8. Properties of Materials.
Density is the amount of mass in a unite volume.
It is measured in kilograms per cubic meter.
Most materials have a high density.
Aluminum alloys (2800 kg/m3) are considerably less dense than steels (7800kg/m3).
Density is important in any application where the material must not be heavy.
Stiffness (rigidity) is the measure of the resistance to deformation such as stretching or bending.
Stiffness is important when a rigid structure is to be made.
Strength is the force per unit area that a material can support without failing.
The yield strength is the stress at which a material first deforms plastically.
The fracture strength is the strength at which it breaks.
Many materials have a higher strength in compression than in tension.
Ductility is the ability of material to deform without breaking.
One of the great advantages of metals is their ability to be formed into the shape that is needed, such as car body parts.
Materials that are not ductile are brittle.
Toughness is the resistance of a material to breaking when there is a crack in it.
Brittle materials have low toughness: glass can be broken along a chosen line by first scratching it with a diamond.
Composites usually have greater toughness than their constituent materials.
The example of a very tough composite is fiberglass that is very flexible and strong.
Creep resistance is the resistance to a gradual permanent change of shape, and it becomes especially important at higher temperatures.
Machine parts, for example, operate at high temperatures and under high tensile forces without gradually extending.
9. Metals and Their Properties. Steel Treatment.
Metals are materials most widely used in industry because of their properties such as density, malleability, ductility.
Metals vary greatly in their properties.
For example, lead is soft and can be made by hand, while iron can only be worked by hammering at red heat.
The regular arrangement of atoms in metals gives them a crystalline structure.
Irregular crystals are called grains.
The properties of the metals depend on the size, shape, orientation, and composition of these grains.
In general a metal with small grains will be harder and stronger than one with coarse grains.
Small amounts of other metals(less than 1 per cent are often added to a pure metal.
This is called alloying and it changes the grain structure and properties of metals.
All metals can be formed by drawing, rolling, hammering, and extrusion, but some require hot –working.
Metals are subject to metal fatigue and to creep causing deformation and failure.
Both effects are taken into account by engineers when designing, for example, airplanes, gas-turbines, and pressure vessels.
The most important method in industry is iron and its alloy-steel (an alloy of iron and carbon).
There are following methods of steel treatment: quenching, tempering and annealing.
Quenching is a heat treatment when metal at high temperature is rapidly cooled by immersion in water or oil.
It makes steel harder and more brittle.
Tempering is re-heating to an intermediate temperature and cooling slowly, reduces its hardness and brittleness.
Annealing is a heat treatment in which a material at high temperature is cooled slowly.
After cooling the metal again becomes malleable and ductile.
All these methods are used to obtain steels with certain mechanical properties.
10. Technological Processes.
Drawing consists of pulling metal through a die.
An example of drawing is wire drawing.
Sheet metal forming is widely used when parts of certain shape and size are needed.
It includes forging, bending, and shearing
Each of these processes may be used alone but often al three are used on one part.
It is used for example to make a root of an automobile from a flat sheet, three edges are gripped and the piece is pulled in tension over a lower die.
Forging is the shaping of a piece of metal by pushing with open or closed dies.
It is usually done hot.
Open-die forging is usually done by hammering a part between two flat faces.
For this operation air or steam hammers are used nowadays.
They control the force and the rate of forming.
Closed-die forging is the shaping of hot metal within the walls of the two dies.
With closed dies, parts can be made to close tolerances.
That’s why these parts don’t require finishing machining.
Two closed forging operations are given special names.
They are upsetting and coining.
Coining involves small strains and is done cold, usually used for forming metal coins.
Upsetting involves the flow of the metal back upon itself.
They are usually used for forming a head of a nail or a bolt.
11. Welding.
Welding is a process when metal parts are joined together by the application of heat, pressure or a combination of both.
The processes of welding can be divided into two main groups: pressure welding when the weld is achieved by pressure and heat welding when the weld is achieved by heat.
Heat welding is the most common welding today.
Welding is necessary in almost all productions where metals are used.
The welding process depends greatly on the properties of the metal, the purpose of their application and the available equipment.
Welding is necessary in almost all productions where metals are used.
Welding processes are classified according to the sources of heat and pressure used: gas welding, arc welding, resistance welding, laser welding and electron-beam welding.
Gas welding is a non-pressure process using heat from a gas flame.
The flame is applied directly to the metal edges to be joined and simultaneously to a filler metal in the form of wire or rod.
Arc welding is the most important welding process for joining steels.
It requires a continuous supply of either direct or alternating electrical current.
This current is used to create an electric arc which generates enough heat to melt metal and create a melt.
Arc welding is faster because the acceleration of heat is high.
The most widely used arc welding processes are shielded metal arc, gas-tungsten arc, gas-metal arc, and submerged arc.
Resistance welding is widely employed in many fields of sheet metal or wire manufacturing and is often used for welds machines especially in automobile industry.
12. Types of Welding.
There are the following types of welding: non-consumable electrodes arc welding, shielded metal arc welding, gas-metal arc welding, submerged arc welding.
As a non-consumable electrodes tungsten or carbon electrodes can be used.
In gas tungsten arc-welding a tungsten electrode is used in place of the metal electrode used in shielded metal arc welding.
This process can be used with nearly all metals and produces a high quality weld.
However, the rate of welding is considerably slower than in other processes.
In shielded metal arc welding, a metallic electrode, which conducts electricity, is coated with flux and connected with a source of electric current
The metal to be welded is connecter to the other end of the same source of current.
This process is used mainly for welding steels.
In gas-metal arc welding a bare electrode is shielded from the air by surrounding it with argon or carbon dioxide gas and sometimes by coating the electrode with flux.
The electrode is fed into the electric arc, and melts off in droplets that enter the liquid metal of the weld seam.
Most metals can be joined by this process.
Submerged arc welding is similar to gas-metal arc welding, but in this process no gas is used to shield the weld.
Instead of gas they use a layer of granular fusible material that covers the weld seam.
This process is also called electroslag welding.
It is very efficient but can be used only with steels.
13. Dies.
Dies are tools for the shaping of solid materials, especially those employed in the press working of cold metals.
In press work dies are used in pairs.
The metal to be firmed, usually a sheet, is placed over the matrix on the press.
The punch is mounted on the press and moved down by hydraulic or mechanical force.
A number of different forms of dies are employed for different operations.
The simplest are piercing dies used for punching holes.
Bending and folding dies are designed to make simple or compound bends.
A combination die is designed to perform more than one of the above operations in one stroke of the press.
A progressive stamp permits successive forming operations with the same die.
In coining metal is forced to flow into matching dies each of which bears an engraved design.
In the manufacture of a wire, a drawplate is usually employed.
This tool is a metal plate containing a number of holes successfully less in diameter and known as a wire dies.
Wire drawing dies are made from extremely hard materials, such as tungsten, carbide or diamonds.
Thread cutting dies are used for cutting threads on bolts or on the outside pipes.
It is usually made of hardened steel in the form of a round plate with a hole in the centre.
The corresponding tool for cutting an inside thread, such as tat inside a nut, is called a tap.
14. Machine-Tools.
Machine tools are used to shape metals and other materials.
Most machine tools are now electrically driven.
All the machine tools have facilities for holding both the workpiece and the tool and for controlling their movements.
Machine tools usually work materials mechanically but other machining methods have been developed lately.
They include chemical machining, spark erosion to machine very hard materials.
Other machining methods include drilling using ultrasound, and cutting by means of a laser beam.
Machine tools are controlled numerically.
Lathe is still most important machine tool.
It produces parts of circular cross section by turning the workpiece on its axes and cutting its surface with its sharp stationary tool.
That allows continuous rotation of the workpiece at a variety of speeds.
In a milling machine the cutter is a circular device with a series of cutting edges on its circumference.
The workpiece is mounted on a table that controls the feed against the cutter.
The table has three possible movements: longitudinal, horizontal, and vertical.
Milling machines are used to make angles, slots, gear teeth, and cuts.
Drilling and boring machines are used to drill a hole, to enlarge a hole, to cut threads for a screw, or to create a smooth hole.
Drilling machines are different in size and function, from portable drills to radial drilling machines.
The shaper is used mainly to produce different flat surfaces.
The planer is intended to produce vertical, horizontal or diagonal cuts.
Grinders remove metal by a rotating abrasive wheel.
Grinding can be carried out on fragile materials that cannot be machined by other conventional devices.
15. Automation. Types of Automation.
Automating is performing certain tasks by machines.
The most familiar example of automated system is an assembly plant for automobiles or other complex products.
Such devices as automatic pilots, automatic telephone equipment and automated control systems are used to perform various operations much faster and better than could be done by people.
Automated manufacturing had several steps in its development.
The first step is mechanization that helped to achieve better production efficiency.
Industrial robots are now used to transfer, manipulate and position light and heavy workpieces performing all the functions of a transfer machine.
The feedback principle is used in all automatic-control mechanisms when machines have the ability to correct themselves.
Using feedback devices machines can start, stop, speed up, slow down, count, inspect, test, compare, and measure.
Computers have greatly facilitated the use of feedback in manufacturing processes.
Another development using automation is the flexible manufacturing system: a computer can be used to monitor and control the operation of the whole factory.
Automation is used in the following fields: in communications (especially in the telephone industry), in the railways (help to monitor the movement and location of trains).
There are several types of automation: fixed automation, programmable automation and flexile automation.
Fixed automation refers to automated machines in which the equipment configuration allows fixed sequence of processing operations.
It is used in the automobile industry, automatic assembly machines, and certain chemical processes.
Programmable automation is a form of automation for producing products in large quantities.
For each new product the production equipment must be reprogrammed and changed over; a numerical –control machine –tool is a good example of programmable automation.
Flexible automation is a kind of programmable automation in which the number of products is limited so that the changeover of the equipment can be changed very quickly and automatically.
Flexible automation allows a mixture of different products to be produced one right after another.
16. Robots in Industry.
Today most robots are used in manufacturing operations.
The application of robots can be divided into three categories: material handling, processing operations, assembly and inspection.
Material handling is the transfer of material and loading and unloading of machines.
Material transfer applications require the robot to move materials or work parts from one another.
Many of these tasks are relatively simple: robot picks up parts from one conveyer and place them on another.
Other transfer operations are more complex, such as placing parts in an arrangement that can be calculated by the robot.
Some robots load and unload parts of machines.
This requires the robot to be equipped wit a gripper that can grasp parts.
In processing operations robot manipulates a tool to perform a process on the work part.
Examples of such applications include spot welding, continuous arc welding and spray painting.
Other operations in this category include grinding and polishing in which a rotating spindle serves as a robot’s tool.
The third application area of industrial robots is assembly and inspection.
The use of robots in assembly is expected to increase because of the high cost of manual labour.
In a typical inspection job, the robot positions a sensor with respect to the work part and determines whether the part answers the quality specifications.
In nearly all industrial robotic applications, the robot provides a substitute for human labour.
There are certain characteristics of industrial jobs performed by humans that can be done by robots:
the operation is repetitive
the operation is hazardous or uncomfortable(for example; spray painting, spot welding, arc welding, and certain machine loading and unloading tasks)
the work piece or tool are too heavy and difficult to handle
the operation allows the robot to be used on two or three shifts.
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