut fixed under the movable jaw. The screw is provided with a collar inside to prevent it from coming out and handle at the outer end. The width of the jaws suitable for common work varies from 80 to 140 mm and the maximum opening being 95 to 180 mm. 9 www.AgriMoon.Com Workshop Practice 2.2. Surface Plate Its specific use is in testing the trueness of a finished surface, testing a try square, providing adequate bearing surface for V- block and angle plates, etc., in scribing work. It is a cast iron plate having a square or rectangular top perfectly planed true and square with adjacent machined faces. The top is finished true by means of grinding and scrapping. This plate carries a cast iron base under it and the bottom surface of the base is also machined true to keep the top surface of the plate in a perfect horizontal plane. 10 www.AgriMoon.Com Workshop Practice 2.3. ‘V’ –Block A ‘V” block serves as a very useful support to the work in marking. It usually works in conjunction with a U-clamp. Round bar is placed longitudinally in the block and the screw in the clamp tightened. Its specific use is in holding the round bars during marking and center drilling their end faces, which are to be held between centers on the lathe. Also it is very suitable for holding round bars in drilling operations when the axis of the drill is to be kept normal to the axis of the bar. 2.4. Simple Scribing block It is principal marking tool in a fitting shop and is made in various forms and sizes. It consists of a cast iron sliding base fitted with a vertical steel rod. The marker is fitted into an adjustable device carrying a knurled nut at one end. By means of the nut the marker can be loosened or tightened to set it at any desired inclination, moved to and fro inside the hole accommodating it or adjust its height along the vertical pillar. Normally it is used in conjunction with either a surface plate or marking table. Its specific use is in locating centers of round rods held in V-block, describing straight lines on work held firmly in its position by means of a suitable device like angle plate and also in drawing a number of lines parallel to a true surface. 2.5. Universal Surface Gauge It consists of a cast base, perfectly planed at the top, bottom and all sides. Two guide pins are provided at the rear end of the base which can be pressed down to project below the base. These pins can be used against the edge of the surface plate or any other finished surface for guiding the instrument during scribing. A swivel bolt is provided at the top of the base in which the spindle is fitted. This spindle can be swung and locked in any desired position by means of the adjusting screw. The scriber is fitted in an adjustable screw on the spindle and is capable of 11 www.AgriMoon.Com Workshop Practice being adjusted at any inclination and height along the spindle. A rocker is provided at the top of the base and it carries an adjusting screw at its rear end. 2.6. Try Square It is better known as engineer’s try square and is a very common tool used for scribing straight lines at right angles to a true surface or testing the trueness of mutually normal surfaces. They are made in different sizes from the steel pieces. 12 www.AgriMoon.Com Workshop Practice It consists of a steel blade fitted into a steel stock of rectangular cross-section. They are well hardened and tempered to suit the need. Both inner and outer surface of the blade are kept truly at right angles to the corresponding surfaces of the stock. 2.7. Bevel gauge Whenever angles other than right angles are required to be tested or set and marked sliding bevel square or bevel gauge is used. It consists of a steel stock of rectangular cross-section carrying a slotted steel blade at its end. This blade can be made to slide, set at any desired angle and secured in that position by means of a screw. 2.8. Files Files of different types are the principal hand tools used by a fitter. All the files, irrespective of their shape, size and grade, essentially consist of two main parts, viz., a toothed blade and a pointed tang, which is fitted in a handle. Files are generally forged out of high carbon steel, followed by cutting of teeth, hardening and tempering etc. Common shapes of the files available are flat, hand, square, pillar, round, half round, triangular, knife edge, etc. These files are manufactured in different varieties and their classification is governed by the following factors: effective length- i.e. excluding the length of tang, shape or form of the cross-section, depth, spacing and cut of teeth Length of the files varies according to the need but the most commonly used lengths range from 10 cm to 30 cm and they cover almost all sorts of filing work done by hand. Length between 10 cm and 15 cm are generally used for fine work, between 15 cm and 25 cm for medium sized work and above 25 cm for all general and large sized jobs. Square file which carried double cut teeth on all the four faces and is normally made tapered for about one-third of its length near the end opposite to the tang. Triangular file which normally carries single cut teeth on all the faces and is made tapered towards the end for about two-third of its length near the tip. The cross-section is an equilateral triangle. 13 www.AgriMoon.Com Workshop Practice Teeth of the files may single cut or double cut. Single teeth are parallel and at angle of 60ºto the center line of the file. Double cut files have two sets of teeth, the overcut teeth are cut at angle of 60º and the uppercut at 75º to 80º to the centre line. Files are also further classified according to the coarseness or spacing between the rows of teeth. 1. Rough (R) with 10 to 4.5 cuts per 10 mm length 2. Bastard (B) with 18 to 6 cuts per 10 mm length 3. Second cut (SC) with 21 to 11 cuts per 10 mm length 4. Smooth (S) with 30 to 15 cuts per 10 mm length 5. Dead smooth (DS) with 35 to 28 cuts per 10 mm length 6. Super smooth (SS) with 63 to 40 cuts per 10 mm length 2.9. Scrapers Scraping is a very important hand operation in bench work employed for obtaining a fine surface finish on the work, particularly for removing convex spots from machined surfaces, and the tools used for doing this operation are known as scrapers. They vary in shape and size, depending upon the specific work for which they are employed. They are usually made from rejected old files. Such files are heated and bent to the desired shape. They are fitted with a wooden handle. 2.10. Chisels There are many verities of chisels used for chipping work by a fitter. Some very commonly used forms are Flat, Cross-cut, Round nose and Diamond point. All the chisels are forged from bar stock of carbon steel, to the desired shape and the cutting edge ground to the correct angle. 14 www.AgriMoon.Com Workshop Practice The forging operation is followed by annealing, hardening and tempering to make chisel body tough and obtain a sharp cutting edge. Full length of the chisel is never hardened, only a small length about the cutting edge (say about 20 to 30 mm) is hardened. The included angle at the cutting edge varies between 40 and 70, depending upon the material on which it is to be used. Approximate values of cutting angles for common materials are as follows: Brass and copper 40 Wrought iron 50 Cast iron and general cutting work 60 Steel (cast) 70 A flat chisel is a general purpose chisel which is most widely used in cutting work, chipping large surface, cutting metal sheets, rods, bar stocks and similar other purposes. Since it cuts the metal in cold state it is also frequently known as cold chisel. A round nose chisel is used for drawing the eccentric hold back to correct centre which has run off-centre during drilling operation. Another specific use of this type of chisel is in cutting oil grooves and channels in bearings and pulley bushes and cleaning small round corners. A cross cut is a comparatively narrow chisel having its cutting edge slightly broader than the blade. It is made to keep the blade free when the chisel is used to cut deep groove into the metal. Normal widths of the cutting edge vary from 3 mm to 12 mm. This chisel is used to cut parallel grooves on large surfaces, before chipping by means of a flat chisel, cutting key ways, etc. A diamond point chisel is a special purpose chisel used for chipping rough plates and cutting cast iron pipes, cutting ‘V’ grooves, chipping sharp corners, squaring up corners of previously cut slots and cleaning angles. 15 www.AgriMoon.Com Workshop Practice 2.11. Hammers The hammer is one of the most widely used fitter’s tools. It is used for striking chisels in chipping and cutting and the punch in marking. All the hammers used in a fitting shop are similar in construction to the smith’s hand hammers, such as ball peen, cross peen, straight peen, etc. The only difference lies in weight. Hammers used in fitting work are comparatively lighter in weight than the smith’s hand hammers. They normally weigh from 0.45 kg to 0.7 kg. Ball peen hammer is the most commonly used hammer. The peen is ball shaped. It is used for riveting, chipping, drawing and laying out. The weight of the hammer varies from 0.11 to 0.91 kg (as per IS standards).. Cross peen hammer resembles the ball peen hammer in shape except that its peen is in wedge shape and at right angles to the eye. This hammer is used for bending and hammering in the corners. Straight peen hammer has a peen in line with the handle and is used for peening or stretching the metal. 2.12. Hack-saw Desired lengths of bar stocks, rods, tubes, iron flats and metal sheets, etc. are always required to be cut in fitting shop. Hack-saw is a common tool used for this purpose. It consists of a metal frame, fitted with a wooden handle, carrying metal clips with wing-nut at its end to hold. The clip carrying the wing nut is threaded so as to stretch the blade to the desired extent. The frame can be either of fixed type, which can accommodate the same length of blades or adjustable type which is capable of accommodating different lengths of blades. Hack saw blades are made of high carbon steel or low alloy steel. Hack saw blade is the main part. Push type blades, those which cut in forward stroke only, are generally used. In these, the teeth always point away from the operator. The blades in common use are generally 0.7 mm thick, 12.7 mm wide and 20 cm to 30 long. About 5 to 7 teeth per cm length of blade from the course group and 8 to 12 teeth per cm from the fine group of teeth. 16 www.AgriMoon.Com Workshop Practice 17 www.AgriMoon.Com Workshop Practice Module 2. Bench work tools and processes Lesson-3 The bench work tools, its uses and processes 3.1. Miscellaneous tools 1. Punch A punch made from a steel rod with a length of 90 to 150 mm and a diameter of 8 to 13 mm is used in bench work for marking purpose and locating centres in more permanent manner. The punch with a tapered point angle of 400 is called a princk punch and that of600 point angle is called a centre punch. 2. Calipers Calipers are the devices used for measuring and transferring the inside or outside dimensions of components. Although gradually they are being replaced by the more accurate and precision instruments and gauges, like micrometers in modern workshops, still they stand as the in general work on account of their cheapness and ease in handling. 3. Screw Drivers It is a very useful hand tool for rotating the screws. It consists of wooden or a plastic handle and steel blade, shaped at the end. The flat end of the tool is inserted into the slot provided on the head of the screw for rotating it. Screw drivers are made in various sizes to suit the corresponding sizes of the slots on the screw heads. Sometimes star headed screw driver is used for star headed screws. 18 www.AgriMoon.Com Workshop Practice 4. Drills Drilling is an important operation carried out in a fitting shop for producing different types and sizes of holes in various materials. There are many forms of drills used for this purpose. The simplest form is a flat drill which is used for wood work. The other important and most widely used is a fluted twist drill. It has a cylindrical body carrying the spiral flutes cut on its surface. Twist drills are usually made of high-speed steel, some cheaper varieties are made of high carbon steel. They are made in different forms to suit the work but the most commonly used types are (i) those having parallel shank and (ii) those having tapered shank,Parallel shank is provided on small sized drills (say up to 12.7 mm) only and those above this size are usually provided with a tapered shank. The twist drill essentially consists of two main parts, a shank which is gripped in the chuck of the drilling machine and the body forms the main cutting unit. Main advantages of using twist drills are: 1. The chips of the metal are automatically driven out of the hole through the spiral flutes. 2. Cutting edges are retained in good condition for a fairly long period. 3. Heavier feeds and speeds can be quite safely employed. 4. For the same size and depth of hole they need less power as compared to other forms of drills. 19 www.AgriMoon.Com Workshop Practice 5. Taps The hand operated taps used in fitting shops are employed for cutting internal threads in cylindrical holes or for cleaning damaged threads in similar parts. A tap consists of a toothed body having flutes (usually 4) cut on its surface, a round shank and a square formation at the end of the shank. The flutes are provided for the same purpose as in case of a twist drill and square formation at the top enables to grip by the tapping handle. All the hand taps of different sizes are usually available in a set of three taps of each size known as taper or rough, second and finish or plug respectively. The main difference between the three taps is the chamfer angle. In the threading operations they are used in the same order as taper, second and plug. When starting tapping care should be taken to start the thread in alignment with the hole. Also the tap should be occasionally rotated back about a turn to break the chips and facilitate their removal. 20 www.AgriMoon.Com Workshop Practice 6. Dies and stocks Dies are used to cut threads on a round bar of a metal, such as the threads on a bolt. It is a round or square block of hardened steel with a hole containing threads and flutes which form cutting edges. Die may be a solid or adjustable type. Solid die has fixed dimensions. An adjustable die may be split type with a split through one side or two piece rectangular type. These types of dies are fitted into special stocks and closed by means of adjusting screws.The size of a die is specified by the outside diameter of the thread to be cut and pitch of the thread. 21 www.AgriMoon.Com Workshop Practice 3.7. Bench work processes: Bench work involves following hand operations to finish the work to desired shape and size with required accuracy. 1. Marking 2.Chipping 3.Sawing 4. Filing 5. Draw filing 6.Threading 7. Grinding Some common bench work processes are described here 1. Marking It is the basic and one of the most important operations in bench work. It should be remembered that how accurately and carefully one tries to perform other operations it will be of no help until and unless the piece has been properly and accurately marked. Sufficient care should be exercised in performing this operation to obtain a desired fitting of the components. Marking on the work can be done by setting out dimensions with the help of a working drawing. The surface to be marked is coated with either the paste of red lead or chalk and allowed to dry. After that, the work is held in a clamp, if it is round. If the work is too thin, it is normally supported against an angle plate keeping the surface to be marked in a vertical plane. Lines in horizontal direction are scribed by means of a scribing gauge. Lines at right angles to this can be drawn easily by first turning the work through 90 and then using the scriber. Lines can easily be marked with the help of a try square. Circles and arcs on flat surfaces are inscribed by means of dividers. 22 www.AgriMoon.Com Workshop Practice After the scribing work is over, indentations on the surface are made, by using the center punch and hammer along the scribed lines and arcs. The punch marks serve as the guide during further operations like filing, chipping and drilling. etc. 2. Chipping It is the operation employed for removing the excess metal by means of cold chisels. To have a properly chipped surface it is essential that the same cutting angle should be maintained throughout the operation. In case the surface is too large it is advisable to cut grooves along the whole surface by means of a cross cut chisel and then chip off the remaining metal. The cutting angles of the chisels differ for different metals. Frequent lubrication and cooling of the cutting edge, while taking heavy cuts for removing large amount of metal, it helps considerably in chipping the metal easily and more effectively. To the correct cutting angle of the chisel, proper gripping of the chisel and the hammer and correct standing position of the operator play a significant part. The chisel should be firmly gripped in one hand leaving about 3 to 5 cm length above the thumb of the hand, and hammer should be held near the end of the handle to ensure more power in the blows. The operator should stand erect with his two feet sufficiently apart to balance his own weight equally on both the feet. The operator should always see the cutting edge of the chisel and not the top of the same. 3. Sawing This operation is performed in fitting shop for cutting different metal pieces to the desired size and shape, usually prior to other operations such as filing, drilling, scraping, etc. It is also employed for cutting metal pieces of required length out of the bar stock. For sawing, the saw blade should be properly fitted, and stretched to have the proper tension, in such a way that the cutting teeth always point away from the operator so as to cut the metal in forward stroke. Sawing should be done steadily and slowly. An average speed of about 50 strokes per minute is a good practice. Sufficient pressure should be exerted in the forward stroke and this be relieved during the backward stroke. It is advisable to use a coolant throughout the operation. A new blade should not be directly used on a hard metal. 4. Filing Similar to the saw blades, most of the files have their teeth pointing away from the operator such that they cut during the forward stroke. The pressure of the hand in filing should also be applied only during the forward stroke and relieved during the return stroke. Beginners particularly should be careful enough to practice correct movement of file. It should always be more in a perfect horizontal plane for obtaining a truly plane and smooth surface. As far as possible, try to use full length of the file during the operation. Moving the file diagonally on a flat surface always yields best results. A coarse pitched file should be employed when enough metal is to be removed, followed by finishing with a smooth file. 5. Draw filing When the surface is to be finally finished by filing only and no other operation, like scraping, is to follow the filing operation, a special method of filing, called Draw filing, is employed for finishing the surface. A flat file of fine cut is used for this operation. It should be ensured before use that the file teeth are free from metal particles, Other wise a numbers of scratches will be produced on the surface. It is usual to employ a file card quite frequently for cleaning the file teeth both before use as well as during use. For draw filing operation the file is held flat on the surface between the two hands. The file must move forward and backward. Flatness and evenness of the surface should be checked quite frequently during the operation. For final finishing, it is a common practice to rub a chalk piece over the entire surface of the file. This helps in producing a finely finished surface. 23 www.AgriMoon.Com Workshop Practice 24 www.AgriMoon.Com Workshop Practice Module 3. Smithy and forging tools & operation Lesson-4 Smithy and forging tools and equipment 4.1. Introduction A smithy’s work involves heating of a metal stock to a desired temperature, enable it to obtain sufficient plasticity, followed by the operations like hammering, bending, pressing etc., to give it the desired shape. This is known as forging. The above operations can either be carried out by hand hammering, by power hammers, or by forging machines. Hand forging is the term used for the process when it is done by hand tools. Similarly, forging done with the help of power hammers is known as power forging, when carried out by means of drop hammers as drop forging, and when by forging machines as machine forging. Applying pressure for shaping the metal, the primary requirement always is to heat the metal to a definite temperature to bring in into the plastic state. This may be done either in an open hearth, known as smith’s forge, or in closed furnace. Small jobs are normally heated in the Smith’s forge and larger jobs in closed furnaces. The Hand forging process is employed for relatively small components, machine forging for medium sized and large articles requiring very heavy blows and drop forging for mass production of identical parts. 4.2. Principal tools and other equipments used in hand forging 4.2.1. Smith’s forge or hearth. It has a robust cast iron or steel structure consisting of 4 leg supports, an iron bottom known as hearth, a hood at the top and tuyere opening into the hearth either from the rear or from the bottom. The hearth carries the coal and provided with fire bricks lining to withstand the extensive heat produced due to the combustion of coal. In the absence of this lining the heat produced, as started above, will directly effect the metal structure of the hearth, so that the body, particularly the bottom and the surrounding walls, may even melt. With the result, the entire structure will collapse and the hearth will no more be useful. Air, under pressure is supplied by the blower, suitably placed somewhere near the forge, through the tuyere opening in the hearth. This blower can either be hand operated or power driven. The latter is preferable, but in the absence of availability of power supply choice of the former has no alternative. If hand blowers are to be used, they are usually mounted at the rear of the forge itself. In case the power driven units are to be employed the blower is suitably placed in one corner of the shop and all the forges are connected with it by means of a well-laid pipe running underground all around the hearths. At suitable points auxiliary pipes are used to connect the tuyere with the main pipe line. A valve is incorporated in the auxiliary pipe, just before the place where it is connected with the tuyere, to control the supply of air to the furnace. The chimney provided at the top enables as easy escape of smoke and gases produced due to the burning of coal. A water tank is provided, in front of the forge, which carries water for the purpose of quenching. These hearths can also be made to have masonry construction provided with all the attachments like chimney, tuyere, blower, water tank, etc. 4.2.2. Anvil To carry out the forging operations successfully, a proper supporting device is needed which should be capable of withstanding heavy blows rendered to the job. 25 www.AgriMoon.Com Workshop Practice An anvil stands as the most appropriate choice for this purpose. Its body is generally made of cast steel, wrought iron or mild steel provided with a hardened top, about 20 to 25 mm thick. This hardened plate is welded to the body on the top. The horn or beak is used in bending the metal or forming curved shapes. The flat step provided, between the top and the horn, is used to support jobs during cutting and is known as chipping block. The flat projecting piece at the back of the anvil is known as tail. It carries a square hole to accommodate the square shank of the bottom part of various hand tools like swages, fuller. It is called a hardie hole. The circular hole provided near the hardie hole is known as pritchel hole. The commonly used size of an anvil weighs approximately 50-150 kg although it is manufactured in various sizes. The top face of the anvil should stand at about 0.75 m from the floor. 4.2.3. Hammer The classification of hammers is largely according to the size and weight of the hammers used in forging. A smith’s hand hammer is a small sized hammer used by the smith himself and the sledge hammer is comparatively larger in size, heavier in weight and is used by the smith’s helper, known as hammer man. The smith’s hand hammer is normally a small sized ball peen hammer. 26 www.AgriMoon.Com Workshop Practice All the hammers are mainly divided into 4 parts; namely peen, eye, cheeks and face. The peen is the top part made slightly tapered from the cheeks and rounded at the top. It gets a particular form known a ball peen hammer. The face is hardened and polished well and is given slight rounding along the circular edges so that the metal surface is not spoiled by the sharp edges when the former is struck by the hammer. The eye is normally made oval or elliptical in shape and accommodates the handle or shaft. For small sized hammers these handles are made of shisham wood or bamboo, but in case of sledge hammers the handles made of solid bamboos. A steel wedge is always forced into the handle after it is fitted into the hammer so as to prevent the slipping of the hammer off the handle during striking. A smith’s hammer is usually a ball peen hammer or a straight peen sledge type hammer of relatively small size. Its weight normally varies between 1.0 kg and1.8 kg. A ball peen hammer is used for all general work and its peen is employed when light blows at a faster speed are needed, such as in fullering a rivet head in a countersunk hole. Sledge hammers are comparatively 3to 4 times heavier than the hand hammers. They are available in varying sizes and weights from 3 kg to 8 kg. They are employed when heavy blows are needed in forging and other operations done on heavy jobs. 27 www.AgriMoon.Com Workshop Practice 4.2.4. Swage Block It is usually a block of cast steel or cast iron carrying a number of slots of different shapes and sizes along its four side faces and through holes from its top face to bottom face. This is used as a support in punching holes and forming different shapes. The job to be given a desired shape is kept on a similar shaped slot, which acts as a bottom swage, and then the top swage is applied on the other side of the job. The holes in the top and bottom face are used in punching. Their use prevents the punch from spoiling by striking against a hard surface after the hole has been punched. 4.2.5. Tongs They are used to hold the jobs in position and turning over during forging operation. They are made of mild steel. 28 www.AgriMoon.Com Workshop Practice Tongs are usually made in two pieces, riveted together to form a hinge. Smaller length on one side of the hinge carries the holding jaws, which are made in different shapes and sizes to suit the corresponding shapes and sizes of the jobs, and the longer portions on the other side of the hinge form the arms which are held in hand by the smith. Overall sizes of the tongs vary according to the size and shape of the job to be held, but the commonly used lengths of the tongs in hand forging vary from 400 mm to 600 mm with the jaws’ opening ranging from 6mm to 55 mm. Tongs are usually named after the inside shapes of the jaws. Flat tongs are used for gripping thin section and small flat pieces. Round hollow tongs, with curved surface inside, are used for holding round work. Hollow tongs with square jaws are used to hold square or hexagonal work. Pick up tongs have their jaws so shaped that even small sections can be easily picked up. They are not used for holding the work. 4.2.6. Chisels Chisels are used to cut metals in hot or cold state. Those which are used for cutting the metal in hot state are termed as hot chisels and the others used for cutting in cold state are known as cold chisels. The main difference between these chisels is in the included angle at the cutting edge. A cold chisel carries an included angle of 600 at the cutting edge and the latter is well hardened and tempered. It is made of high carbon steel. A hot chisel can be made of medium carbon steel as there is no need of hardening. It is used to cut the metal in plastic state. The included angle of its cutting edge is 300. 4.2.7. Punches Punches are tapered tools made in various shapes and sizes. They are used for producing holes in red hot jobs. A larger tapered punch is called a drift. The job is placed on the anvil and the punch is hammered through it up to about half its depth. In is then turned over and the punch made to pass through it.Completion of this operation in two stages prevents the job from splitting and full to bursting. 29 www.AgriMoon.Com Workshop Practice 4.2.8.Flatters These are also known as smoothers.They are made of high carbon steel and consist of a square body, fitted with a handle, and a flat square bottom. They are used for leveling and finishing a flat surface after drawing out or any other forging operation. 4.2.9. Set Hammer It is made of tool steel and hardened. It is not used for striking purpose. Its construction is also similar to that of a flatter but is smaller in size and it does not carry an enlarged bottom face. It is used for finishing corners, formed by two adjacent surfaces at right angles. The job is supported on the anvil and the tool is hammered from the top. 30 www.AgriMoon.Com Workshop Practice 4.2.11. Fullers These tools are made of high carbon steel in different sizes to suit the various types of jobs. They are usually used in pairs, consisting of a top and a bottom filler. Their working edges are normally rounded. They, are employed for making necks by reducing the cross-section of a job and also in drawing out. 31 www.AgriMoon.Com Workshop Practice 4.2.12. Swages Like fullers, they are also made of high carbon steel in two parts called the top and bottom swages. Their working faces carry circular grooves to suit the size of the work. They are available in various sizes. The top swage carries is a handle and the bottom swage a square shank to fit the hardie hole of the anvil during the operation. They are used for increasing the length of a circularrod or for finishing the circular surface of a job after forging. 32 www.AgriMoon.Com Workshop Practice Module 3. Smithy and forging tools and operation Lesson 5. Smithy and forging operation 5.1. Fuels Used in Furnaces Many types of fuels are used in the furnace employed in forging work. All these fuels can be broadly classified into three groups, as follows: 1. Solid fuels – such as coal, coke, charcoal etc. 2. Liquid fuel – they include different types of fuel oils. 3. Gaseous fuels – natural gas and producer gas. 5.2. Forging Materials The materials possessing the ability to sustain substantial plastic deformation without fracture even in presence often sile stresses can be forged easily. Wrougt iron, low and medium carbon steels, low alloy steels, aluminium, magnesium and copper alloys are common forgeable materials. Austenite and marten site stainless steels, nickel alloys can be forged with some difficulty. 5.3. Forging Temperatures Forging materials must be heated to a temperature at which it will possess high plastic properties both at the beginning and at the end of the forging process. If the forging operation is finished at lower temperature, this leads to cold hardening and cracks. With excessive heating, the forgings suffer oxidization and much metal is wasted. Approximate temperatures for forging the following common metals at the beginning and at the end of forging process are as under: The temperature of heating steel for hand forging can be estimated by the heat colour of the heated steel are given in the following table : 33 www.AgriMoon.Com Workshop Practice 5.4. Forging Operations: For giving desired shapes to the products the following operations are used in a smithy shop. 1. Upsetting 2. Drawing out or drawing down. 3. Cutting 4. Bending. 5. Punching and drifting. 6. Setting down and finishing. 5.4.1. Upsetting or Jumping Upsetting is the process through which the cross-section of a metal piece is increased with a corresponding reduction in its length. When a metal is sufficiently heated, so that it acquires the plastic stage, it becomes soft. If some pressure is applied to it the metal tends to increase in its dimensions at right angles to the direction of application of force with a corresponding reduction in its dimensions parallel to the line of action of the said force. The particular part in the bar shape, where said increase in the cross-section is desired, is heated till it acquires a fully plastic state. The hot portion of the bar is then kept on the anvil face and the bar hammered at the top. 34 www.AgriMoon.Com Workshop Practice Couldn't load plugin. Hammering in this operation is done either by the smith himself, if the job is small, by means of a hand hammer or by his helper in case of big jobs, when heavy blows are needed, by means of a sledge hammer. 5.4.2.Drawing Out This process is also known as drawing down. It is exactly a reverse process to that of upsetting in the sense, it is employed when a reduction in thickness, width or both of a bar is desired with a corresponding increase in its length. The desired effect is possible to be obtained by the use of either the peen of a cross peen hammer, a set of fullers or a pair of swages (for round bars only). The process of heating and cooling the length, not required to be drawn, is the same as in case of upsetting, but the selection of the above tools is governed by the shape of the cross-section of the stock, the amount by which the increase in length is desired and also the required finished shape of the job. 5.4.3.Cutting Cutting of metals in hot or cold state is done by means of hot or cold chisels respectively. This operation is required in removing extra metal from the job before finishing it, cutting required lengths of pieces from a stock, splitting a metal piece into two at a desired location and similar other requirements. Enough care should be taken while cutting cold steel, since there is every likelihood of the chips flying off in different directions and cause injuries. Also, more power and time is taken in cold cutting as compared to hot chiseling. If very thick section is being cut, even cracks may sometimes occur. Cold cutting is, therefore, preferred for the thin sections only, such as rods of thin sections and sheets, etc., (usually below 20 mm thickness). Especially alloy steels should never be cut cold. 35 www.AgriMoon.Com Workshop Practice For hot cutting of steel, it should be heated to red heat in the furnace and then cut. The usual temperature for hot cutting is 850 0C to 950 0C. 5.4.4.Bending Bending of bars, flats and other similar stock material is usually done in a smithy shop. This can be done to produce different types of bent shapes such as angles, ovals and circles, etc. Any desired angle can be made through this operation. For making a right angle bend that particular portion of the stock, which is to be subjected to bending, is heated and jumped on the outer surface. This operation is carried out on the edge of the anvil or on the perfectly square edge of a rectangular block. After bending, the outside bulging is finished by means of a flatter and the inside one by means of a set hammer. Curved shapes of bends are formed on the horn of the anvil. For mass production of articles made through bending, particularly when dimensional accuracy is a must, jigs and fixtures are designed to help in performing this operation quickly and efficiently. This results in a considerable saving of time and labour. 5.4.5.Punching and drifting Punching and drifting are used for producing and finishing holes and preparatory for producing other shapes.Punching should be done in two stages. In the first stage the work piece is kept flat on the anvil and holes performed half way through. Then job is turned upside down. The application of punching, producing the slot a number of holes are punched and the remaining excess material is cut out using a chisel. The slot may then be finished hot drifting or may be finished by filing when cold. 5.4.6 Setting down It is a localized drawing down or swaging operation. Usually the work is fullered at the place where the setting down is effected by the set hammer. 5.5 Forging processes The processes of reducing a metal billet between flat dies or in closed impression dies to obtain a part of predetermine size and shape are smith forging and impression die forging respectively.Depending on the equipments utilized they are further sub-divided as under. 5.5.1 Smith die forging It is also known as flat die or open die forging. It is simple,relatively inexpensive and allows the production of large verities of shapes.The final shape of the forging depends largely on the skill of the smith. A. Hand forging: Hand forging is employed only to shape a small number of light forgings mainly in repair shops. This is done by hammering the piece of metal, when it is heated to proper temperature, on an anvil. A hand hammer or a sledge hammer is used for striking. B. Power forging: Large machine parts which cannot be forged by hand forging, use of power hammers and presses is employed to do the job. 36 www.AgriMoon.Com Workshop Practice i. Hammer forging: machines which work on forging by blow are called hammers. The heavy falling part of the hammer is called the ram and the rigid support is in the form of anvil block. The power hammer may be a gravity fall type or a higher striking velocity type such as mechanical hammer,air and steam hammer etc. these hammers are available with different ram weight and different blows rate per minute. Press forging: Forging presses for smithy work are usually of the hydraulic type. In press forging, pressure or squeeze is applied to the raw material and intensity of this pressure increase as the plastic metal resists deformation. As the pressure applied squeezes the metal slowly compared to blow hammer, more time is available for the flow of metal being forged. 5.5.2. Impression die forging: It is employed for more complex shapes of greater accuracy, large quantities of identical forgings as well as for special items with quality and economy reasons. a) Drop forging: Three types of drop hammers are used in making drop forgings. They are board or gravity type, air lift hammer and power drop hammer also, called steam hammer. b) Press forging: It is done in presses rather than with hammers. The action is relatively slow squeezing instead of delivering heavy blows. This allows the gases to escape from the forging. Machine or upset forging: Forging of the ring or rod types with all kinds of heads and shoulder, such as bolts, nuts, washers, collers, pinions gear, blanks etc can be conveniently produced in forging machines. Large number of small identical items can be machine forged. 37 www.AgriMoon.Com Workshop Practice Module 4. Heat treatment Lesson-6 Heat treatment process:Hardening, tempering, annealing and normalizing 6.1. Introduction Steel and other alloys have a large number of applications in engineering practice under varying conditions, requiring different properties in them. At one place they may be subjected to bending while at the other to twisting. They may be required to withstand various types of stresses and as tool materials to have hardness, specially red hardness, combined with toughness along with anon-brittle cutting edge. They may be required to bear static or dynamic loads,revolve at extremely high speeds, operate in highly corrosive media, carry an extremely hard skin with a tough core, subjected to fatigue and creep, etc.Such varying condition of their applications require these materials to possess specific properties of the required order to successfully serve under these conditions. But, a material may lack in some or all of these properties either fully or partially. These deficiencies are fulfilled through the process of heat treatment. Generally all steels can be heat treated as per need. Aluminium is the only non-ferrous metal which can be effectively heat treated. The process of heat treatment involves heating of solid metals to specified (recrystalisation)temperatures holding them at that temperature and then cooling them at suitable rates in order to enable the metals to acquire the desired properties to the required extents. All this take place because of the changes in size, form,nature and the distribution of different constituents in the micro-structure of these metals. All heat treatment processes, therefore, comprise the following three stages of components: 1. Heating the metal to a predefined temperature. 2. Holding it at that temperature for sufficient time so that the structure of the metal becomes uniform throughout. 3. Cooling the metal at a predetermined rate in a suitable media so as to force the metal to acquire a desired internal structure and thus, obtain the desired properties to the required extent. All this takes place because of the changes in size,form, nature and the distribution of different constituents in the micro- structure of these metals. 6.2. Purpose of Heat Treatment Metals and alloys are heat treated in order to achieve one or more of the following objectives: 1. To relieve internal stresses set up during other operations like casting, welding, hot and cold working, etc. 2. To improve mechanical properties like hardness,toughness, strength, ductility, etc. 3. To improve machinability 4. To change the internal structure to improve their resistance to heat, wear and corrosion. 38 www.AgriMoon.Com Workshop Practice 5. To effect a change in their grain size. 6. To soften them to make suitable for operations like cold rolling and wire drawing. 7. To improve their electrical and magnetic properties. 8. To make their structure homogenous so as to remove coring and segregation. 9. To drive out trapped gases. In order to understand the complete mechanism of heat treatment it is essential to know the internal structure, phase transformation, etc. fully. However, a brief review is given: 6.3. Classification of heat treatment processes Various heat treatment processes can be classified as follows: 1. Annealing. 2. Normalizing. 3. Hardening. 4. Tempering. 5. Case hardening. 6. Surface hardening. 7. Diffusion coating. 6.3.1. Annealing Annealing is indeed one of the most important heat treatment processes. The internal structure of the metal gets stabilized through this process. This heat treatment is given to the metal so as to achieve one on more of the following objectives: 1. To refine the grains and provide homogenous structure. 2. To relieve internal stresses set up during earlier operations. 3. To soften the metal and, thus, improve its machinability. 4. To effect changes in some mechanical,electrical and magnetic properties. 5. To prepare steel for further treatment or processing. 6. To drive out gases trapped during casting. 7. To produce desired macro structure. 39 www.AgriMoon.Com Workshop Practice Different type of annealing processes can be classified as follows: 1. Full annealing. 2. Process annealing. 3. Spheroidise annealing. 4. Diffusion annealing. 5. Isothermal annealing. 6.3.2. Full annealing The main objectives of this type of annealing are to soften the metal, relieve its stresses and refine its grain structure. It is also known as high temperature annealing. In this process complete phase recrystallisation takes place and,therefore, all imperfections of the previous structure are wiped out. This involves heating of steel to a temperature about 30o to 50oabove the higher critical point for hypoeutectoid steels, and by the same amount above the lower critical point for hyperuectoid steels, holding it at that temperature for sufficient time to allow the internal changes to take place and then cooling slowly. The steel gets softened by this process,together with an appreciable amount of increase in its ductility and toughness. Table: Annealing temperatures for carbon steels Cooling is done by allowing approximately 3 to 4 minutes time at elevated temperatures per mm thickness of the largest section. High temperature cooling is usually done in the furnace itself by lowering of temperature at the rate of 10 to 30o C below the lower critical temperature. The specimen is then air cooled down to the room temperature. This process makes a course pear litic structure which is quite soft and ductile. An alternate method of cooling after soaking is to embed the metal in a non-conducting material like sand, lime, mica, ash, etc. 40 www.AgriMoon.Com Workshop Practice 2. Process annealing The purpose of process annealing is to remove the ill effects of cold working and often the metal so that its ductility is restored and it can be again plastically deformed or put to service without any danger of its failure due to fracture. It is also known a slow temperature annealing or sub-critical annealing or commercial annealing.The process is extremely useful for mild steels and low carbon steels and is cheaper and quicker than full annealing. Also, less scale is produced during this process. The main out put of this process is increased ductility and plasticity, improved shock resistance, reduced hardness, improved machinability and removal of internal stresses. During cold working operations like cold- rolling, wire drawing, a metal gets severely strain-hardened. Due to this, the metal is heated to a temperature, generally in the range of 550oC to 650oC, held there for enough time to allow recrystallisation of cold worked metal and,thus, softening to take place and then cooled at a slower rate (normally in air). 3. Spheroidise annealing The main purpose of spheroidise annealing is to produce a structure of steel which consists of globules or well dispersed spheroids of cementite in ferrite matrix. Following are the main methods through which the above objective can be obtained: 1. High carbon steels: Heating the steel to a temperature slightly above the lower critical point (say between 730oC to 770oC,depending upon the carbon percentage), holding it at that temperature for sufficient time and than cooling it in the furnace to a temperature 600oCto 550oC, followed by slowly cooling it down to room temperature instill air. 2. Tool steels and high-alloy steels: Heating to a temperature of 750oC to 800oC, or even higher, holding at that temperature for several hours and then cooling slowly. 4. Diffusion annealing The purpose of diffusion annealing is to remove the heterogeneity in the chemical composition of steel ingots and heavy castings This process is mainly used before applying full annealing to steel castings. In this process , the metal is heated to a temperature between 1100oC to 1200oC,where diffusion occurs and grains are homogenized. The metal piece being treated is held at the diffusion temperature for a short time to allow complete diffusion and than cooled down to between 800oC to 850oC by keeping it inside the shut off furnace for a period of about 6 to 8 hours. Then it is removed from the furnace and cooled in air down to the room temperature. Then full annealing is performed. 5. Isothermal annealing The isothermal annealing consists of heating steel to austenite state and then cooling it down to a temperature of about 630oC to 680oCat a relatively faster rate. It is followed by holding it at this constant temperature (i.e isothermal) for some time and then cooling it down to the room temperature at a rapid rate. During the isothermal holding full decomposition to pearlite structure takes place and that is why the process is known as isothermal annealing. Because of the two rapid coolings the total annealing time is considerably reduced. Normalizing: 41 www.AgriMoon.Com Workshop Practice The normalizing process is similar to annealing in sequence but vary in the heating temperature range, holding time and the rate of cooling. Heating temperature of steel is 40oC to 50oCabove the higher critical point, held at that temperature for a relatively very short period of time (about 15 min.) and then cooled down to room temperature in still air. This heat treatment is commonly used as the final heat treatment for such articles which are supposed to be subjected to higher stress during operation. Due to this treatment internal stress caused during previous operations are removed, internal structure is refined to fine grains and mechanical properties of steel are improved. This process also improves the impact strength, yield point and ultimate tensile strength of steels. As compared to the annealed steels of the same composition the normalized steels will be less ductile but stronger and harder. For improvement of the mechanical properties normalizing process should be preferred and to attain better machinability, softening and greater removal of internal stress annealing process should be employed. Hardening: This process is widely applied to all cutting tools, all machine parts made from alloy steels, dies and some selected machine parts subjected to heavy duty work. In hardening process steel is heated to a temperature within the hardening range, which is 30oC to 50oC above the higher critical point for hypoeutectoid steels and by the same amount above the lower critical point for hypoeutectoid steels, holding it at that temperature for sufficient time to allow it to attain austenitic structure and cooled rapidly by quenching in a suitable medium like water, oil or salt both. In the process of hardening the steel is developed in such controlled conditions,by rapid quenching, that the transformation is disallowed at the lower critical point and by doing so we force the change to take place at a much lower temperature. By rapid cooling the time allowed to the metal is too short and hence transformation is not able to occur at the lower critical temperature. Tempering: A hardened steel piece, due to martensitic structure, is extremely hard and brittle, due to which it is found unsuitable for most practical purposes. So a subsequent treatment is required to obtain a desired degree of toughness at the cost of some strength and hardness to make it suitable for use. It is especially true in case of the tools. This is exactly what is mainly aimed at through tempering of steel. This process enables transformation of some martensite into ferrite and cementite. The exact amount of martensite transformed into ferrite plus cementite will depend upon the temperature to which the metal is reheated and the time allowed for the transformation. The process involves reheating the hardened steel to a temperature below the lower critical temperature, holding it at that temperature for sufficient time and then cooling it slowly down to the room temperature. When the hardened steel is reheated to a temperature between 100oC to 200oCsome of the interstitial carbon is precipitated out from martensite to form acarbide called epsilon carbide. This leads to the restoration of BCC structure in the matrix. Further heating to between 200oC 400oCenables the structure to transform to ferrite plus cementite. Further heating to between 400oC and 550oC leads to the nucleation and growth of a new ferrite structure, rendering the metal weaker but more ductile.If steel is heated above 550oC the cementite becomes spheroidised,and if heating is continued even beyond the structure will revert back to the stable martensite. As such, if a good impact strength is desired reheating should not extend beyond 300o to 350oC. The section thickness of the components being treated also have a decisive effect on the results. Heavy components and thicker sections required longer tempering times then the lighter and thinner ones. 42 www.AgriMoon.Com Workshop Practice Types of tempering: On the basis of the ranges of temperatures to which the components are reheated for tempering, the tempering procedures are classified as follows: 1. Low temperature tempering. This treatment results in reduction of internal stresses and improvement in toughness and ductility without any appreciable loss in hardness. The heating range for this type of tempering is from 150oC to 250oC. The different colours appearing on the surface of the metal are indicative of the approximate temperature attained by it. Carbon tool steels, low alloy tool steels, case carburized and surface hardened parts, measuring tools, etc are tempered by this method. Approximate temperatures, corresponding colours and the tools for whose tempering they are used are given in following table. Approximate tempering temperatures and temper colours for tools : 2. Medium temperature tempering. This process involves reheating the component to a temperature range between 350oCto 450oC, holding at that temperature for sufficient time and then cooling it to room temperature. This method of tempering is used to increase the toughness of steel but reduces the hardness. It also increases the ductility and decresess the strength. It is mainly used for articles where a high yield strength, coupled with toughness, is a major requirement and subjected to impact loading, like coils and springs, hammers, chisels, etc. 43 www.AgriMoon.Com Workshop Practice 3. High temperature tempering. The process involves reheating the hardened steel to a temperature between 500oC to 650 o, holding it there for a certain time and then cooling it down to the room temperature.This process enables the steel attaining high ductility while retaining enough hardness. This provides a micro-structure which carries a useful combination of good strength and toughness with complete elimination of internal stresses.E.g.Crankshafts, connecting rods and gears Tempering baths: Mainly following three types of tempering baths are used for tempering of steel parts and cutting tools: Lead bath : Lead or lead alloy bath may be used for tempering steel parts. The parts are preheated and then immersed in the bath, which is already heated to the tempering temperature. Once the parts reach the tempesing temperature they are taken out and cooled to attain the required temper. Oil bath: Oil baths can be employed for various temperature ranges. Mineral oils are commonly used for these baths. Light oil baths are used for temperatures upto 230 oC only. Heavy oil baths can be used for heating range from343 oC to 370 oC. For oil heating the bath temperature is first raised to the required tempering range and then partially heated component is immersed in it. If the temperature of the bath falls below the required level both the bath and the immersed component can be heated together to the tempering temperature. After the component has reached the required temperature it is removed and immersed in a tank of caustic soda, followed by quenching in a hot water bath. Salt bath : Salt baths, carrying liquid nitrates or nitrates plus nitrites, are used for higher temperatures. The salts used for these baths are generally chlorides and fluorides. These baths are very widely used for tempering of high speed steels.They can be used for temperature range upto 540 oC to 600 oC. From efficiency and economy points of view salt bath can not be used below 173o. 44 www.AgriMoon.Com Workshop Practice Module 4. Heat Treatment Lesson-7 Metal cutting 7.1. Introduction All metal cutting operations basically involve forcing a cutting tool with one or more cutting edges progressively through the excess material on the work piece. The work piece and the tool are securely held in a machine tool and its accessories while power is supplied to provide relative motion between the tool and the work piece. This results in removal of any excess material interfering with the relative motion in the form of chips. Metals are cut primarily to produce surfaces of desired shape, accuracy or surface finish depending upon their use. Since all machining involves considerable amount of labour, cost and loss of material as chips, machining should never be overdone to the extent of producing surfaces which are more accurate or better finished than those required for proper functioning of the product. 7.2. The Machine Tool Machining is done with the help of power driven non-portable machines known as machine tools in order to perform its function. The machine tool must incorporate means for holding the workpiece and the tools and for providing relative motion between the tool and the workpiece. The form of surface produced in a particular machine tool depends upon the shape of the cutting tool, the path of the tool as it traverses through the material or both. If the tool moves past the workpiece in a linear path as in shaping or vice versa as in planning a straight cut plane surface is produced. On the other hand, if either the tool (boring) or workpiece (turning) is rotating and the other unit is travelling in a definite path relative to the axis, a surface of revolution is generated. Machining operations are named and classified according to the shape of the cutter, nature of relative movement, shape of the generated surface and the type of finish. Performance wise a machine tool is expected to satisfy the following requirements: 1. It must have a high efficiency. 2. It must be possible to produce the specified dimensional accuracy, surface finish and form consistently and preferable independent of operator’s skill. 3. It should have a high enough production rate corresponding to latest development in technology. 7.2.1. Types of Machine Tools 45 www.AgriMoon.Com Workshop Practice Machine tools of different types and sizes have been developed because of variation in shape of the surface to be machined, size of the workpiece, surface accuracy desired and quantity required to be produced. Broadly speaking machine tools may be classified into three major categories: 1. General purpose machine tools. 2. Production machine tools. 3. Special purpose machine tools. 1. General purpose machine tools General purpose machine tools are machine tools like lathe, drilling machine and milling machine that are designed to deal with a variety of work and can perform a reasonably large number of operations within their range. A lathe, for example, can be used to do turning, knurling, threading or tapering on a job held between centres, turning, drilling, boring or facing on a job held in a chuck, turning, boring or facing on a job held on the face plate or boring in work held on the carriage using a boring bar held in the spindle. It can also be used for milling, grinding ore relieving with suitable attachments. Similarly a milling machine can be used for plain milling, slotting, angular cutting, indexing or helical milling operations. General purpose machine tools are useful in smaller machine shops and repair shops or for small quantity production. 2. Production Machine Tools Production machine tools are designed to increase the rate of production and to reduce the manufacturing costs. Features like multiple spindle heads, multi-tool turrets and specially designed fixtures are incorporated in these machine tools to reduce the non-productive time or to combine more than one operation. Typical examples of this category are capstan and turret lathes, automatic screw machines, multi-spindle drilling machines and production milling machines. They are used for medium size production and batch work. 3. Special Purpose Machine Tools Special purpose machine tools are machines which have been designed for some specific purpose and perform only one or a limited number of operations. Machines of this category include cam shaft grinders, gear generators and piston turning lathes. Special purpose machines, in most cases, perform operations that may be done on basic machines but for larger quantities they are much more economical than standard machine tools. 7.3. Cutting Tools Metal cutting tools may be classified as single-point, double-point or multi-point tools depending upon the number of active cutting edges on the tool. Tools used on lathe, shaper or planer have a single cutting edge and are called single-point tools. Drilling tools have two cutting edges while milling cutters, in general, have more than two cutting edges. Grinding is a more general form of 46 www.AgriMoon.Com Workshop Practice multi-point cutting in that the abrasive grains taking part in cutting do not have any fixed geometry or orientation. As far as their metal cutting action is concerned a double or multi-point tool behaves just the same way as a combination of so many single-point tools. The properties of cutting tools are thus discussed with reference to single point tools of the type used on lathe for turning. 7.4. Tool Materials In order for a machining operation to proceed at a fast rate with minimum tool and machining cost the cutting tool material must satisfy certain basic requirements. The more important of these requirements are given below: 47 www.AgriMoon.Com Workshop Practice 7.4.1. Hot Hardness: This represents the capacity of the tool to retain its cutting ability and hardness at the high temperatures developed at the chip-tool interface. To be effective the tool material must remain harder than the work material at all temperatures. Hot hardness of the tool material becomes more significant as the cutting speed is increased or the hardness of the metal to be machined becomes higher. 7.4.2. Wear Resistance: The life of a tool is determined by the wear developed on its cutting face due to motion of the chip and on its flanks due to contact with the machined surface. In order for the tool to continue to perform its duties satisfactorily it is important that the wear characteristics of its material relative to that of the work piece are such that excessive tool wear does not occur during the machining process. 7.4.3. Toughness: Toughness is necessary to enable the tool to withstand cutting forces, to absorb shock and to prevent the chipping of the cutting edge. The tool must not become so hard that it becomes brittle. Toughness is particularly important for tools like milling cutters which are subjected to impact loading due to interrupted cutting. 7.4.5. Low Friction: The co-efficient of friction between the tool material and the chip should be low. This is important for reducing tool forces, keeping chip-tool interface temperature low, increasing tool life and improving surface finish. 7.4.6. Thermal Conductivity: A material with a high thermal conductivity can conduct heat away from the chip tool interface faster. This results in a lower chip-tool interface temperatures, less interface welding and longer tool life. 7.4.7. Cost: This includes the cost of material, cost of grinding and the cost of replacement when the tool is worn out. A cheap material that requires frequent stopping of the machine for tool changing may prove much costlier in the long run compared to the one which has a higher initial cost but can be operated for a longer time at a higher speed. The properties of the tool material as outlines above are often contradictory and inter-dependent. For example, a material that has a good wear resistance will not generally have high toughness. There is no single tool material that satisfies all the requirements specified above. 7.5. Commonly available tool materials and their characteristics are discussed below: 7.5..1 Carbon Tool Steels: Carbon tool steels contain carbon in amount ranging from 0.90 to 1.20 percent. These steels are relatively cheap and the tools are relatively easy to make and harden. With 48 www.AgriMoon.Com Workshop Practice proper heat treatment these steels can attain hardness as much as any of the high speed alloys but they begin to lose their hardness at around 300oC. Cutting tools of carbon steels are limited to show speeds and light duty work. Carbon steels are used for machining soft materials like wood and for hand tools like files and chisels. 7.5.2. High Speed Steels: High speed steels were so named because they could cut at speeds higher than those of carbon steels. The name is misleading because the speeds at which these materials cut are actually much lower than those used for many other materials like carbides and stellites that are now available. High speed steels have excellent hardenability and can retain their hardness up to 650oC. They are relatively tough and moderately priced. They can be shaped easily. As such high speed steels are commonly used for drills, reamers; counter bores, milling cutters and single point tools. One of the oldest and the most common variety of high speed steels is 18-4-1. It contains 18 percent tungsten, 4 percent chromium, 1 percent vanadium and about 0.5 to 0.75 percent carbon. It is considered to be one of the best all purpose tool steels. Many high speed steels use molybdenum to replace tungsten partially or completely because one part of molybdenum can replace two parts of tungsten. Molybdenum high speed steels such as 6-6-4-2 containing 6 percent tungsten, 6 percent molybdenum, 4 percent chromium and 2 percent vanadium with about 0.6 percent carbon have excellent toughness and cutting ability. Cobalt is sometimes added to high speed steels to improve their red-hardness. These super high speed steels are used for heavy cutting operations involving higher cutting pressures and temperatures on the tool but are too costly for general purpose work. One composition of these super high speed steel alloys contains 20 percent tungsten, 4 percent chromium, 2 percent vanadium, and 12 percent cobalt. High speed steels have one major disadvantage in that they require lot of care in heat treatment. Rather complex heat treatment cycles are used to develop the most favourable properties. 7.5.3. Cast Non-Ferrous Alloys: These are alloys containing principally chromium cobalt and tungsten with smaller percentages of one or more carbide forming elements like tantalum, molybdenum and boron but no iron. They also contain 1 to 4 percent carbon. A typical alloy of this type known as stellite contains 30 to 35 percent chromium, 43 to 48 percent cobalt, 17 to 19 percent tungsten and about 2 percent carbon. Cast non-ferrous alloys are able to maintain good cutting edges up to 900oC. Compared with high speed steels they can be used at twice the cutting speeds. They have a good resistance to cratering. They can take a good polish which helps metal from sticking on the tool face and forming the built- up-edge. They are also corrosion resistant. But they are brittle, can be machined only by grinding and do not respond to heat treatment. Intricate tools can only be made by casting and grinding. 7.5.4. Carbides: Carbide cutting tool inserts principally consist of tungsten carbide particles held together by cobalt or nickel as binder. Straight tungsten carbide tools containing about 94 percent tungsten carbide and 6 percent cobalt are used for machining cast iron and most other materials. 49 www.AgriMoon.Com Workshop Practice They cannot be used for machining steel because the chips tend to stick to the tool. Tantalum, Titanium carbides are added in steel cutting tungsten carbide grades in addition to increasing their cobalt content to overcome this difficulty. A typical analysis of a steel cutting grade may contain 82 percent tungsten carbide, 10 percent titanium carbide and 8 percent cobalt. Such a carbide has very low coefficient of friction and thus has less tendency for sticking. Carbide tools are made by powder metallurgy techniques. They have a high initial cost but can be used at speeds which are two-to-three times those for cast nonferrous alloys. They can retain their cutting edges up to 1200 oC. They are very hard and have a high compressive strength but they are brittle and cannot withstand impact loading. Grinding is difficult and can only be done with silicon carbide or diamond wheels. Because of these reasons carbide tools are generally used as brazed or throw-away inserts. Even they have to be rigidly clamped. The need to provide high rotational speeds and yet assure extreme rigidity has led to considerable improvement in the design of machine tools used with these inserts. 7.5.6. Ceramics: Ceramics, sintered oxides, or cemented oxides are essentially aluminum oxide powder along with additives of titanium, magnesium or chromium oxide with a binder processed by powder metallurgy in the form of tool inserts. These inserts are either clamped into a tool holder or bonded to it. Ceramics are harder than other materials discussed so far and retain their hardness up to 1100 oC. They have a low coefficient of friction and a good resistance to cratering. The surface finish produced by ceramics is comparable with that produced by carbides but ceramics consume about 20 percent lesser power. The use of ceramic tools is limited only by their brittleness and the lack of rigidity and speed range on the conventional machine tools. 7.5.7. Diamond: Diamond is the hardest known material and can be used for machining at very high cutting speeds up to 25 m/s. Because of its high cost diamond is justified only when machining hard materials which are difficult to cut with other tool materials or for applications where very high accuracy and surface finish are desired. Diamond is also brittle, does not conduct heat well and can take only light cuts. Typical applications are precision boring of holes and machining of highly abrasive materials like fiber glass. Diamonds are also used for dressing grinding wheels and in finishing operations like lapping, honing and super finishing. When uses as cutting tools diamonds must be held very rigidly to avoid shock loading. 7.6. Cutting Parameters: Cutting speed, Feed and depth of cut: Cutting speed, feed and depth of cut are the parameters which determined the relative motion of the tool and work piece in a cutting operation and represent the rate at which excess material is removed per unit time. A proper selection of these parameters is essential for efficient machining. 7.6.1. Cutting speed 50 www.AgriMoon.Com Workshop Practice The cutting speed is defined as the surface rate of travel of the cutting edge relative to the work piece. It is expressed in meters per second. The amount of heat generated at the chip tool interface during a machining operation and the life of the tool are directly influenced by the cutting speed. The speed selected for any operation depends on the work material, cutting tool material, cutting fluid used and the type of cut. Lower speeds are used for harder materials and for heavier roughing cuts while finishing cuts in softer materials can be taken at much higher speeds. 7.6.2. Feed Feed is defined as the rate at which the cutting tool advances along or into the surface of the work piece. For machines in which either the job or the tool rotates, feed is expressed in millimeters per revolution of the rotating member. For machines in which the work piece or tool reciprocates feed is expressed as millimeters per stroke. Other methods of expressing feed include feed in millimeters per second or millimeters per tooth of the cutter. Feed has an important influence on the tool forces and surface finish. Lower feed values have to be used when machining with higher speeds, harder work pieces, less rigid machine tools, lesser supply of cutting fluid or a blunt tool. 7.6.3. Depth of cut Depth of cut is the normal distance from the original surface to the surface being exposed by the tool. It is measured in millimeters. Either tool or work piece may be moved to give depth of cut depending upon the machine. It must be pointed out that the direction of feed and depth of cut must be established carefully with reference to the type of operation. On a lathe for example, the longitudinal movement of the tool along the length of the bed constitutes feed motion in plain turning but a depth motion in facing. Similarly the motion at right angles to the bed axis with the help of cross slide constitutes a depth motion for turning but a feed motion for facing. 7.7. Cutting Fluids 7.7.1. Functions of the cutting fluid Cutting fluids are used in metal cutting primarily for two reasons: 1. To reduce friction at the tool work and tool chip contact zones lubricating action. 2. To dissipate the heat generated during the cutting process – cooling action. In addition, cutting fluids also help in washing away the chips from the cutting zone and in lubricating some of the moving parts of the machine. 51 www.AgriMoon.Com Workshop Practice The lubricating action of the cutting fluids reduces forces, increase tool life, reduces the tendency to form built-up-edge and improves surface finish. Since in metal cutting the ratio of real area of contact to the apparent area of contact is very close to unity and contact pressures are very higher, there is no possibility of fluid film existing between the surfaces in contact. The lubricating action of the cutting fluid is primarily due to the formation of a low shear strength film in the metal surface which can be easily sheared. The formation of such films takes time. Therefore, lubricating action of cutting fluids is not very predominant for high speed machining operations such as grinding. Again, the chemical properties of the cutting fluid are more important than its physical properties. Additives are often added to the fluids to improve their lubricating properties. The cooling action of the cutting fluid helps carry away the heat generated during cutting and hence helps in retaining the strength of the tool. Cooling the work piece also helps in maintaining the dimensional accuracy by reducing the distortion caused due to heat. It also makes work handling easier. Amongst all the fluids, water based fluids are the most efficient for cooling because of their high specific heat and thermal conductivity. Compared to oils, water based fluids are two to three times faster. But water is likely to cause corrosion of machine parts. Anti-corrosive additives are mixed with water based coolants to control this corrosive action. 7.6.3. Types of cutting fluids The cutting fluids commonly used may be divided into : (i) neat oils (ii) water soluble oils (iii) synthetic coolants and (iv) gaseous fluids. (i) Neat oils: Neat oils or straight cutting oils are mineral oils, vegetable oils or combination of these two. Neat oils can be further divided into straight mineral oils, compounded oils for E.P. oils. Straight minerals oils without any additives are suitable only for light loads and hence are used for machining nonferrous metals like aluminum and magnesium. (ii) Water soluble oils: Water soluble oils are blends of mineral oils, emulsifying agents, and coupling agents. For use these oils are mixed with water to form a water emulsion. Water provides the cooling effect and the oil is used for its lubricating properties. (iii) Synthetic coolants: Synthetic coolants are non-petroleum products which are blended with water in the ratio of 50 to 250 parts of water for each part of the chemical. They have cooling properties better than soluble oils and are used chiefly for grinding. (iv) Gaseous fluids: One of the major problems in cutting fluid application is the difficulty for the cutting fluid to actually reach the cutting zone during machining. The effectiveness of the cutting fluid can be considerably increased by supplying the cutting fluid in the form of a gas. Mist is the most commonly used gaseous fluid. In a mist cooling system, compressed air is used to atomize the coolant. 7.6.4. Selection of cutting fluids for different operations The type of cutting fluid to be used depends upon the work material and the characteristics of the machining process. No single cutting fluid can be specified as the best of meet all requirements. The following general guide lines may be used. 52 www.AgriMoon.Com Workshop Practice a) Effect of work piece material Cast Iron: Cast iron is generally cut without any cutting fluid. The graphite flakes in the structure of cast iron help in its easy machining. Sometimes cast iron is also cut with water soluble oils or using compressed air. The use of compressed air necessitates and exhaust system to remove the dust caused by blowing of fine iron particles. Wrought iron: lard oil or water soluble oil Steel: 1. Low and medium carbon steels: Water soluble oil 2.High carbon and nickel chromium alloy steel: Heavy duty soluble oils Stainless steel: Heavy duty soluble oil or neat oil, with chlorine Aluminium: Soluble oil or kerosene. Brass, Bronze: Worked dry or with paraffin or lard oil. Magnesium alloys: Low viscosity inactive fatty mineral oils. 53 www.AgriMoon.Com Workshop Practice Module 5. Welding Lesson-8 Electric arc welding 8.1. INTRODUCTION Welding is process of joining similar metals by application of heat with or without application of pressure and addition of filler material. Such a welded joint has continuous homogeneous material of the similar composition and properties of the parts being joined together. All the engineering branches and metal industries extensively make use of welding processes in one or other form. Types of welding: Welding methods may be broadly classified in two general groups. I. Plastic welding: It is also known as pressure welding. Metal pieces to be joine