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INTRODUCTION
Spark testing is a method of determining general classification of ferrous materials. It normally entails taking a piece of metal, usually scrap, and applying it to a grinding wheel in order to observe the sparks emitted. These sparks can be compared to a chart or to sparks from a known test sample to determine the classification. Spark testing also can be used to sort ferrous materials, establishing the difference from one another by observing whether the spark is the same or different. Spark testing is used because it is quick, easy, and inexpensive. Moreover, test samples do not have to be prepared in any way, so, often, a piece of scrap is used. The main disadvantage to spark testing is its inability to identify a material positively; if positive identification is required, chemical analysis must be used..The spark comparison method also damages the material being tested, at least slightly. Spark testing most often is used in tool rooms, machine shops, heat treating shops, and foundries.
In 1909, Max Bermann, an engineer in Budapest, was the first to discover that spark testing can be used reliably to classify ferrous material. He originally claimed to be able to distinguish different types of ferrous materials based on percent carbon and principal alloying elements. Moreover, he claimed to achieve an accuracy of 0.01% carbon content.
1. About Metals, Non Metals and Alloy
1.1 Metal
A metal is a solid material that is typically hard, opaque, shiny, and features good electrical and thermal conductivity. Metals are generally malleable —that is, they can be hammered or pressed permanently out of shape without breaking or cracking—as well as fusible (able to be fused or melted) and ductile (able to be drawn out into a thin wire).91 of the 118 elements in the periodic table are metals
1.2 Non metal
Non – metals may be solids, liquids or gases at room temperature. They do not have luster (Exceptions – Diamond and Iodine) they generally have low density. They are usually bad conductors of heat and electricity. (Exception – carbon in the form of gas carbon and graphite) Non-metals are not malleable and ductile. They are brittle when solid. They can be crushed into powder. They have different types of structures with covalent and van-der-Walls’ bonds Non- metals are generally soft (Exception: Diamond). Non- metals usually have low tensile strength.
1.3 Alloy
An alloy is a mixture of two or more elements in which the main component is a metal. Most pure metals are either too soft, brittle or chemically reactive for practical use. Combining different ratios of metals as alloys modifies the properties of pure metals to produce desirable characteristics. The aim of making alloys is generally to make them less brittle, harder, resistant to corrosion, or have a more desirable color and luster. Of all the metallic alloys in use today, the alloys of iron (steel, stainless steel, cast iron, tool steel, alloy steel) make up the largest proportion both by quantity and commercial value. Iron alloyed with various proportions of carbon gives low, mid and high carbon steels, with increasing carbon levels reducing ductility and toughness. The addition of silicon will produce cast irons, while the addition of chromium, nickel and molybdenum to carbon steels (more than 10%) results in stainless steels.
Alloys specially designed for highly demanding applications, such as jet engines may contain more than ten elements
.2. GRINDING MACHINE
Grinding is the process of removing metal by the application of abrasives which are bonded to form a rotating wheel. When the moving abrasive particles contact the work piece, they act as tiny cutting tools, each particle cutting a tiny chip from the work piece. It is a common error to believe that grinding abrasive wheels remove material by a rubbing action; actually, the process is as much a cutting action as drilling, milling, and lathe turning. The grinding machine supports and rotates the grinding abrasive wheel and often supports and positions the work piece in proper relation to the wheel. The grinding machine is used for roughing and finishing flat, cylindrical, and conical surfaces; finishing internal cylinders or bores; forming and sharpening cutting tools snagging or removing rough projections from castings and stampings; and cleaning, polishing, and buffing surfaces. Once strictly a finishing machine, modem production grinding machines is used for complete roughing and finishing of certain classes of work. Grinding machines have some special safety precautions that must be observed. From the simplest grinding machine to the most complex, grinding machines can be classified as utility grinding machines, cylindrical grinding machines. and surface grinding machines. The average machinist will be concerned mostly with floor-mounted and bench-mounted utility grinding machines, buffing machines and reciprocating Surface grinding machines.
Utility grinding machine is intended for offhand grinding where the work piece is supported in the hand and brought to bear against the rotating grinding abrasive wheel.
The accuracy of this type of grinding machine depends on the operator’s dexterity. skill, and knowledge of the machine’s capabilities and the nature of the work. The utility grinding machine consists of a horizontally mounted motor with a grinding abrasive wheel attached to each end of the motor shaft. The electric-motor-driven machine is simple and common. It may be bench-mounted or floor-mounted. Generally, the condition and design of the shaft bearings as well as the motor rating determine the wheel size capacity of the machine. Suitable wheel guards and tool rests are provided for safety
and ease of operation
2.2. Floor Mounted Utility Grinding Machine
The typical floor-mounted utility grinding machine stands waist-high and is secured to the floor by bolts. The floor mounted utility grinding machine shown in Figure 5-1mounts two 12-inch-diameter by 2-inch-wide grinding abrasive wheels. The two wheel arrangement permits installing a coarse grain wheel for roughing purposes on one end of the shaft and a fine grain wheel for finishing purpose son the other end this saves the time that would be otherwise consumed in changing wheels. Each grinding abrasive wheel is covered by a wheel guard to increase the safety of the machine. Transparent eye shields. Spark arresters. and adjustable tool rests are provided for each grinding wheel. A tool tray and a water pan are mounted on the side of the base or pedestal. The water pan is used for quenching carbon steel cutting took as they are being ground. Using the 12-inch wheel, the machine provides a maximum cutting speed of approximately 5.500 SFPM. The 2-HP electric motor driving this machine has a maximum speed of 1750 RPM.
2.3. Bench Type Utility Grinding Machine
Like the floor mounted utility grinding machine, one coarse grinding wheel and one fine grinding wheel are usually mounted on the machine for convenience of operation. Each wheel is provided with an adjustable table tool rest and an eye shield for protection. On this machine, the motor is equipped with a thermal over-load switch to stop the motor if excessive wheel pressure is applied thus preventing the burning out of the motor. The motor revolve at 3.450 RPM maximum to provide a maximum cutting speed for the 7 inch grinding wheels of about 6,300 surface feet per minute (SFPM). Here in our experiment we have used Floor mounted Grinding machine.
2.4. GRINDING WHEELS
STANDARD TYPES OF GRINDING WHEELS
Grinding wheels come in many different sizes, shapes, and abrasives . Some of the various types are listed below.
Straight
Straight wheels, numbers 1, 5, and 7, are commonly applied to internal, cylindrical, horizontal spindle, surface, tool, and offhand grinding and snagging. The recesses in type
numbers.5 and 7 accommodate mounting flanges. Type number 1 wheels from 0.006-inch to l/8-inch thick are used for cutting off stock and slotting.
Cylinder
Cylinder wheels, type number 2, may be arranged for grinding on either the periphery or side of the wheel.
Tapered
Tapered wheels, type number 4, take tapered safety flanges to keep pieces from flying if the wheel is broken while snagging.
Straight Cup
The straight cup wheel, type number 6, is used primarily for surface grinding, but can also be used for offhand grinding of flat surfaces. Plain or beveled faces are available.
Flaring Cup
The flaring cup wheel, type number 11, is commonly used for tool grinding. With a resinous bond, it is useful for snagging. Its face may be plain or beveled.
Dish
The chief use of the dish wheel, type number 12, is in tool work. Its thin edge can be inserted into narrow places, and it is convenient for grinding the faces of form-relieved milling cutters and broaches.
Saucer
The saucer wheel, type number 13, is also known as a saw gummer because it is used for sharpening saws.
2.5. ABRASIVE MATERIALS
The abrasive grains are the cutting took of a grinding wheel. They actually cut small pieces or chips off the work as the wheel rotates. The shape of each grain is irregular with several sharp cutting edges. When these edges grow dull, the forces acting on the wheel tend to fracture the abrasive grains and produce new cutting edges. Most grinding wheels are made of silicon carbide or aluminum oxide, both of which are artificial abrasives. Silicon carbide is extremely hard but brittle. Aluminum oxide is slightly softer but is tougher than silicon carbide. It dulls more quickly, but it does not fracture easily therefore it is better suited for grinding materials of relatively high tensile strength.
Abrasive grains are selected according to the mesh of a sieve through which they are sorted. For example, grain number 40 indicates that the abrasive grain passes through a sieve having approximately 40 meshes to the linear inch. A grinding wheel is designated coarse, medium, or fine according to the size of the individual abrasive grains making up the wheel. The abrasive particles in a grinding wheel are held in place by the bonding agent. The percentage of bond in the wheel determines, to a great extent, the “hardness” or “grade” of the wheel. The greater the percentage and strength of the bond, the harder the grinding wheel will be. “Hard” wheels retain the cutting grains longer, while “soft” wheels release the grains quickly. If a grinding wheel is “too hard” for the job, it will
glaze because the bond prevents dulled abrasive particles from being released so new grains can be exposed for cutting. Besides controlling hardness and holding the abrasive, the bond also provides the proper safety factor at running speed. It holds the wheel together while centrifugal force is trying to tear it apart. The most common bonds used in grinding wheels are vitrified, silicate, shellac, resinoid, and rubber. A vast majority of grinding wheels have a vitrified bond. Vitrified bonded wheels are unaffected by heat or cold and are made in a greater range of hardness than any other bond. They adapt to practically all types of grinding with one notable exception: if the wheel is not thick enough, it does not withstand side pressure as in the case of thin cutoff wheels.
Silicate bond releases the abrasive grains more readily than vitrified bond. Silicate bonded wheels are well suited for grinding where heat must be kept to a minimum, such as grinding edged cutting tools. It is not suited for heavy-duty grinding. Thin cutoff wheels are sometimes made with a shellac bond because it provides fast cool cutting.
Resinoid bond is strong and flexible. It is widely used in snagging wheels (for grinding irregularities from rough castings), which operate at 9,500 SFPM. It is also used in cutoff wheels. In rubber-bonded wheels, pure rubber is mixed with sulfur. It is extremely flexible at operating speeds and permits the manufacture of grinding wheels as thin as 0.006 inch for slitting nibs. Most abrasive cutoff machine wheels have a rubber bond. The grade of a grinding wheel designates the hardness of the bonded material.
A soft wheel is one on which the cutting particles break away rapidly while a hard wheel is one on which the bond successfully opposes this breaking away of the abrasive grain. Most wheels are graded according to hardness by a letter system. Most manufacturers of grinding abrasive wheels use a letter code ranging from A (very soft) to Z (very hard). Vitrified and silicate bonds usually range from very soft to very hard, shellac and resinoid bonds usually range from very soft to hard, and rubber bonds are limited to the medium to hard range.
The grade of hardness should be selected as carefully as it illustrates sections of three grinding abrasive the grain size. A grinding abrasive wheel that is too soft wheels with different spacing of grains. If the grain and bond will wear away too rapidly, the abrasive grain will be materials in each of these are alike in size and hardness, the discarded from the wheel before its useful life is wheel with the wider spacing will be softer than the wheel realized. On the other hand, if the wheel is too hard for with the closer grain spacing. Thus, the actual hardness of the job, the abrasive particles will become dull because the grinding wheel is equally dependent on grade of hardness the bond will not release the abrasive grain, and the and spacing of the grains or structure wheel’s efficiency will be impaired.
2.6. ABRASIVE WHEEL STRUCTURE
Bond strength of a grinding wheel is not wholly dependent Every grinding wheel is marked by the manufacturer with a upon the grade of hardness but depends equally on the stencil or a small tag. The manufacturers have worked out a structure of the wheel, that is, the spacing of the grain or its standard system of markings density. The structure or spacing is measured in number of grains per cubic inch of wheel volume.
Conditions under which grinding wheels are used vary considerably, and a wheel that is satisfactory on one machine may be too hard or soft for the same operation on another
machine. The softer and more ductile the material, the coarser the grain size. The larger the amount of stock to be removed, the coarser the grain size. The finer the finish desired, the finer the grain size. The harder the material, the softer the wheel. The smaller the arc of contact, the harder the grade should be. The arc of contact is the arc, measured along the periphery of the wheel, that is in contact with the work at any instance. It follows that the larger the grinding wheel, the greater the arc of contact and, therefore, a softer wheel can be used. The higher the work speed with relation to the wheel speed, the milder the grinding action and the harder the grade should be. The better the condition of the grinding machine and spindle bearings, the softer the wheel can be. The softer tougher, and more ductile the material, the wider the grain spacing. The finer the finish desired, grain spacing should be. Surfacing operations require open structure (wide grain spacing).
Cylindrical grinding and tool and cutter grinding are best performed with wheels of medium structure. Thin cutoff wheels and other wheels subject to strains require resinoid, shellac, or rubber bonds. Solid wheels of very large diameters require a silicate bond. Vitrified wheels are usually best for speeds up to 6,500 SFPM and resinoid, shellac, or rubber wheels are best for speeds above 6,500 SFPM. Resinoid, shellac, or rubber bonds are generally best where a high finish is required.
Grinding wheels wear unevenly under most general grinding operations due to uneven pressure applied to the face of the wheel when it cuts. Also, when the proper wheel has not been used for certain operations, the wheel may become charged with metal particles, or the abrasive grain may become dull before it is broken loose from the wheel bond. [n these cases it is necessary that the wheel be dressed or trued to restore its efficiency and accuracy. All wheels do not emit the same tone; a low tone does not necessarily mean a cracked wheel. wheels are often filled with various resins or greases to modify their cutting action, and resin or grease deadens the tone. Vitrified and silicate wheels emit a clear metallic ring. Resin, rubber, and shellac bonded wheels emit a tone that is less clear. Regardless of the bond, the sound of a cracked wheel is easy to identify.
The abrasive stick dresser comes in two shapes: square for hand use, and round for mechanical use. It is often used instead of the more expensive diamond dresser for dressing shaped and form wheels. It is also used for general grinding wheel dressing.
3. Metals used in Experiment
Cold roll steel is metal that has been made by rolling it out at room temperature. When they cold roll a sheet it is to get a better finish or more strength. Cold roll steel sheets offers a variety of outstanding properties including easy formability and a smooth, clean surface and are used in automobiles, appliances, furniture and many other everyday items.
Alloying steel with tungsten, chromium, molybdenum, etc produces alloy steel known as high speed steel (HSS). It can retain the hardness up to 600°c. it can operate at cutting speed 2-3 times greater than that of carbon steels
It is basically an alloy of carbon and silicon with iron. it contains carbon(2.5-3.8)%,silicon(1.1-2.8)%, Manganese(0.4-1)%,Phosphorous-0.15%. Cast iron is a hard and brittle material. It has excellent vibration damping property. It is mainly use for machine tool structure, piston rings, cylinder block of IC engine cylinder block of IC engine etc.
The chief ore of aluminum is bauxite. It is a silver white light metal having a specific gravity 2.7 and melting point 658°c. It has good electrical conductivity and high resistance to corrosion. It is extensively used in air craft and automobile components.
Stainless Steel is made by using chromium. The minimum amount of chromium used to make stainless steel is 10.5%, it is the chromium that makes the steel stainless. Chromium also improves the corrosion resistance by forming a chromium oxide film on the steel. Other elements used to make stainless steel are nickel, nitrogen and molybdenum.
Mild steel is a carbon steel typically with a maximum of 0.25% carbon and 0.4-0.7% manganese, 0.1-0.5% silicon and some traces of other elements such as phosphorus, it may also contain lead or sulphur. Mild steel is a general term for a range of low carbon steels having good strength and can be bent, worked or can be welded into an endless variety of shapes for uses from vehicles to buildings.
The basic ingredient of most carbide is tungsten carbide. Carbide suitable for steel machining consists of 82% tungsten carbide, 10% titanium carbide and 8% cobalt. Carbides have very high hardness over a wide range of temperatures and have relatively high thermal conductivity.
Brass is an alloy of copper and zinc. It is used for decoration for its bright gold-like appearance, for applications where low friction is required such as locks, gears, bearings, doorknobs, ammunition casings and valves, for plumbing and electrical applications and musical instruments. Brass has low melting point of 900° to 940°. The density of brass is approximately 0.303 lb/cubic inch, 8.4 to 8.73 grams per cubic centimeter.
It is one of the most widely used non ferrous metals in industry. It is soft and ductile metal with a reddish brown appearance .its specific gravity is 8.9 and melting point 1053°c. It is largely used in electrical cables and wires, electric machinery and appliances and making coins.
4. PROCEDURE
1. Firstly collect the metals samples and keep them together.
2. Clean the metal pieces with the help of Sand paper, in order to remove the oxides, dirt present in the surface.
3. Switch on the power supply of the Stationery Grinding Wheel.
4. Take any one piece from the given metal samples.
5. Hold the work piece tightly and move the piece towards the grinding wheel.
6. When spark occurs, observe the pattern of the spark carefully.
5. Spark
5.1. Spark test of CR-sheet
