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Note for Basic Manufacturing process - BMP by Mohan Suram

  • Basic Manufacturing process - BMP
  • Note
  • AVN Institute of Engineering and Technology - AVNIET
  • Mechanical Engineering
  • B.Tech
  • 8 Topics
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SURAM MOHAN KUMAR, (Ph.D) 1 Assistant Professor, Dept. of ME, AVNIET, Hyderabad, Telangana, India MANUFACTURING PROCESS (Production Technology) COMPLETE NOTES UNIT-I INTRODUCTION Manufacture of a machine part by heating a metal or alloy above its melting point and pouring the liquid metal/alloy in a cavity approximately of same shape and size as the machine part is called casting process. After the liquid metal cools and solidifies, it acquires the shape and size of the cavity and resembles the finished product required. The department of the workshop, where castings are made is called foundry. The manufacture of a casting requires: (a) Preparation of a pattern, (b) Preparation of a mould with the help of the pattern, (c) Melting of metal or alloy in a furnace, (d) Pouring of molten metal into mould cavity, (e) Breaking the mould to retrieve the casting, (f) Cleaning the casting and cutting off risers, runners etc., (this operation is called ‘fettling’), and (g) Inspection of casting. Castings are made in a large number of metals and alloys, both ferrrous and non-ferrous. Grey cast iron components are very common; steel castings are stronger and are used for components subject to higher stresses. Bronze and brass castings are used on ships and in marine environment, where ferrous items will be subjected to heavy corrosion. Aluminium and aluminium-magnesium castings are used in automobiles. Stainless steel castings are used for making cutlery items. Casting is an economical way of producing components of required shape either in small lots or 1

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SURAM MOHAN KUMAR, (Ph.D) 2 Assistant Professor, Dept. of ME, AVNIET, Hyderabad, Telangana, India in larger lots. However, castings are less strong as compared to wrought components produced by processes such as forging etc. However castings offer the possibility of having slightly improved properties in certain part of the casting by techniques such as use of chill etc. In casting process, very little metal is wasted. PATTERNS Patterns are replicas of the casting required. It is similar in shape and size to the final product, but not exactly. Usually, the mould is prepared in wet sand, to which some binder is added to hold sand particles together. The pattern is then withdrawn from inside the sand mould in such a manner that the impression/ cavity made in the mould is not damaged or broken in anyway. Finally molten metal is poured into this cavity and allowed to solidify and cool down to room temperature. PATTERN ALLOWANCES Since most metals shrink in volume, when solidifying from liquid state and again on cooling, it is obvious, that the pattern should be made slightly larger than the size of finished casting. This difference in size of the pattern is called shrinkage allowance. For cast iron, this allowance is 1% and for aluminium, it is about 1.6%. On many occasions, castings produced in the foundry shop are machined subsequently. The object of machining is to get exact sizes and better surface finish on the component. If such is the case, a layer of 1.5–2.5 mm thick material has to be provided all round the casting. This is done by making the pattern suitably bigger than the casting. This increase in size of pattern is called “machining allowance”. Another important allowance provided on patterns is called draft allowance. It facilitates withdrawal of pattern from the mould. It is provided on vertical surfaces. The idea is to give an inclination of 2–3 degrees to vertical surfaces, so that while lifting the pattern, the upper surface is wider and withdrawing the pattern with draft provided will not damage the sand mould. On inner vertical surfaces, draft is provided in such a way that top surface is narrower and bottom portion of pattern is wider. Apart from the above allowances, some other allowances are sometimes given to compensate for inherent distortion or bending of castings. Sharp corners and bends are also radius sized while making a pattern. 2

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SURAM MOHAN KUMAR, (Ph.D) 3 Assistant Professor, Dept. of ME, AVNIET, Hyderabad, Telangana, India Patterns are usually made of good quality wood. Wood is easy to work, acquires good smooth surface and properly seasoned wood retains its size. It is also relatively cheap and abundant. However, if a very large number of castings are required, metal patterns may be used. Usually, they are made of aluminium-magnesium alloys. TYPES OF PATTERNS (i) Solid or single piece pattern: Such patterns are made in one piece and are suitable only for very simple castings. There is no provision for runners and risers etc. Moulding can be done either in the foundry floor (called pit moulding) or in a moulding box. There is no difficulty in withdrawing the pattern from the mould as the broadest portion of the pattern is at the top. As an example, if a cylindrical pin with a circular head has to be cast, a one piece pattern shown in Fig. 6.1 will be adequate. One-half of the impression in the mould will be made by using piece no. 1 in one moulding box and the other half of the impression will be made by using piece no. 2 in a second moulding box. After withdrawing the pattern halves from the respective moulding boxes, the two boxes will be assembled and clamped together, so that the complete impression is available for pouring the metal. 3

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SURAM MOHAN KUMAR, (Ph.D) 4 Assistant Professor, Dept. of ME, AVNIET, Hyderabad, Telangana, India The two pattern halves are provided with locating dowels, so that one-half may sit on the other half in the exact position required with no mismatch. Also two tapped holes are provided on the flat mating surface of each part. These tapped holes are used to provide a grip to lift the pattern halves from the sand without damaging the mould-impression. The line along which the pattern is divided into halves is called “parting line” and it usually follows the broadest cross-section of the casting. Deciding where the parting line should be is a matter of considerable skill and experience. Some of the more complicated castings may require pattern to be split in three or even more pieces. (iii) Loose piece pattern: In some cases, the casting may have small projections or overhanging portions. These projections make it difficult to withdraw the pattern from the mould. Therefore these projections are made as loose pieces. They are loosely attached to the main part of the pattern and the mould is made in the usual way. When the main pattern is withdrawn from the mould, the loose pieces slip off and remain behind in the mould. After removing the main body of the pattern, the loose pieces are taken out by first moving them laterally and then lifting them through the space vacated by the main pattern. The method is illustrated in Fig. below (iv) Match plate pattern: Match plate is a metal plate, usually made of aluminium. The two halves of the split pattern are mounted on this match plate one on either side. While fixing them to the match plate, care is taken so that there is no mismatch. These patterns are used in conjunction with mechanically operated moulding machines. Bottom side of match plate pattern is used for making the bottom half of the mould impression in one moulding box (known as the drag). The upper side of the match plate pattern is used for making the mould impression in another moulding box. Finally, the two moulding boxes are kept on top of each other, the bottom box is known as the drag, whereas the top one is called the cope. 4

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