ManufacturingTechnologyforMechatronics.pdf

Course Material for Unit - I Name of the Course : Manufacturing Technology for Mechatronics Name of the Unit : Casting and Welding Name of the Topic : Introduction to casting, Patterns, Types, Pattern...

Course Material for Unit - I Name of the Course : Manufacturing Technology for Mechatronics Name of the Unit : Casting and Welding Name of the Topic : Introduction to casting, Patterns, Types, Pattern materials and Allowances.  Objectives: To study the concept of casting technology. 1. Outcomes: Upon successful completion, the student should be able to learn the process of metal casting. 2. Pre-requisites: To have a basic knowledge of Fundamentals of Basic Mechanical Engineering. 1. A pattern is generally made up of how many pieces? a) 1 b) 2-3 c) 4-5 d) 6 2. How does pattern vary in size with casting? a) Pattern is larger in size b) Casting is larger in size c) Both have same size d) Size depends on other factors 3. A pattern carries which allowance for internal and external surfaces? a) Shrinkage allowance b) Machining allowance c) Distortion allowance d) Draft allowance 4. What is draft allowance also known as? a) Shake allowance b) Contraction allowance c) Taper Allowance d) Rapping Allowance 5. Machining allowance does not depend on which of the following factor? a) Solidifying contraction b) Machining method c) Shape and size of casting d) Casting method 6. How much does the distortion allowance vary? a) 1mm to 10mm b) 2mm to 20mm c) 1mm to 15mm d) 2mm to 15mm 7. Shrinkage allowance does not depend on which of the following factor? a) Moulding method b) Casting dimension c) Pouring temperature of molten metal d) Amount of finish required 8. A pattern is shaken by striking it with a wooden piece. A negative allowance is provided for this. Which allowance is it? a) Machining Allowance b) Rapping Allowance c) Distortion Allowance d) Contraction Allowance 9. The following figure depicts which allowance? a) Machining Allowance b) Shrinkage Allowance c) Draft Allowance d) Shake Allowance 10. In order the get a smooth casting, the size of the sand particles should be a) Coarse b) Fine c) Moderate d) Large 3. Metal Casting 3.1 Introduction Virtually nothing moves, turns, rolls, or flies without the benefit of cast metal products. The metal casting industry plays a key role in all the major sectors of our economy. There are castings in locomotives, cars trucks, aircraft, office buildings, factories, schools, and homes. Figure some metal cast parts. Metal Casting is one of the oldest materials shaping methods known. Casting means pouring molten metal into a mold with a cavity of the shape to be made, and allowing it to solidify. When solidified, the desired metal object is taken out from the mold either by breaking the mold or taking the mold apart. The solidified object is called the casting. By this process, intricate parts can be given strength and rigidity frequently not obtainable by any other manufacturing process. The mold, into which the metal is poured, is made of some heat resisting material. Sand is most often used as it resists the high temperature of the molten metal. Permanent molds of metal can also be used to cast products.  Figure: Metal Cast parts Advantages The metal casting process is extensively used in manufacturing because of its many advantages. 1. Molten material can flow into very small sections so that intricate shapes can be made by this process. As a result, many other operations, such as machining, forging, and welding, can be minimized or eliminated. 2. It is possible to cast practically any material that is ferrous or non-ferrous. 3. As the metal can be placed exactly where it is required, large saving in weight can be achieved. 4. The necessary tools required for casting molds are very simple and inexpensive. As a result, for production of a small lot, it is the ideal process. 5. There are certain parts made from metals and alloys that can only be processed this way. 6. Size and weight of the product is not a limitation for the casting process. Limitations 1. Dimensional accuracy and surface finish of the castings made by sand casting processes are a limitation to this technique. Many new casting processes have been developed which can take into consideration the aspects of dimensional accuracy and surface finish. Some of these processes are die casting process, investment casting process, vacuum-sealed molding process, and shell molding process. 2. The metal casting process is a labour intensive process 3.2 Steps in Making Sand Castings There are six basic steps in making sand castings: 1. Patternmaking 2. Core making 3. Molding 4. Melting and pouring 5. Cleaning Pattern making The pattern is a physical model of the casting used to make the mold. The mold is made by packing some readily formed aggregate material, such as molding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mold cavity, which is ultimately filled with metal to become the casting. If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities. Core making Cores are forms, usually made of sand, which are placed into a mold cavity to form the interior surfaces of castings. Thus the void space between the core and mold-cavity surface is what eventually the casting becomes. Molding Molding consists of all operations necessary to prepare a mold for receiving molten metal. Molding usually involves placing a molding aggregate around a pattern held with a supporting frame, withdrawing the pattern to leave the mold cavity, setting the cores in the mold cavity and finishing and closing the mold. Melting and Pouring The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, and the molten metal is transferred to the pouring area where the molds are filled. Cleaning Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from the casting. Burned-on sand and scale are removed to improve the surface  appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Inspection of the casting for defects and general quality is performed. Pattern The pattern is the principal tool during the casting process. It is the replica of the object to be made by the casting process, with some modifications. The main modifications are the addition of pattern allowances, and the provision of core prints. If the casting is to be hollow, additional patterns called cores are used to create these cavities in the finished product. The quality of the casting produced depends upon the material of the pattern, its design, and construction. The costs of the pattern and the related equipment are reflected in the cost of the casting. The use of an expensive pattern is justified when the quantity of castings required is substantial. Functions of the Pattern 1. A pattern prepares a mold cavity for the purpose of making a casting. 2. A pattern may contain projections known as core prints if the casting requires a core and need to be made hollow. 3. Runner, gates, and risers used for feeding molten metal in the mold cavity may form a part of the pattern. 4. Patterns properly made and having finished and smooth surfaces reduce casting defects. 5. A properly constructed pattern minimizes the overall cost of the castings. Pattern Material Patterns may be constructed from the following materials. Each material has its own advantages, limitations, and field of application. Some materials used for making patterns are: wood, metals and alloys, plastic, plaster of Paris, plastic and rubbers, wax, and resins. To be suitable for use, the pattern material should be: 1. Easily worked, shaped and joined 2. Light in weight 3. Strong, hard and durable 4. Resistant to wear and abrasion 5. Resistant to corrosion, and to chemical reactions 6. Dimensionally stable and unaffected by variations in temperature and humidity 7. Available at low cost The usual pattern materials are wood, metal, and plastics. The most commonly used pattern material is wood, since it is readily available and of low weight. Also, it can be easily shaped and is relatively cheap. The main disadvantage of wood is its absorption of moisture, which can cause distortion and dimensional changes. Hence, proper seasoning and upkeep of wood is almost a pre-requisite for large-scale use of wood as a pattern material.  Figure 2: A typical pattern attached with gating and riser system Pattern Allowances Pattern allowance is a vital feature as it affects the dimensional characteristics of the casting. Thus, when the pattern is produced, certain allowances must be given on the sizes specified in the finished component drawing so that a casting with the particular specification can be made. The selection of correct allowances greatly helps to reduce machining costs and avoid rejections. The allowances usually considered on patterns and core boxes are as follows: 1. Shrinkage or contraction allowance 2. Draft or taper allowance 3. Machining or finish allowance 4. Distortion or camber allowance 5. Rapping allowance Shrinkage or Contraction Allowance All most all cast metals shrink or contract volumetrically on cooling. The metal shrinkage is of two types:  Liquid Shrinkage: it refers to the reduction in volume when the metal changes from liquid state to solid state at the solidus temperature. To account for this shrinkage; riser, which feed the liquid metal to the casting, are provided in the mold.  Solid Shrinkage: it refers to the reduction in volume caused when metal loses temperature in solid state. To account for this, shrinkage allowance is provided on the patterns. The rate of contraction with temperature is dependent on the material. For example steel contracts to a higher degree compared to aluminium. To compensate the solid shrinkage, a shrink rule must be used in laying out the measurements for the pattern. A shrink rule for cast iron is 1/8 inch longer per foot than a standard rule. If a gear blank of 4 inch in diameter was planned to produce out of cast iron, the shrink rule in measuring it 4 inch would actually measure 4 -1/24 inch, thus compensating for the shrinkage. The various rate of contraction of various materials are given in Table 1. Table 1: Rate of Contraction of Various Metals Material Dimension Shrinkage allowance (inch/ft) Grey Cast Iron Up to 2 feet 0.125 2 feet to 4 feet 0.105 over 4 feet 0.083 Cast Steel Up to 2 feet 0.251 2 feet to 6 feet 0.191 over 6 feet 0.155 Aluminum Up to 4 feet 0.155 4 feet to 6 feet 0.143 over 6 feet 0.125 Magnesium Up to 4 feet 0.173 Over 4 feet 0.155 Draft or Taper Allowance By draft is meant the taper provided by the pattern maker on all vertical surfaces of the pattern so that it can be removed from the sand without tearing away the sides of the sand mold and without excessive rapping by the moulder. Figure 3 (a) shows a pattern having no draft allowance being removed from the pattern. In this case, till the pattern is completely lifted out, its sides will remain in contact with the walls of the mold, thus tending to break it. Figure 3 (b) is an illustration of a pattern having proper draft allowance. Here, the moment the pattern lifting commences, all of its surfaces are well away from the sand surface. Thus the pattern can be removed without damaging the mold cavity. Figure 3 (a) Pattern Having No Draft on Vertical Edges  Figure 3 (b) Pattern Having Draft on Vertical Edges Draft allowance varies with the complexity of the sand job. But in general inner details of the pattern require higher draft than outer surfaces. The amount of draft depends upon the length of the vertical side of the pattern to be extracted; the intricacy of the pattern; the method of molding; and pattern material. Table 2 provides a general guide lines for the draft allowance. Table 2: Draft Allowances of Various Metals Pattern material Height of the Draft angle Draft angle given surface (inch) (External surface) (Internal surface) 1 3.00 3.00 1 to 2 1.50 2.50 Wood 2 to 4 1.00 1.50 4 to 8 0.75 1.00 8 to 32 0.50 1.00 1 1.50 3.00 1 to 2 1.00 2.00 Metal and plastic 2 to 4 0.75 1.00 4 to 8 0.50 1.00 8 to 32 0.50 0.75 Machining or Finish Allowance The finish and accuracy achieved in sand casting are generally poor and therefore when the casting is functionally required to be of good surface finish or dimensionally accurate, it is generally achieved by subsequent machining. Machining or finish allowances are therefore added in the pattern dimension. The amount of machining allowance to be provided for is affected by the method of molding and casting used viz. hand molding or machine molding, sand casting or metal mold casting. The amount of machining allowance is also affected by the size and shape of the casting; the casting orientation; the metal; and the degree of accuracy and finish required. The machining allowances recommended for different metal is given in Table 3. Table 3: Machining Allowances of Various Metals Metal Dimension (inch) Allowance (inch) Up to 12 0.12 Cast iron 12 to 20 0.20 20 to 40 0.25 Up to 6 0.12 Cast steel 6 to 20 0.25 20 to 40 0.30 Up to 8 0.09 Non ferrous 8 to 12 0.12 12 to 40 0.16 Distortion or Camber Allowance Sometimes castings get distorted, during solidification, due to their typical shape. For example, if the casting has the form of the letter U, V, T, or L etc. it will tend to contract at the closed end causing the vertical legs to look slightly inclined. This can be prevented by making the legs of the U, V, T, or L shaped pattern converge slightly (inward) so that the casting after distortion will have its sides vertical ( (Figure 4). The distortion in casting may occur due to internal stresses. These internal stresses are caused on account of unequal cooling of different section of the casting and hindered contraction. Measure taken to prevent the distortion in casting include: i. Modification of casting design ii. Providing sufficient machining allowance to cover the distortion affect iii. Providing suitable allowance on the pattern, called camber or distortion allowance (inverse reflection) Figure 4: Distortions in Casting Rapping Allowance Before the withdrawal from the sand mold, the pattern is rapped all around the vertical faces to enlarge the mold cavity slightly, which facilitate its removal. Since it enlarges the final casting made, it is desirable that the original pattern dimension should be reduced to account for this increase. There is no sure way of quantifying this allowance, since it is highly dependent on the foundry personnel practice involved. It is a negative allowance and is to be applied only to those dimensions that are parallel to the parting plane. Types of Pattern Patterns are of various types, each satisfying certain casting requirements. 1. Single piece pattern 2. Split or two piece pattern 3. Match plate pattern Single Piece Pattern The one piece or single pattern is the most inexpensive of all types of patterns. This type of pattern is used only in cases where the job is very simple and does not create any withdrawal problems. It is also used for application in very small- scale production or in prototype development. This type of pattern is expected to be entirely in the drag and one of the surface is is expected to be flat which is used as the parting plane. A gating system is made in the mold by cutting sand with the help of sand tools. If no such flat surface exists, the molding becomes complicated. A typical one- piece pattern is shown in Figure 6. Figure 6: A Typical One Piece Pattern Split or Two Piece Pattern Split or two piece pattern is most widely used type of pattern for intricate castings. It is split along the parting surface, the position of which is determined by the shape of the casting. One half of the pattern is moulded in drag and the other half in cope. The two halves of the pattern must be aligned properly by making use of the dowel pins, which are fitted, to the cope half of the pattern. These dowel pins match with the precisely made holes in the drag half of the pattern. A typical split pattern of a cast iron wheel Figure 7 (a) is shown in Figure 7 (b). Figure 7 (a): The Details of a Cast Iron Wheel Figure 7 (b): The Split Piece or Two Piece Pattern of a Cast Iron Wheel Test after completion 1. Which of the following is not used for the formation of the molding sand? a) Silica Sand b) Binders c) Additives d) Coal 2. The ____________ is responsible for the impact strength in the molding sand. a) Aggregates b) Refractoriness c) Impurities d) Permeability 3. The property ensures the removal of excess sand in the mould box. a) Adhesiveness b) Cohesiveness c) Green strength d) Compressive strength 4. Binding property of the sand increases because of the property. a) Cohesiveness b) Collapsibility c) Flow ability d) Permeability  5. Which of the following is the most important property of the molding sand in cores? a) Dry Strength b) Green strength c) Collapsibility d) Cohesiveness Conclusion  Flask: A metal or wood frame, without fixed top or bottom, in which the mold is formed. Depending upon the position of the flask in the molding structure, it is referred to by various names such as drag – lower molding flask, cope – upper molding flask, cheek – intermediate molding flask used in three piece molding.  Pattern: It is the replica of the final object to be made. The mold cavity is made with the help of pattern.  Parting line: This is the dividing line between the two molding flasks that makes up the mold.  Molding sand: Sand, which binds strongly without losing its permeability to air or gases. It is a mixture of silica sand, clay, and moisture in appropriate proportions.  Facing sand: The small amount of carbonaceous material sprinkled on the inner surface of the mold cavity to give a better surface finish to the castings. Demo Videos http://youtube.com/watch?v=tB2ga9mISks References 1. Sharma, P.C., A textbook of Production Technology – Vol I and II, S.Chand and Company Ltd., New Delhi, 1996. 2. Rao, P.N., Manufacturing Technology, Vol I and II, Tata McGraw Hill Publishing Co., New Delhi, 1998. 3. Chapman W.A.J., Workshop Technology Vol. I and II, Arnold Publisher, New Delhi, 1998. 4. Hajra Choudhary, S.K. and Hajra Choudhary, A.K., Elements of Manufacturing Technology, Vol II, Media Publishers, Bombay, 1988. 5. Jain, R.K., Production Technology, Khanna Publishers, New Delhi, 1988. 6. Kalpakjian, Manufacturing and Technology, Addison Wesley Longman Pvt., Singapore, 2000.  Answers to the assignments with full explanation Assignment 1 1. Sprue: The passage through which the molten metal, from the pouring basin, reaches the mold cavity. In many cases it controls the flow of metal into the mold. 2. Runner: The channel through which the molten metal is carried from the sprue to the gate. 3. Gate: A channel through which the molten metal enters the mold cavity. 4. Chaplets: Chaplets are used to support the cores inside the mold cavity to take care of its own weight and overcome the metallostatic force. 5. Green-sand molds - mixture of sand, clay, and water; “Green" means mold contains moisture at time of pouring.  Course Material for Unit - I Name of the Course : Manufacturing Technology for Mechatronics Name of the Unit : Casting and Welding Name of the Topic : Moulding – types – Moulding sand, Gating and Risering, Cores and Core making.  Objectives: To study the concept of casting technology. 1. Outcomes: Upon successful completion, the student should be able to learn the process of metal casting. 2. Pre-requisites: To have a basic knowledge of Fundamentals of Basic Mechanical Engineering. 1. For melting of cast iron, which of the following furnace is used? a) Rotary furnace b) Cupola furnace c) Pit furnace d) Electric furnace 2. Why is sand bed prepared for melting of metal in Cupola furnace? a) It provides a refractory bottom b) It soaks the moisture from the metal c) It conducts the heat faster and uniformly d) None of the mentioned 3. What is the function of slag hole in a cupola furnace? a) To tap the slag generated b) To generate the slag c) To allow outflow of gases d) All of the mention 4. Which type of sand is used in shell moulding? a) Black sand b) Wet and fine sand c) Dry and fine sand d) Any of the sand 5. Which is the most widely used resin in shell moulding? a) Phenol formaldehyde b) Ethanol formaldehyde c) Phenol d) Ethanol 6. The machine used to mix shell mould particles consistently is a) Mixer b) Cupola furnace c) Mueller d) Mixer & Cupola furnace 7. Which of the following statement is true? a) The size of the casting can be adjustable b) The size of the casting is not limited c) The size of the casting obtained by shell moulding is limited d) None of the mentioned 8. Cylinder and cylinder heads for air cooled IC engines are casted using which technique? a) Precision investment casting b) Shell moulding c) Permanent mould casting d) Die casting 9. In precision investment casting, to make the mould the prepared pattern is dipped into a slurry. From which material is this slurry is made up of? a) Ethyl silicate b) Sodium silicate c) Both Ethyl and Sodium silicate d) Neither Ethyl nor Sodium silicate 10. Cores used in PMC are usually made up of? a) Metal b) Collapsible material c) Sand d) All of the mentioned 3. Core and Core Prints Castings are often required to have holes, recesses, etc. of various sizes and shapes. These impressions can be obtained by using cores. So where coring is required, provision should be made to support the core inside the mold cavity. Core prints are used to serve this purpose. The core print is an added projection on the pattern and it forms a seat in the mold on which the sand core rests during pouring of the mold. The core print must be of adequate size and shape so that it can support the weight of the core during the casting operation. Depending upon the requirement a core can be placed horizontal, vertical and can be hanged inside the mold cavity. A typical job, its pattern and the mold cavity with core and core print is shown in Figure 5. Figure 5: A Typical Job, its Pattern and the Mold Cavity Classification of casting Processes Casting processes can be classified into following FOUR categories: 1. Conventional Molding Processes a. Green Sand Molding b. Dry Sand Molding c. Flask less Molding 2. Chemical Sand Molding Processes a. Shell Molding b. Sodium Silicate Molding c. No-Bake Molding 3. Permanent Mold Processes a. Gravity Die casting b. Low and High Pressure Die Casting 4. Special Casting Processes a. Lost Wax b. Ceramics Shell Molding c. Evaporative Pattern Casting d. Vacuum Sealed Molding  e. Centrifugal Casting Green Sand Molding Green sand is the most diversified molding method used in metal casting operations. The process utilizes a mold made of compressed or compacted moist sand. The term "green" denotes the presence of moisture in the molding sand. The mold material consists of silica sand mixed with a suitable bonding agent (usually clay) and moisture. Advantages 1. Most metals can be cast by this method. 2. Pattern costs and material costs are relatively low. 3. No Limitation with respect to size of casting and type of metal or alloy used Disadvantages Surface Finish of the castings obtained by this process is not good and machining is often required to achieve the finished product. Sand Mold Making Procedure The procedure for making mold of a cast iron wheel is shown in (Figure 8(a),(b),(c)).  The first step in making mold is to place the pattern on the molding board.  The drag is placed on the board ((Figure 8(a)).  Dry facing sand is sprinkled over the board and pattern to provide a non-sticky layer.  Molding sand is then riddled in to cover the pattern with the fingers; then the drag is completely filled.  The sand is then firmly packed in the drag by means of hand rammers. The ramming must be proper i.e. it must neither be too hard or soft.  After the ramming is over, the excess sand is leveled off with a straight bar known as a strike rod.  With the help of vent rod, vent holes are made in the drag to the full depth of the flask as well as to the pattern to facilitate the removal of gases during pouring and solidification.  The finished drag flask is now rolled over to the bottom board exposing the pattern.  Cope half of the pattern is then placed over the drag pattern with the help of locating pins. The cope flask on the drag is located aligning again with the help of pins ( (Figure 8 (b)).  The dry parting sand is sprinkled all over the drag and on the pattern.  A sprue pin for making the sprue passage is located at a small distance from the pattern. Also, riser pin, if required, is placed at an appropriate place.  The operation of filling, ramming and venting of the cope proceed in the same manner as performed in the drag.  The sprue and riser pins are removed first and a pouring basin is scooped out at the top to pour the liquid metal.  Then pattern from the cope and drag is removed and facing sand in the form of paste is applied all over the mold cavity and runners which would give the finished casting a good surface finish.  The mold is now assembled. The mold now is ready for pouring (see ((Figure 8 (c) ) Figure 8 (a) Figure 8 (b) Figure 8 (c) Figure 8 (a, b, c): Sand Mold Making Procedure Molding Material and Properties A large variety of molding materials is used in foundries for manufacturing molds and cores. They include molding sand, system sand or backing sand, facing sand, parting sand, and core sand. The choice of molding materials is based on their processing properties. The properties that are generally required in molding materials are: Refractoriness It is the ability of the molding material to resist the temperature of the liquid metal to be poured so that it does not get fused with the metal. The refractoriness of the silica sand is highest. Permeability During pouring and subsequent solidification of a casting, a large amount of gases and steam is generated. These gases are those that have been absorbed by the metal during melting, air absorbed from the atmosphere and the steam generated by the molding and core sand. If these gases are not allowed to escape from the mold, they would be entrapped inside the casting and cause casting defects. To overcome this problem the molding material must be porous. Proper venting of the mold also helps in escaping the gases that are generated inside the mold cavity. Green Strength The molding sand that contains moisture is termed as green sand. The green sand particles must have the ability to cling to each other to impart sufficient strength to the mold. The green sand must have enough strength so that the constructed mold retains its shape. Dry Strength When the molten metal is poured in the mold, the sand around the mold cavity is quickly converted into dry sand as the moisture in the sand evaporates due to the heat of the molten metal. At this stage the molding sand must possess the sufficient strength to retain the exact shape of the mold cavity and at the same time it must be able to withstand the metallostatic pressure of the liquid material. Hot Strength As soon as the moisture is eliminated, the sand would reach at a high temperature when the metal in the mold is still in liquid state. The strength of the sand that is required to hold the shape of the cavity is called hot strength. Collapsibility The molding sand should also have collapsibility so that during the contraction of the solidified casting it does not provide any resistance, which may result in cracks in the castings. Besides these specific properties the molding material should be cheap, reusable and should have good thermal conductivity. Molding Sand Composition The main ingredients of any molding sand are: Base sand, Binder, and Moisture Base Sand Silica sand is most commonly used base sand. Other base sands that are also used for making mold are zircon sand, Chromite sand, and olivine sand. Silica sand is cheapest among all types of base sand and it is easily available. Binder Binders are of many types such as: 1. Clay binders, 2. Organic binders and 3. Inorganic binders Clay binders are most commonly used binding agents mixed with the molding sands to provide the strength. The most popular clay types are: Kaolinite or fire clay (Al2O3 2 SiO2 2 H2O) and Bentonite (Al2O3 4 SiO2 nH2O) Of the two the Bentonite can absorb more water which increases its bonding power. Moisture Clay acquires its bonding action only in the presence of the required amount of moisture. When water is added to clay, it penetrates the mixture and forms a microfilm, which coats the surface of each flake of the clay. The amount of water used should be properly controlled. This is because a part of the water, which coats the surface of the clay flakes, helps in bonding, while the remainder helps in improving the plasticity. A typical composition of molding sand is given in (Table 4).  Table 4: A Typical Composition of Molding Sand Molding Sand Constituent Weight Percent Silica sand 92 Clay (Sodium Bentonite) 8 Water 4 Gating System The assembly of channels which facilitates the molten metal to enter into the mold cavity is called the gating system (Figure17). Alternatively, the gating system refers to all passage ways through which molten metal passes to enter into the mold cavity. The nomenclature of gating system depends upon the function of different channels which they perform. Down gates or sprue Cross gates or runners In gates or gates The metal flows down from the pouring basin or pouring cup into the down gate or sprue and passes through the cross gate or channels and ingates or gates before entering into the mold cavity. Figure 17: Schematic of Gating System  Goals of Gating System The goals for the gating system are To minimize turbulence to avoid trapping gasses into the mold To get enough metal into the mold cavity before the metal starts to solidify To avoid shrinkage Establish the best possible temperature gradient in the solidifying casting so that the shrinkage if occurs must be in the gating system not in the required cast part. Incorporates a system for trapping the non-metallic inclusions. Riser Riser is a source of extra metal which flows from riser to mold cavity to compensate for shrinkage which takes place in the casting when it starts solidifying. Without a riser heavier parts of the casting will have shrinkage defects, either on the surface or internally. Risers are known by different names as metal reservoir, feeders, or headers. Shrinkage in a mold, from the time of pouring to final casting, occurs in three stages. 1. during the liquid state 2. during the transformation from liquid to solid 3. during the solid state First type of shrinkage is being compensated by the feeders or the gating system. For the second type of shrinkage risers are required. Risers are normally placed at that portion of the casting which is last to freeze. A riser must stay in liquid state at least as long as the casting and must be able to feed the casting during this time. Functions of Risers Provide extra metal to compensate for the volumetric shrinkage Allow mold gases to escape Provide extra metal pressure on the solidifying mold to reproduce mold details more exact Design Requirements of Risers 1. Riser size: For a sound casting riser must be last to freeze. The ratio of (volume / surface area)2 of the riser must be greater than that of the casting. However, when this condition does not meet the metal in the riser can be kept in liquid state by heating it externally or using exothermic materials in the risers. 2. Riser placement: the spacing of risers in the casting must be considered by effectively calculating the feeding distance of the risers. 3. Riser shape: cylindrical risers are recommended for most of the castings as spherical risers, although considers as best, are difficult to cast. To increase volume/surface area ratio the bottom of the riser can be shaped as hemisphere. Test after completion 1. Another name of Gravity Die Casting is? a) Centrifugal casting b) Permanent mould casting c) Precision investment casting d) Hot chamber process 2. In PIC, any wax remnants are dissolved with the help of hot vapors of a solvent. Which solvent is used for this purpose? a) Ethanol b) ChloroFluoroCarbon c) TriChloroEthylene d) Any of the above 3. Which of the following is a limitation of die casting, but is overcome in vacuum die casting? a) The air left in the cavity when the die is closed b) The moisture left in the cavity when the die is closed c) The air left in the cavity when the die is open d) The moisture left in the cavity when the die is open 4. Pattern used in shell moulding is normally made up of? a) Wax b) Metal c) Wood d) Plastic  5. The casting process that does not require a core to produce a hollow casting is? a) Shell moulding b) Hot-chamber die casting c) Permanent mould casting d) True centrifugal casting Conclusion  When it is desired that the gas forming materials are lowered in the molds, air-dried molds are sometimes preferred to green sand molds.  In skin drying a firm mold face is produced. Shakeout of the mold is almost as good as that obtained with green sand molding. The most common method of drying the refractory mold coating uses hot air, gas or oil flame.  Skin drying of the mold can be accomplished with the aid of torches, directed at the mold surface.  Core: A separate part of the mold, made of sand and generally baked, which is used to create openings and various shaped cavities in the castings.  Pouring basin: A small funnel shaped cavity at the top of the mold into which the molten metal is poured. Demo Videos http://youtube.com/watch?v=M95bhPrDwA0 References 1. Sharma, P.C., A textbook of Production Technology – Vol I and II, S.Chand and Company Ltd., New Delhi, 1996. 2. Rao, P.N., Manufacturing Technology, Vol I and II, Tata McGraw Hill Publishing Co., New Delhi, 1998. 3. Chapman W.A.J., Workshop Technology Vol. I and II, Arnold Publisher, New Delhi, 1998. 4. Hajra Choudhary, S.K. and Hajra Choudhary, A.K., Elements of Manufacturing Technology, Vol II, Media Publishers, Bombay, 1988. 5. Jain, R.K., Production Technology, Khanna Publishers, New Delhi, 1988. 6. Kalpakjian, Manufacturing and Technology, Addison Wesley Longman Pvt., Singapore, 2000.  Answers to the assignments with full explanation Assignment 2 1. Riser: A column of molten metal placed in the mold to feed the castings as it shrinks and solidifies. Also known as “feed head”. 2. Vent: Small opening in the mold to facilitate escape of air and gases. 3. Sand Casting (Green sand mould) is simply melting the metal and pouring it into a preformed cavity, called mold, allowing (the metal to solidify and then breaking up the mold to remove casting. In sand casting expandable molds are used. So for each casting operation you have to form a new mold. 4. The cavity in the sand mold is formed by packing sand around a pattern, separating the mould into two halves. The mold must also contain gating and riser system For internal cavity, a core must be included in mold. 5. A new sand mold must be made for each part:  Pour molten metal into sand mold  Allow metal to solidify  Break up the mold to remove casting  Clean and inspect casting.  Course Material for Unit - I Name of the Course : Manufacturing Technology for Mechatronics Name of the Unit : Casting and Welding Name of the Topic : Special Casing Process – Shell Investment, Die casting and Centrifugal Casting.  Objectives: To study the concept of casting technology. 1. Outcomes: Upon successful completion, the student should be able to learn the process of metal casting. 2. Pre-requisites: To have a basic knowledge of Fundamentals of Basic Mechanical Engineering. 1. Hollow casting is the other name of which of the following special casting process? a) Slush casting b) Vacuum die casting c) Precision investment casting d) Squeeze casting 2. Squeeze casting method was developed in which country? a) Japan b) India c) America d) Russia 3. The tundish is a a) Pouring vessel b) Riser c) Type of core d) Machine name 4. Which of the type of centrifugal casting methods is used to produce ‘non- symmetrical’ shaped castings? a) True centrifugal casting b) Semi centrifugal casting c) Centrifuging d) Non centrifugal casting 5. True centrifugal casting method is usually used to make a) Bent pipes b) Hollow pipes c) Bolts d) Nuts 6. Pressure range for low pressure die casting is a) 0.3-1.5 bars b) 0.5-2 bars c) 2-6 bars d) up to 8 bars 7. Which of the following forces provides continuous pressure on the metal in centrifugal casting? a) Spring force b) Centrifugal force c) Gravitational force d) Frictional force 8. Which of the following methods of casting is best suited for casting of hollow pipes and tubes? a) Investment casting b) Permanent mould casting c) Die casting d) Centrifugal casting 9. Which of the following parts is provided in between the mould and casing to reduce the vibrations? a) Steel balls b) Metallic roller c) Viscous fluid d) Grease 10. Which of the following types of centrifugal casting process is used for the casting, whose shape of casting is not axi- symmetric? a) True centrifugal casting b) Semi centrifugal casting c) Centrifuging d) Full centrifugal casting 3. Investment casting process The investment casting process begins with the production of wax replicas of the desired castings. These replicas, called patterns, are injection molded in metal dies. A pattern must be manufactured for each casting to be produced. A number of patterns (depending on size and complexity) are attached to a central wax stick, or sprue, to form a casting cluster, or assembly. After some initial pre-dips, which thoroughly clean the wax, the assemblies are immersed, or “invested,’’ then into a bed of extremely fine sand to form a shell. The first critical layers are often applied by hand. Between each layer the ceramic is allowed to dry. The later, heavier layers are often applied by automated equipment or special shell building robots. Enough layers must be applied to build a shell strong enough to withstand subsequent operations. After the shell is completely dry, the wax is melted out in a high pressure steam autoclave, leaving a hollow void within the mold, which exactly matches the shape of the assembly. Prior to casting, the shells are fired in an oven where intense heat burns out any remaining wax residue and prepares the mold for the molten metal. In the conventional gravity pouring method, metal is poured into the shell through a funnel shaped pour cup and flows by gravity down the sprue channel, through the gates and into the part cavities. As the metal cools, the parts, gates, sprue and pouring cup become one solid casting. After the casting has cooled, the ceramic shell is broken off and the parts are cut from the sprue using a high speed friction saw. After minor finishing operations, the castings, which are identical in configuration to the wax patterns which shaped them, are ready for certification and shipment to the customer. Advantages: Parts of great complexity and intricacy can be cast. Close dimensional control and good surface finish. Wax can usually be recovered for reuse. Disadvantages: Many processing steps are required. Relatively expensive process. 4. Shell moulding process Steps in shell-moulding: Shell-mould casting yields better surface quality and tolerances. The process is described as follows the 2-piece pattern is made of metal (e.g. aluminium or steel), it is heated to between 175oC – 370oC, and coated with a lubricant, e.g. silicon spray. Each heated half-pattern is covered with a mixture of sand and a thermoset resin/epoxy binder. The binder glues a layer of sand to the pattern, forming a shell. The process may be repeated to get a thicker shell. The assembly is baked to cure it. The patterns are removed, and the two half-shells joined together to form the mould; metal is poured into the mould. When the metal solidifies, the shell is broken to get the part. Smoother cavity surface permits easier flow of molten metal and better surface finish on casting. Advantages: Better surface finish Better dimensional tolerances. Reduced machining Less foundry space required.  Semi skilled operators can handle the process The process can be mechanized. Disadvantages: The raw materials are relatively expensive. The process generates noxious fumes which must be removed. The size and weight range of castings is limited. Applications: Crankshaft fabrication Steel casting parts, fittings Moulded tubing fabrication Hydraulic control housing fabrication Automotive castings (cylinder head and ribbed cylinder) fabrication. 5. Centrifugal casting process Casting, or reforming materials by heating, melting and molding can be traced back in history six thousand years. As civilization progressed and the use of metals became more advanced, the technology of casting metals advanced as well. As foundry industries began to demand higher yields and better physical properties from cast metal products, casting processes became more specialized. The centrifugal casting method was developed after the turn of the 20th century to meet the need for higher standards. Spinning molds generate centrifugal force on molten metal to position the metal within a mold. As the molten metal solidifies from the outside in, a casting with dense, close grain structure is created. As a result of close grain structure the centrifugal process offers products with better physical properties than castings made using the static casting method. Other advantages of products made by the centrifugal process are: Smoother surface Lighter weight Thinner walls. Combining proper mold design, mold coatings, mold spinning speeds, pouring speeds, cooling rates and metal chemistry results in castings with higher yields, fewer impurities and greater strength. Advantages: Formation of hollow interiors in cylinders without cores. Less material required for gate. Fine grained structure at the outer surface of the casting free of gas and shrinkage cavities and porosity. Disadvantages: More segregation of alloy component during pouring under the forces of rotation. Contamination of internal surface of castings with non-metallic inclusions. Inaccurate internal diameter. 6. Full Mold Process / Lost Foam Process / Evaporative Pattern Casting Process The use of foam patterns for metal casting was patented by H.F. Shroyer on April 15, 1958. In Shroyer's patent, a pattern was machined from a block of expanded polystyrene (EPS) and supported by bonded sand during pouring. This process is known as the full mold process. With the full mold process, the pattern is usually machined from an EPS block and is used to make primarily large, one-of-a kind castings. The full mold process was originally known as the lost foam process. However, current patents have required that the generic term for the process be full mold. In 1964, M.C. Flemmings used unbounded sand with the process. This is known today as lost foam casting (LFC). With LFC, the foam pattern is moulded from polystyrene beads. LFC is differentiated from full mold by the use of unbounded sand (LFC) as opposed to bonded sand (full mold process). Foam casting techniques have been referred to by a variety of generic and proprietary names. Among these are lost foam, evaporative pattern casting, cavity less casting, evaporative foam casting, and full mold casting. In this method, the pattern, complete with gates and risers, is prepared from expanded polystyrene. This pattern is embedded in a no bake type of sand. While the pattern is inside the mold, molten metal is poured through the sprue. The heat of the metal is sufficient to gasify the pattern and progressive displacement of pattern material by the molten metal takes place. The EPC process is an economical method for producing complex, close-tolerance castings using an expandable polystyrene pattern and unbonded sand. Expandable polystyrene is a thermoplastic material that can be moulded into a variety of complex, rigid shapes. The EPC process involves attaching expandable polystyrene patterns to an expandable polystyrene gating system and applying a refractory coating to the entire assembly. After the coating has dried, the foam pattern assembly is positioned on loose dry sand in a vented flask. Additional sand is then added while the flask is vibrated until the pattern assembly is completely embedded in sand. Molten metal is poured into the sprue, vaporizing the foam polystyrene, perfectly reproducing the pattern. In this process, a pattern refers to the expandable polystyrene or foamed polystyrene part that is vaporized by the molten metal. A pattern is required for each casting. Process Description ((Figure 12) 1. The EPC procedure starts with the pre-expansion of beads, usually polystyrene. After the pre-expanded beads are stabilized, they are blown into a mold to form pattern sections. When the beads are in the mold, a steam cycle causes them to fully expand and fuse together. 2. The pattern sections are assembled with glue, forming a cluster. The gating system is also attached in a similar manner. 3. The foam cluster is covered with a ceramic coating. The coating forms a barrier so that the molten metal does not penetrate or cause sand erosion during pouring. 4. After the coating dries, the cluster is placed into a flask and backed up with bonded sand. 5. Mold compaction is then achieved by using a vibration table to ensure uniform and proper compaction. Once this procedure is complete, the cluster is packed in the flask and the mold is ready to be poured.  Figure 12: The Basic Steps of the Evaporative Pattern Casting Process Advantages The most important advantage of EPC process is that no cores are required. No binders or other additives are required for the sand, which is reusable. Shakeout of the castings in unbonded sand is simplified. There are no parting lines or core fins. Test after completion 1. Which of the following moulds or moulding is also known as sodium silicate process. a) Shell moulding b) Permanent moulding c) Slush moulding d) CO2 moulding 2. How much percentage of sodium silicate (Na2SiO3) is added to the sand mixture in Co2 moulding? a) 0 to 2 % b) 2 to 6 % c) 6 to 10 % d) 10 to 14 % 3. How much time (in a minute) is usually required for the passing of Co2 through the mould? a) One b) Two c) Five d) Seven 4. Which of the following additives are added to the sand in Co2 moulding for the improvement in collapsibility of the sand? a) Copper oxide b) Wood flour c) Aluminium oxide d) Oil 5. Which of the following defect is not a gas defect? a) Blow holes b) Air inclusions c) Run out d) Pin hole porosity  Conclusion  The molding sand is a mixture of fine grained quartz sand and powdered bakelite. There are two methods of coating the sand grains with bakelite. First method is Cold coating method and another one is the hot method of coating.  In the method of cold coating, quartz sand is poured into the mixer and then the solution of powdered bakelite in acetone and ethyl aldehyde are added. The typical mixture is 92% quartz sand, 5% bakelite, 3% ethyl aldehyde. During mixing of the ingredients, the resin envelops the sand grains and the solvent evaporates, leaving a thin film that uniformly coats the surface of sand grains, thereby imparting fluidity to the sand mixtures.  In the method of hot coating, the mixture is heated to 150-180 o C prior to loading the sand. In the course of sand mixing, the soluble phenol formaldehyde resin is added. The mixer is allowed to cool up to 80 – 90 o C. This method gives better properties to the mixtures than cold method.  The process in which we use a die to make the castings is called permanent mold casting or gravity die casting, since the metal enters the mold under gravity. Some time in die-casting we inject the molten metal with a high pressure.  The permanent mold process is also capable of producing a consistent quality of finish on castings Demo Videos http://youtube.com/watch?v=I2yM1WmHzRs References 1. Sharma, P.C., A textbook of Production Technology – Vol I and II, S.Chand and Company Ltd., New Delhi, 1996. 2. Rao, P.N., Manufacturing Technology, Vol I and II, Tata McGraw Hill Publishing Co., New Delhi, 1998. 3. Chapman W.A.J., Workshop Technology Vol. I and II, Arnold Publisher, New Delhi, 1998. 4. Hajra Choudhary, S.K. and Hajra Choudhary, A.K., Elements of Manufacturing Technology, Vol II, Media Publishers, Bombay, 1988. 5. Jain, R.K., Production Technology, Khanna Publishers, New Delhi, 1988. 6. Kalpakjian, Manufacturing and Technology, Addison Wesley Longman Pvt., Singapore, 2000.  Answers to the assignments with full explanation Assignment 3 1. The root of the investment casting process, the cire perdue or “lost wax” method dates back to at least the fourth millennium B.C. The artists and sculptors of ancient Egypt and Mesopotamia used the rudiments of the investment casting process to create intricately detailed jewelry, pectorals and idols. 2. The investment casting process alos called lost wax process begins with the production of wax replicas or patterns of the desired shape of the castings. A pattern is needed for every casting to be produced. The patterns are prepared by injecting wax or polystyrene in a metal dies. A number of patterns are attached to a central wax sprue to form a assembly. 3. The mold is prepared by surrounding the pattern with refractory slurry that can set at room temperature. The mold is then heated so that pattern melts and flows out, leaving a clean cavity behind. The mould is further hardened by heating and the molten metal is poured while it is still hot. When the casting is solidified, the mold is broken and the casting taken out. 4. The basic difference in investment casting is that in the investment casting the wax pattern is immersed in a refractory aggregate before dewaxing whereas, in ceramic shell investment casting a ceramic shell is built around a tree assembly by repeatedly dipping a pattern into a slurry (refractory material such as zircon with binder). 5. After each dipping and stuccoing is completed, the assembly is allowed to thoroughly dry before the next coating is applied. Thus, a shell is built up around the assembly. The thickness of this shell is dependent on the size of the castings and temperature of the metal to be poured.  Course Material for Unit - I Name of the Course : Manufacturing Technology for Mechatronics Name of the Unit : Casting and Welding Name of the Topic : Special welding – Laser, Electron Beam, Ultrasonic, Electro slag, Friction welding and electrical resistance welding.  Objectives: To study the concept of casting technology. 1. Outcomes: Upon successful completion, the student should be able to learn the process of metal casting. 2. Pre-requisites: To have a basic knowledge of Fundamentals of Basic Mechanical Engineering. 1. Upon which of the following parameters does the current intensity in arc welding depend? a) Stability of arc b) Electrode diameter c) Gap between the electrode and parent metals d) Thickness of parent metals 2. In which of the following welding processes we use two non-consumable electrodes? a) MIG b) TIG c) Atomic hydrogen d) Submerged arc 3. Which of the following brazing process is good for mass scale joining? a) Furnace b) Induction c) Dip d) Torch 4. For grey cast iron, which of the following welding methods is preferable? a) MIG b) Submerged arc c) Gas flame d) Electric arc 5. Due to which of the following reasons, flux is not used in atomic hydrogen welding? a) The burning hydrogen shields the molten metal b) Two electrodes are coated which gradually release the flux c) The filler rod is coated with flux d) One of the two electrodes is coated which releases the flux 6. In resistance welding, between the electrodes, the nature of current and voltage parameters being used? a) high current, high voltage b) low current, high voltage c) low current, low voltage d) high current, low voltage 7. Which of the following welding process in which heat is produced for welding by a chemical reaction? a) Resisting welding b) Thermit welding c) Forge welding d) Gas welding 8. The maximum diameter of electrodes being used in submerged arc welding? a) 30 mm b) 20 mm c) 15 mm d) 10 mm 9. In arc welding, arc is created between the electrode and work by? a) Contact resistance b) Flow of voltage c) Flow of current d) Electrical energy 10. The coating material used for the electrode is termed as? a) Flux b) Slag c) Protective layer d) Deoxidizer 3. Electro slag welding process Electro slag welding process which produces coalescence through electrically melted flux which melts both the filler metal and the surface of the work piece to be melted. Welding is initiated on a starting block at the bottom of the vertically positioned joint. Flux poured around the electrode is converted to slag that floats on a layer of molten metal confined in the joint by water cooled copper shoes that slides on the sides. The heat of the fusion is provided by resistance heating in the slag. The welding dams and head move upward as weld metal solidifies and new metal is fed in by the wire electrodes. The consumable wire electrode may be solid or flux coated but most of the shielding is provided by an argon and CO2 gas mixture injected into the gap. The heat is furnished by an electrical arc between the electrode and metal pool. It is used particularly for thick plates and structures for turbine shafts, boiler parts. Advantages of Electro slag welding: High deposition rate Low slag consumption Low distortion Unlimited thickness of work piece. Disadvantages of Electro slag welding: Coarse grain structure of the weld Low toughness of the weld Vertical position possible only.  Applications: Construction of bridges, pressure vessels. Construction of thick walled and large diameter pipes. Construction of thick storage tanks and ships. 4. Resistance seam welding Seam welding is a method of making a continuous joint between two over lapping pieces of sheet metal. The normal procedure for making seam welding is to place the work between the wheels which serve as producing continuous welds. As pressure is applied, the drive started and the welding current switched on. Then at the same time, the over lapping surfaces of metal are forced together as fast as they are heated. A coolant is applied to conserve the electrodes and cool the work rapidly to speed the operation. The materials that may be seam welded include most of those that may be spot welded. Steel plates 10mm thick have been seam welded to hold about 200kg/cm2 pressure. Advantages of resistance seam welding: No filler metal required. High production rates possible. Lends itself to mechanization and automation. Lower operator skill level than for arc welding. Good repeatability and reliability. Disadvantages of resistance seam welding: High initial equipment cost. Limited to lap joints for most seam welding processes. 5. Friction stir welding The frictional energy generated when two bodies slide on each other is transformed into heat; when the rate of movement is high and the heat is contained in a narrow zone, welding occurs. In practically one part is firmly held while the other is rotated under simultaneous application of axial pressure. The temperature rises, partially formed welded spots are sheared, surface films are disrupted, and the rotation is suddenly arrested when the entire surface is welded. Some of the softened metal is squeezed out into a flash, but it is not fully clear whether melting takes place. The heated zone being very thin, dissimilar metals are easily joined. Limitations: At least one of the parts must be rotational. Flash must usually be removed. Upsetting reduces the part lengths (which must be taken into consideration in product design) Applications: Shafts and tubular parts. Industries: automotive, aircraft, farm equipment, petroleum and natural gas. 6. Ultrasonic welding process Ultrasonic welding is the joining or reforming of thermoplastics through the use of heat generated from high-frequency mechanical motion. It is accomplished by converting high-frequency electrical energy into high-frequency mechanical motion. That mechanical motion, along with applied force, creates frictional heat at the plastic components’ mating surfaces (joint area) so the plastic material will melt and form a molecular bond between the parts. The two thermoplastic parts to be assembled are placed together, one on top of the other, in a supportive nest called a fixture. A titanium or aluminium component called a horn is brought into contact with the upper plastic part. A controlled pressure is applied to the parts, clamping them together against the fixture. The horn is vibrated vertically 20,000 Hz(20kHz) or 40,000 Hz(40kHz) times per second, at distances measured in thousandths of an inch(microns), for a predetermined amount of time called weld time. Through careful part design, this vibratory mechanical energy is directed to limited points of contact between the two parts. The mechanical vibrations are transmitted through the thermoplastic materials to the joint interface to create frictional heat. When the temperature at the joint interface reaches the melting point, plastic melts and flows, and the vibration is stopped. This allows the melted plastic to begin cooling. The clamping force is maintained for a predetermined amount of time to allow the parts to fuse as the melted plastic cools and solidifies. Once the melted plastic has solidified, the clamping force is removed and the horn is retracted. The two plastic parts are now joined as if moulded together and are removed from the fixture as one part. Advantages: Fast, economical and easily automated. Mass production can be made.  Increased flexibility and versatility. Possibility to join large structures. Disadvantages: Large joints cannot be weld in a single operation. Specifically designed joints are required. Tooling costs for fixtures are high. Test after completion 1. Which of the following welding process in which two pieces to be joined are overlapped and placed between two pointed electrodes? a) Seam welding b) Resistance welding c) Projection welding d) Spot welding 2. Which of the following gases are used in Tungsten inert gas welding? a) Helium and neon b) Hydrogen and oxygen c) Argon and helium d) Carbon dioxide and hydrogen 3. Which kind of resistance is experienced in upset butt welding? a) Electric resistance b) Magnetic resistance c) Thermal resistance d) Air resistance 4. Which of the following can be easily be welded from flash butt welding process? a) Tin b) Lead c) Cast irons d) Carbon steel 5. Electrodes used in spot welding are made up of which material? a) Only Copper b) Copper and tungsten c) Copper and chromium d) Copper and aluminium Conclusion  Resistance welding processes are pressure welding processes in which heavy current is passed for short time through the area of interface of metals to be joined. These processes differ from other welding processes in the respect that no fluxes are used, and filler metal rarely used.  All resistance welding operations are automatic and, therefore, all process variables are pre-set and maintained constant. Heat is generated in localized area which is enough to heat the metal to sufficient temperature, so that the parts can be joined with the application of pressure. Pressure is applied through the electrodes.  Spot welding electrodes of different shapes are used. Pointed tip or truncated cones with an angle of 120° - 140° are used for ferrous metal but with continuous use they may wear at the tip.  Domed electrodes are capable of withstanding heavier loads and severe heating without damage and are normally useful for welding of nonferrous metals. The radius of dome generally varies from 50-100 mm.  Most of the industrial metal can be welded by spot welding, however, it is applicable only for limited thickness of components. Demo Videos http://youtube.com/watch?v=TlhGTSDfQxc  References 1. Sharma, P.C., A textbook of Production Technology – Vol I and II, S.Chand and Company Ltd., New Delhi, 1996. 2. Rao, P.N., Manufacturing Technology, Vol I and II, Tata McGraw Hill Publishing Co., New Delhi, 1998. 3. Chapman W.A.J., Workshop Technology Vol. I and II, Arnold Publisher, New Delhi, 1998. 4. Hajra Choudhary, S.K. and Hajra Choudhary, A.K., Elements of Manufacturing Technology, Vol II, Media Publishers, Bombay, 1988. 5. Jain, R.K., Production Technology, Khanna Publishers, New Delhi, 1988. 6. Kalpakjian, Manufacturing and Technology, Addison Wesley Longman Pvt., Singapore, 2000. Answers to the assignments with full explanation Assignment 4 1. Gas metal arc welding (GMAW) is the process in which arc is struck between bare wire electrode and work piece. The arc is shielded by a shielding gas and if this is inert gas such as argon or helium then it is termed as metal inert gas (MIG) and if shielding gas is active gas such as CO2 or mixture of inert and active gases then process is termed as metal active gas (MAG) welding. 2. Direct current flat characteristic power source is the requirement of GMAW process. The electrode wire passing through the contact tube is to be connected to positive terminal of power source so that stable arc is achieved. If the electrode wire is connected to negative terminal then it shall result into unstable spattery arc leading to poor weld bead. Flat characteristic leads to self-adjusting or self- regulating arc leading to constant arc length due to relatively thinner electrode wires. 3. GMA welding requires consumables such as filler wire electrode and shielding gas. Solid filler electrode wires are normally employed and are available in sizes 0.8, 1.0, 1.2 and 1.6 mm diameter. Similar to submerged arc welding electrode wires of mild steel and low alloyed steel, are coated with copper to avoid atmospheric corrosion, increase current carrying capacity and for smooth movement through contact tube. 4. Tungsten Inert Gas (TIG) or Gas Tungsten Arc (GTA) welding is the arc welding process in which arc is generated between non consumable tungsten electrode and work piece. The tungsten electrode and the weld pool are shielded by an inert gas normally argon and helium. 5. The tungsten arc process is being employed widely for the precision joining of critical components which require controlled heat input. The small intense heat source provided by the tungsten arc is ideally suited to the controlled melting of the material.  Course Material for Unit - II Name of the Course : Manufacturing Technology for Mechatronics Name of the Unit : Mechanical Working of Metals Name of the Topic : Hot and Cold Working: Rolling, Forging, Wire Drawing, Extrusion – types – Forward, backward and tube extrusion  Objectives: To study the various ways of working of metals. 1. Outcomes: Upon successful completion, the student should be able to explain the concept of different metal forming operations. 2. Pre-requisites: To have a basic knowledge of Fundamentals of Basic Mechanical Engineering. 1. Which of the following processes is mainly used for making the connecting rods? a) Casting b) Deep drawing c) Rolling d) Forging 2. Which of following is necessary in order to have a good set of rolls? a) Short horizontal distances b) Small lead-in flanges c) A smooth flow of material d) Less number of forming passes 3. Which of the following is true in case of “too quick” forming (too few passes)? a) It makes the tooling and the process uneconomical b) It makes process easier c) It distorts the product d) Eliminate the need of a skilled operator 4. What is “leg length” or “leg height”? a) The maximum vertical dimension of the profile b) Total length of the product c) Excess length of the product d) Maximum width of the section 6. What will be the elongation if a 1in high section is formed in four passes in a mill having a horizontal distance (between passes) of 14in? a) 0.04% b) 0.16% c) 0.63% d) 0.50% 7. What is camber in roll forming? a) Deviation of the strip edge from a straight line in horizontal plane b) Deviation of the strip edge from a straight line in vertical plane c) Difference in theoretical and actual elongation of the strip d) Waviness of the strip 8. When an asymmetrical section is roll formed, the finished product will have a) Camber b) Cross-bow c) A twist d) Waviness 9. In which process the cross section of the metal is reduced by forcing it to flow through a die under high pressure? a) Forging b) Forming c) Extrusion d) Welding 10. Which of the following is a type of extrusion process? a) Direct b) Indirect c) Impact d) All of the mentioned 3. Hot Working: Plastic deformation of metal carried out at temperature above the recrystallization temperature, is called hot working. Under the action of heat and force, when the atoms of metal reach a certain higher energy level, the new crystals start forming. This is called recrystallization. When this happens, the old grain structure deformed by previously carried out mechanical working no longer exist, instead new crystals which are strain free are formed. In hot working, the temperature at which the working is completed is critical since any extra heat left in the material after working will promote grain growth, leading to poor mechanical properties of material. In comparison with cold working, the advantages of hot working are 1. No strain hardening. 2. Lesser forces are required for deformation. 3. Greater ductility of material is available, and therefore more deformation is possible. 4. Favourable grain size is obtained leading to better mechanical properties of material. 5. Equipment of lesser power is needed. 6. No residual stresses in the material. Some disadvantages associated in the hot-working of metals are: 1. Heat energy is needed. 2. Poor surface finish of material due to scaling of surface. 3. Poor accuracy and dimensional control of parts. 4. Poor reproducibility and interchangeability of parts. 5. Handling and maintaining of hot metal is difficult and troublesome. 6. Lower life of tooling and equipment. 4. Cold Working: Plastic deformation of metals below the recrystallization temperature is known as cold working. It is generally performed at room temperature. In some cases, slightly elevated temperatures may be used to provide increased ductility and reduced strength. Cold working offers a number of distinct advantages, and for this reason various cold-working processes have become extremely important. Significant advances in recent years have extended the use of cold forming, and the trend appears likely to continue. In comparison with hot working, the advantages of cold working are 1. No heating is required. 2. Better surface finish is obtained. 3. Better dimensional control is achieved; therefore no secondary machining is generally needed. 4. Products possess better reproducibility and interchangeability. 5. Better strength, fatigue, and wear properties of material. 6. Directional properties can be imparted. 7. Contamination problems are almost negligible. Disadvantages associated with cold-working processes are: 1. Higher forces are required for deformation. 2. Heavier and more powerful equipment is required. 3. Less ductility is available. 4. Metal surfaces must be clean and scale-free. 5. Strain hardening occurs (may require intermediate annealing). 6. Undesirable residual stresses may be produced. Cold forming processes, in general, are better suited to large-scale production of parts because of the cost of the required equipment and tooling.  5. Wire Drawing Wire drawing is primarily the same as bar drawing except that it involves smaller- diameter material that can be coiled. It is generally performed as a continuous operation on draw bench like the one shown in figure. Large coil of hot rolled material of nearly 10mm diameter is taken and subjected to preparation treatment before the actual drawing process. The preparation treatment for steel wire consists of:  Cleaning. This may be done by acid pickling, rinsing, and drying. Or, it may be done by mechanical flexing.  Neutralization. Any remaining acid on the raw material is neutralized by immersing it in a lime bath. The corrosion protected material is also given a thin layer of lubricant. To begin the drawing process, one end of coil is reduced in cross section up to some length and fed through the drawing die, and gripped. A wire drawing die is generally made of tungsten carbide and has the configuration shown in figure. for drawing very fine wire, diamond die is preferred. Small diameter wire is generally drawn on tandem machines which consist of a series of dies, each held in a water cooled die block. Each die reduces the cross section by a small amount so as to avoid excessive strain in the wire. Intermediate annealing of material between different states of wire may also be done, if required. Tube Drawing The diameter and wall thickness of tubes that have been produced by extrusion or other processes can be reduced by tube drawing process. The process of tube drawing is similar to wire or rod drawing except that it usually requires a mandrel of the requisite diameter to form the internal hole. Tubes as large as 0.3m in diameter can be drawn. This is also called as tube sinking. In tube sinking method there is no control over the thickness of the tube. To overcome this drawback, mandrels are used in the process. In this method, the mandrel is fixed and attached to a long support bar to produce inside diameter and wall thickness during the process. 6. Extrusion Extrusion is the process by which long straight metal parts can be produced. The cross- sections that can be produced vary from solid round, rectangular, to L shapes, T shapes. Tubes and many other different types. Extrusion is done by squeezing metal in a closed cavity through a tool, known as a die using either a mechanical or hydraulic press. Extrusion produces compressive and shear forces in the stock. No tensile is produced, which makes high deformation possible without tearing the metal. The cavity in which the raw material is contained is lined with a wear resistant material. This can withstand the high radial loads that are created when the material is pushed the die. Cold Extrusion: Cold extrusion is the process done at room temperature or slightly elevated temperatures. This process can be used for most materials – subject to designing robust enough tooling that can withstand the stresses created by extrusion. Examples of the metals that can be extruded are lead, tin, aluminium alloys, copper, titanium, molybdenum, vanadium, steel. Examples of parts that are cold extruded are collapsible tubes, aluminium cans, cylinders, gear blanks. The advantages of cold extrusion are:  No oxidation takes place.  Good mechanical properties due to severe cold working as long as the temperatures created are below the re-crystallization temperature.  Good surface finish with the use of proper lubricants. Hot Extrusion: Hot extrusion is done at fairly high temperatures, approximately 50 to 75% of the melting point of the metal. The pressures can range from 35-700MPa (5076-101,525psi). Due to the high temperatures and pressures and its detrimental effect on the die life as well as other components, good lubrication is necessary. Oil and graphite work at lower temperatures, whereas at higher temperatures glass powder is used. Typical parts produced by extrusions are trim parts used in automotive and construction applications, window frame members, railings, aircraft structural parts. Direct Extrusion or Forward Extrusion The equipment consists of a cylinder or container into which the heated metal billet is loaded. One end of the container, the die plate with necessary opening is fixed. From the other end plunger or ram compresses the metal billet against the container walls and die plate, thus the forcing it to flow of metal in the forward direction through the die opening. Acquiring the shape of the opening the extruded metal is then carried by the metal is then carried to the metal handling system as it comes out of the die. A dummy block which is a steel disc of about 40mm thick with a diameter slightly less than container is kept between the hot billet and the ram to protect it from heat and pressure. In direct extrusion, the problem of friction prevalent because of the relative motion between heated metal billet and cylinder walls. To reduce this friction lubricants are to be used. To reduce the damage to equipment, extrusion is finished quickly and the cylinder is cooled before further extrusion. Indirect extrusion or backward extrusion In order to completely overcome the problem, the backward hot extrusion as shown in figure, in this process the metal is confined fully by the cylinder, the ram which houses the die also compresses the metal against the container, forcing it to flow backward to the die in the hollow plunger or ram. It is termed backward because of the opposite direction of the flow of the metal. Thus the billet in the container remains stationary and hence produces no friction. Also the extrusion pressure is not affected by the length. In the extrusion press since the friction is not loss. The figure of the backward extrusion is shown. 7. Forging Forging is a process in which material is shaped by the application of localized compressive forces exerted manually or with power hammers, presses or special forging machines. The process may be carried out on materials in either hot or cold state. When forging is done cold, processes are given special names. Therefore, the term forging usually implies hot forging carried out at temperatures which are above the recrystallization temperature of the material. Forging is an effective method of producing many useful shapes. The process is generally used to produce discrete parts. Typical forged parts include rivets, bolts, crane hooks, connecting rods, gears, turbine shafts, hand tools, railroads, and a variety of structural components used to manufacture machinery. The forged parts have good strength and toughness; they can be used reliably for highly stressed and critical applications. A variety of forging processes have been developed that can be used for either producing a single piece or mass – produce hundreds of identical parts. Some common forging processes are: 1. Open – die hammer forging 2. Impression – die drop forging 3. Press Forging 4. Upset Forging 5. Swaging 6. Rotary Forging 7. Roll forging Open – Die Hummer Forging. It is the simplest forging process which is quite flexible but not suitable for large scale production. It is a slow process. The resulting size and shape of the forging are dependent on the skill of the operator. Fig 2.1 Open die forging does not confine the flow of metal, Fig 2.1. The operator obtains the desired shape of forging by manipulating the work material between blows. Use may be made of some specially shaped tools or a simple shaped die between the work piece and the hammer or anvil to assist in shaping the required sections (round, concave, or convex), making holes, or performing cut – off operations. This process is most often used to make near – final shape of the part so that some further operation done on the job produces the final shape. Impression – Die Drop Forging (Closed – Die Forging) The process uses shaped dies to control the flow of metal. The heated metal is positioned in the lower cavity and on it one or more blows are struck by the upper die. This hammering makes the metal to flow and fill the die cavity completely. Excess metal is squeezed out around the periphery of the cavity to form flash. On completion of forging, the flash is trimmed off with the help of a trimming die. Most impression – die sets contain several cavities. The work material is given final desired shape in stages as it is deformed in successive cavities in the die set. The shape of the cavities cause the metal to flow in desired direction, thereby imparting desired fibre structure to the component. Auto – Forging: This is a modified form of impression – die forging, used mainly for non – ferrous metals. In this a cast preform, as removed from the mold while hot, is finish – forged in a die. The flash formed during die forging is trimmed later in the usual manner. As the four steps of the process – casting, transfer from mold to the forging die, forging, and trimming are in most applications completely mechanized, the process has acquired the name Auto – forging. Coining: It is a closed – die forging process used mainly for minting coins and making of jewellery. In order to produce fine details on the work material the pressures required are as large as five or six times the strength of the material. Lubricants are not employed in this process because they can get entrapped in the die cavities and, being incompressible, prevent the full reproduction of fine details of the die. Net - shape Forging (Precession Forging) Modern trend in forging operation is toward economy and greater precision. The metal is deformed in cavity so that no flash is formed and the final dimensions are very close to the desired component dimensions. There is minimum wastage of material and need for subsequent machining operation is almost eliminated. The process uses special dies having greater accuracies than those in impression – die gorging, and the equipment used is also of higher capacity. The forces required for forging are high. Aluminium and magnesium alloys are more suitable although steel can also be precision – forged. Typical precision – forged components are gears, turbine blades, fuel injection nozzles, and bearing casings. Because of very high cost of toolings and machines, precision forging is preferred over conventional forging only where volume of production is extremely large. Press Forging Press forging, which is mostly used for forging of large sections of metal, uses hydraulic press to obtain slow and squeezing action instead of a series of blows as in drop forging. The continuous action of the hydraulic press helps to obtain uniform deformation throughout the entire depth of the work piece. Therefore, the impressions obtained in press forging are more clean. Press forgings generally need smaller draft than drop forgings and have greater dimensional accuracy. Dies are generally heated during press forging to reduce heat loss, promote more uniform metal flow and production of finer details. Hydraulic presses are available in the capacity range of 5 MN to 500 MN but 10 MN to 100MN capacity presses are more common. Upset Forging Upset forging involves increasing the cross – section of a material at the expense of its corresponding length. Upset – forging was initially developed for making bolt heads in a continuous manner, but presently it is the most widely used of all forging processes. Parts can be upset – forged from bars or rods upto 200 mm in diameter in both hot and cold condition. Examples of upset forged parts are fasteners, valves, nails, and couplings. The process uses split dies with one or several cavities in the die. Upon separation of split die, the heated bar is moved from one cavity to the next. Upon completion of upsetting process the heading tool comes back and the movable split die releases the stock. Upsetting machines, called up setters, are generally horizontal acting. When designing parts for upset – forging, the following three rules must be followed.  1. The length of unsupported bar that can be upset in one blow of heading tool should not exceed 3 times the diameter of bar. Otherwise bucking will occur. 2. For upsetting length of stock greater than 3 times the diameter the cavity diameter must not exceed 1.5 times the dia of bar. 3. For upsetting length of stock greater than 3 times the diameter and when the diameter of the upset is less than 1.5 times the diameter of the bar, the length of un – supported stock beyond the face of die must not exceed diameter of the stock. Roll Forging This process is used to reduce the thickness of round or flat bar with the corresponding increase in length. Examples of products produced by this process include leaf springs, axles, and levers. The process is carried out on a rolling mill that has two semi – cylindrical rolls that are slightly eccentric to the axis of rotation. Each roll has a series of shaped grooves on it. When the rolls are in open position, the heated bar stock is placed between the rolls. With the rotation of rolls through half a revolution, the bar is progressively squeezed and shaped. The bar is then inserted between the next set of smaller grooves and the process is repeated till the desired shape and size are achieved. Test after completion 1. Which of the following is true about the extrusion process? a) Structure is homogeneous b) No time is lost in changing the shape c) Service life of extrusion tool is too high d) Its leading end is in good shape as compared to rolling 2. In which extrusion process the direction of flow of metal is in same direction as that of ram? a) Direct b) Indirect c) Impact d) Hydrostatic 3. In direct extrusion process at higher temperature which of the following is used to avoid friction? a) Oil b) Lubricants c) Molten glasses d) Wax 4. Which of the following is not used because of the problem of handling extruded metal coming out through moving ram? a) Direct b) Indirect c) Impact d) Hydrostatic 5. Which of the following is not a cold extrusion process? a) Cold extrusion forging b) Impact extrusion c) Hydrostatic extrusion d) Cold rolling Conclusion  There are four basic production processes for producing desired shape of a product. These are casting, machining, joining (welding, mechanical fasteners, epoxy, etc.), and deformation processes.  Deformation processes exploit a remarkable property of metals, which is their ability to flow plastically in the solid state without deterioration of their properties.  With the application of suitable pressures, the material is moved to obtain the desired shape with almost no wastage.  The required pressures are generally high and the tools and equipment needed are quite expensive. Large production quantities are often necessary to justify the process.   When a force is applied to deform it or change its shape, a lot of changes occur in the grain structure. These include grain fragmentation, movement of atoms, and lattice distortion. Slip planes develop through the lattice structure at points where the atom bonds of attraction are the weakest and whole blocks of atoms are displaced. Demo Videos http://youtube.com/watch?v=dNbVsmVgOnM References 1. Sharma, P.C., A textbook of Production Technology – Vol I and II, S.Chand and Company Ltd., New Delhi, 1996. 2. Rao, P.N., Manufacturing Technology, Vol I and II, Tata McGraw Hill Publishing Co., New Delhi, 1998. 3. Chapman W.A.J., Workshop Technology Vol. I and II, Arnold Publisher, New Delhi, 1998. 4. Hajra Choudhary, S.K. and Hajra Choudhary, A.K., Elements of Manufacturing Technology, Vol II, Media Publishers, Bombay, 1988. 5. Jain, R.K., Production Technology, Khanna Publishers, New Delhi, 1988. 6. Kalpakjian, Manufacturing and Technology, Addison Wesley Longman Pvt., Singapore, 2000. Answers to the assignments with full explanation Assignment 1 1. When metal is formed in cold state, there is no recrystallization of grains and thus recovery from grain distortion or fragmentation does not take place. As grain deformation proceeds, greater resistance to this action results in increased hardness and strength. The metal is said to be strain hardened. There are several theories to explain this occurrence. 2. The amount of deformation that a metal can undergo at room temperature depends on its ductility. The higher the ductility of a metal, the more the deformation it can undergo. Pure metals can withstand greater amount of deformation than metals having alloying elements, since alloying increases the tendency and rapidity of strain hardening. Metals having large grains are more ductile than those having smaller grains. 3. When metal is deformed in cold state, severe stresses known as residual stresses are set up in the material. These stresses are often undesirable, and to remove them the metal is heated to some temperature below the recrystalline range temperature. In this temperature range, the stresses are rendered ineffective without appreciable change in physical properties or grain structure. 4. In this process, the diameter of a rod or a tube is reduced by forcing it into a confining die. A set of reciprocation dies provides radial blows to cause the metal to flow inward and acquire the form of the die cavity. The die movements may be of in – and – out type or rotary. The latter type is obtained with the help of a set of rollers in a cage, in a similar action as in a roller bearing. The work piece is held stationary and the dies rotate, the dies strike the work piece at a rate as high as 10 - 20 strokes per second. 5. In tube swaging, the tube thickness and / or internal dia of tube can be controlled with the use of internal mandrels. For small – diameter tubing, a thin rod can be used as a mandrel; even internally shaped tubes can be swaged by using shaped mandrels.  Course Material for Unit - II Name of the Course : Manufacturing Technology for Mechatronics Name of the Unit : Mechanical Working of Metals Name of the Topic : Sheet Metal Operations: Blanking – blank size calculation, draw ratio, drawing force, Piercing, Punching, Trimming, Stretch forming and Shearing.  Objectives: To study the various ways of working of metals. 1. Outcomes: Upon successful completion, the student should be able to explain the concept of different metal forming operations. 2. Pre-requisites: To have a basic knowledge of Fundamentals of Basic Mechanical Engineering. 1. Which of the following processes of metal forming is best suited for making products like aircraft wings and window frames? a) Forging b) Rolling c) Drawing d) Stretch forming 2. In which of the following process frictional loss is eliminated at the billet container interface? a) Direct b) Indirect c) Impact d) Hydrostatic 3. In which of the following process fluid medium is used to apply the load on the billet? a) Direct b) Indirect c) Impact d) Hydrostatic 4. Which of the following components is mainly manufactured by performing metal forging? a) Piston b) Engine block c) Connecting rod d) Crankcase 5. Which of the following metal forming processes performs squeezing out of material through a hole? a) Forging b) Rolling c) Drawing d) Extrusion 6. Which of the following processes is not the type of bulk forming process in the metal forming? a) Bending b) Rolling c) Forging d) Extrusion 7. Which of the following manufacturing processes is mainly considered for producing the components of very high strength? a) Casting b) Forging c) Extrusion d) Rolling 8. Which of the following metal forming processes is best suitable for making the wires? a) Forging b) Extrusion c) Drawing d) Rolling 9. Which of the following methods of forming is not the part of electromagnetic forming? a) Compression b) Expansion c) Shearing d) Counter forming 10. Which of the following is the main advantage of using the electromagnetic forming process? a) High speed b) Low maintenance c) Applicable to all materials d) No spring-back 3. Shearing operations: Sheet metal subjected to shear stress developed between a punch and a die is called shearing. Shearing usually starts with formation of cracks on both the top and bottom edges of the work piece.  These cracks meet each other and separation occurs. Shearing process has three important basic stages. 1. Plastic deformation. 2. Fracture and 3. Shear. The shearing operations are: 1. Blanking 2. Punching 3. Piercing 4. Power shearing 5. Cutting off 6. Parting 7. Notching 8. Slitting 9. Lancing 10. Nibbling 11. Trimming 12. Shaving 13. Perforating The following figures show different types of shearing operations. Shearing of sheet metal between two cutting edges: a. Just before the punch contacts work; b. Punch begins to push into work, causing plastic deformation Shearing is a process for cutting sheet metal to size out of a larger stock such as roll stock.  Shears are used as the preliminary step in preparing stock for stamping processes, or smaller blanks for CNC presses.  The shearing process produces a shear edge burr, which can be minimized to less than 10% of the material thickness. The burr is a function of clearance between the punch and the die, and the sharpness of the punch and the die. Blanking  Blanking is the operation of cutting a flat shape from the sheet metal.  The Portion which is removed is the required part is called as blank and the operation is called as blanking. Punching o Similar to blanking except cut piece is scrap, called a slug. o A slug (the material punched out) is produced in punching operations but not in piercing work. Piercing o It is “forming a hole in sheet metal with a pointed punch with no metal fallout (slug)”. o In this case, a significant burr or deformed sharp edge is created on the bottom side of the material being pierced. o Piercing is the operation of cutting internal features (holes or slots) in stock, without forming slug scrap. Power shearing o This operation is carried out on power shearing machines where the sheet metal is cut between the movable upper cutting blade and fixed lower cutting blade. Cutting off o In this operation a piece is removed from a strip by cutting along a single line. Parting o In parting operation the sheet is sheared into two or more pieces and the scrap is removed between the two pieces to part them. Notching o Notching refers to removing pieces of desired shapes from the edge. Slitting o Shearing operations carried out by means of pair of circular blades called as slitting. o It is the operation of marking an unfinished cut through a limited length only. o A slit edge normally has a burr, which may be plastically folded over the sheet surface by rolling the sheet between two rolls. Lancing o It is an operation of cutting on one side and bending on the other side to form a sort of tab (or) louver. o In this operation no metal is being removed. Nibbling o In a nibbler machine a small straight punch moves up and down rapidly into a die. o A sheet is fed through the gap and many over lapping holes are made. Trimming o It is the operation of cutting and removing unwanted excess metal from the periphery of a previously formed/forged/cast component. Shaving o The rough edges of a blanked part are removed by cutting thin strip of metal along the edge on the periphery. Perforating o This process is used to make multiple holes which are small in diameter and close together, in a flat work material. 4. Stretch forming