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GRINDING:Grinding can also considered as a machining process i.e. process of removing metal but in smaller volume as compared with other machining process.To grind means TO ABRADE to wear by FRICTION or TO SHARPENIn grinding the material is removed by means of rotating abrasive wheel. In this process, the material removal in the form of small chips by mechanical action of ABRASIVE particulars bonded together in the grinding wheel.

Grinding Wheel & Workpiece interactionGrinding Wheels Grinding wheel consists of hard abrasive grains called grits, which perform the cutting or material removal, held in the weak bonding matrix. A grinding wheel commonly identified by the type of the abrasive material used.

THE SPECIAL FEATURES OF GRINDING PROCESS ARE(GRINDING HAS FOLLOWING ADVANTAGES)Abrasive are mineral crystal with harden much higher than that of work piece.Grinding wheel has self sharpening character.This means that dull or worn out grains of the grinding wheel during the operation are removed either by fracture or teasing of the bond which result into exposure of fresh new grains.The grain geometry is highly random and the time between the chip and an abrasive grain is very small.The effective rake angle of abrasive grains is highly negative.

Grinding Wheels The conventional wheels include Aluminium oxide and Silicon carbide wheels while Diamond and CBN (cubic boron nitride) wheels fall in the category of superabrasive wheel

Grinding Wheel & Workpiece interactionABRASIVE: An abrasive is a substance which is hard material used for grinding and polishing operations. These are small particles bonded together in different shapes, size of grinding wheels.ABRASIVE MAY BE CLASSIFIED IN TWO PRINCIPLE GROUPS:Artificial Natural1.Natural:Following are the natural abrasives.A)Sand stone or solid quartzB)EmeryC)CorundumD)DiamondARTIFICIAL: These days, almost all grinding wheels made of artificial abrasives because there composition can be controlled and quality will be better. These artificial abrasives, grinding wheels possess better cutting properties and higher efficiencies than the natural abrasivesArtificial Abrasives:These abrasives include: Aluminium oxide, silicon carbide, Diamond (pure form of carbon), boron carbide and cubic boron nitride. The manufactured abrasives have well defined and controlled properties of hardness, toughness and type of structure.Aluminium Oxide: Aluminium oxide, Al2O3 abrasive is composed chiefly of crystalline aluminium oxide. It is obtained by melting material rich with this oxide (bauxite ore which is mainly aluminium hydroxide) in an electric furnace. Iron chips and coke are added to combine with and remove the impurities. The refined aluminium oxide comes out from the furnace in a large lump called a Pig. It crashed and rolled in to small grains. Then this treated magnetically to remove the ferrous impurities and washed.Properties:Aluminium oxide is is softer than silicon carbide. It is tough and not easily fractured. It is extensively used for grinding materials of high tensile strength such as most steels, H.S.S., ferrous alloys, non-ferrous cast alloys and annealed malleable and ductile iron.Silicon Carbide: This is a chemical compound of silicon and carbon. It is produced by fusing quartz sand and powder coke in electric furnace. Sawdust and salt are also added to the said mixture. In the furnace at the temperature around 2300o C, The silicon sand combines with carbon of the coke to form silicon carbide. The sawdust burns and leaves pores to let the gases escape. The salt helps remove impurities. After the process has run, the furnace is cooled. The core of loosely knit silicon carbide crystal are broken into individual grains.Two types of silicon carbides are available:Pure silicon carbide of green variety and black or grey. Green silicon carbide is better abrasive than black silicon carbide. Both these silicon carbide have a higher hardness than artificial aluminium oxide. Being extremely brittle, SiC is not recommended for grinding hard material. Silicon carbide wheels are used for grinding non-ferrous metals, non-metallic elements and cast irons.EFFICIENCY OF ABRASIVE PARTICLES: Depends on1. Purity2. Uniformity in composition3. Hardness4. Toughness: If this is not tough, the abrasive particles will fracture readily and wheel wear will be excessive.5.Sharpness of fracture: The better cutting action is obtained by sharp edged abrasives. The natural abrasive give rounded edges and are therefore not efficient in cutting.CUTTING ACTION IN GRINDING:

From this fig. it is clear that wheel consists of abrasive particles bounding material and voids. The projecting abrasives particles act like a cutting tool tips and remove metal.A properly selected grinding wheel exhibits self- sharpening action. As cutting proceeds, the abrasive particles at the cutting edge become dull and eventually these become cracked along the planes due to resistance offered by work piece material which resists the cutting action. The process continues till the abrasive grains get worn down till the level of bond.Grain or Grit:The term Grain or Grit denotes the approximate size of the abrasive particles and gives an idea of the coarseness of fineness of the grinding wheel. A grinding may have the abrasive particles of the same size or different sizes. The farmer is known as straight wheel and the latter compound wheel.The choice of grain size or grit depends upon many factors viz. quality of finish required, the amount of stock material to be removed and physical properties of the material to be ground.The coarse grit wheels will remove the stock at a faster rate and finer finish will always required a finer grit.Coarse grit wheels are more suitable for grinding soft and ductile materials. Finer grit wheels are suitable for hard ad brittle materials.The grit or grain size of an abrasive is denoted by a number representing the number of meshes/inch of the screen through which the grain of crushed abrasive are passed for grading.The following are standard numbers representing different grains sizes are:Grit DesignationGrain size or Grit No.1. Coars10, 12, 14, 16, 20, 242.Medium30, 36, 46, 54, 603.Fine80, 100, 120, 150, 1804.Very fine220, 240, 280, 320, 400, 500, 800First three are used in grinding work and 4th one is used in honing work

MANUFACTURING OF GRINDING WHEELS:The abrasive particle are first crushed to powder form and passed over magnetic separators for removing iron impurities.Then these are washed with water to remove foreign elements like dust etc. then with chemical compound to grease.The particles are then graded according to their grain size by passing through proper sieves.Abrasive particles are then mixed with proper bonding material (the bonding material is based on abrasive and the process used ) and moulded in proper shape.After drying it, it is backed (heating). After baking the binding material holds the particles together.It is then cut and given a final shape.It is tested for crash, leakage and balance.Tight control during manufacturing in terms of density control, mix flow and geometrical accuracy result in more uniform and consistent wheel.

BOND MATERIALS OR BOND (BONDING PROCESS) OR MOULDING PROCESS:In order to give an effective and continuous cutting action. It is necessary that the grains of abrasive material should be held firmly together to form a series of cutting edges.The material employed for holding them is known BOND The main bonds are:Vitrified2. Silicate3. Oxychloride 4. Resinoid 5. Shellac6. RubberVITRIFIED BOND:It is a clay bond. The basic material is FELSPAR which is fusible clay. The flux and refractories are added to it and mixed thoroughly. This mixture together with the abrasive grains is fed into revolving drums containing water. After mixing it become paste. The paste is then placed in moulds to get the shape of the wheel and air dried at room temperature. Now, these wheels fed into a klin and allowed them to remain there for a few days at 1260 degree C. This process is known as fusing.After this wheels are trimmed to the required size.Advantages:It is made porous wheel and enable quicker material removal.It is not effected by water, oil, acid or climatic condition.The bond is very hard and acts as an abrasive also.The structure of the wheel is uniform due to wet mixing of the different constituents.Dis-advantages:The process of manufacturing is very slow.Crack may develop in large size wheel during fusing.Proper control during fusing becomes difficult.High temp. in the kiln tends to make the abrasive grains weak.SILICATE BOND:Its basic material is silicate of soda. The process of making the bonds is the same as in the case of vitrified bond but here the oven carries a temp. of about 260 degree C. This low temp. help in fairly high tensile strength. Wheels possessing this bond are light grey in colour.Advantages:It is a more rapid process than vitrified bond.Because of moderate temp. there is no tendency to weaken the grains.When wet grinding is performed with these wheels the soda acts as lubricants.The cutting action of the wheel is smoother and cooler.Dis-advantages:Extra hard wheels can not be produced with this bond.Harder grades of this bond do not provide a free cutting action.

OXYCHLORIDE BOND:It is mixture of oxide and chloride of magnesium and setting in cold state. The process of manufacturing is similar to the above two but NO HEATING but cooling is required on account of cold setting hardness. The bound provides cool cutting action, but grinding is usually done dry.RESINOID BOND:

These wheels are manufactured from a mixture of abrasive grains, synthetic resins and some compounds. This mixture is filled in moulds and then fed into the furnace at 200 degree C. After this shape and size thus produced.It is synthetic organic compound, which is enough strong and flexible. It provides sharp cutting action and enables a high rate of stock removal at high speeds. Resinoid bonded wheels are vastly employed for cutting bar stocks, fine grinding of cams, precision grinding of rolls etc.SHELLAC BOND:Shellac bonded wheels are produced by mixing the bond and the abrasive grains in a steam heated mixes. After this mixture is moulded under pressure in steam heated moulds. These wheels are cool cutting and are vastly used on hardened tool steel and thin section. They also help in producing high surface finish. They can run safely in water but use of oil or caustic soda should be avoided.RUBBER BOND:It is composed of hard vulcanized rubber. The process consists of passing of rubber and sulphur through the mixing rolls and adding the abrasive grains slowly as the above two constituents pass through the roll. Adding of abrasive grains continues till the required proportion is achieved. The mixture is passed through another set of rolls to obtain the required thickness. The wheels are then cut and placed in pre-heated moulds and vulcanized under pressure. These wheels are very strong. They are mainly used where a very high class surface finish with close dimensional accuracy is a primary requirement.Selection of Grinding Wheel:The proper selection of grinding wheel is very important for getting good result. Selection mainly depends upon the following factors.Constant FactorsVariable FactorsCONSTANT FACTORS:Work material: If the work material hard then the soft grinding wheel is used and vise-versa.Amount of material to be removed.Condition of grinding machines: The softer grade of wheel is used on robust and heavy machine.Finish and accuracy required on the job.

VARIABLE FACTORS:Wheel SpeedWork SpeedThe following are the details as to how the various factors influence the choice of wheel based on above factors. The work material will influence the following element.Abrasive MaterialGrain size or Grit NumberGrade (Strength of bond)Structure

ABRASIVE:The choice of right abrasive determine by the type of material to be ground. SiC is the best suited abrasive for brittle and hard materials e.g. Grey cast iron, iron castings, tungsten carbide, hard steel, porcelain and other ceramic substances. This is also used for low tensile strength materials e.g. Bronze, Brass, Copper, Aluminum. Where Al2O3 is used for tough material having high tensile strength like mild steel, tough bronze, wrought iron etc.GRAIN SIZE:For softer material use coarse grain size and for harder material use fine grain size. The coarser grain is used for high rate of material removal. The fine grain is used where the surface finish is important.GRADE:The hard materials and materials having high strength offer more resistance to the wheel. If the hard grade of wheel is used then wheel will get blunt soon. Therefore, better result on such materials, the abrasive particles should breaks and fall quickly. For softer material high or harder grade i.e. good bond is used.STRUCTURE:This represents the void between the abrasive and is influenced by the work material. If the material is harder then dense structure is desirable. For softer materials, the open structure is prescribed as the rate of metal removal is high and the size of the chip is also long. The structure is denoted by number from 1 to 15 or so.VARIABLE FACTORS:Wheel Speed and Work SpeedWheel speed offers the grade to a considerable extent and for higher wheel speed, soft grade wheel should be used. If the higher work speed it is desirable to use harder wheel and vise-versa.Condition of Grinding: In dry grinding condition with hard wheel the heat generation is more and thus soft wheel is required and vise-versa.The affects of various working parameters on grinding wheel:

PARAMETER EFFECTSLarge work speed v More wheel wearLarge wheel speed V Reduce wheel wearLarge value of N Reduce wheel wearLarger depth of cut td More wheel wearLarger wheel diameter Reduce wheel wearLarge work diameter Reduce wheel wear The above parameters can be justified if we write the maximum chip formula: D Diameter of cylindrical wheel.d diameter of cylindrical work piecetd depth of cutV is the velocity of the grinding wheel.v is the velocity of the work piece.

Specifications of Grinding Wheels Compositional specification Type of grit material Grit size Bond strength of the wheel, commonly known as wheel hardness Structure of the wheel denoting the porosity i.e. the amount of inter grit spacing Type of bond material Other than these parameters, the wheel manufacturer may add their own identification code prefixing or suffixing (or both) the standard code.

PrefixGrain TypeGrain sizeGradeWheel StructureBond TypeSuffixAluminium Oxide (A )Silicon Carbide (C)B:ResinoidE: ShellacR: Rubber S: Silicate V: VitrifiedSpecifications of Grinding Wheel1-4:Dense5-8:Normal916:OpenA-G:SoftH-P:MediumQ-Z:Hard10-24: Coarse30-60: Medium80-180:Fine220-600:Very fineALPHNUMERIC SYSTEM FOR GRINDNG WHEEL SPECIFICATION

Specifications of Grinding Wheels A 60 K 5 V The letter A denotes that the type of abrasive is Aluminium oxide. In case of silicon carbide the letter C is used. The number 60 specifies the average grit size in inch mesh. For a very large size grit this number may be as small as 6 where as for a very fine grit the designated number may be as high as 600. The letter K denotes the hardness of the wheel, which means the amount of force required to pull out a single bonded abrasive grit by bond fracture. The letter symbol can range between A and Z, A denoting the softest grade and Z denoting the hardest one. The number 5 denotes the structure or porosity of the wheel. This number can assume any value between 1 to 20, 1 indicating low porosity and 20 indicating high porosity.

The operating faults in grinding wheel AndTrouble shooting (How to remove them)There are two common faults which occur with grinding wheel.(i) Glazing(ii) LoadingGLAZING:After continuous use of the grinding wheel the sharp points (cutting points) of the abrasive grains become DULL. They lose their cutting ability and sharpness and are worn-out.Due this, this wheel face becomes smooth and it instead of biting (penetrating) into work material, gives a sort of rubbing action only.The above phenomenon, which makes the un-useful, is called GLAZING of grinding wheel. Causes of GLAZING:

This is due to incorrect dressing.Wrong wheel selectionUsing slow traverse and high work speed.Remedies of GLAZING: Keeping the wheel sharpSelection of the wheel should be correct i.e. use of softer wheel or coarser grit must be decided as per the material to be ground.Reducing the wheel speed.Loading:Another problem associated with the grinding wheel in operation is the adherence (attached) of the cut particles of the work material to the face of the grinding wheel. These particles occupy the open space between the cutting points. Due to this, the sharpness of the cutting point is lost and the face becomes, smooth depriving the wheel of its cutting ability. This phenomenon is known as LOADING OF WHEEL and the effected wheel is called LOADED WHEEL.CAUSES:Forcing the cut too fast.Forcing the cut too deep.Using the wheel to grind material softer than for which it is suited.REMEDIES:Fine grained and soft bonded wheel do not load as readily as opposite wheel.Increase in speed may help to affect a cure.

TRUEING AND DRESSING OF THE GRINDING WHEELS: Produced surface cannot be truer than the trueness of the grinding wheel producing it. It is also known as that full cutting capacity of the grinding wheel cannot be used if its periphery has been dull or got clogged with some foreign material. Due to these reasons the operation of truing and dressing is performed.The truing operation is done to make the periphery of the wheel concentric with its axis, make its sides true and to recover the lost shape of its face.Dressing of the wheel is done to recover the proper cutting action of the wheel face by removing the layer of drilled grains or grains clogged with foreign material.How many times a wheel is to be trued or dressed will depends upon the type of the work, operators skill and wheel fitness.The following the common devices used for dressing of grinding wheel.W heel dressersAbrasive sticksAbrasive wheelsDiamond dresserCrush dressing fixtureWHEEL BALANCING:In order to get a good surface finish, prevent vibrations and chatter and void undue wear on machine parts, it is necessary that the grinding wheel should have good balance before it is used.Larger the diameter of the wheel and higher the operating speed more important is the balancing. Balancing is done in the static position of the wheel by shifting the position of the weights on one of the flanges of the wheel. The wheel is mounted on a suitable mandrel or arbor and arbour and placed on balancing fixture for finding out the direction in which the weights are to be shifted.GRINDING WHEEL WEAR OR OCCURANCE ATTRITION AND FRACTURE OF THE ABRASIVE GRAINS: VOLUME OF WORK MATERIAL REMOVEDGrinding Ratio Defined, G = VOLUME OF WHEEL WORN AWAYThis is employed to adjudge the wear and performance characteristic of grinding wheel. The large volume of G would signify easier grinding where as, the low volume would mean poor grind ability. There are various factors which effect the grinding wheel.e.g. 2v D x td tmax. = td ; V N d x D PARAMETER EFFECTSLarge work speed v More wheel wearLarge wheel speed V Reduce wheel wearLarge value of N Reduce wheel wearLarger depth of cut td More wheel wearLarger wheel diameter Reduce wheel wearLarge work diameter Reduce wheel wear WEAR MECHANISM OF GRINDING WHEEL:

If wheel wear is plotted against the volume of work material removed, refer above fig. Here that the wear curve is quite similar to that of the other cutting tool. Also, there is an initial breakdown period followed by a region of uniform wear rate. Finally, the wheel again starts breaking down rapidly. The middle zone is the real usable which determines the life of the wheel.Once the period of uniform wear rate ends, the wheel should be reconditioned before using it again. The reconditioning of a wheel is done by a process commonly known as wheel dressing. THERMAL ASPECT OF GRINDING:It is observed that most of the energy supplied during grinding is converted in to heat. A good portion of the total heat energy passes on to the chip, work and the grinding wheel. Because the chip is very small and poor thermal properties of the total heat going to them is rather small.It means the surface temperature of the work piece can be very high. This high temperature leads to development of thermal stresses in the work piece and also thermal cracking. This HIGH TEMPERATURE PROMOTES ATTRITION and FRACTURE of the abrasive grains. The surface temperature in grinding is proportional to specific grinding energy.