WORKSHOP PRACTICE I WORKSHOP PRACTICE THEORY DEPARTMENT OF PRODUCTION ENGINEERING BIRLA INSTITUTE OF TECHNOLOGY, MESRA, RANCHI

Transcription

1 WORKSHOP PRACTICE I WORKSHOP PRACTICE THEORY DEPARTMENT OF PRODUCTION ENGINEERING BIRLA INSTITUTE OF TECHNOLOGY, MESRA, RANCHI

2 FITTING SHOP

3 INTRODUCTION TO FITTING SHOP Introduction: This is the process of finishing the job using different hand tools to obtain the desire surface is finish and accuracy. Inspite of the increasing use of automatic machines in the workshop these days, the fitting shop is indispensable. Although majority of work can be finished to a good degree of accuracy in a reasonable time through various machining operations they still require some operations to be done on them to finish the job by hand. Bench work usually refers to the production of an article by hand on the bench. Fitting is the assembling of parts and removing metals for secure fit. Fitting Operations [1] Chipping: The job whose material is to be chipped away is held in the bench vice firmly. The cold chisel and hammer are the tools used for this process. The chisel is held with right hand especially with second and third fingers the index being relaxed. The chisel is put on the work at some inclination. The inclination depends upon the cutting angel of chisel and the depth of cut required. The shaft of the hammer is held at the end and taken up to sufficient height to provide power in the stroke. Chisel is put on the job and struck by the hammer to chip of the metal. When large volume of metal is to be removed frequently the lubrication of the cutting edge is advisible for a long tool life. [2] Filing: Fig: Chipping Filing is the most important operation after the chipping operation. This is used to remove burr and clean the surface. The movement of file matters much in the effectiveness of this operation. The job to be filed is held in the bench vice. The handle of the file is gripped with right hand. The end of the file is held with the left hand for light work the end of the file is held with the thumb and finger and for heavy work i.e. when large material is to be removed, the end is held with the hand to apply sufficient pressure. The pressure is applied in the forward stroke and relieved in return stroke, the file diagonally yield good results. As

4 far as possible the full length of file should be used. Usually not more than 0.6 m. m. of tooling allowance should be left for filling. Fig: Filing These are three types of filing method: (1)Cross filing: In this method the file strokes run alternately from right to left and from left to right. This is the commonest from filling. In this the whole of the file surface moves across the whole of the work surface. It is used for removing large metal from work. (2)Straight filing: In this method the file is pressed forward approximately at right angles to the length of the work. On the back stroke the file is lifted from the work surface. This used for filling the work piece having the width of surface lesser than the width of file. (3)Draw filing: In this the file is held at right angles to the work surface. The file is not held from the handle but the blade is held with hands close together and gripped with thumbs. The file is moved up and down the left of the work. It does not remove material but gives smoother surface. [3] Marking:

5 Marking is the most important operation of the fitting shop. The accuracy of all the operations depends upon the accuracy of marking. All the lines marked on any job act as guide lines for the fitter to obtain the desired shape with accuracy. The surface of the work to be marked out is coated with either chalk or with copper sulphate solution and is allowed to dry. Then the work is held either in v block if it is round or laid on surface plate if it is flat. If the work is very thin, it is kept vertical with the help of an angle plate. The horizontal line on the work is marked using scribing block. The line perpendicular to this horizontal line can be marked by turning the work through 90*and then using scriber. After the lines have been scribed on the surface the indentations on the surface are made using centre punch and hammer along the scribed lines or arcs. Punch marks serves as a guide to the operator while filling, sawing etc. The metal is removed to half circumference of the punch mark to ensure that the work has not gone undersize. (4) Sawing: This operation is used for cutting the tubes, rods, plates etc. to the desired size. The Hacksaw blade is fitted in to the frame in the two pins projecting in the frame and then tightened with the help of wing nut at the leading pin. The blade is fixed with its teeth facing forward so that cutting is made in the forward stroke. The saw after fitting into the frame is placed on the work surface to be cut. The handle is held with the right hand and the other end of the saw is gripped firmly with the left hand. The pressure is applied on the forward stroke; which is the cutting stroke. The sawing should begin with a backward stroke. While cutting the tubes first a file is notched on the tube surface and cut to a point near the inner wall, then turned the tube forwardly to such an extent that the saw will still be guided in the saw kerf and cut to a point near the inner wall. Again it is turned to the appropriate angle and procedure is repeated till the tube is divided.

6 Fig: Types of hacksaw (5) Drilling: Drilling is the operation of producing circular holes in the job. The pillar drilling machine is used for drilling and the tool is called drill bit the job to be drilled is held firmly in a vice or in any other clamping device on the table of the machine. Cylindrical jobs are held using gripping plate and v block. The socket containing drill is fitted in the machine and is lowered with the help of a handle to touch the centre mark made by the punch. The point of the drill should be exactly over the centre mark made. The machine is started and drill is gradually pressed against the job to produce the hole. Coolant should be frequently used to avoid the overheating of tool. (6) Scraping: This is the operation for removing for convex surfaces left after filling. Obviously it follows the filling operation and removes small amount of material. The job is held in vice and scraper is held with both the hands at some inclination to the job surface and moved forward.

7 Fig: Scraping FITTING TOOLS (1)Holding Tools: These are the tools used for holding the job to be worked. (a) Bench Vice: It is the most commonly used holding device, called Engineer s parallel jaw bench vice or fitter s vice. It essentially consists of a cast iron body, a fixed jaw and a movable jaw made of cast steel, a handle, a square threaded screw and nut made of mild steel. Two cast steel plants called the jaw plates are fixed with the two jaws by set screws. They are provided with serrations to increase the grip on the job. The bench vice is firmly fixed to the work bench. The size of the vice is designated by the width of the jaw which is from 80 to 140 mm and its maximum opening is from 95 to 180 mm. For gripping the work piece in the vice the jaws of the vice are opened by with drawing the movable jaw by rotating screw. The job is held with one hand between the jaws and tightened by rotating the handle by the other hand. Fig: Bench Vice

8 (b)leg Vice: Hand vice is secured firmly at the bench top. Its leg is attached to the leg of the bench and its end goes into the hole made at the floor. Hence this construction of the leg vice is becomes very much rigid and can be used for heavy work. The only disadvantage with leg is that when the job is gripped its jaws come close in a V shape and don t provide as firm grip as parallel bench vice. The leg vice is used by black smiths but can also be very well used for hammering, chipping and cutting in the fitting shop. (c)hand Vice: Hand vice is used for gripping small object. It consists of two steel legs hinged together at the bottom, two steel jaws at the top of the vice. One spring is used just above the hinge of the legs to keep the two legs separated. A screw is fastened to one leg and goes through the other leg. At the end of the screw a wing nut is used for tightening of loosening the grip of the vice. This vice is used for griping the small objects such as keys, rivets, and screws etc. which are too small to be gripped in a bench vice. The vice, gripping the object to be worked is held in one hand the tool is held with the other hand while working. (d)pipe Vice: Pipe vice is used for holding the round section, pipes, tubes etc. It has got a vertical movable screw which moves up and down while tightening and loosening. (e)pin vice: Pin vice consists of a handle and a collect chuck. The chuck contains jaws are operated by rotating the handle. Pin vice is used for hobing the objects of very small diameter e. g. wire, pins etc. (2)Marking Tools: (a)surface plate: It is a solid rectangular plate made of grey cast iron available in different size. The base of the plate is also machined properly to have a perfect horizontal. The top of the plate is grinded to obtain a perfectly plane surface. Surface plate is used for testing the trueness of the finished surface and also for the marking purpose. In marking process surface plate is used a base for V block and angle plate etc. The surface plate is put on a table of sufficient height for use. The different available sizes of surface plate are 1.5*5m, 1.5*3 m, 2*2 m and 2*4 m. The marking surface should be protected from dirt and rust to preserve its accuracy.

9 Fig: Surface Plate (b)scribers: Scribers are the pieces of hardened steel. These are pointed at both the ends. It is used for scribing the line on the metal. (c)try square: Fig: Scriber It consists of a steel blade fitted in to a steel stock also known as the beam. Both inner and outer edges of the blades are kept at perfectly right angles to the stock. This is similar to the carpenter s try square constructions but is more accurate. Try squares are used to check whether any surface is perpendicular to the true square or not. It is also to mark a line perpendicular to the true surface. The accuracy of a try square is very important for a fitter. Hence try square should not be used for striking or supporting purpose and its accuracy is frequently checked. Fig: Try Square (d)punch: Punch is a marking tool made of hardened steel. Punch is used for marking the marks on the job in a more permanent manner than that of scriber. After marking with scribers, the punch is placed on the scribed line and hammered to mark a point as that line. Several such points are made on he scribed line. Usually two types of punches are used in fitting shop (1)Prick punch It consists of a tapered portion sharply pointed having the angle of 40 degree. It is used for marking small marks on the line.

10 (2)Centre Punch It is used for making the prick punch hole larger. Usually the centres for drilling are marked using centre punch. Its angle at the tip is usually 60 degree. Fig: Punch (c) V Block: V block is steel block having a V shaped groove. It is used for marking or working on round shaped objects, so that the objects can t move side wise and also does not rotate easily. The V block is placed over the surface plate and the job to be marked is placed over the upper V of the block. The scribing block is also placed over the surface plate and is used for making lines on the job. The accuracy of marking depends upon the accuracy of the edges and surfaces of V block. Fig: V Block (f) Angle plate: Angle plate has two mutually perpendicular surface and several holes on the surface. It is used for making the jobs when they are placed in vertical position. The angle plate is placed over the surface plate and the job to be marked is placed in vertical position on the angle plate and is held firmly in this position using clamps and bolts. The scribing block is used for making the job.

11 Fig: Angle Plate (3)Cutting Tools: (a) Hack Saw: Hack saw contains a metallic frame. At one end of the frame a wooden handle is provided for gripping the saw while at the other end a wing nut is used for tightening the hacksaw blade. The frames are also of two types. First is solid frame whose length is fixed, second is adjustable frame. As the name implies the length of the adjustable frame can be changed to accommodate the different sizes of blades. Hack saw blades are made of either high carbon steel or alloy steel. Usually the push type blades are used which cut the metal in forward stroke. Hence, the teeth of the saw point away from the operator. While fitting the blade in the frame care should be taken to fit the blade such that teeth are pointed forward. The blade may be all hard type, which is hard throughout or may be flexible type. All hard blades are used for cutting hard material e.g., cast iron alloy steel etc. Flexible type blades are used for general purposes. The thickness of the blade is about 0.7 mm, the width is about 12.7 mm and length of the blade various from 20 to 30 cm. The blades of the coarse group contain 5 to 7 teeth per cm, where as the fine group contain 8 to 12 teeth per cm length. Set is provided on the blade teeth by blanding the teeth alternate outward and inward. This setting of blades gives a cut wider than the thickness of the blades so that while cutting the blade is not in contact and friction is less. Fig: Sawing (b)chisels: Chisels are used for chipping operation. They are made of carbon steel. The chisels are made in the forging shop. The round bar of carbon steel is heated to the

12 plastic state. It is then made hexagonal or octagonal as required. Then the cutting edge is made at one end of this bar. After the desired shape and cutting angles are formed, it is hardened and tempered. In fitting shop, cold chisels are used, whose cutting angles vary from 40 to 70 degree depending upon the material to be cut. The common types of chisels are Fig: Chisel (1)Flat chisels flat chisels is used for general chipping operation. It may be used for chipping large surfaces, cutting metal sheets, bars etc. (2)Cross cut chisel cross cut chisel is also known as cape chisel. It is used for cutting grooves in large surfaces prior to using flat chisel. It is used for cutting keyways. (3)Half round chisel Half round chisel is used for oil ways and also to make a small pilot hole, prior to drilling a hole. (4)Diamond point chisel Diamond point chisel is used for cutting vee grooves, cleaning corners and squaring small holes. (5)Side chisel Side chisel is used particularly in removing the surplus metal in cutter ways and slots. (4)Files: Fig: Types of Chisel Files are most widely used tool of the fitting shop. The different parts of a file are handle, tang, heel, face, point and edge. The files are made of high carbon steel or

13 tungsten steel by forging. Then the teeth are cut, hardened and tempered. The files used in fitting shop are classified on the basis of following factors. (1)Length of file: the length of file from heel to the point is called the effective length of files as shown in fig. the files are available from 10 to 40 cm. the files of length 1o to 15 cm are used for fine work between 15 to 25 cm. for medium sized jobs and above 25 cm. long files are used for large sized jobs. (2)Shape: the files are produced in different shapes of cross section to be used for filling the jobs of different shapes. The common types of files are (a)flat file It is most commonly used file which is tapered in width and thickness. It is double cut in face and single cut at edges. (b) Hand file: It is parallel in width but tapered in thickness, it has one safe edge, which does not have teeth. Other files classified on the basis of shapes are Square file square section Pillar file narrow rectangular section Round file round section Half round file Triangular file half round section Triangular section Knife edge file (3) Teeth: the files can also be classified on the basis of the spacing, the depth of teeth and cut of teeth. They are of two types. (a) Single cut In this type the teeth are cut in parallel row across the face of the file and make an angle of 0 degree with the centre line of the file. They are used where small amount of material is to be removed but a good surface finish is desired. (b)double cut this type of file has two types of teeth across the face of the file. One similar to the single cut file and another inclined by 75 to 80 degree so the centre line of file. This is used to remove more material. The single cut and double cut files are further divided according to the coarseness of the files which actually depends upon the spacing of teeth. They are given below in the increasing degree of smoothness. (a) rough (b) bastard (c) sec (d) smooth (e) dead smooth (f) super smooth. The number of teeth of files per unit length increases from (a) to (f).

14 (5)Scrapers: Scrapers are used for scraping which is actually a finishing process of fitting shop. They contain very hard cutting edge and are made of forged steel. They are forged to the desired shape and then cutting edge is made at the end of the place. The cutting edge is hardened and tempered to make it able to scrap the material and to make the surface of the job plane. Fig: Types of Scraper Fig: Scraping

15 1. LOOSE CLOTHING SHOULD BE AVOIDED. 2. WEAR SHOES WHILE WORKING IN THE SHOP. SAFETY PRECAUTIONS FOR FITTING SHOP 3. TOOLS SHOULD BE PLACED IN THEIR RESPECTIVE PLACE AFTER PROPER CLEANING. 4. ALWAYS USE THE RIGHT TOOL FOR THE JOB. 5. KEEP THE CUTTING TOOLS SEPARATE FROM THE MEASURING TOOLS. 6. SHARP EDGE OF THE CUTTING TOLS SHOULD BE COVERED WHEN THEY ARE NOT IN USE. 7. MAKE CETAIN THAT ALL TOOL HANDLES ARE SECURELY ATTACHED BEFORE USING THEM. 8. ALWAYS WEAR GOGGLES WHEN CHIPPING METAL AND WHEN GRINDING EDGE ON TOOL. 9. HOLD DRIVING TOOLS CORRCTLY SO THAT THEY WILL NOT SLIP OFF THE WORK SURFACE 10. MAKE SURE THAT WORK TO BE OUT, SHEARED, FILED ETC. IS STEADIED AND SEWCURE, TO PREVENT THE TOOL FROM SLIPPING.

16 FOUNDRY SHOP

17 INTRODUCTION TO FOUNDRY SHOP Many methods like machining, casting, forging, rolling, extrusion etc. are used for making and giving finishing to components by engineers. Casting is probably the oldest manufacturing process of producing an engineering article. It is estimated that the art of making metal castings has been in existence for more than 5000 years. The earlier axe heads were cast in open moulds made of stone. Some castings were probably made of gold, brass, copper, silver and bronze. A life sized portrait head of cast bronze from Mesopotamia dates back to about 2250 BC and still survive. The first foundry dates back to the Shang dynasty of China BC. Casting is the process of manufacturing an engineering component by filling up a cavity with molten metal and allowing it to solidify. It is the fastest and most economical process of producing components. It is due to the fact that there is no limit to the size and shape of the article to be produced by casting. It also offers one of the easiest and most economical methods of producing intricate parts. Foundries can be classified into two main categories, (a) ferrous foundries (b) non ferrous foundries. Ferrous foundries can again be divided into (i) cast iron foundries (ii) malleable iron foundries and (iii) steel foundries. In making a casting the following basic steps are involved: a) Pattern making b) Mould and core making c) Melting and pouring the metal d) Cleaning and testing the casting A foundry generally consists of the following main sections: a) Pattern making shop b) Sand mixing/preparation c) Molding section d) Core making section

18 e) Mould assembly and handling f) Melting shop g) Pouring h) Shaking out section i) Fettling and finishing j) Heat treatment shop k) Inspection The following raw materials are used in a foundry: a) Metals and alloys b) Fuels for melting c) Fluxes for melting d) Refractories e) Sand and binding materials Foundry tools and equipments For performing different operations like lifting, shifting, casting, repairing or assembling an item in a foundry, a variety of hand tools and equipments is needed. Depending upon size, shape and number of castings to be produced the foundry tools can be broadly classified as given below. (a) (b) (c) (d) (e) (f) (g) Hand tools: Molding tools, core making tools, pattern making tools etc. Container: Molding boxes, core boxes, ladles etc. Finishing and cleaning: Files, chippers, chisels, grinders, sand and shot blasting machine Mechanical tools: Molding machines, core making machines, power riddles, sand mixers etc. Sand testing and conditioning equipment, permeability meter, mechanical sieve, hydraulic press, hardness tester, rammers etc. Metal melting: Cupola, crucible, tilting furnace, oil fired furnace etc. Testing: Destructive and non destructive testing machines.

19 The various hand tools are used in a foundry for making moulds. A brief description of various hand tools is given below. 1. Shovel: It consists of a square iron blade fitted with a D shaped wooden handle as shown in the figure. It is used for transferring sand to the sand mixer and pouring sand in the molding flask. 2. Strike off bar: It is a wooden or metallic bar having a true edge. It is used for removing the surplus sand after ramming has been completed. 3. Hand riddle: It is also known as a sieve. It consists of a wooden frame fitted with a screen of standard wire mesh at its bottom. It is used for removing foreign materials such as nails, shot metals, splinters of wood etc. from the molding sand. Different types of riddles are denoted by different numbers like 8, 10, 12 etc. 4. Vent wire: It is a thin steel rod or wire having a pointed edge at one end and a wooden handle or a bent loop at the other end. It is used for making small holes called vents in the rammed sand moulds to permit easy escape of gases and steam generated during cooling of the heated metal. 5. Trowels: Trowels are made of iron and are provided with a wooden handle. The three types of commonly used trowels are i. Finishing trowel ii. iii. Square end trowel Heart shaped trowel Trowels are used for i. Finishing flat surfaces for molds ii. Cutting ingots iii. Making joints iv. Repairing molds v. Giving shape 6. Slicks: Slicks are used for repairing and provide finishing to mold surface and edges after the pattern has been withdrawn. The commonly used slicks in a foundry are i. Heart and leaf slick ii. iii. iv. Square and heart slick Spoon and bead slick Heart and spoon slick

20 v. Leaf and spoon slick Fig. (a) Heart and leaf slick (b) Square and heart slick (c) Spoon and bead slick (d) Heart and spoon slick (e) Leaf and spoon slick Fig. : Rammers (a) Floor (b) Pein (c) Hand (d) Butt Figure: Lifters

21 Fig. (a) Shovel (b) Strike off bar (c) Riddle (d) Vent wire Fig. Trowels (a) Finishing (b) Square end (c) Heart shaped 7. Lifter/cleaner: The lifters are made of thin sections of steel of various width and lengths with one end bent at right angles. The lifter is a finishing tool used for patching deep sections of a mould and removing loose sand from pockets of the mold. 8. Rammers: Rammers are used for packing and ramming the sand in the molding box. Depending upon the type of casting there are many types of rammers. These are generally made of wood or steel. The commonly used rammers in a foundry are i. Pein rammer ii. iii. Hand rammer Floor rammer

22 iv. Butt rammer v. Pneumatic rammer 9. Swab: It is a small brush having long hemp fibres. A swab is used for applying water around the edges of a pattern. This prevents the sand edges from crumbling when the pattern is removed from the mold. A bulb swab has a rubber bulb to hold the water and a soft hair brush at the open end. 10. Sprue pin: A sprue pin is also known as a rubber peg. It is a tapered wooden or iron rod, which when embedded into top part of the mold known as cope and later withdrawn produces a cavity known as down gate or runner. Through the runner the molten metal is poured into the mold. 11. Draw spike: It consists of a tapered steel rod having a loop or ring at one end and a sharp 12. Smoothers and corner slicks: These are tools used for giving a finishing to and repairing of round and square comers and edges. (ii) Round and flat surfaces. According to their use and shape, they are given different names. Most commonly used tools for giving finishing and repairing are inside square, flat, pipe, button, half round comer and egg shaped. 13. Clamps: Clamps are made of steel and are used for holding and clamping the molding boxes in proper position. Fig. Swabs Fig. (a) Sprue pin (b) Draw spike Molding box: A molding box or flask is a container that holds the rammed sand rigidly and allows the molten metal to solidity after casting in a mould cavity.

23 Molding boxes are generally made in two parts. These are held in position by location pins. The top part is called the cope and bottom is called the drag. If the mould is made in three parts, the intermediate part is called the check. Molding boxes are made of wood, cast iron or steel. Wooden boxes are used for limited production. Boxes made of fabricated steel are light, robust and can withstand impact In general a molding box must be capable of withstanding rough handling. Figure: Molding flasks Selection of a molding box regarding its shape and size mainly depends upon the type of product to be cast. Since it is not possible to make as many molding boxes as the number of components, it is always better to standardize the size of molding boxes. Various types of molding boxes used in the foundry are (i) Box type flask (ii) Tapered slip flask (iii) Snap flask (iv) Wooden molding box. All the above flasks can be categorized under one head called removable flasks. Other type of flasks used is permanent and light flasks. Removable flasks are used for small to moderate size molding like match plate molding and cope and drag molding. I. Box type flask: Box type flasks are also known as tight or permanent flasks. These are generally made of metal and are used for small and medium sized castings. A box type flask is removed from the rammed mould only after the solidification of the casting is complete. These flasks can be rectangular or circular in shape, but the former are commonly used. 2. Tapered slip flask: A tapered slip flask is simply known as a slip flask. It has tapered sides having an angle of 4 o for removal of the load. A cam actuated retractable shelf (also known as sand strip) is attached to the cope. It can be lifted for removal of pattern from the

24 molded sand. Many moulds can be prepared from a single flask of this type for pouring the molten metal. 3. Snap flask: It consists simply of a frame work fitted with a hinge at one corner and a fastener at the diagonally opposite corner. Unlike the box flask it can be opened after ramming by removing the fasteners and moving it around the hinge. It is highly suitable for mass production of small castings. 4. Wooden molding box: These are used for produci1lg relatively large sized castings in small quantities. On the ends are fitted wooden handles for easy handling of flasks. The sides of the flask are held together by strong cross bars or Ribs.

25 Figure: Different stages of preparation of mould Molding is a cavity in a molding box, formed by a pattern. It is similar in shape and size to that of the actual casting plus some allowance as discussed in the chapter of pattern making. Molding is the process of making moulds. Foundry is the shop where casting of the job is done in a molding box followed by its cleaning by sand blasting, etc. Different stages in preparation of a mould are shown in Figure. A mould is generally made of heat resistant materials. Silica sand is the most commonly used molding material as it is abundantly available. It is cheap can be packed easily into any desired shape. It can also withstand very high temperatures and doesn't react with the molten metal. Clay and molasses are added as binding materials in a molding sand. Water provides strength and plasticity to sand and is added in varying proportions to molding sand. SAFETY PRECAUTIONS FOR FOUNDRY SHOP [1] DRESS PROPERLY FOR THE WORK TO BE DONE. [2] REFRAIN FROM PRACTICAL JOKES. [3] ALWAYS STICK THE SHOVEL IN SAND PILE. [4] DONOT EXERCISE EXCESSIVE WEIGHT. ASK FOR ASSISTANCE WHEN LIFTING OR ROLLING A HEAVY MOULD. [5] USE THE VENT WIRE WITH GREAT CARE. AVOID YOUR STICKING FINGERS OR HAND FROM THE SHARP POINT. [6] WEAR SAFETY GOGGLES WHEN TAPPING FURNACES OR HANDLING MOLTEN METAL.

26 [7] DONOT PERMIT WATER TO COLLECT ON THE FLOOR AROUND A FURNACE. [8] DONOT THROW DAMP OR WET METAL INTO FURNACE CRUCIBLE OR LADLE. [9] BLOW AIR THROUGH CRUCIBLE FURNACE BEFORE ATTEMPT TO LIGHT IT. IT WILL PREVENT EXPLOSION CAUSED BY ACCUMULATED GAS IN THE FURNACE. [10] DONOT WEDGE PIECES OF METAL INTO A CRUCIBLE WHEN CHARGING IT. THE METAL WILL EXPAND WHEN HEATED AND WILL CRACK THE CRUCIBLE. [11] LADLES SHOULD BE THROUGHLY DRY AND WARM BEFORE METAL IS POURED INTO THEM. [12] DONOT PLACE YOUR FACE DIRECTLY OVER SPRUES OR RISER. [13] STORE CRUCIBLE IN A WARM OR DRY PLACE. [14] IF YOU HAVE AN ACCIDENT, HOWEVER SLIGHT, REPORT IT TO YOUR INSTRUCTOR IMMEDIATELY. [15] SAFETY FIRST, LAST AND ALWAYS SHOULD BE THE FOUNDRY SLOGAN.

27 MACHINE SHOP INTRODUCTION TO MACHINE SHOP All manufacturing processes can be classified broadly under three categories: Metal cutting, Metal forming, and Metal casting. Metal cutting is done in various machine tools present in the machine shop. Machine tools are kind of machines on which metal cutting or metal forming processes are carried out. The functions of machine tools are holding the workpiece, holding the tool, moving the tool or the work piece or both relative to each other and supply energy required to cause metal cutting. Every machine tool has a primary cutting tool for metal removal.

28 The various types of machine tools are Lathe machine, Milling machine, Drilling machine, Shaping machine, Planning machine, Broaching machine and Grinding machine. In this lab work you will be introduced to two types of machine tools namely Lathe machine and shaper machine. In any metal cutting the three most important machining parameters are: cutting speed, feed, and depth of cut. Cutting Speed: Cutting speed is the distance traveled by the work surface in unit time with reference to the cutting edge of the tool. For example, if workpiece is rotating past the cutting tool, it is the peripheral speed of the wokpiece. The cutting speed, v is simply referred to as speed and usually expressed in mm/min. A lower or higher cutting speed may be chosen depending on variations in depth of cut (large depth of cut requires lower cutting speed) and feed (lower feed requires higher cutting speed). Feed: The feed is the distance advanced by the tool into or along the workpiece each time the tool point passes a certain position in its travel over the surface. In case of turning, feed is the distance that the tool advances in one revolution of the workpiece. On a shaper, the feed is the distance that the workpiece is moved relative to the tool for each cutting stoke. Feed f is usually expressed in mm/rev. Sometimes it is also expressed in mm/min and is called feed rate. The surface finish produced by machining depends on the feed used. A low feed, in general produces fine surface finish. Depth of cut: It is the distance through which the cutting tool is plunged into the workpiece surface. Thus it is the distance measured perpendicularly between the machined surface and the unmachined (uncut) surface or the previously machined surface of the workpiece. The depth of cut d is expressed in mm.

29 CENTRE LATHE Of the many standard and special types of turning machines that have been built, the most important, most versatile, and most widely recognized is the engine lathe/centre lathe. The basic engine lathe, which is one of the most widely used machine tools, is very versatile when used by a skilled machinist. However, it is not particularly efficient when many identical parts must be machined as rapidly as possible. Although the standard engine lathe is not a high production machine, it can be readily tooled up for many one piece or short run jobs. It is also possible to modify the basic machine for many higher production applications. The modern engine lathe provides a wide range of speeds and feeds which allow optimum settings for almost any operation. There have been advances in headstock design to provide greater strength and rigidity. This allows the use of high horse power motors so that heavy cuts with carbide tools are practical. To utilize this high power without losing accuracy, new lathes incorporate heavier beds, wider hardened ways, and deeper sectioned carriages.

30 COMPONENTS OF CENTRE LATHE AND THEIR FUNCTIONS: Bed: The bed of the lathe is its backbone. It must be rigid enough to resist deflection in any direction under load. The bed is made of cast iron or a steel weldment, in a box or I beam shape, and is supported on legs, a cabinet, or a bench. The Bed forms the base of a Lathe machine. It provides a heavy rigid frame on which all the other basic components are mounted. The headstock and the tailstock are located at either end of the bed and the carriage rests over the Lathe bed and slides over it.

31 Ways: The ways of the lathe are the flat or V shaped surfaces on which the carriage and the tailstock are moved left and right. Each has its separate pair of ways, often one flat surface, for stability, and one V way for guidance in a perfectly straight line. These ways are hardened and scraped or ground to close tolerances. The basic accuracy of movement of the carriage depends on the ways. Headstock: The headstock is the powered end and is always at the operator s left. This contains the speed changing gears and the revolving, driving spindle, to which any one of several types of work holders is attached. The center of the spindle is hollow so that long bars may be put through it for machining.. A live centre, a face plate, collet or a chuck can be fitted to the spindle nose to hold and drive the work. Headstock spindle can be driven by a stepped pulley and a belt or by transmission gears in the headstock. The Lathe with a stepped pulley drive is generally called belt drive Lathe. The gear driven Lathe is referred to as geared head Lathe. Headstock belt drive and back Gear arrangement Nose of the head stock, where various work holding devices may be fitted

32 Tailstock: The tailstock is located on the inner ways at the right end of the bed. This has two main uses: 1. It supports the other end of the work when it is being machined between centers, and 2. It holds a tool for performing operations such as drilling, reaming The tailstock is non rotating but on hardened ways, it can be moved, to the left or right, to adjust to the length of the work. It can also be offset for cutting small angle tapers. Carriage: The carriage can be moved left or right either by hand wheel or power feed. This provides the motion along the Z axis. During this travel turning cuts are made. Carriage consists of the following parts: (1) Saddle, (2) Cross slide, (3) Compound slide or compound rest, (4) Tool post, and (5) Apron. Saddle: The saddle is an H shaped casting that fits over the bed and slides along the bed ways. It carries the cross slide and tool post.

33 Cross Slide: The cross slide is mounted on the carriage and can be moved in and out (X axis) perpendicular to the carriage motion. This is the part that moves when facing cuts are made with power feed, or at any time a cut must be made square with the Z axis. This, or the compound, is also used to set the depth of cut when turning. The cross slide can be moved by its hand wheel or by power feed. Compound Rest: The compound rest is fitted on the top of the cross slide, is used to support the cutting tool. It can be swiveled to any angle for taper turning operations and is moved manually. It can be moved in and out by its hand wheel for facing or for setting the depth of cut. It can also be rotated 360 degrees and fed by its hand wheel at any angle. The compound does not have any power feed but it always moves longitudinally with the cross slide and the carriage. Tool Post: The tool post is mounted on the compound rest. This can be any of several varieties but in its simplest form is merely a slotted cylinder, which can be moved, left or right in the T slot in the compound and clamped in place. It can also be rotated so as to present the cutter to the work at whatever angle is best for the job. Apron: The apron attached to the front of the carriage, holds most of the control levers. These include the levers, which engage and reverse the feed lengthwise (Z axis) or crosswise (X axis) and the lever which engages the threading gears. The apron is fastened to the saddle, houses the gears and mechanisms required to move the carriage and crossslide automatically. The apron hand wheel can be turned manually to move the carriage along the Lathe bed. This hand wheel is connected to a gear that meshes in a rack fastened to the Lathe bed. The automatic feed lever engages a clutch that provides the automatic feed to the carriage Feedrod: The feedrod is a long shaft that has a keyway. The power is transmitted from the lathe spindle to the apron gears through a feedrod via a large number of gears. The feedrod

34 is used to move the carriage or crossslide for turning, facing and all other operations except thread cutting. Leadscrew: The leadscrew is powered by gears from the head stock and is used for providing specific accurate mechanized movement to the carriage for cutting threads on the workpiece. The leadscrew has a definite pitch. A splint nut is used to engage the leadscrew with the carriage. In some lathes, the leadscrew performs the functions of feed rod and there is no separate feed rod. Apron mechanism: Apron mechanism is used for transferring rotary motion of the feed rod and the lead screw into feed motion of the carriage. Both automatic longitudinal and crossfeed can be provided to the carriage by gears and clutch engagements. The mechanism is so designed that when the half nut is engaged with the lead screw, the automatic feed motion from the feedrod is disconnected. There is an interlocking device when prevents simultaneous engagement of the carriage with the feed shaft and leadscrew and saves the machine from any probable damage. This arrangement of the apron is called fool proof mechanism. The half nut or split nut mechanism: The half nut makes the carriage to engage or disengage with the leadscrew. It comprises of a pair of half nuts capable of moving in or out of mesh with the lead screw. The half nut can be engaged with the lead screw by means of a lever provided on the apron. This mechanism is called half nut mechanism. The half nut or split nut is used only for thread cutting. CUTTING TOOLS FOR LATHES Tools used in lathe machine are of various materials: High speed steel, cemented carbide, ceramic etc. Different types of tools are used in lathe for different operations as shown in the figure below: Work Holding Devices: In lathe work the three most common work holding methods are: Held in a chuck: Three jaw self centering chuck, Four jaw independent chuck, magnetic chuck and hydraulic chuck.

35 Held between centers Held in a collet Chuck Three jaw self centering chuck and four jaw independent chuck TYPES OF OPERATONS THAT CAN BE PERFORMED ON CENTRE LATHE:

36 Various turning operations: (a) chamfering, (b) parting, (c) threading, (d) boring, (e) drilling, (f) knurling. 1) Turning: Turning is the most commonly used operation in Lathe. By turning operation excess material from the work piece is removed to produce a cylindrical or cone shaped surface. Two of the common types of turning are: i. Straight turning: In this operation the work is held in the spindle and is rotated whole the tool is fed past the work piece in a direction parallel to the axis of rotation. The surface generated is a cylindrical surface. ii. Taper turning: A taper may be defined as a uniform increase or decrease in diameter of a work piece measured along its length. In a Lathe taper turning is an operation to produce a conical surface by gradual reduction in diameter from a cylindrical job. Taper turning can be done by the following ways; a) By a form tool. b) By setting over the tailstock. c) By swiveling the compound rest. d) By taper turning attachment. e) By compound feed. 2) Facing: Facing is an operation for generating flat surface at the ends of a work piece. In this operation the feed given is in a direction perpendicular to the axis of rotation. 3) Chamfering: It is an operation of beveling the extreme end of a work piece. This done to remove unwanted metal projections at the ends and to protect end of the work piece from being damaged and to have a better look. 4) Parting: In. this operation a flat nose tool is used to cut the work piece, with feed in the direction perpendicular to the axis of rotation In parting operations the workpiece rotates while the tool carries out a radial feed movement. A parting tool is deeper and narrower than a turning tool. It is designed for making narrow grooves and for cutting off parts.

37 5) Knurling: Knurling is process of embossing a diamond shaped pattern on the surface of the work piece. The purpose of knurling is to provide an effective gripping surface on a work piece to prevent it from slipping when operated by hand. Knurling is done with a special tool called knurling tool. This tool consists of a set of hardened steel rollers in a holder with teeth cut on their surface in definite pattern. 6) Grooving or Recessing Operations: Grooving or recessing operations is the operation of reducing the diameter of a workpiece over a very narrow surface. Grooving or recessing operations, sometimes also called necking operations, are often done on workpiece shoulders to ensure the correct fit for mating parts. When a thread is required to run the full length of the part to a shoulder, a groove is usually machined to allow full travel of the nut. Grooving the workpiece prior to cylindrical grinding operations allows the grinding wheel to completely grind the workpiece without touching the shoulder. A grooving tool is similar to a parting tool. 7) Drilling/reaming/ boring: These are operations to accurately make holes on a workpiece. These operations use the tailstock of the lathe. The tool is held on the tailstock and is fed toward the rotating work piece. A lathe can also be used to drill holes accurately concentric with the centerline of a cylindrical part. First, install a drill chuck into the tail stock. Make certain that the tang

38 on the back of the drill chuck seats properly in the tail stock. Withdraw the jaws of the chuck and tap the chuck in place with a soft hammer. Move the saddle forward to make room for the tailstock. Move the tailstock into position, and lock it in place (otherwise it will slide backward as you try to drill). Before starting the machine, turn the spindle by hand. You've just moved the saddle forward, so it could interfere with the rotation of the lathe chuck. Always use a center drill to start the hole. You should use cutting fluid with the center drill. It has shallow flutes (for added stiffness) and doesn't cut as easily as a drill bit. Always drill past the beginning of the taper to create a funnel to guide the bit in. The drill chuck can be removed from the tail stock by drawing back the drill chuck as far as it will easily go, then about a quarter turn more. Boring is an operation in which a hole is enlarged with a single point cutting tool. A boring bar is used to support the cutting tool as it extends into the hole. Because of the extension of the boring bar, the tool is supported less rigidly and is more likely to chatter. This can be corrected by using slower spindle speeds or by grinding a smaller radius on the nose of the tool. 8) Thread cutting: Thread cutting is one of the most important operations performed on Lathe. The principle of thread cutting is to produce a helical groove on a cylindrical or conical surface by feeding the tool longitudinally when the job is revolved between centers or by a chuck. The longitudinal feed should be equal to the pitch of the thread to be cut. For different kinds of threads different tool has to be used. THE SHAPER MACHINE The shaper is a relatively simple machine. It is used fairly often in the tool room or for machining one or two pieces for prototype work. The metal working shaper was developed by James Nasmyth in Shaper is a machine tool to produce flat surfaces. The flat surfaces may be horizontal, vertical or inclined. The shaper uses single point tools and cuts only in straight lines. The shaper handles relatively small work. The cutting stroke of the shaper is made by moving the tool bit attached to the ram. The shaper cut only in one direction, so that the return stroke is lost time. However, the return stroke is made at up to twice the speed of the cutting stroke. The shaper is a relatively simple machine. It is used fairly often in the tool room or for machining one or two pieces for prototype work. Tooling is simple, and shapers do not always require operator attention while cutting.

39 1. Table support, 2. Table, 3. Clapper block, 4.Apron clamping bolt, 5. Downfeed hand wheel, A horizontal shaper 6. Swivel base degree graduations, 7. Position of stroke adjustment, 8. Ram block locking COMPONENTS handle, OF A 9. SHAPER Ram, 10. MACHINE: Column, 11. Driving Pulley, 12. Base, 13. Feed disc, 14. Pawl mechanism, 15. Elevating screw Ram: The ram slides back and forth in dovetail or square ways to transmit power to the cutter. The starting point and the length of the stroke can be adjusted.

40 Toolhead: The toolhead is fastened to the ram on a circular plate so that it can be rotated for making angular cuts. The toolhead can also be moved up or down by its hand crank for precise depth adjustments. Attached to the toolhead is the tool holding section. This has a tool post very similar to that used on the engine lathe. The block holding the tool post can be rotated a few degrees so that the cutter may be properly positioned in the cut. Clapper Box: The clapper box is needed because the cutter drags over the work on the return stroke. The clapper box is hinged so that the cutting tool will not dig in. Often this clapper box is automatically raised by mechanical, air, or hydraulic action. Table: The table is moved left and right, usually by hand, to position the work under the cutter when setting up. Then, either by hand or more often automatically, the table is moved sideways to feed the work under the cutter at the end or beginning of each stroke. Saddle: The saddle moves up and down (Y axis), usually manually, to set the rough position of the depth of cut. Final depth can be set by the hand crank on the tool head. Column: The column supports the ram and the rails for the saddle. The mechanism for moving the ram and table is housed inside the column. Tool holders: Tool holders are the same as the ones used on an engine lathe, though often larger in size. The cutter is sharpened with rake and clearance angles similar to lathe tools, though the angles are smaller because the work surface is usually flat. These cutters are fastened into the tool holder, just as in the lathe, but in a vertical plane. Work Holding: Work holding is frequently done in a vise. The vise is specially designed for use in shapers and has long ways which allow the jaws to open up to 14 inches or more; therefore quite large work pieces can be held. The vise may also have a swivel base so that cuts may be made at an angle. Work which, due to size or shape, cannot be held in the vise, is clamped directly to the shaper table in much the same way as parts are secured on milling machine tables.

41 Shaper Size: The size of a shaper is the maximum length of stroke which it can take. Horizontal shapers are most often made with strokes from 175 to 900 mm long, though some smaller and larger sizes are available. The length of the stroke indicates, in addition to the general size of the machine, the size of a cube that can be held and planed in the shaper. Therefore the cross feed and the maximum height that can be accommodated is approximately equal to the stoke length. In addition to the stoke length, other particulars like cutting to return stroke ratio, individual motor drive, number and amount of feed etc. Drive Mechanisms Shapers are available with either mechanical or hydraulic drive mechanisms. Figures show diagrams of both shaper drive mechanisms. Mechanical Drive or Crank and Slotted Link Mechanism The less expensive shaper, the one most often purchased, uses a mechanical drive. Most shaping machine uses Crank and Slotted link mechanism, although sometimes Whitworth quick return mechanism is also used. Crank and Slotted Link Mechanism The bull gear is driven by a pinion which is connected to the motor shaft through a gear box with four, eight or more speeds available. The speed of the bull gear may be changed by different combination of gearing or by simply shifting the belt on step cone pulley. Bolted to the centre of the bull gear is a radial slide (not shown in figure). This slide carries a sliding block, into which the crank pin is fitted. The crank pin connects the slotted link and the sliding block. The slotted link which is known as rocker arm is pivoted at the bottom end attached to the frame of the column. The upper end of the rocker arm is connected to the

42 ram. As the bull gear rotates causing the crank pin to rotate, the sliding block fastened to the crank pin will rotate on the crank pin circle, and at the same time move up and down the slot in the slotted link giving it a rocking movement which is communicated to the ram. Thus the rotary motion of the bull gear is converted to reciprocating motion of the ram. The principle of quick return motion: When the link is in the position AB and AC, the ram will be in the extreme positions. The forward cutting stroke therefore takes place when the crank rotates through angle D 1 MD 2 and the return stroke takes place when the crank rotates through the angle D 1 LD 2. The angular velocity of the crank pin being constant the return stroke is therefore completed in shorter time than forward stroke. Therefore this mechanism is known as quick return mechanism. time Therefore, Cutting Return time Cutting time to return time ratio varies within 2:1 and the practical limit is 3:2. Adjustment of length of stroke: The distance of the sliding block from the centre of the bull gear can be varied radially within the radial slide. If the sliding block is brought inwards i.e. towards the centre the length of stroke will be reduced, and vice versa. Adjustment of the position of stroke: The position of the ram relative to the work can also be adjusted. The rocker arm is connected to a screw on the ram. The position of this connection can be varied on this screw. So the ran can be positioned adjusting this by means of a lever and wheel provided on top of the ram.

43 SAFETY PRECAUTIONS FOR MACHINE SHOP 1. NEVER ATTEMPT TO OPERATE ANY MACHINE UNLESS YOU ARE FULLY AWARE OF THE MEANS OF STOPPING THE MACHINE IMMEDIATELY, SHOULD ANY EMERGENCY ARISE. 2. NEVER ENGAGE ANY OPERATING LEVER OR CONTROL UNLESS YOU KNOW IN ADVANCE, ITS DIRECTION OR WHAT ITS EFFECT WILL BE. 3. NEVER IDLY PLAY WITH CONTROL HANDLES ETC. ON ANY MACHINE WHETHER IT IS RUNNING OR STANDING. 4. UNDER NO CIRCUMSTANCES DISTRACT THE ATTENTION OF ANY OTHER OPERATOR WHILST HIS MACHINE IS UNDER POWER. 5. NEVER LEAN AGAINST OR REST THE HANDS ON ANY MOVING PART, EVEN THOUGH THE MOVEMENT BE SLOW, AS YOU MAY EASILY BE TRAPPED UNEXPECTEDLY AT SOME POINT OF ITS TRAVERSE. 6. NEVER OIL OR MAKE ANY ADJUSTMENTS TO A MACHINE, WHICH IS IN MOTION. 7. NEVER START A MACHINE OR ENGAGE A FEED UNTIL YOU ARE CERTAIN THAT THE WORK IS ADEQUATELY SECURED. 8. NEVER LEAVE WRENCHES OR KEYS IN POSITION WHEN SETTING UP OR REMOVING WORK. 9. NEVER CHANGE A BELT POSITION WHILST IT IS RUNNING. 10. ALWAYS REMOVE CHIPS WITH A BRUSH OR OTHER SUITABLE MEANS AND NEVER WITH HANDS OR FINGERS. NEVER ATTEMPT TO BLOW THEM AWAY. YOU MAY EASILY BLOW THEM INTO THE WORKING PARTS OF MACHINE AND FAR MORE EASILY AND LIKELY RECEIVE SOME IN THE EYES. 11. WHEN YOU LEAVE THE MACHINE ALWAYS TURN OFF THE POWER.

44 12. BEFORE PUTTING ON THE POWER, BE SURE THAT THE WORK IS SECURELY FASTENED. 13. WHILE THE MACHINE IS RUNNING NEVER THROUGH THE BACK GEAR. 14. DO NOT WEAR LOOSE GARMENTS WHILE WORKING ON A MACHINE.

45 WELDING SHOP INTRODUCTION TO WELDING SHOP The American Welding Society has made each welding process definition as complete as possible so that it will suffice without reference to another definition. The AWS definition for a welding process is "a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone and with or without the use of filler material".

46 Welding may be also defined as joining of two pieces of metals by application of heat, with or without application of pressure and with or without addition of filler materials. Welding finds huge application in industry as a most efficient method of joining metals. Welding is extensively used in construction or manufacture of automobile cars, aircrafts, electronic equipments, ships and bridges etc. The heat source may be: an electric arc: as in electric arc welding. a gas flame: as in gas welding. electric resistance heating: as in resistance welding. Classification of welding: All welding processes can be classified under two broad headings: Plastic Welding or pressure welding. Fusion Welding or non pressure welding Plastic Welding or pressure welding: In plastic welding, the pieces of metal to be joined are heated to a plastic state and then forced together by external pressure. This procedure is used in resistance welding, thermit welding where pressure is required. Fusion Welding or non pressure welding: In fusion welding the metals to be joined are heated to a molten state and are allowed to solidify together into an inseparable joint. This includes all arc welding and gas welding processes. Applications of Welding: Welding is used in industry as an effective tool in the form of Regular method of production of metal structures and components for automobile, aircraft, railway and many other construction industries. An easy and effective method of on site repairs and maintenance or rebuilding of broken parts of a machine or structure. Industries, where welding finds most extensive use, are given below: Automobile and Transport industry wherein cars, trucks, jeeps and many other transportation machines and equipments are fabricated. Material handling equipment such as overhead cranes, jib cranes and tower cranes are manufactured by welding along with their auxiliary equipment like trolleys, lifting aids and gadgets. Rail Road industry wherein major fabrication and welding is involved in the production of locomotive under frames, bogies, trolleys, railway bridges, electrification network, signaling equipment, lighting towers, platform sheds and godowns, storage tanks and bodies and frames of railway coaches. Bridge construction industry utilizes welding as the most popular means of joining steel bridge components or structural, in a factory or at the construction site.

47 Joining of steel reinforcement for cast in place concrete bridges is also done by welding. Ship building industry involves welding in the construction of ship body or structures including decks, supporting girders and framework, platforms and many other structures. Aircraft industry involves considerable use of welding for joining aircraft components of alloy steels, stainless steels and aluminium alloys. Besides the fabrication of the aircraft body, frames, mounts, fuel tanks, ducts and fittings, welding is used for the production of allied equipment that help aircraft operations and maintenance like material handling systems, transport means for man and luggage, sheds, fuel storage tanks and many such structures. Chemical and Petroleum industry make good use of welding for the fabrication of plant and machinery, stainless steel vessels and storage tanks besides many other structures. Pressure Vessels and Tanks are used in various industries for storing of fuel, and other liquids. These are made by welding together the bent steel plates. Oil, gas and water storage tanks are also steel fabricated. Manufacturing of Machine tools and production tools include mass production of machine tool frames, columns, beds and other auxiliary supports, press and die equipment for cold and hot forming of steel and non ferrous metals. These involve welding as a major means of fabrication. Tools and Safety Equipments in welding: Goggles: Goggles with glasses are used to protect the eyes of the welder from the light sparks produced during welding. Face Shield: A face shield is also used to protect the eyes of the welder from the light sparks produced during welding. It is normally held in hand. Chipping Hammer: A chipping hammer is used to remove slags which form during welding.

48 Ground Clamp: It is connected to the end of the ground cable. It is normally clamped to the welding table or the job itself to complete the electric circuit. Wire brush: The wire brush is used to clean the surface to be welded. Electrode Holder: It is a device used for mechanically holding the electrode and conducting current to it. Edge preparation: To obtain sound welds, it is desirable that weld should completely penetrate the metal thickness. The heat will not be able to melt the joint edges to their entire thickness it thick plates are to be welded. Complete becomes more important in case of butt joints. Hence to obtain good butt joints edge preparation is required. Edge preparation may of the following types:

49 Types of welded joints: Types of welding positions: A Classification of common welding processes:

50 ARC WELDING: Arc welding is a group of welding processes wherein coalescence is produced by heating with an electric arc or arcs, mostly without the application of pressure and with or without the use of filler metal depending upon the base plate thickness. Equipments used in Arc Welding: Arc welding power source. Electrodes or welding rods. Electrode holder Cables and cable connectors. Earthing clamp. Chipping hammer. Safety goggles Apron. Hand gloves. Arc Welding Power Source: During arc welding process a power source is required to maintain the arc between electrode and the base metal. The power source may be: D.C Generator, A.C transformer with D.C rectifier, A.C. transformer.

51 D.C Generator: D.C Generator is run either by an electric motor or diesel engine. This generators supplies voltage in the range of 15 to 50 volts and output current is in the range of 200 to 600 ampere. When D.C. current is used the polarities will be fixed. The two types of polarities that are used are: Direct Current Straight polarity (DCSP): When work is positive and electrode is negative. Direct Current Reverse Polarity (DCRP): When work is negative and electrode is positive. As the heat generated is split into two parts in the ratio of 66% at the positive pole and 33% at the negative pole. Therefore for welding thin materials the work is made negative and electrode positive (DCRP is used). For welding heavy AC Trans former: AC transformer changes high voltage and low amperage to low voltage and high amperage. The main advantage of the transformer over generator is low cost and ease of operation. Since there are no moving parts in the equipment the operation is noiseless. The disadvantage of the transformer is that the polarity cannot be fixed. AC Transformer with DC rectifier: In this machine the power supplied is first stepped down by means of a transformer to the required voltage and then silicon controlled rectifiers are used to convert AC to DC. Principles of Metal Arc Welding/ Shielded Metal Arc Welding (SMAW): Shielded Metal Arc Welding (SMAW) is one of the oldest, simplest and most versatile of all welding techniques and about 50% of all industrial application and maintenance welding is performed by this. The process is generally used in construction, ship building, and for maintenance work. The SMAW process is best suited for work piece of about 3 to 19 mm, although this range can easily be extended using multi pass. Shielded Metal Arc Welding uses a coated consumable electrode. This coating produces a gas shield and slag to protect the weld from the atmosphere. Thus this welding process is called shielded metal arc welding.

52 To start the arc, initially the welder has to briefly touch the tip of the electrode to the work piece to complete the circuit. Then the electrode is removed to a slight distance from the work piece, this initiates an arc. The arc so produced generates intense heat which melts the tip of the electrode, the coating and a portion of the adjacent base metal. This molten metal is allowed to solidify together to form a solid inseparable join. As the coating on the electrode melts and vaporizes, it forms a protective atmosphere that stabilizes the arc and protects the molten and hot metal from contamination. Fluxing constituents unites with any impurities in the molten metal and form slag which floats on the surface of the molten metal. This slag coating protects the hot molten metal from oxidation and slows down the cooling rate to prevent formation of hard brittle structure. The slag solidifies over the metal and is then easily chipped from the weld when it is cooled. In Shielded Metal Arc Welding both AC and DC can be used. Arc welding Procedure: The surface to be welded is cleaned and the edges of the plates may be filed for perfect joint and more strength. The welding rod is held in the electrode holder and the ground clamp is clamped to the plate to be welded. The electric arc produced melts the welding rod and joins the two metal plates. Maintain a gap of 3mm between the plate and the welding rod. Complete the welding process by removing the slag using chipping hammer. Symbolic representation of different weld joint: Symbolic representation of square butt weld Symbolic representation of lap weld Symbolic representation of tee fillet weld

53 Symbolic representation of single vee weld Symbolic representation of double vee weld SAFETY PRECAUTIONS FOR WELDING SHOP [16] INSTRUCTIONS AND PRINTED RULES COVERING OPERATIONS OF EQUIPMENT SUPPLIED BY THE MANUFACTURER SHALL BE STRICTICALLY FOLLOWED. [17] WORKING AREA ANF FLOOR SHOULD BE KEPT CLEAN. [18] BEFORE STARTING WELDING ENSURES THAT THE WELDING EQUIPMENT IS ADEQUITELY EARTHED.