Since these operations are time consuming and expensive, it is essential to try to eliminate or at least minimize their need when designing the product and choosing the specific casting method. Besides, possibilities should also be explored for automation of these operations.

Sand cores are removed from castings usually by mechanical shaking, hydro blasting or at times by chemical dissolution depending upon the size, complexity and hardness of the core. Gates and risers are either knocked off or cut-off by an abrasive wheel or a power saw. Alternatively, they are removed by melting them away with an oxy-acetylene flame. The casting surface, from where the gates and risers have been removed, is generally rather rough. Small projections may easily be chipped off with the help of either hand tools or pneumatic chisels. To remove fins, flash and sand that might be sticking to the surface of castings, a process called tumbling is employed. In the tumbling process, castings together with abrasive material (in the form of broken grinding wheels), cleansing fluid and metal shots are loaded in a horizontal barrel and given a slow rotary motion. The castings rub and strike against each other as well as against the abrasive grinding wheel pieces. Alternatively, castings may be subjected to sand blasting, shot blasting or vibratory finishing process.

The word ‘plastics’ is mostly used as a synonym for ‘polymers', although plastics constitute only a subset of polymeric materials. In fact, the word plastic is from the Greek word plastikos which means ‘able to be moulded and shaped'. Plastics can be moulded, cast, formed, machined and joined by welding operations. They have many unique and diverse useful properties due to which they are increasingly replacing metals and alloys for manufacturing of components that are used in a wide range of applications.

The casting processes which use expendable moulds can be grouped into two categories:

• 1. Casting processes that employ multiple-use patterns: Under this category, the main casting processes are as follows:

• • Sand mould casting and its variants like Green-sand mould casting, skin-dried sand mould casting, dry-sand mould casting, sodium silicate bonded sand mould casting and shell mould casting

• • Plaster mould casting

• • Ceramic mould casting

• 2. Casting processes that employ single-use patterns: Under this category, the main casting processes are as follows

• • Investment casting

• • Full-mould casting

One thing common among all the expendable mould casting processes described in the previous chapter is that all of them require a new mould for every casting. Such a wasteful proposition is acceptable if only a limited number of identical parts are to be produced; but when parts are required in large quantities, great savings can be made and are actually made by using moulds that are re-usable again and again, that is, permanent moulds, or dies.

The question arises as to what the permanent mould should be made of so that it is able to withstand the melting point of metals. For casting of non-ferrous metals and cast iron (melting point up to 1200°C), we use dies made of steel. In case of casting of cast iron, frequent smoothing (redressing) of the die profile is necessary. For casting of steels (melting point nearly 1550°C), we use graphite dies, although machining of complex profiles in graphite dies is difficult. For this reason, non-ferrous die casting is much more common than steel die casting.

Metal melting is an important activity of casting operation that directly affects the quality of castings. It is done in a furnace and there are many requirements which the furnace must meet. These are as follows:

• 1. It should provide an adequate amount of molten metal.

• 2. It should provide molten metal at the desired temperature.

• 3. It should be able to hold molten metal for an extended period of time without

• deterioration of quality.

• 4. It should not pollue the environment.

• 5. It should be economical to operate and maintain. Foundries use many types of furnaces – pit furnace, open hearth furnace, arc furnace, induction furnace, cupola furnace and so on.

The basic features of a casting system are shown in Figure 7.1. As the molten metal is poured into the pouring basin, it flows through the other parts of the gating system (sprue, runner and gates) and finally fills the mould cavity. The design of the gating system should ensure the following:

• • The necessary amount of molten metal is able to flow through the system.

• • The flow of molten metal into the mould cavity is at an appropriate speed and nonturbulent.

• • While flowing through the gating system, the molten metal

A component can be generally produced by several manufacturing processes, each offering its own advantages and limitations; the challenge lies in selecting the most economical and suitable process for a given component. Casting is one of the oldest and most popular methods of producing metallic products, whereby liquid metal is poured into a cavity known as the mould, and allowed to solidify and cool. The casting thus produced is almost an exact replica of the mould. This casting is cleaned and, if required, machined to the desired dimensions. Moulds are generally prepared in sand but can just as easily be made of metals or non-metals.

The term manufacture, first coined during the 1560s, is derived from two Latin words manus (hand) and factus (make). Meaning ‘made by hand', this term describes the fabrication methods that were used in earlier times. Today, most manufacturing operations are, however, accomplished by mechanized and automated equipment under human supervision. In simpler terms, manufacturing can be considered as the process of converting materials into products by combining one material with an other, or by changing their shape, properties and/or appearance with the application of physical and chemical processes.

Manufacturing has always been important to humans. From the technological point of view, manufacturing is significant as the principles of science are applied to create and provide all the necessary goods and products for society. From the point of view of economics also, manufacturing is vital since it adds value to materials by changing their size, shape and properties.

Manufacturing is a value adding activity, in which materials are converted into products, thereby adding value to the original material. Therefore, the aim of an organization engaged in manufacturing is to add value in the most efficient manner, using the minimum amount of manpower, material, money, time and space. Proper selection of materials and processes is vital for minimizing waste and maximizing efficiency. The sequence and location of operations must be organized in such a manner that permits smooth and controlled flow of material through the various stages of manufacturing.

This book has been written from the material used by the author for teaching two undergraduate courses related to manufacturing processes over several years at the University of Roorkee/Indian Institute of Technology Roorkee. The book aims to provide a descriptive introduction to the large number of manufacturing processes currently available. The subject matter has been dealt with in simple language along with line diagrams. The book covers recently developed manufacturing processes in addition to traditional processes that have been used and refined over the past several decades. An important feature of the book is that a section on product design considerations has been given in many of the manufacturing process chapters.

The book is designed to cover metal casting processes, metal forming and shaping processes (including high energy rate forming processes and powder metallurgy) and metal joining processes. It also includes a chapter on plastics and processing of plastics. At the end of each chapter, problems have been given to test the students’ grasp of the subject matter.

There are a large number of items of daily use that are made by sheet metal forming and cutting operations. Some examples are almirahs, filing cabinets, fan wings, bodies of automobiles, refrigerators and water coolers, computer desktop bodies, household appliances and utensils, beer and soft drink cans. Sheet metal parts (also called stampings) have several advantages over those made by casting or by forging, such as low cost, light weight, high strength, good dimensional accuracy, good surface finish, and possibility of a large variety of shapes. Typical sheet metal thickness varies from 0.5 mm to 6 mm. When the thickness is less than 0.5 mm, it is considered as a leaf or foil but when it is more than 6 mm, the stock is generally referred to as a plate rather than a sheet. The most commonly used sheet material is mild steel or low carbon steel because it is low cost and has good strength and formability characteristics, although for aircraft and aerospace applications, aluminium and titanium are the common sheet materials. Sheet metal processing is generally carried out at room temperature (cold working), although some processes may be carried out in the warm state of the sheet metal especially when it is brittle or when the deformation required is large. The tooling used for sheet metal processing is called a die-and-punch, and the machine tool on which these processes are carried out is called the stamping press.

An arc is generated when there is an electrical discharge that flows between two adjacent metal objects that are not actually touching each other. This discharge can be sustained through a path of ionized gaseous particles called plasma – the temperatures generated being over 15,000°C inside the arc and in excess of 6,000°C at the surface of the arc. Electrical currents involved in the generation of the arc are of the order of 1,000 amps, although voltages are quite low and in the range of 30 to 80 V.

Several manufacturing processes are available to produce a part. Each of these processes has different degrees of suitability for the same part. While selecting an appropriate manufacturing process, not only economy but other aspects such as the desired life, strength, surface quality, appearance, repairability and use of the product must also be given due considerations.

Metal working or metal forming is a process wherein the desired shape and size of a product are obtained by deforming the metal plastically. Stresses experienced by the material during the deformation process are greater than the yield strength but less than the fracture strength of the material. Molten metal is first cast into ingots, slabs, rods or pipes, which are then converted to wrought structures by deformation processes. These processes exploit a remarkable property of metals, which is the ability to flow plastically in the solid state without any major loss to any of their other properties. With the application of suitable pressures, the material is moved or displaced to obtain the desired shape with almost no wastage. The pressures required are generally high and a major part of the input energy is efficiently utilized to enhance the material strength by strain hardening.

In this chapter, certain special welding processes will be described. These processes are considered special because they cannot be classified as arc, oxy-fuel or resistance welding processes. Each of these processes use a unique technology and has special applications.

19.1 Electron Beam Welding (EBW)

Electron beam welding (EBW) is a fusion welding process that uses the heat resulting from impingement of a narrow focused beam of high velocity electrons on the workpiece to be welded. When electrons from the beam impact the surface of a solid, they collide with the particles of the solid and lose their kinetic energy. In fact, the electrons ‘travel’ a very small distance (a few hundredths of a millimeter) below the surface before their kinetic energy is transferred into heat. This distance is proportional to their initial energy and inversely proportional to the density of the solid.

Resistance welding can be considered as a solid state welding, although it is generally also classified as a fusion welding process. In the same way as any other fusion welding process, resistance welding also requires both heat and pressure in the weld area to create a satisfactory joint. All metals have finite electrical resistance that resists the flow of electrical current and, in doing so, generates heat. The resistance welding process utilizes thermal energy obtained from the flow of electrical current through the electrical resistance of the workpieces and the interface between them. By external means, pressure is applied that is varied with the progression of the weld cycle. Initially, a small amount of pressure is applied to hold the workpieces in contact, thereby controlling the electrical resistance at the interface. As the current is passed through the workpieces, a rise in temperature takes place at the interface due to the presence of high electrical resistance. As the proper temperature is attained, the pressure is increased to make the weld.

Forging is a deformation process in which the material is shaped by the application of localized compressive forces exerted either manually or with power hammers, presses or special forging machines. In other words, the pressures applied may be either gradual or by sudden impact. The process may be carried out on materials in either hot or cold state. When forging is done cold, processes have been given special names. Therefore, the term ‘forging’ usually implies hot forging carried out at temperatures which are above the recrystallization temperature of the material. Table 10.1 gives the hot forging temperature for some important metal alloys.

Various types of equipment and systems are used in a rolling mill. The major equipment is obviously the mill stand in which the material is actually rolled between two or three rotating rolls. Apart from the mill stand along with its own different mechanisms and systems for rotating the rolls, positioning the rolls to effect amount of reduction on the stock, to guide the stock in perfect position and orientation with respect to the rolls, etc., various other equipment and systems are necessary based on the rolling process (hot or cold), size and shape of input stock, specifications of product mix, desired quality of the products, and handling of the rolling stock up to finished product stage. These are called auxiliary equipment.

In this section, for each of these equipment and systems the functions, constructional features and specifications and in some cases the basic design features and calculations have been discussed. The design aspects will be limited to mechanical design. Electrical drive, controls and instrumentation plays a very important role in modern rolling mills. A brief section has been devoted to these but their design aspects are beyond the scope of the present treatise.

Mill Stand Components and Mechanisms

Most type of rolling mill stands have certain common basic components, mechanisms and systems. These are:

1. (i) Work rolls between which rolling takes place.

2. (ii) Backup rolls to support work rolls, excepting in case of 2-Hi, 3-Hi mills or cluster mills.

3. (iii) Roll supporting bearing and bearing blocks (chocks).

4. (iv) Roll positioning (or screw-down) mechanism which alters the gap between the work rolls.

5. (v) Roll balancing arrangement which keeps the set of top work and backup rolls in position and does not allow the top work and back-up rolls to come down and touch bottom work roll when there is no stock in roll bite.

6. (vi) Mill housings (generally a pair for normal 2-Hi, 3-Hi, or 4-Hi mills) within whose windows the roll bearing chocks are supported and allowed to be slided up and down during screw down adjustment.

7. (vii) Base Plates (also called shoes) in the form of two girders onto which the housings are fixed. The base plates are mounted on the foundation.