Mechinopedia For Mechanical Engineers

 Bars of all shapes, rods, sheets, and strips are commonly finished by cold-rolling . Foil is made of the softer metals in this way. Two main reasons for cold - rolling metals are to get smooth bright surface finish and improved physical properties. If the object is only to give a clean, smooth finish to the metal, only a superficial amount of rolling will be needed. On the other hand, where it is desirable that the tensile strength , stiffness , and hardness be increased substantially , and that the section thickness be appreciably reduced, then higher roll pressures and deeper kneading are necessary. Cold-rolling also improves machinability by conferring the property of  brittleness , a condition which is conductive to smooth tool finishes and well-broken chips.  As a preliminary step to the cold-rolling operation, sheets, strips etc. of hot-rolled steel are immersed in an acid solution to remove the scale and washed in water and then dried. The cleaned steel is passed...

The working of metals at temperatures below their re-crystallization temperature is defined as cold-working. Sometimes this working temperature is well above room conditions as in cold finishing of hot-rolled products. Most of the cold-working processes, however, are performed at room temperature.  When materials is cold - worked the resulting change in material shape brings about marked changes in the grain structure. The effect of the cold-working on the structure of steel is to distort or elongate the grain in the direction of the flow of metal. Much greater pressures are needed for cold-working than for hot-working. The metal, being in a more rigid state, is not permanently deformed until stress exceeding the elastic limit is passed. Severe stress, known as residual stress is, therefore, set up inside the metal during cold-working. This effect of cold - working is reduced or minimized by heating the metal into the re-crystallization or annealing range, and finally the metal is ...

  Extrusion is a process in which a heated billet or slug of metal is forced by high pressure through an orifice that is shaped to provide the desired form to the finished part. A everyday analogy is the squeezing of toothpaste from a collapsible tube. Because of the large forces required in extrusion, most metals are extruded hot under conditions where the deformation resistance of  the metal is low. How ever, cold extrusion is possible for many metals and is rapidly taking an important commercial position. Most commercial metals and their alloys , such as steel, copper, aluminum , magnesium, and nickel, are directly extruded at elevated temperatures. Rods, tubes, molding trim, structural shapes, brass cartridges, lead - converted cables, Aircraft parts, flooring strips, and many hardware items such as window sash, door trim, etc., are typical products of extrusion.  The reaction of the extrusion billet with the container and die results in high compressive stresses whic...

 Parts that have circular cross-sections can be made by spinning them from sheet metal. The principle of metal spinning . A heated circular blank of sheet metal is lightly held against a chuck by the pressure of a freely rotating pad on the lathe tail stock. This chuck may be made of plaster, wood or metal and is revolved on the spindle of a lathe. A rounded stick or roller is pressed against the revolving piece and moved in a series of sweeps. This displaces the metal in several steps to conform to the shape of the chuck. Once the operation is started, considerable frictional heat is generated which aids in maintaining the metal at a plastic state,  Spinning is a highly specialized art. To avoid excessive thinning of the metal, the pressure of the forming tool (rounded stick) should be directed sometime backward toward the tail stock as well as forward toward the head stock. During "spinning on air" (not against the chuck), with large parts made from relatively thin metal, a...

Deep drawing is defined as a process for the making of cup - shaped parts from flat sheet - metal blanks. The blank is first heated to provide necessary plasticity for working. The heated blank is then placed in the position over the die or cavity. The punch descends and pushes the metal through the die to form a cup. So this process is also known as cupping . The process may be continued through a series of successively smaller dies and punches to obtain cup-shaped pieces of  the desired size and wall thickness. On many occasions, several dies of successively decreasing diameter is set up on a bench known as hot-draw bench.  Seamless tubing and cylinders made in this way are used primarily for thick walled cylindrical tanks.    Check out other hot working processes

Drawing is pulling the metal through a die or a set of dies to achieve a reduction in a diameter. The material to be drawn is reduced in diameter for a short distance at one end by swaging, to permit it into the die orifice and gripped in the jaws. The process requires a very large forces in order to pull the metal through the die. To reduce the frictional force between the die and the metal the die is kept well lubricated.  Check out other hot working processes

Piercing is employed to produce seamless tubing which is the natural form from which is made any thin-walled round objects. Seamless tubing is a popular and economical raw stock for machining because it saves drilling and boring of parts. The process of making hot-pierced tubing consists of  passing a hot-rolled billet between two conical-shaped rolls and over a mandrel which assists in piercing and controls the size of  the hole as the billet is forced over it. The solid billet is uniformly heated to about 1100*C, and the piercing action is actually started previous to placing it between the rolls by drilling, punching, or piercing with a oxygen a hole to about 25mm deep. It it is then pushed into the two piercing rolls which impart axial as well as rolling movement to the billet and force it over the mandrel. The alternate squeezing and bulging of the billet open up a seam in its center. The first pass makes a rather thick-walled tube, which is again passed over a tapered ...

Hot Rolling : Rolling is the most rapid method of forming metal into desired shapes by plastic deformation in between rolls. The crystals are elongated in the direction of rolling, and they start to reform after leaving the zone of stress, but in cold rolling they retain substantially the shape given to them by the action of paths.  In deforming metal between rolls, the work is subjected too high compressive  stresses from the squeezing action of the rolls and to surface shear stresses as a result of  a friction between the rolls and the metal. The frictional forces are also responsible for drawing the metal into the rolls. In the hot rolling process, metal in a hot plastic state is passed between two rolls revolving at the same speed but in opposite direction.  As the metal passes through the rolls, it is reduced in thickness and increased in length. The forming of bars, plates, sheets, rails, and other structural sections requires many passes through plain or groov...

Hot working is the initial step in mechanical working of most metals and alloys. Hot working combines the working and annealing processes by deforming metal above the recrystallisation temperature at which new grains are formed. Since most metals and alloys have relatively high recrystallisation temperatures, they must be worked at high temperatures. Each metal, of course, has a characteristic hot-working temperature range over which hot working may be performed. The upper limit of working temperature depends on metal composition, prior deformation, and impurities within the metal. Obviously, the upper limit for working must be somewhat below the melting or burning point. The finishing temperature , however, is usually just above the minimum recrystallisation temperature at which reheating is desired before further hot working. In addition to mere change of shape, hot working has profound effects on metal characteristics or properties. The changes in structure from hot working improve...

Subsequent to casting, further shaping operations are frequently desirable either to produce a new shape or to improve the properties of the metal. Shaping in the solid state may be divided into "non-cutting shaping" or "non-chipping shaping" such as forging, rolling, pressing, etc., and "cutting shaping" or "chipping shape giving" including the machining operations of  various machine tools. Non-cutting shaping are referred to as mechanical working processes.   Mechanical working of metal is simply a plastic deformation performed to change dimensions, properties and /or surface condition by means of mechanical pressure. Mechanical working may be either hot working or cold working depending on temperature, and strain rate such that recovery processes take place simultaneously with the deformation. Plastic deformation above recrystallization temperature, but below the melting or burning point, is hot work . Some metals, such as lead and tin, ...

In the case of the permanent pyrometer , Segar cones are used as temporary means for determining approximate temperatures. The cones are pyramidal in shape, made from mixtures of kaolin , lime, feldspar, magnesia, quartz and boric acid , each cone melting at a predetermined temperature arranged in steps from 600 to 2000*C . When a furnace temperature is required, several of these cones, within the required temperature range, are put in the furnace and observed. The temperature is judged from which the cone collapses, and which remain unaffected by the heat of the furnace. For example, to verify an approximate temperature of  825*C , cones having melting points of 700, 750, 800 and 850*C are put in the furnace and the temperature is ascertained from their condition after sufficient time has elapsed for them to be effected.

It is sometimes desirable to measure the temperature of  a fuel bed or other body which may be at or above red heat. For this purpose, a thermocouple is frequently not the best means either because the temperatures are too high or the position is in accessible. Under such circumstances , optical pyrometers are used, and these may be employed at any temperatures up to about 4000*C. This type of  instrument is frequently used by the steel smelters to ascertain the temperature of the furnace while melting steel. It compares the intensity of  light being emitted from the furnace with that of some standard source. This instrument may be calibrated so that it reads in degrees of temperature, instead of in units of  electric current. In practice, the telescope is sighted on the hot objected and focused. The lamp filament will then be seen against the red hot background. Current is supplied to a filament from a battery through an adjustable resistance and is gradually incr...

The thermoelectric pyrometer may be employed at any temperature up to about 1200*C for those made of  " base metals "  and to 1500*C for " noble metals ". Base metal couples are made from nickel-chromium wire with nickel-aluminum wire for temperatures up to 1200*C ; nickel - chromium with constant wire up to 800*C. Base-metal couples oxidize more readily than noble-metal couples and they have, therefore, a much shorter life than the other. A noble-metal couple consists of one-wire of pure platinum and the other of an alloy of platinum and rhodium. This is used for temperatures up to 1500*C. The wires in the noble - metal couple do not oxidize readily. The operation of thermoelectric pyrometers is based on the principle that when two wires of different metals are joined to form a complete electric circuit, and one junction is at a higher temperature  than the other. an electric current flows in the circuit. The magnitude of the current depends up on the metal used ...

In all heat-treating practices and in a number of different situations, some equipment are necessary for measuring temperatures. Mercury boils at 357*C, and thus cannot be used above this temperature. Most of  the heat-treatments involve temperatures well above 357*C, so this places a severe limitation on the use of mercury thermometers. They may be suitable for heat treatments which are carried out below this temperature.  A scientific and reliable method of  measuring temperatures is by an instrument called pyrometer.  Two types of pyrometers in common use are : 1. Thermoelectric pyrometers. 2. Optical Pyrometers.

Liquid baths are often used for heating steel in the heat - treating processes. They are usually cylindrical and consist of a sheet steel casing lined with insulating bricks faced with refractory material. This forms the heating chamber in which is suspended a steel pot. The materials which serve as the heating media are usually salt, lead, and oil. Heating may be by gas, oil or electricity. In the case of gas, low-pressure forced- drought fan is coupled with the gas supply and provides a high flame temperature. In the electrically -  heated types, there are externally-heated types, and immersed electrode type.  The externally - heated bath furnaces are used for quenching and tempering up to about 700*C. The electrical resistance elements are located in the walls of furnace and heat the outside wall of the metal pot by radiation. A refractory lining is made to withstand the high temperature and prevent rapid loss of heat. The immersed  electrodes salt furnaces are used fo...