Bicol University College of Engineering

Legazpi City

Properties of Metal &

Machining and Finishing Processes

Submitted by:

Sarah Mae B. Ajon BSEM - 2

Submitted to:

Engr. Victor M. Florece Professor

I. Properties of Metals The distinguishing characteristics or qualities that are used to describe a

substance such as metal are known as its physical properties. Those physical properties which describe the behavior of a metal when it is subjected to particular types of mechanical usage are called mechanical properties.

The chemical properties of metal refer to the characteristics and behavior of the atomic structure in a metal. The structure of the atom, in particular the configuration of the electron cloud, is responsible for the obvious physical differences between metals and nonmetals. Metals have a characteristic luster, are opaque, can be hammered and drawn into various shapes, and conduct electricity. Nonmetal elements, on the other hand, are often gases, and, if solid, nonmetals are generally brittle, sometimes transparent, and do not conduct electricity.

A. Physical Properties

These properties are related to the atomic structure and density of the material. 1. Physical State

Most metals are solid at room temperature but some exceptions are there.

Gallium, mercury, cesium, and rubidium are the only metal elements that melt near room temperature.

Melting Gallium Gallium metal has a melting point of 30° C (86° F), which is lower than our body temperature. In this photo, a sample of gallium melts in a person’s hand. Lester V. Bergman/Corbis

2. Melting Point and Boiling Point

The melting point is the temperature at which a substance passes from a solid state to a liquid state while the boiling point is the temperature at which the substance passes from the liquid state to the gaseous state. Melting and boiling points of metals depend upon the strength of bonds present in the molecules of

metals. Metals with week bonds have low melting and boiling points where as metals having strong bonds have high melting and boiling points.

3. Luster

Metals have lustrous surface. This is due to the presence of free electrons in the atoms of metals. These free electrons absorb energy and jump to the higher orbits. When these return, electrons radiates energy and this gives metals luster. It is somewhat related to reflectivity, which is the ability of a material to reflect light or heat. 4. Heat and Electrical Conductivity

Heat and electrical conductivity is the ability of a material to conduct or transfer heat or electricity. 5. Coefficient of Linear Expansion

The coefficient of linear expansion is the increase in length of a body for a given rise in temperature. The increase is the changed length of a rod for each degree that the temperature is increased. Metal expands when heated and contracts when cooled. It increases not only in length, but also in breath and thickness. The increase in unit length when a solid is heated one degree is called the coefficient of linear expansion. 6. Magnetic Susceptibility

Magnetic susceptibility is the ability of a material to hold a magnetic field when it is magnetized. 7. Specific Gravity

Specific gravity is the ratio of weights between two objects of equal volume, one of which is water. B. Mechanical Properties

Metals are generally very strong and resistant to different types of stresses. Though there is considerable variation from one metal to the next, in general metals are marked by such properties as hardness, the resistance to surface deformation or abrasion; tensile strength, the resistance to breakage; elasticity, the ability to return to the original shape after deformation; malleability, the ability to be shaped by hammering; fatigue resistance, the ability to resist repeated stresses; and ductility, the ability to undergo deformation without breaking.

1. Strength

The strength of a material is the property of resistance to external loads or stresses while not causing structural damage. Ultimate strength is the unit stress, measured in pounds per square inch, developed in the material by the maximum slowly applied load that the material can resist without rupturing in a tensile test. The strength of metals and alloys depends upon two factors: the strength of the crystals of which the metals are constructed, and the tenacity of adherence between these crystals.

The strongest substance known is tungsten-molybdenum; titanium and nickel

follow in order of strength of commercially pure metals. Pure iron is much weaker, but, when alloyed with the chemical element known as “carbon" to make steel, it may then become stronger than any of the pure metals except tungsten. Strength and plasticity are considered the two most important properties that a metal can possess.

a. Tensile Strength

Tensile strength is the ability of a metal to resist being pulled apart by opposing forces acting in a straight line (figure A). It is expressed as the number of pounds of force required to pull apart a bar of material 1 inch wide and 1 inch thick. The tensile test is the one most often used to measure the strength of metals. Pure molybdenum has a high tensile strength and is very resistant to heat. It is used principally as an alloying agent in steel to increase strength, hardenability, and resistance to heat.

b. Shear Strength

Shear strength is the ability of a material to resist being fractured by opposing forces acting in a straight line but not in the same plane (figure B).

c. Compressive Strength

Compressive strength is the ability of a material to withstand pressures acting on a given plane (figure C).

2. Elasticity

Elasticity is the ability of material to return to its original size, shape, and dimensions after being deformed (figure D). Any material that is subjected to an external load is distorted or strained.

Elastically stressed materials return to their original dimensions when the load

is released, provided that the load is not too great. Distortion or deformation is in proportion to the amount of the load, up to a certain point. If the load is too great, the material is permanently deformed, and, when the load is further increased, the material will break. The property of regaining the original dimensions upon removal of the external load is known as elasticity. (a) The elastic limit is the point at which permanent deformation begins. (b) The yield point is the point at which a definite deformation occurs with little or no

increase in load. (c) The yield strength is the number of pounds per square inch required to produce

deformation to the yield point.

3. Ductility

Ductility is the capacity of a material, such as copper, to be drawn or stretched under tension loading and permanently deformed without rupture or fracture. Specifically, the term denotes the capacity to be drawn from a larger to a smaller diameter of wire. This operation involves both elongation and reduction of area (figure E).

4. Malleability

Malleability is the property of a metal to be deformed or compressed permanently without rupture or fracture. Specifically, it means the capacity to be rolled (figure F) or hammered into thin sheets. The property of malleability is similar to but not the same as that of ductility, and different metals do not possess the two properties in the same degree. Lead and tin are relatively high in order of malleability; however, they lack the necessary tensile strength to be drawn into fine wire. Most metals have increased malleability and ductility at higher temperatures. For example, iron and nickel are very malleable when heated bright red.

5. Plasticity

Plasticity is the ability of a metal, such as gold, silver, or lead, to be

deformed extensively without rupture. This property, together with strength, is considered to be the two most important properties that a metal can possess.

6. Fatigue Resistance

Fatigue (materials), in metals, is a progressive deterioration that ultimately results in the breaking of the metal. Fatigue is caused by repeated application of stress to the metal, and the deformation of a material or object as a result of the stress is known as creep. The fatigue strength of a typical steel alloy is about 50 percent of the ultimate strength and 75 percent of the elastic strength but may be considerably lower, particularly for the strongest heat-treated steels.

If the elastic strength of a steel beam is about 45,000 kg (about 100,000 lb), it could withstand a continuous stress of about 41,000 kg (about 90,000 lb) for centuries, with no measurable yielding. A stress of about 36,000 kg (about 80,000 lb) alternately applied and withdrawn, however, would probably cause fatigue failure after a few million applications.

Fatigue is not important in civil engineering structures, in which stress is generally continuous, but in an engine turning at 3000 rpm, any stress to which an engine part is subjected will often be applied millions of times within a few hours of operation. Fatigue failures account for an overwhelming majority of all structural failures in cyclic devices such as engines, and design engineers must consider fatigue strength, rather than elastic strength or ultimate strength, in their calculations.

Alternation of stress will produce failure more rapidly than repetition of stress. Alternations of stress mean the alternate tension and compression on any material. The definition of fatigue is the failure of metals and alloys that have been subjected to repeated or alternating stresses too small to produce a permanent deformation when applied statically. 7. Toughness

Toughness is a combination of high strength and medium ductility.

Toughness is the ability of a material or metal to resist fracture, plus the ability to resist failure after the damage has begun. In short, a tough metal, such as a cold chisel, is one that can withstand considerable stress, slowly or suddenly applied, and that will deform before failure.

Toughness has been defined by some metallurgists as having the property of

absorbing considerable energy before fracture and, therefore, involves both ductility and strength. Toughness is a measure of the total energy absorbing capacity of the material, including the energy of both elastic and plastic deformation under a gradually applied load.

Generally speaking, toughness applies to both strength and plasticity. Thus, a

very easily deformed substance of low strength would not be considered tough, nor would a material of high strength, but with little plasticity, such as hardened tool steel. The true tough metal is one that will rapidly distribute within itself both the stress and resulting strain caused by a rapidly applied load.

8. Hardness

Hardness is the ability of a solid substance to resist surface deformation or abrasion. In metallurgy and engineering, hardness is determined by impressing a small ball or cone of a hard material on the surface to be tested and measuring the size of the indentation. Hard metals are indented less than soft metals. This test to determine the hardness of metal surfaces is known as the Brinell test, named after the Swedish engineer Johann Brinell, who invented the Brinell machine for measuring the hardness of metals and alloys.

It takes a combination of hardness and toughness to withstand heavy

pounding. The hardness of a metal is directly related to its machinability, since toughness decreases as hardness increases. Steel can be hardened by heat-treating it. The object of heat-treating steel is to make the steel better suited, structurally and physically.

9. Brittleness

The term "brittleness" implies sudden failure. It is the property of breaking without warning; that is, without visible permanent deformation. It is the reverse of toughness in the sense that a brittle piece of metal has little resistance to rupture after it reaches its elastic limit. Brittleness can also be said to be the opposite of ductility, in the sense that it involves rupture with very little deformation. In many cases, hard metals are brittle; however, the terms should not be confused or used synonymously.

10. Corrosive Resistance

Corrosive resistance is the resistance to eating away or wearing by the atmosphere, moisture, or other agents, such as acid.

11. Corrosion Fatigue

Failure by corrosion fatigue is a fatigue failure in which corrosion has lowered the endurance limit by the formation of pits which act as centers for the development of fatigue cracks. Moreover, when any protective film that has been placed on the metal is broken by fatigue stresses, corrosion spreads through the cracks in the film and produces pits which act as stress raisers.

If a metal member exposed to fatigue is also exposed to corrosive agencies,

such as a damp atmosphere or oil that has not been freed from acid, the stress necessary to cause failure is lowered. It is interesting to note that the unit stress of an extremely strong heat treated alloy steel that is subjected to corrosion fatigue will be no greater than that of a relatively weak structural steel. The importance of protecting the surfaces of fatigue members against corrosion by galvanizing, plating, etc., is obvious.

12. Abrasion Resistance. Abrasion resistance is the resistance to wearing by friction.

13. Machinability

Machinability is the ease or difficulty with which a material lends itself to being machined.

C. Chemical Properties

In early attempts to explain the electronic configurations of the metals, scientists cited the characteristics of high thermal and electrical conductivity in support of a theory that metals consist of ionized atoms in which the free electrons form a homogeneous sea of negative charge. The electrostatic attraction between the positive metal ions and the free-moving and homogeneous sea of electrons was thought to be responsible for the bonds between the metal atoms. Free movement of the electrons was then held to be responsible for the high thermal and electrical conductivities. The principal objection to this theory was that the metals should then have higher specific heats than they do.

Metallic Bonding Silver, a typical metal, consists of a regular array of silver atoms that have each lost an electron to form a silver ion. The negatively charged electrons distribute themselves throughout the entire piece of metal and form nondirectional bonds between the positive silver ions. This arrangement, known as metallic bonding, accounts for the characteristic properties of metals: they are good electrical conductors because the electrons are free to move from one place to another, and they are malleable (as shown here) because the positive ions are held together by nondirectional forces. A force applied to a malleable substance shifts the positions of the atoms without breaking the bonds that hold them together. © Microsoft Corporation. All Rights Reserved.

1. High reactivity

Metals are highly reactive due to less ionization energy and bigger size of atoms. The most reactive metal is francium (Fr). 2. Reaction with oxygen:

Metals react with oxygen to form basic oxides which convert red litmus to blue litmus for example

4Na + O2 → 2Na2O Sodium oxygen sodium oxide

When these oxides are mixed with water these form bases.

Na2O + H2O → 2NaOH Sodium Oxide water sodium hydroxide

3. Electropositive nature:

Metals are electropositive in nature because these form positively charged ions by losing valence electrons.

Na → Na+ + e-

Mg → Mg2+ + 2e-

4. Reaction with Hydrogen:

Some metals react with Hydrogen to form metal hydrides. Example:

2 Na + H2 → 2NaH 5. Reaction with acids:

Metals react with acids to produce hydrogen gas. Example:

Zn + 2HCl → ZnCl2 + H2

Mg + 2HCl → MgCl2 + H2

Minerals of the Metals The lithium –containing mineral is spodumene (LiAlSi2O6); the beryllium containing mineral is beryl. The minerals containing the rest of the alkaline earth metals are the carbonates and sulfates. The minerals for Sc, Y, and La are the phosphates. Some metals have more than one type of important mineral. For example, in addition to sulfide, iron is found as oxides hematite (Fe2O3) and magnetite (Fe3O4); and aluminum, in addition to oxide, is found in beryl (Be3Al2Si6O18). Technetium is a synthetic element.

II. Machining and Finishing Processes

Machining is resorted to depending on the shape of the component and when close dimensional tolerances or surface finishes are required. The desired shape is obtained by removal of metal from the work piece. The finishing process is the final stage of manufacture.

A. Boring

Boring is a process in which the internal surfaces of revolution are generated using a traversing tool. It is used to enlarge and finish holes accurately. This may be done on a lathe or a milling machine.

If boring is done in a lathe, the workpiece is held in the rotating chuck. Where higher accuracy is required, machines specially designed for boring are used. In such machines the workpiece is clamped to a table and the cutting tool rotates and traverses the depth of the bore. Internal bores of undercarriage struts and hydraulic system

jacks are examples of components bored in horizontal boring machines.

Boring is a machine operation in which the work is in contact with a single point tool. A workpiece may be held in a 3, 4, or 6 jaw chuck and bullets.

Jig Boring

Jig boring machine is a vertical boring machine designed for precision boring of components mainly used for tooling applications and for certain aero-engine components such as casings.

B. Broaching

Broaching is an operation that completes the cutting in one stroke or cut. Metal is removed by a rotating multiple tool cutter called a broach, against the workpiece surface. The cutter is pushed through or pulled through a hole or across a surface.

Broaching can be done on both internal and external surfaces. The teeth of a broaching tool are equally surfaced so that as the tool

advances into the workpiece, each tooth removes a specified amount of metal. Close dimensional tolerances and surface finish are achieved by broaching. Slots, keyways and serrations in levers, shafts are usually broached.

C. Drilling

Drilling is a process of producing holes, countersinking and spot facing. It is an economical way of removing large amounts of metal to create semi-precision round hole or cavity. The cutting tool, called drill is usually held in the machine rotating spindle and the drill is forced into the workpiece. Drilling allows a person to make holes through boards, metals, and other materials. It is used for the last removal of stock on preparation for other operations like boring, reaming or tapping.

The Drill Press

The drill press is a machine used to hold drill bits which will produce cylindrical holes. It is used for producing cylindrical holes as well as reaming, boring, counter-boring, counter-sinking, honing, lapping and tapping. There are three major types:

Sensitive drill (light drilling) Upright drill (heavy duty drilling) Radial arm drill press (large,

heavy workpieces)

The sensitive drill press is a high speed machine that drills very small holes. It is used for light duty work. It has an extremely precise quill and spindle, and is capable of speeds of over 40,000 rpm. Its maximum drill size is of 1/32 of an inch.

D. Facing

Facing is a lathe operation in which the cutting tool removes metal from the end of the workpiece or a shoulder. It is a machine operation where the work is rotated against a single point tool. A workpiece may be held in a 3, 4, or 6 jaw chucks, collets or a faceplate.

E. Shaping

It is an operation used to produce flat surfaces. When the cutting tool reciprocates while the work is fed towards

the tool removing material on each stroke, it is called shaping.

Shaper

The shaper is used primarily to produce flat surfaces. The tool slides against the stationary workpiece and cuts on one stroke, returns to its starting position, and then cuts on the next stroke after a slight lateral displacement. In general, the shaper can produce almost any surface

composed of straight-line elements. It uses a single-point tool and is relatively slow, because it depends on reciprocating (alternating forward and return) strokes. For this reason, the shaper is seldom found on a production line. It is, however, valuable for tool and die rooms and for job shops where flexibility is essential and relative slowness is unimportant because few identical pieces are being made. There are three types of shapers:

Horizontal / plain or universal Vertical (slotter and key seater) Special

F. Milling

In this, metal is removed by rotating a multiple tool cutter. The cutting action is intermittent unlike in turning or boring. The workpiece is clamped to the table of the machine and the milling cutter is fixed to the rotating spindle. Milling operations are

used for machining flat surfaces or cubic components, and curved surfaces of complex shapes.

Milling Machine

In a milling machine, a workpiece is fed against a circular device with a series of cutting edges on its circumference. The workpiece is held on a table that controls the feed against the cutter. The table conventionally has three possible movements: longitudinal, horizontal, and vertical; in some cases it can also rotate. Milling machines are the most versatile of all machine tools. Flat or contoured surfaces may be machined with

excellent finish and accuracy. Angles, slots, gear teeth, and recess cuts can be made by using various cutters.

A horizontal milling machine uses a rotating tool to produce flat surfaces. It is used for heavy stock removal. The spindle is mounted on a horizontal position and is available in different size tables.

A vertical milling machine uses a rotating tool to produce flat surfaces. It is a very flexible, light-duty machine. The spindle is mounted on a vertical position.

G. Tapping

Tapping is the process of cutting a thread inside a hole so that a cap screw or bolt can be threaded into the hole. Also, it is used to make threads on nuts. Tapping is done with a tool called a "Tap". Tapping may be done by:

hand lathe machine milling machine tapping machine

H. Turning

Turning is a lathe operation in which the cutting tool removes metal from the outside diameter of a workpiece. A single point tool is used for turning. A workpiece may be held in a 3, 4, or 6 jaw chuck, collets or may also be held between centers.

Turning operations generate external surfaces of revolution on rotating workpieces using a traversing tool. The typical parts which are machined by turning were shafts, axles, housings, conical parts, etc. It is the first operation prior to boring, and is more economical than milling and a better tolerance and finish is achieved.

Lathe

A lathe, the oldest and most common type of turning machine, holds and rotates metal or wood while a cutting tool shapes the material. The tool may be moved parallel to or across the direction of rotation to form parts that have a cylindrical or conical shape or to cut threads.

The lathe is used for producing cylindrical work. The workpiece is rotated while the cutting tool movement is controlled by the machine. The lathe may be used for: boring, drilling, tapping, turning, facing, threading, polishing, grooving, knurling, and trepanning.

I. Reaming

Reaming is a finishing operation for enlarging a drilled hole as to obtain close tolerance and good surface finish. The cutting tool is called a reamer. It removes a small amount of metal from a hole already drilled. Machinists may use hand or machine reamers depending on the job they are performing.

J. Grinding

It is a metal cutting process that uses an abrasive tool called grinding wheel. Grinding processes remove very small chips in very large numbers by cutting the action of many small individual abrasive grains. The cutting elements of grinding wheel are grains of abrasive material having high hardness and high heat resistance.

They have sharp edges and are held together by bonding materials. Grinding provides high accuracy and good surface finish. Therefore it is used as finishing operation. This process removes comparatively little material usually from 0.25 mm to 0.5 mm. Tolerances as small as 0.0025 mm can be obtained by commercial grinding.

During grinding temperature rises with the increased wear of grains which may lead to distortion of work piece structural changes and crack formation in the ground surface. Hence an abundant flow of coolant is commonly used in grinding. The coolant also slows down softening of wheel bond which is due to heating.

Grinding wheels usually consist of particles of a synthetic abrasive, such as silicon carbide or aluminum oxide, mixed with a vitrified or resinoid bonding material. Grinding can be coarse or fine, depending on the size of the grit used in the grinding wheel. Metal and glass can be ground to a mirror finish and an accuracy of 0.0000025 cm. On a production basis, hundreds of millions of parts per year are routinely ground to an accuracy of 0.001 cm. There are two types of grinding:

Non-precision Grinding

It is a cutting technique used when the grinding does not need to be accurate. Non-precision grinding is a free-hand operation done on a pedestal or bench grinder.

Precision Grinding

Precision grinding is a cutting technique used when close tolerances and very smooth finishes are required. Precision grinding allows very small amounts

of material to be removed from a workpiece. This is extremely useful in acquiring smooth finishes.

Grinding Machine

Grinding machine machines metal parts with an abrasive wheel which can grind to close tolerances. Grinding machines can produce parts of the identical size, shape, and finish quality. There are various types of grinding machines:

Plain Surface grinders Rotary Surface grinders Tool & Cutter grinders Universal grinders Internal grinders

K. Honing

Honing is an abrading process mostly used for finishing internal cylindrical surface such as drilled or bored holes. Removal of metal by honing involves the use of a number of bonded abrasives stones called hones. Honing stones are formed by bonding abrasives like aluminum oxide or silicon carbide in vitrified or resinoid bond.

Material like sulfur, resin or wax can be added to the ponding agent to improve the cutting action. It is similar to lapping where abrasive sticks are mounted in a rotating tool. It is

capable of accuracies of less than 1/10,000th of one inch.

Honing improves the accuracy and finish of automobile cylinder bores, hydraulic cylinders, and similar parts. The honing machine consists of four fine-grain abrasive stones attached to an expandable tool that is then slowly revolved and oscillated inside the cylinder until the desired finish and diameter are obtained.

L. Lapping

Lapping is the process of producing an extremely accurate highly finished surface. Lapping is carried out by means of lapping shoes called laps. The laps are made up of soft cast iron, copper, lead and brass. The lap material is always softer than the material to be finished.

Fine abrasive particles are charged (caused to become embedded) into the lap, and the two parts are

then rubbed together with irregular strokes. Silicon carbide, aluminum oxide and diamond dust are the commonly used lapping powders. Oil and thin greases are used to spread the abrasive powders. As the charged lap is rubbed against work piece surface, the abrasive particles in the surface of the lap remove small amount of material from the work piece surface. Thus it is the abrasive that does the cutting and the soft lap is not worm away. The material removed by lapping is usually less than 0.025mm.

lap

M. Buffing

It is also a surface finishing process and is used to produce a lustrous surface of attractive appearance. There is a very little amount of material is removed. In this process also the work piece is brought in contact with the revolving wheel. Buffing wheels are made up of felt or cotton. Powdered abrasives are applied to the surface of the wheel. The abrasive may consist of iron oxide, chromium oxide, emery etc. In this way very less amount of material is removed.

N. Polishing

It is surface finishing process by which scratches and tool marks are removed with a polishing wheel. The work piece is brought in contact with the revolving wheel that has been charged with a very fine abrasive. Polishing wheels are made of canvas, leather or paper. Tolerances of 0.025mm or less can be obtained in machine polishing.

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