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Portions 3rd test 1.Milling machine- • Dividing heads Types, Indexing Methods, Pblms. 2.Grinding- • Abrasives,types, Properties of good abrasives, • Bonds-Types of bonds, • Grinding Vs other operations, • Types of grinding- Surface,internal,Centre & Centreless grinding, • Advantages of Centreless grinding.
CH- Grinding • Process of removing material by abrasive action of a revolving wheel on surface of a work piece in order to bring it to required shape & size. • The metal removed will be in smaller volume. • To grind means “to abrade”, to wear away by ‘friction’ or “to sharpen”. • Tool used: Grinding wheel • (Sharp crystals –abrasives bonded) • In all the grinding processes ,the material from the workpiece is removed either by feeding the workpiece against the rotating wheel or by forcing the rotating wheel against the workpiece depending on the arrangement and features of the machine. • Wheel can be single piece or several blocks joined together.
Common abrasive tools 1.Stones & Sticks: Abrasive grains are bonded in the form of stones & sticks of various shapes & sizes. Used in hand sharpening of tools,cleaning, polishing and hand finishing of various components like dies,jigs etc. 2.Coated abrasives: A single layer of abrasive grains is applied on a backing of paper or cloth by means of a suitable adhesive or bond 9glue or resin). Used for polishing outside & inside surfaces of machined articles,removing burrs on castings. 3.Polishing grains: These are loose grains of abrasives & are coated with adhesive on the periphery of flexible wheel bodies made of cloth or leather.
Used in finishing operation called lapping. 4.Grinding wheels: Binding material (bond) holds the grains together. Used in grinding operation. Wheel Materials. 1.Abrasives 2.Bond Abrasives are material of Grinding wheel which does the cutting. They are extremely hard materials consisting of very small particles called grains which carry sharp cutting edges & corners. Types 1.Natural . 2.Artificial or Manufactured.
Natural: Obtained directly from mines Eg: sand stone (for wood working tools),emery, quartz, diamond(grinding process). Artificial: Manufactured under controlled conditions in closed electric furnaces in order to avoid the impurities & active proper temperature for chemical reaction to take place. 1.Silicon carbide SiO2 +3C TO BE CONTD
Grinding is a process of removing excess material from the workpiece by the mechanical action of abrasive particles that are held together by an adhesive, generally in the form of a solid wheel. The wheel known as grinding wheel is rotated at high speeds, and when the surface of the rotating wheel is brought in contact with the workpiece, material is removed in the form of fine chips. The depth of cut is obtained by the downward movement of the grinding wheel, or in some machines the worktable carrying the workpiece is raised towards the grinding wheel. • On a microscopic scale, the chip formation in grinding is similar to other manufacturing process like turning, milling etc. But, the process removes very little material in the form of fine chips when compared to that of turning, milling or drilling, and this makes it to be called a 'finishing process'.
Since the material from the workpiece is removed in the form of fine chips, the process tends to be slower towards achieving the desired size and finish. For this reason, other manufacturing methods like turning, milling etc., are used to bring the workpiece close to its final size and then the workpiece is ground to achieve the desired surface finish and tolerance. Applications of Grinding • Grinding is employed to produce a workpiece of high accuracy and surface finish. Automobile camshafts, jet engine rotors, bearings and various other parts are machined and fmished to accurate size and shape. Materials from rubber to hardest carbides and even composite materials can be machined by grinding process. • Grinding is also used to cut steel and masonry block; sharpen knives, drill bits and many other tools; or to clean and prepare surfaces for painting and plating.
Reasons for selecting grinding process Grinding operation is selected when: • (a) the workpiece material is too hard to be machined economically by other manufacturing process like turning, milling etc. • (b) the required surface finish and dimensional tolerance cannot be easily achieved by other manufacturing process. Applications of Grinding • Grinding is employed to produce a workpiece of high accuracy and surface finish. Automobile camshafts, jet engine rotors, bearings and various other parts are machined and fmished to accurate size and shape. Materials from rubber to hardest carbides and even composite materials can be machined by grinding process. • Grinding is also used to cut steel and masonry block; sharpen knives, drill bits and many other tools; or to clean and prepare surfaces for painting and plating.
Practically, a grinding wheel has three main components: • (i) the abrasive particle, which does the actual cutting. • (ii) the bond, which holds the abrasive particles together to form the wheel and • (iii) fillers or additives that produce pores (free space or voids) within the wheel. • Additives or fillers contribute to the cutting characteristics of the grinding wheel.They create pores or spaces within the bond that allow room for small chips of metal and worn out abrasive particles generated during the grinding process. The open spaces also provide pathways that carry coolants used to reduce the heat generated due to the cutting action. The coolants also flush away the chips and worn out abrasive particles thereby improving the cutting characteristics of the abrasive grains. Without adequate spacing between the abrasive particles, the wheel can become loaded with chips and cease (discontinue or stop) to cut properly.
TYPES OF ABRASIVES. • An abrasive is a tiny, irregular shaped hard particle that acts as a cutting edge in grinding wheel. When several hundred or thousands of such particles are bonded together to form a grinding wheel, the effect they produce is quite significant. Two types of abrasives are used in, grinding wheels. They are: • Natural abrasives and • Artificial abrasives. • a) Natural abrasives • Natural abrasives are those, which are available in nature (they are mined from the earth) and include sand stone, diamond, corundum, emery and garnet. • Sand stone (solid quartz) is natural silica and was probably the earliest abrasive used to smooth and sharpen weapons. • Diamond, a crystalline form of carbon is the hardest material ever known and was used in ceramic grinding-or final polishing. Its use was restricted due to high hardness and cost.
Corundum, a relatively pure aluminum oxide (Al2O3) is the hardest known natural abrasive after diamond. • Emery is natural mineral containing corundum (aluminum oxide) and small amounts of iron oxide. Once used extensively by the industry, it occurs today as coated' abrasives that are used in home workshops. • Other natural abrasives include garnet, an alumino-silicate material; feldspar, used in household cleansers; calcined clay, lime and silica. • Natural abrasives possesses higher percentage of impurities and non-uniform properties like hardness, toughness etc. The variations in properties and composition of impurities depend on the specific geological source. Due to this, natural abrasives have been largely replaced by artificial abrasives.
Although diamond and aluminum oxide are naturally occurring materials, they are also produced artificially. • b) Artificial abrasives • Artificial abrasives are manufactured abrasives that are free from impurities and possess properties (hardness, toughness etc.) better than those of natural abrasives. Some of the common artificial abrasive materials include aluminum oxide (Al2O3), silicon carbide (SiC), boron carbide (B4C),synthetic diamond and cubic boron nitride (CBN). • • Aluminum oxide is refined from bauxite ore in an electric furnace (Lid appears in a variety of forms determined by the refining process. It is the most widely preferred abrasive • material in industries and is used for grinding ferrous alloys, high tensile strength alloys like carbon steels, HSS (high speed steel) etc. It is also used for grinding wood.
Silicon carbide, a fused abrasive harder than aluminum oxide is typically used for grinding non-ferrous materials like brass, aluminum, titanium etc. It is not used for ferrous materials, because the high solubility of carbon and silicon in iron would result in a reaction of silicon carbide with the iron base alloy resulting in poor grinding performance. It is also used in non-metal applications such as in wood and leather industries. Synthetic diamond is manufactured in a special high-temperature, high-pressure process and subsequently treated to make a variety of abrasive grains for use in grinding nonferrous materials and ceramics. It is best suited for grinding non-ferrous metals, ceramics,glass, stone and building materials. But diamond is not useful in grinding steel or ferrous alloys, because carbon or diamond readily dissolves or reacts with iron. Diamond pastes are useful in ferrous polishing or lapping applications where heat and reactivity are not a factor.
Cubic boron nitride (CBN) commonly known by its trade name Borazon is an abrasive material second in hardness only to the synthetic diamond. Like diamond, CBN is produced in a high temperature, high pressure process and is selected as an abrasive material for hard and difficult-to-grind materials like steels. BONDING PROCESS • A bond or bonding material is an adhesive substance that binds or holds the abrasive particles together to form a grinding wheel. This process of binding the abrasive particles with the help of a bonding material is known as bonding process.A good bond is an essential requirement in making grinding wheels, since it determines the force required to dislodge the abrasive particles from the wheel. The most commonly used bonding materials are,
a) Vitrified bond Vitrified bond, also called ceramic bond is composed of clay and other ceramic substances. The abrasive particles are mixed with the bonding material with sufficient water and formed into a wheel, usually by pressing in suitable moulds. The wheel is then dried to remove moisture and later tired to high temperatures of approximately 1250-1325°C. This makes the bonding material to become hard and strong. Firing to high temperatures causes the bonding material to form a glass-like (vitreous) structure and hence the name vitrified bond.
Vitrified bonded grinding wheels are strong, rigid and porous and are not affected by fluids (water, oil and acids). They can be stored for longer periods. However, they are brittle and have poor shock resistance. Vitrified bonded grinding wheels are the most commonly used and are denoted by the letter 'V' as a standard designation for specification.
b) Resinoidbond • A resinoid bond is a synthetic resin or a plastic that becomes hard on heating. The abrasive particles are mixed with the resins, either in powdered or liquid form along with certain additives and then pressed into the shape of a grinding wheel. The wheel is cured at a temperature of approximately 180°C and then cooled to room temperature. • Resinoid bonded grinding wheels have good strength and are more elastic than vitrified bonds.However, its resistance to heat and chemical action is low. Hence, prolonged exposure to grinding fluids should be avoided. Also, they cannot be stored for longer periods, as the strength of the bond decreases. Resinoid bond is the most frequently used bond next to vitrified bond and is designated by the letter 'B'. It is used in foundries, welding and billet shops, in cut-off and thread cutting operations, and also in high-speed applications (they generate less heat than vitrified bonded wheels).
Rubber bond Rubber bond is composed of pure rubber and sulfur that acts 'as a vulcanizing" agent. The bonding material is mixed with the abrasive particles and then passed between hot rolls to form it into sheets of desired thickness. Wheels are cut from these sheets and then heated under pressure to vulcanize the rubber. Thin wheels can be produced in this manner and such wheels can be used like cutting saws for cut-off operation in foundries and machine shops. Rubber bonded grinding wheels are used for grinding saws and also in those applications where surface finish with high dimensional accuracy is a primary requirement. Rubber bonded wheels are resilient,and more dense than resinoid bond, but are less heat resistant. Also, they cannot be stored for longer periods, since the strength of the bond decreases.Off all the bonds, rubber bond is the most flexible. It is designated by the letter 'R'.
d) Shellac bond • Shellac bond is an organic bond made of shellac: a resin like substance. The grinding wheel is made by mixing the abrasive particles with shellac in a steam-heated mixer. The mixture is then pressed or rolled into the desired shape and baked for several hours at about 160°C.Shellac bonded grinding wheels produce good surface finish, and hence preferred for parts like • camshafts and mill rolls. But, the bond is not suitable for heavy-duty work. Shellac bonded grinding wheels are designated by the letter 'E' .
e) Silicate bond Silicate bond is made of sodium silicate and used in those operations that generate less heat.The abrasive particles are mixed with sodium silicate and formed into wheels of desired shape and size by pressing it in suitable moulds. It is then baked at about 260°C for a day or more. • Grinding wheels made from silicate bond are soft and wear more easily. Hence, these wheels are used for grinding the edges of tools where the heat must be kept to a minimum. Silicate bonded grinding wheels are designated by the letter'S'.
SURFACE GRINDING MACHINE • Surface grinding machines are used primarily to produce and finish flat surfaces. However, with the use of special fixtures and form dressing devices, angular and formed surfaces can also be finished. Surface grinding machines are further classified into four types. They are: • a) Horizontal spindle type with reciprocating table. • b) Vertical spindle type with reciprocating table. • c) Horizontal spindle type with rotary table. • d) Vertical spindle type with rotary table. Specification of surface grinding machine • The size of the surface grinding machine is specified by the dimensions of the working area of the table.
Centerless Grinding Machine • Centerless type of cylindrical grinding machine is used for grinding surfaces of long, slender rods that cannot be held and rotated between centers. In other words, the workpiece is not rotated between centers and hence the name centerless grinding. Principle of Centerless Grinding Machine • In centerless grinding, the workpiece is supported by the combination of grinding wheel, workrest blade and the regulating wheel. The grinding wheel is larger in diameter and rotates at high speeds when compared to the regulating wheel. Both the wheels rotate in clockwise direction, while the workpiece driven by the regulating wheel rotates in the counter-clock-wise direction. • The grinding operation is performed by the grinding wheel only. The function of the regulating wheel is to control (slow) the speed of rotation of the workpiece, so that it does not rotate at the same speed as the grinding wheel.
The regulating wheel also controls the longitudinal motion of the work piece or rate of feeding of the work piece past the grinding wheel. Based on the above two reasons, the regulating wheel is made to rotate at much slower speeds than that of the grinding wheel. The regulating wheel usually carries a rubber bond and sometimes can contain some hard grain material to achieve good traction between the work- piece and the regulating wheel. For most applications, the centerline of the grinding wheel and the regulating wheel are in the same planet, at equal heights above the machine bed. But the centerline of the workpiece should be higher than the centerline of the regulating wheel. This forms the critical relationship that helps centerless grinding to perform excellent roundness of the work piece. The workpiece rests on a blade mounted on the work rest. The top surface of the blade is inclined towards the regulating wheel in order to ensure that the workpiece is fixed between the blade and the regulating • wheel. The angle may vary from 0° - 45°, but a 30° angle provides the best results.
Truing of grinding wheel is carried out to restore concentricity and correct geometric shape of the wheel that has been lost due to non-uniform wear. Generally, a sharp pointed diamond tool is used to remove a layer of the surface (abrasive particles) from the grinding wheel to obtain the desired shape.
CH-GEAR CUTTING OPERATIONS Gear drives possess a very prominent role in mechanical power transmission. A gear is an important machine element which is used for transmission of power or motion or both from one shaft to the other. It is normally a round blank carrying projections or teeth along its periphery of a cylinder or a cone, or on elliptical discs which enable a positive drive. They are mounted on the axles or shafts and keyed to them. Gears are vastly employed to form mechanisms for transmission of power from one part to other in a machine and to effect change in speed or torque or both of one part with respect to other.