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Chapter :4

Abrasive Machining

1Grinding is a metal cutting operation performed by means of a rotating abrasive tool called grinding wheel.Grinding process consists of removing material from the work piece by the use of a rotating wheel that has a surface composed of abrasive grains. Grinding is considered to be the most accurate of the existing machining processes.Grinding processes are used when high accuracies, close dimensional tolerances, and a fine surface finishes are required. Grinding processes also allow for high production rates.This allows for a lowered cost of production. Hard materials can also be machined.

ABRASIVE PROCESSES : GRINDING2Grinding is the most common form of abrasive machining.It is a material cutting process which engages an abrasive tool whose cutting elements are grains of abrasive material known as grit.These grits are characterized by sharp cutting points, high hot hardness, chemical stability and wear resistance.The grits are held together by a suitable bonding material to give shape of an abrasive tool.

ABRASIVE PROCESSES : GRINDING

3Advantages : Dimensional accuracy.Good surface finish.Good form and locational accuracy.Applicable to both hardened and unhardened material.

Applications :Surface finishing.Slitting and parting. Descaling, Deburring. Stock removal (abrasive milling).Finishing of flat as well as cylindrical surface.Grinding of tools and cutters and re-sharpening of the same.

ABRASIVE PROCESSES : GRINDING4Grinding is done on the surfaces of almost all conceivable shapes and materials of all kinds.Grinding can be classified broadly in to two groups :Rough or Non-Precision grinding.Precision grinding.Non-precision grinding: The common forms are called, snagging and off-hand grinding. Both are done primarily to remove stock that can not be taken off as conveniently by other methods.The work is pressed hard against the wheel or vice versa.The accuracy and surface finish are of secondary importance.

KINDS OF GRINDING5Precision grinding : Precision grinding is concerned with producing good surface finishes and accurate dimensions. The wheel or work both are guided in precise path.

Three types of precision grinding exists :

External cylindrical grindingInternal cylindrical grindingSurface grindingForm grinding

KINDS OF GRINDING6External cylindrical grinding produces a straight or tapered surface on work piece.The work piece must be rotated about its own axis between centers as it passes lengthwise across the face of revolving grinding wheel.Internal cylindrical grinding produces internal cylindrical holes and tapers.The work pieces are chucked and precisely rotated about their own axis.The grinding wheel or in the case of small bore holes, the cylinder wheel rotates against the sense of rotation of the work piece.

KINDS OF GRINDING

7Surface grinding produces flat surface.The work may be ground by either the periphery or by the end face of the grinding wheel.The work piece is reciprocated at constant speed bellow or on the end face of the grinding wheel.

Form grinding is done with specially shaped grinding wheels that grind the formed surfaces as in grinding gear teeth, threads, spindle shafts, holes, and spheres, etc.

KINDS OF GRINDING

8Grinding machines according to the quality of surface finish, may be classified as : Rough Grinders.Precision Grinders.Rough Grinders : Rough grinders are those grinding machines whose chief work is the removal of stock with out any reference to the accuracy of the result.These are mainly of the following types : Floor stand and bench grinders.Portable and flexible shaft grinders.Swing frame grinders.Abrasive belt grinders.

GRINDING MACHINES9Schematic illustration of a horizontal spindle surface grinder

GRINDING MACHINES

10Precision Grinders : these grinders are finish the parts to a very accurate dimensions.Classification :

GRINDING MACHINESSurface GrindersReciprocating Table Horizontal spindle Vertical Spindle(b) Rotating TableHorizontal spindleVertical SpindleTool and Cutter GrindersUniversalSpecialSpecial Grinding MachinesCylindrical Grinders(a) Centre type (plain)(b) Centre type ( universal)(c) Centreless

Internal Grinders(a) Chucking (Plain & Universal)(b) Planetary(c) Centreless

11Plain Centre Type Cylindrical GrinderFigure illustrates schematically this machine and various motions required for grinding action.The machine is similar to a centre lathe in many respects.The workpiece is held between head stock and tailstock centres. A disc type grinding wheel performs the grinding action with its peripheral surface.Both traverse and plunge grinding can be carried out in this machine as shown in Fig.

GRINDING MACHINES

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GRINDING MACHINES

Plain Centre Type Cylindrical Grinder13Universal Centre Type Cylindrical GrinderThese grinders are usually used in tool rooms for grinding tools.A universal machine has the following additional features : The head stock spindle may be alived or dead, so that the work can be held and revolved by chuck as well as ground between centres.The head stock can be swiveled at an angle in a horizontal plane.The wheel head and slide can be swiveled and traversed at any angle.The wheel can also be arranged for internal grinding by the addition of an auxiliary wheel head to revolve small wheels at high speeds.

GRINDING MACHINES14Centreless GrindersCenterless grinding is a method when the workpiece is supported by a blade instead of by centers or chucks.Two wheels are used.The larger one is used to grind the surface of the workpiece and the smaller wheel is used to regulate the axial movement of the workpiece.Types of centerless grinding include:through-feed grinding, in-feed/plunge grinding, and internal centerless grinding.

GRINDING MACHINES

15Centreless GrindersThrough-feed grinding : The work is passed completely through the space between the grinding wheel and the regulating wheel.In-feed/plunge grinding : The regulating wheel is drawn back so that the work piece may be placed on the work rest blade.Endfeed grinding : Used to produce taper, either the grinding wheel or regulating wheel or both are formed to a taper. The work is fed lengthwise between the wheels and is ground as it advances until it reaches the end stop.

GRINDING MACHINES

16Internal GrindersThese are used to finish straight, tapered, or formed holes to the correct size, shape and finish.The depth of cut depends up on the diameter of the hole being ground and may vary from 0.02 to 0.05mm in roughing and from 0.002 to 0.01mm in finishing operations.There are three types of internal grinders : Chucking grindersPlanetary grindersCentreless grinders

GRINDING MACHINES

17Internal GrindersChucking grinders :In this the work piece is chucked and rotated about its own axis to bring all parts of the bore or other surfaces to be ground in contact with the grinding wheel.Checking grinders are best applied whenever the work itself can be conveniently chucked and rotated.

GRINDING MACHINES

18Internal GrindersChucking grinders :According to general construction there are three types of internal grinders:The wheel is rotated but has no longitudinal movement while the work is slowly rotated and transverse back and forth.The work head is mounted on a longitudinal slide at the left end of the machine so that the work piece may be traversed past.The grinding wheel, and the wheel head is mounted on a cross-slide at the right end so that the wheel may be fed for depth of cut.

GRINDING MACHINES

19Internal GrindersChucking grinders :According to general construction there are three types of internal grinders:The wheel is rotated at the same time reciprocated back and forth through the length of the hole.The work is rotated slowly but has no lateral and cross movement.In this type of machine the design traverses the wheel head and cross feed the work.

GRINDING MACHINES20Internal GrindersChucking grinders :According to general construction there are three types of internal grinders:The grinder has two wheels side by side mounted on a horizontal overhead bar.One has a small wheel to grind the hole, and the other has a large wheel for facing in the same setup and squaring with the hole. This is known as internal and face grinder.

GRINDING MACHINES21Internal GrindersChucking grinders :Internal grinders of chucking type may be classified as :Plain internal grinder : the work head can be swiveled to grind a straight hole tapers up to 450 included angle.The wheel head is moved into and away from the hole and can be cross fed into the work.Universal grinder : it is basically the same as a plain internal grinder, the work head is mounted on a cross slide as in the wheel head, and can be swiveled through a 900 angle.

GRINDING MACHINES22Internal GrindersPlanetary :In a planetary grinder the work is mounted on the reciprocating table and is not reveled.Instead, the grinding wheel is given rotary and planetary motions to grind cylindrical holes. Planetary grinding is usually limited to large work pieces that can not be conveniently rotated by chuck.

GRINDING MACHINES23Internal GrindersCentreless grinders :In internal centerless grinding, the work is supported by three rolls.One is the regulating roll, and the other is a pressure roll to hold the work piece firmly against the support and regulating rolls.The grinding wheel contacts the inside diameter of the work piece directly opposite to the regulating roll, thus assuming a part of absolutely uniform thickness and concentricity.The pressure roll is mounted to swing aside to permit loading and unloading.

GRINDING MACHINES

24Surface grindersSurface grinding is employed to finish plane or flat surfaces.They are also capable of grinding irregular, curved, convex and concave surfaces.Conventional surface grinders may be classified in tot two classes :One class has reciprocating tables for work ground along the straight lines.Other covers the machines with rotating work tables for continuous rapid grinding.

GRINDING MACHINES

25Types of Surface grindersHorizontal spindle reciprocating tableHorizontal spindle rotary tableVertical spindle reciprocating tableVertical spindle rotary table

GRINDING MACHINES

26Types of Surface grindersHorizontal spindle reciprocating table:These machines use the circumference of straight grinding wheels, and are able to deal with a wide range of work needing super finish and extremely fine limits of accuracy.They yield a greater out put and take off metal faster than similar machines using cup-shaped, segmental or annular wheels.If a rotating work table is used, a finish comprising concentric circles can be obtained.

GRINDING MACHINES

27Types of Surface grindersVertical spindle flat grinding machines:These machines remove metal faster when using a cup, cylindrical or segmental wheel than when using a straight wheel.They have a great precision, and if strong and rigidly built can grind to extremely fine limits.

GRINDING MACHINES

28Types of Surface grindersDisc grinding machines:In it one disc of abrasive type is mounted on a vertical spindle so that the plan of the disc is horizontal, the work resting on the surface of a flat, rotating carrier or work table.

GRINDING MACHINES

29Tool and cutter grinder machine Tool and cutter grinders are used mainly to sharpen and recondition multiple tooth cutters like reamers, milling cutters, drills, taps, hobs and other type of tools used in the shop.

They are classified according to the purpose of grinding as :

Universal tool and cutter grinderSingle - purpose tool and cutter grinders

GRINDING MACHINES

30Tool and cutter grinding machine Universal tool and cutter grinders are particularly intended for sharpening of miscellaneous cutters. Single - purpose tool and cutter grinders are used for grinding tools such as drills, tool bits, etc. in large production plants and in large amount of grinding work is necessary to keep production tools in proper cutting conditions.Tools can be ground uniformly and with accurate cutting angles.

GRINDING MACHINES31Universal tool and cutter grinders These are used for grinding tools such as drills, tool bits, etc. in large production plants and in large amount of grinding work is necessary to keep production tools in proper cutting conditions.Tools can be ground uniformly and with accurate cutting angles.Principal parts : BaseSaddleTableHeadstock and tailstockWheel headGrinding wheel

GRINDING MACHINES32Universal tool and cutter grinders Base : Base gives rigidity and stability to the machine. It is heavy, rigid and box type.Saddle : the saddle is mounted directly on the top of the base.It moves on anti-frictional ball bearings on hardened ways.The column supporting the wheelhead is mounted on the saddle and it can be moved up and down and swiveled to either side.The saddle is also provides the means for moving the work forward and backward.Table : it rests and moves on the top base which is mounted over the saddle.The top of the base contains the gears mechanism which control the table movement.The work table is mounted on the sub-table which has T-slots for mounting the work and attachments used on the machines.The table can be swiveled to enable taper grinding.

GRINDING MACHINES33Universal tool and cutter grinders Head stock and tail stock : These are mounted on either side of the table similar to those on a cylindrical grinding.The work piece is positioned between the centers and driven exactly as in a cylindrical grinder.Wheel head : The wheelhead is mounted on a column on the back of the machine.Grinding wheel : Three types of grinding wheels.The straight or disc shaped wheel.The cup type in either the straight or flaring form.The dish type.

GRINDING MACHINES34PRINCIPLE GRINDING OPERATIONS Taper surfacesFormed surfacesGear teethThreaded surfaces

GRINDING MACHINES

35Abrasive : It is a substance that is used for grinding and polishing operations.It should be pure and have uniform physical properties of hardness, toughness, and resistance to facture to be useful in manufacturing grinding wheels.Abrasives may be classified in to 2 groups : NaturalArtificial or manufactured

ABRASIVE WHEELS36Natural Abrasive : Sand stone, Emery, Corundum, Diamonds are the natural abrasives.The sand stone is used only for sharpening wood working tools.Emery is a natural aluminum oxide.It contains from 55 to 65 % alumina, the remainder consist of iron oxide and other impurities.Corundum is a natural aluminum oxide.It contains from 75 to 95 % alumina, the remainder consist of impurities.Diamonds of less than gem quality are crushed to produce abrasive grains for making grinding wheels to grind cemented carbide tools an to make lapping compounds.

ABRASIVE WHEELS37Artificial Abrasive : SiC, Al2O3Silicon carbide (SiC) is manufactured from 56 parts of silica sand, 34 parts of powdered coke, 2 parts of salt, 12 parts of saw dust in a long, rectangular electric furnace of the resistance type that is built up of loose brick work.Sand furnishes silicon, coke furnishes carbon, saw dust make the charge porous, salt helps to fuse it.There are two type of silicon carbide abrasives : Green grit : consists of 97% SiC.Black grit : consists at least 95%SiC.Aluminum oxide (Al2O3) manufactured by heating mineral bauxite, hydrated aluminum oxide clay containing silica, iron oxide, titanium oxide, etc., mixed through ground coke and iron borings in a arc type electric furnace.

ABRASIVE WHEELS38Bonding materials and processing:

Verified bond used for making verified grinding wheels.Silicate bond for making silicate wheelsShellac bond for making elastic wheelsResinoid bonds for making resinoid wheelsRubber bond used for making vulcanized wheels.Oxychloride bond for making Oxychloride wheels.

BONDINGS and BONDING PROCESSES39

TYPES OF GRINDING WHEELS

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LAPPINGLapping is an abrading or finishing process used to produce geometrically true surfaces, correct minor surface imperfections,improve dimensional accuracy, or provide a very close fit between two contact surfaces.Very thin layers of metal (0.005 to 0.01 mm) are removed in lapping.

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LAPPINGCharacteristics of lapping process: Use of loose abrasive between lap and the workpiece.Usually lap and workpiece are not positively driven but are guided in contact with each other.Relative motion between the lap and the work should change continuously so that path of the abrasive grains of the lap is not repeated on the workpiece.

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LAPPING

(a) Schematic illustration of the lapping process.(b) Production lapping on flat surfaces. (c) Production lapping on cylindrical surfaces.

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LAPPINGAbrasives of lapping: Al2O3 and SiC, grain size 5~100m Cr2O3 , grain size 1~2 m B4C3, grain size 5-60 m Diamond, grain size 0.5~5 V Vehicle materials for lapping Machine oil Rape oil Grease Technical parameters affecting lapping processes are: Unit pressure The grain size of abrasive Concentration of abrasive in the vehicle Lapping speed

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LAPPING Hand lapping: Hand lapping of flat surface is carried out by rubbing the component over accurately finished flat surface of master lap usually made of a thick soft close-grained cast iron block.Abrading action is accomplished by very fine abrasive powder held in a vehicle.Manual lapping requires high personal skill because the lapping pressure and speed have to be controlled manually.Lapping Machine : Machine lapping is meant for economic lapping of batch qualities.In machine lapping, where high accuracy is demanded, metal laps and abrasive powder held in suitable vehicles are used.Bonded abrasives in the form wheel are chosen for commercial lapping. Machine lapping can also employ abrasive paper or abrasive cloth as the lapping medium. 45

LAPPINGProduction lapping of both flat and cylindrical surfaces are illustrated in Figures.

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LAPPING Lapping Machine : In this case cast iron plate with loose abrasive carried in a vehicle can be used.Alternatively, bonded abrasive plates may also be used.Centreless roll lapping uses two cast iron rolls, one of which serves as the lapping roller twice in diameter than the other one known as the regulating roller. During lapping the abrasive compound is applied to the rolls rotating in the same direction while the workpiece is fed across the rolls.This process is suitable for lapping a single piece at a time and mostly used for lapping plug gauges, measuring wires and similar straight or tapered cylindrical parts.

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LAPPING Lapping Machine : Centreless lapping is carried out in the same principle as that of centreless grinding. The bonded abrasive lapping wheel as well as the regulating wheel are much wider than those used in centreless grinding.This technique is used to produce high roundness accuracy and fine finish, the workpiece requires multi-pass lapping each with progressively finer lapping wheel.This is a high production operation and suitable for small amount of rectification on shape of workpiece.Therefore, parts are to be pre-ground to obtain substantial straightness and roundness.The process finds use in lapping piston rings, shafts and bearing races.

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LAPPINGAdvantages It can lap any type of material of any shape, if it is flat.As the parts are not clamped and no heat is generated, there is no warping.It produces no burrs, rather removes those left in the earlier process.Disadvantages Large variables involved and are to be control in proper way to achieve the requirement.High skill is required.Flatness, surface finish and polished surface are not obtained all at the same time.49

HONINGHoning is grinding or a abrading process mostly for finishing round holes by means of bonded abrasive stones, called hones.In honing the material is removed by abrasive sticks (Al2O3 and SiC) mounted on a mandrel or fixture.By floating action between the work and tool, the pressure exerted in the tool is transmitted equally to all sides.The honing tool is given a slow reciprocating motion as it rotates.This action results in rapid removal of stock and at the same time generation of a straight and round surfaces.

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HONING

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HONINGWhen honing is done manually the tool is rotated, and the work piece is passed back and forth over the tool.Honing is done on general purpose machines, such as the lathe, drilling press, and portable drills.There are two general types of honing machines : horizontal and vertical.

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HONINGSchematic illustration of the super finishing process for a cylindrical part:(a) cylindrical micro-honing; (b) centerless micro-honing.

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HONINGAdvantages Correction of geometrical accuracy :Out of roundnessTaperAxial distortion.Dimensional accuracy.Relatively high productivity as compared to lapping.Holes of any diameter or length may be honed.Disadvantages Tough non-ferrous metals cause glazing or clogging of the voids of the abrasive sticks and thus they are difficult to hone.54

POLISHINGPolishing is surface finish operation performed by a polishing wheel for the purpose of removing appreciable metal to take out scratches, tool marks, pits and other defects from the rough surfaces.In polishing usually accuracy of size and shape of the finished surface is not important, but some times tolerances of 0.025 mm or less can be obtained in machine polishing.The polishing wheels are made of leather, papers, canvas, felt or wool.The polishing method is very similar to grinding, and the work may be pressed by hand to wheels mounted on floor stand grinders.The polishing machines may be broadly classified as : endless belt machine and coated abrasive wheels.

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POLISHINGPolishing is surface finish operation performed by a polishing wheel for the purpose of removing appreciable metal to take out scratches, tool marks, pits and other defects from the rough surfaces.In polishing usually accuracy of size and shape of the finished surface is not important, but some times tolerances of 0.025 mm or less can be obtained in machine polishing.The polishing wheels are made of leather, papers, canvas, felt or wool.The polishing method is very similar to grinding, and the work may be pressed by hand to wheels mounted on floor stand grinders.The polishing machines may be broadly classified as : endless belt machine and coated abrasive wheels.

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BUFFINGBuffing is used to give much higher, lustrous, reflective finish that can not be obtained by polishing.The buffing process consists of applying a very fine abrasive with a rotating wheel.Buffing wheels are made of felts passed and glued layers of duck or other cloth, and also of leather.The abrasive is mixed with a binder and is applied either on the buffing wheel or on the work.The buffing wheel rotates with a high peripheral speed up to 40 m/sec.The abrasive may consists of iron oxide, chromium oxide, emery, etc.The binder is paste consisting of wax, mixed with grease, paraffin and kerosene.

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