Mechinopedia For Mechanical Engineers

 The different steps involved in cupola furnace operation process are :   1. Preparation Of Cupola .   The First Operation in preparing a cupola is to clean out the slag and refuse on the lining and around the tuyeres from the previous run. Any bad spots or broken bricks are repaired with a daubing mixture of fire clay and silica sand or ganister . The preparation of the sand bottom in cupola is begun as soon as the patching of the lining has been completed. The bottom doors are raised and held in this position by metal props. The bottom sand is introduced through the charging door and is rammed well around the lining and across the intersection of the bottom doors. This layer of sand is built up to a height of 100 to 200 mm above the cast iron door. The surface of the sand bottom is sloped from all direction towards the tapping hole so that the molten metal can be drained completely from the cupola at any time. An opening about 35 mm diameter is provided for th...

Various types of melting furnaces are used in different foundry shops, depending upon the quantity of metal to be melted at a time, and the nature of work that is carried out in the shop.  The primary objective in cupola is to produce iron of desired composition, temperature and properties at the required rate in the most economical manner. Besides, this furnace has many distinct advantages over the other types, e.g., simplicity of operation, continuity of production, and increased output coupled with a high degree of efficiency.  Description Of a Cupola : The Cupola Furnace consists of a vertical, cylindrical steel sheet, 6 to 12 mm thick, and lined inside with acid refractory bricks or acid tamping clay. The refractory bricks or the tamping clay used consist of  silicon oxide acid (SiO2) and alumina (Al2O3). The lining is generally thicker in the lower where the temperatures encountered are higher than in the upper region. The shell is mounted either on a brick work fo...

Continuous Casting is used in general for the production of rods, pipes, sheet metal and other articles known as semi-finished products in an uninterrupted process. The main feature of  the process is the pouring of molten steel through a tower nearly 300m high ; this replaces the casting of ingots, the removal of molds from ingots, the re-heating of ingots, and their primary rolling. To understand what this shortcut means, one has only to go over the principal operations in a conventional plant.  In a conventional plant, after pig iron has been turned into steel- say, first, in a Bessemer Converter and then in what is known as an open hearth furnace- the white-hot molten steel flows into a huge ladle ; The ladle is carried by a crane and the molten steel is poured from it into a number of molds to form ingots, i.e., pieces of steel of  manageable size for further processing ; the ingots are then carried to a mill for rolling ; They are first re-heated and then put bet...

In the centrifugal casting , molten metal is poured into mold while they are rotating. The metal falling into the center of mold at the axis of rotation is thrown out by the centrifugal force under sufficient pressure towards the periphery, and the contaminants or impurities present being lighter in weight are also pushed towards the center. This is often machined out any way. Solidification progresses from the outer surface inwards, thus developing an area of weakness in the center of wall. This is caused by the meeting of the grain boundaries at final solidification and the entrapment of impurities in the central section. The grain is refined and the castings are completely free from any porosity defect by the forced movement of the molten metal, thus making dense and sound castings which are less subject to directional variations that static castings. The use of gates, feeders, and cores is eliminated, making the method less expensive and complicated. Hollow cylindrical bodies such...

 While in Sand castings the molds are destroyed after solidification of  castings, the molds are reused repeatedly in the permanent mold castings. This requires a mold material that has a sufficiently high melting point to withstand erosion by the liquid metal at pouring temperature, a high enough strength not to deform in repeated use, and high thermal fatigue resistance to resist premature crazing ( the formation of thermal fatigue cracks) that would leave objectionable marks on the finished castings. Finally, and ideally, it should also have low adhesion. The material used for making molds (dies) may be cast iron ,  although alloy steels are most widely used. For higher-melting alloys such a brasses and ferrous alloys , the mold steel must contain large proportion of stable carbides. More recently refractory metal alloys, particularly molybdenum alloys, have found increasing application. Graphite molds can also be used for steel although only for relatively simple shap...

 In this method, a Vacuum is created by withdrawing air from the mold space. Subsequently molding sand is sucked in and the cavity is filled up. The sand can there after be rammed in the pattern. the process is used for casting iron , steel and aluminum . The weight of  casting ranges from 200 gm to 120 kg. Mold is used once and is made from wet artificial crashed sand. The process is different from vacuum molding where plastic sheet is vacuum molded to the contour of  the pattern, back filled with fine grained binder free quartz sand and sealed with another plastic sheet. The mold is constantly connected to a vacuum source before, during and after the casting.  Advantages of the suction molding process are : 1. Optimum mold compaction around the pattern.  2. Decreasing hardness of compacted sand from inside to the outside.  3. High surface quality. 4. Reduced cleaning. Disadvantages of  the process are : 1. High Cost Of Manufacturing . 2. Change - O...

 In ceramic molding, a thick slurry, consisting of  specially developed ceramic aggregates and a liquid chemical binder (alcohol based silicon ester) is poured over the reusable split and gated metal pattern which is usually mounted on a match plate. The slurry fills up all cavities and recesses by itself and no ramming or vibration of  the mold is required. The pattern is withdrawn after it sets in about 3 to 5 minutes. The ceramic mass is then removed from the flask. treated with a hardener to promote chemical stabilization and heated to about 980*C in a furnace to remove the liquid binder. The mold is then ready for pouring molten metal.   Advantages of this Process are : 1. Highest precision and extremely high finish are obtained. 2. Suitable for all types of cast metals including highly reactive metals such as titanium and uranium. 3. The castings do not normally require any riser, venting or chilling as the cooling rate is very slow.  4. Any ordinary ...

 In this method, the mold is prepared in gypsum or plaster of Paris . In practice, The plaster of Paris is mixed with talc, asbestos, fibers, silica flour, and a controlled amount of water to form a slurry. This plaster slurry is poured over the metallic pattern confined in a flask. The mold is vibrated and the slurry allowed to set. The pattern is removed after about  30 minutes when the setting is complete and the mold is dried. and backed by slowly heating it to about 200*C in a conveyor oven. Inserts and cores are placed, cope and drag matched by guide pins. Molten metal is then poured into the mold. Finally, the casting is cooled in the mold, shaken out and the mold is destroyed. Castings are then trimmed of gates, sprues and flash. Advantages of Plaster Molding are : 1. Warping and Distortion of thin sections can be avoided since plaster has no chilling              tendency due to  low rate of heat conductivity. 2. A high degree o...

 In this method, a polystyrene foam pattern, complete with sprues, bottom gates, runners, and risers, and coated with a suitable mold wash is rammed up in a flask filled up with no bake type of sand. For small-sized castings furan or alkyd isocyanate while for medium and large-size castings CO2, ferro silicon may be used. When the molten metal is poured through the sprue, the heat from the molten metal vaporize the pattern and consequently displacement of the pattern material takes place by the molten metal. The amount of gas produced is so small that it can easily escape through the sand without causing any back pressure. However, shapes with high dimensional accuracy can be cast quickly and inexpensively by this method.  The advantages of this process are : 1. High Dimensional Accuracy is obtained. 2. Defects such as blow holes and pin holes are eliminated. 3. There is no limitation as regards size, shape or complexity. 4. Suitable and economical for large-scale production...

 In recent years, pneumatic comparators have been extensively used specially in automatic size control. They are cheap, independent of the contact pressure, and simple to operate. Besides, this form of comparator is free from mechanical wear. However, pneumatic comparators are sensitive to temperature and humidity changes and their accuracy may be influenced by the surface roughness of the parts being checked. The magnification of this type of  comparator is as high as 10,000.  The operation is based on the fact that if air under constant pressure escapes by passing through two orifices the air pressure in space between them is  dependent on the cross-sectional area of  the orifices. If one of them is kept uniform, the pressure will vary according to the size, of the other. In the diagram, The pipe l expands in tube 11 and maintains a constant head of water H , excess air escaping as bubbles. Air from 11 passes through control jet 3, and finally escapes through ...

  Optical Comparators have a high degree of  precision and magnification is obtained with the help of  light beams which have the advantage of being straight and weightless. Optical comparators , therefore, suffer less wear during usage than the mechanical type. Optical comparators are used to magnify parts of very small size and of complex configuration that require accurate and enlarged profile. So they are widely employed in the inspection of many small parts such as needles, saw teeth , gear teeth , screw threads , etc. Since they check work to definite tolerances , they can be used to study wear on tools or distortion caused by heat treatment. In an optical or projection comparator which is designed on the same principle as the ordinary projection lantern, an object is placed before a light source , and the shadow of the profile is projected at some enlarged scale on a screen where it is compared to a master chart or drawing. Interchangeable projection l...

Electrical Comparators are used as a means of  detecting and amplifying small movements of a work contacting elements. An electrical comparator consists essentially of  a pick-up head or transducer for converting a displacement into a corresponding change in current and a meter or recorder connected in the circuit to indicate the electrical change, calibrated to show in terms of displacement. Generally, an amplifier is needed to provide the requisite sensitivity and to match the characteristics of different parts of  the circuit. Electrical comparators can be classified according to the electrical principle used in the pick-up head. Most of the comparators use either a differential transformer, an inductance bridge, a strain gauge or a capacitor as a means of detecting movement of the gauging element. There are different types of  electrical comparators. One kind called an electrolimit gauge is used to check or measure the outside diameter of a  roll. The objec...

 A Mechanical comparator employs mechanical means for magnifying the small movement of the measuring stylus brought about due to the difference between the standard and the actual dimension being checked. Mechanical comparators operating essentially on the same principle are available in different designs. They are : Sigma ,  Mercer, Mikrokator, etc. The method of  magnifying small movement of the stylus in all mechanical comparators is effected by means of levers, gear-trains or a combination of these elements. Mechanical comparators are available having magnifications from 300 to 5000 to 1, and used mostly for inspection of  small parts machined to close limits.  Dial Indicators may be classified as mechanical comparators as they have multiplying mechanism which greatly magnifies on the dial any movement of the indicator.       Dial Indicators . The essential parts of the instrument is like a small clock with a plunger projecting at the bo...

Comparators are instruments which derive their name from the fact that they are used for simple and accurate comparison of parts as well as working gauges and instruments with standard precision gauge blocks. Sometimes they act as gauge themselves. Comparators are designed in several types to meet various conditions, but comparators of  every type incorporate some kind of magnifying device to magnify how much a dimension deviates, plus or minus, from an ideal. Most comparators indicate actual units of measurement, but some only indicate whether a dimension deviates within a given tolerance range. They also provide rapid means of inspecting very small articles made in large quantities.  The comparators are classified according to the principles used for obtaining suitable degrees of magnification of the indicating device. The common Types are  * Mechanical Comparators.  * Electrical Comparators.  * Optical comparators.  * Pneumatic Comparators.