Grinding Machine or Grinder

Grinding Machine, Purpose of grinding, Advantages of grinding, Disadvantages of grinding, Applications of grinding, Types of grinding, Surface grinding methods, Classification of grinding machines, Grinding wheel, Abrasives, Bond and bond materials, Grits, Grade, Structure, Selection of grinding wheels, Maintenance of grinding wheel

Introduction to grinding:

• Grinding is metal cutting operation performed by means of a rotating abrasive wheel that acts as a tool.

• This is used to finish workpieces which must show a high surface quality, accuracy of shape and dimension.

• The art of grinding goes back many centuries. Over 5,000 years ago, the egyptains abraded and polished building stones to hairline fits for the pyramids, columns and statues were shaped and finished with a globular stone which abraded the surface.

• Mostly grinding is the finishing operation because it removes comparatively little metal, 0.25 to 0.50mm in most operations and the accuracy in dimensions is in the order of 0.000025 mm.

• Grinding is also done to machine materials which are too hard for other machining methods that use cutting tools.

• Many different types of grinding machines have now been developed for handling various kinds of work to which the grinding process is applicable.

Purpose of grinding:

• To bring the component to close size.

• To obtain required surface finish.

• To machine very hard surfaces.

Advantages of grinding:

• The surface finish obtained is superior.

• One of the most dimensionally accurate machining processes.

• Hardened components can be machined.

• Grinding permits to machine light and delicate parts due to requiring little pressure.

• Surface unevenness can be eliminated.

• Various kinds of works can be grinded.

• Designed geometrical configuration can be achieved.

• It removes little material of order 0.25 to 0.50mm.

Disadvantages of grinding:

• Work surface gets over heated.

• Wheel wear results duplicate parts do not have the same dimensions.

• Grinding is not suitable for heavy metal removing.

• For some complicated parts grinding needs attention.

• Achieving true parallelism requires skill.

• Achieving concentricity needs attention.

• Achieving true parallelism requires skill.

• Achieving concentricity needs attention.

Applications of grinding:

• All cast components require grinding.

• Welded components require grinding.

• Crank cases, forged components.

Types of grinding:

Grinding operation can be classified into two different categories:

(i) On the basis of quality of grinding;

• Rough grinding or non precision grinding.

• Precision grinding.

(ii) On the basis of surface to be ground;

• External grinding

• Internal grinding

• Surface grinding

• Form grinding.

Rough grinding:

• The rough grinding are snagging and off hand grinding where the work is held in the operators hand.

• The work piece is pressed against the wheel.

• Rough grinding used where, the accuracy and surface finish are secondary factor.

• Some of the application of rough grinding are, trimming the surface left by sprues and risers on casting, grinding part line on casting, removing flash on forgings, excess metal on welding etc.

Precision grinding:

• Precision grinding concerned with producing good surface finishing and high degree of accuracy.

• The wheel or work both are guided in precision paths.

• Precision grinding is followed after rough grinding.

 Surface grinding methods 

External cylindrical grinding:

• External cylindrical grinding produces a straight or tapered surface on a workpiece, shown on figure below.

• Workpiece must be rotated about its own axis between centres as it passes lengthwise across the face of a revolving grinding wheel.

Internal cylindrical grinding:

• Internal cylindrical grinding produces internal cylindrical holes and tapers, shown in figure below.

• The workpieces are chucked and precisely rotated about their own axis.

• The grinding wheel rotates against the sense of rotation of the workpiece.

Surface grinding:

• Surface grinding produces flat surface.

• The work may be ground by either the periphery or by the end face of the grinding wheel.

• The workpiece is reciprocated at a constant speed below or on the end face of the grinding wheel.

Form grinding:

• Form grinding is done with specially shaped grinding wheel.

• That grind the formed surfaces as in grinding gear teeth, threads, splined shafts, holes, etc.

Centreless grinding:

• In this process, the work is supported between grinding wheel and regulating wheel on work rest blade.

• The principle of centreless grinding is shown in figure.

• Used when long slender bars are to be finished.

 Classification of grinding machines 

The grinding machine can be classified as;

• Cylindrical grinding machine

(i) External cylindrical grinder

(a) Centre type

(b) Centreless type

(ii) Internal cylindrical grinder

(a) Centre type

(b) Centreless type

• Surface grinding machines

(i) Vertical spindle

(a) Reciprocating table

(b) Rotary table

(ii) Horizontal spindle

(a) Reciprocating table

(b) Rotary table

• Tool and cutter grinding machines

• Special grinding machines.

Cylindrical grinding machine:

A cylindrical grinding machine is similar to the lathe, but on which a grinding wheel is mounted instead of the single point cutting tool.

Plain milling cutters are divided into;

• Base

• Table

• Headstock

• Tailstock

• Cross feed, and

• Wheel head.

Base:

• The base or bed is the main casting that rests on the floor and supports the parts mounted on it.

• On the top of the base are precision horizontal ways set at right angles for the table to slide on.

• The base also houses the table-drive mechanism.

Table:

• There are two tables-lower table and upper table.

• The lower table slides on ways on the bed provides traverse of the work past the grinding wheel.

• It can be moved by hand or power within desired limits.

• The upper table that is pivoted at its center is mounted on the top of the sliding table.

• It has T-slots for securing the headstock and tailstock and can be positioned along the table to suit the length of the work.

• The upper table can he swivelled and clamped in position to provide adjustment for grinding straight or tapered work as desired.

• Setting for tapers upto ±100 can be made in this way.

• Steep tapers are ground by swivelling the wheel head.

• Adjustable dogs are clamped in longitudinal slots and they are provided at the side of the lower or sliding table and set up to reverse the table at the ends of the stroke.

Headstock:

• The headstock supports the workpiece by means of a dead center and drives it by means of a dog, or it may hold and drive the workpiece in a chuck.

Tailstock:

• The tailstock can be adjusted and clamped in various positions to accommodate different lengths of workpieces.

Wheel head:

• The wheel head carries a grinding wheel and its driving motor is mounted on a slide at the top and rear of the base.

• The wheel head may he moved perpendicularly to the table ways, by hand or power, to feed the wheel to the work.

Cross feed:

• The grinding wheel is fed to the work by hand or power as determined by the engagement of the cross-feed control lever.

• On cylindrical grinding machines, the operation may he stopped automatically when the workpiece has been finished to size.

• In one method it uses an automatic caliper type gauging attachment to measure the workpiece and stop the operation at the proper time.

Working of cylindrical grinding machine:

• The workpiece is usually held between dead centers and rotated by a dog and driver on the face plate as shown in figure.

• The work may also be rotated about its own axis in a chuck.

• There are four movements involved in a cylindrical centre-type grinding,

(i) The work must revolve

(ii) The wheel must revolve

(iii) The work must pass the wheel

(iv) The wheel must pass the work.

• They are equipped with a mechanism which enables the grinding wheel to be fed in automatically towards the work for successive cuts.

• Hand feed is employed only in adjusting the wheel or starting the cut.

• A provision is also made for varying the longitudinal movement of the work or the wheel.

• Hydraulic rather than mechanical controls are preferred on grinding machines to cause minimum vibration.

• Figure illustrates schematically this machine,

• The machine is similar to a centre lathe in many respects.

• A disc type grinding wheel performs the grinding action using the peripheral surface.

• Both traverse and plunge grinding can be carried out in this machine.

Traverse grinding:

• The work is reciprocated as the wheel feeds to produce cylinders longer than the width of the wheel face.

Plunge grinding:

• The work rotates in affixed position as the wheel feeds in to produce cylinders of a length equal to the width of the wheel.

• The important advantage is that cylindrical shapes can be produced as easily as straight cylinders in a single "plunge" of the wheel simply by forming the periphery of the wheel.

Surface grinding machines:

• Surface grinding machines are generally used for grinding flat surfaces. These machines are similar to milling machine in construction as well as in motion.

• Some types of surface grinders are also capable of producing contour surface with formed grinding wheel.

• Basically there are four different types of surface grinding machines characterized by the movement of their tables and the orientation of grinding wheel spindles as follows,

(i) Horizontal spindle and reciprocating table

(ii) Vertical spindle and reciprocating table

(m) Horizontal spindle and rotary table

(iv) Vertical spindle and rotary table.

• The majority of surface grinders are of the horizontal table type. In the horizontal type of machine, grinding is normally done by the periphery of the wheel.

• The area of contact is small, and the speed is uniform over the grinding surface.

• Small grain wheels can be used, and the finest finishes obtained.

• The vertical type, surface grinders apply the face or side of the wheel, and cupped cylindrical, or segmented wheels are used.

• The area of contact may be large, and stoke can be removed rapidly. But a crisscross pattern of grinding scratches are left on the work surface.

Horizontal spindle reciprocating table surface grinder:

• The block diagram of a straight wheel horizontal spindle reciprocating table surface grinder is shown in figure.

• 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 figure.

Base:

• The base has a column at the back for supporting the wheel head. The base also contains the drive mechanisms.

Table:

• The table is fitted to the saddle on carefully machined guide ways. It reciprocates along ways to provide the longitudinal feed.

• T-slots are provided in the table surface for clamping workpieces directly on the table or for clamping grinding fixtures or a magnetic chuck.

• On some machines, the table can also be moved in or out from the vertical column which supports the wheelhead. This movement is known as cross- feed.

Wheelhead:

• The wheelhead is mounted on the column secured to the base.

• It has ways for the vertical slide which can be raised or lowered with the grinding wheel only manually by rotating a hand wheel to accommodate workpieces of different heights and to set the wheel for depth of cut.

• Horizontal, crosswise movement of the wheel slide with the wheel, actuated by hand or by hydraulic drive, accomplishes the cross feed of the wheel.

• The grinding wheel rotates at constant speed - it is powered by a special built - in motor.

Operation:

• The workpiece reciprocates under the wheel, and the wheel or the table feeds axially between passes to produce a fine flat surface.

• Wheel downfeed determines depth of cut and final height of the piece from the table to the wheel.

• The amount of feed must only be equal to a few hundredth of a millimeters.

(i) For example, steel is rough ground with a depth of cut between 0.02 and 0.05mm and finish-ground with a depth of cut of 0.005 to 0.01 mm.

(ii) In the case of grey cast-iron the depth of cut in rough grinding may be anything between 0.08 and 0.15 mm and in finish grinding between 0.02 and 0.05 mm.

Horizontal spindle rotary table surface grinder:

• Surface grinding in this machine is shown in figure.

• In principle, the operation is same as that for facing on the lathe.

• This machine has a limitation in accommodation of workpiece and therefore does not have wide spread use.

• The swiveling the worktable, concave or convex or tapered surface can be produced on individual part as illustrated in figure.

Vertical spindle reciprocating table surface grinder:

• This grinding machine with all working motions is shown in figure.

• The grinding operation is similar to that of face milling on a vertical milling machine.

• In this machine a cup shaped wheel grinds the workpiece over its full width using end face of the wheel as shown in figure.

• This brings more grits in action at the same time and consequently a higher material removal rate.

Vertical spindle rotary table surface grinder:

• The block diagram of a vertical spindle rotary table surface grinder is shown in figure.

• The principal elements of this machine consists of a magnetic chuck which can be moved under or away from the wheel for unloading and loading the work.

• Slide which is mounted on horizontal bed ways.

• The grinding spindle is mounted vertically on the face of a column and rotates in a fixed position feeding only along its axis.

• The rotary table travels beneath the wheel as grinding wheel rotates.

• This combination of table travel and table rotation exposes the entire surface of the workpiece to the wheel and eliminates the need for any lateral movement of the wheel.

Tool and cutter grinding machine:

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

• With various attachments they can also do light surface, cylindrical, and internal grinding to finish such items and jig, fixture, die and gauge details and sharpen single point tools.

• They are classified according to the purpose of grinding, into two groups,

(i) Universal tool and cutter grinders

(ii) Single-purpose tool and cutter grinders.

Universal tool and grinders:

• Universal tool and cutter grinders are particularly intended for sharpening of miscellaneous cutters.

• Single-purpose grinders are used for grinding tools such as drills, tool-hits, etc.

• In large production plants where large amount of grinding work is necessary to keep production tools inpropercutting condition.

• In addition, tools can be ground uniformly and with accurate cutting angles.

• A typical tool and cutter grinder is shown in figure below,

• The universal tool and cutter grinder has the following principal parts;

Base:

• The saddle is mounted directly on the top of the base.

• It is heavy, rugged and box-type.

• It carries all other parts of the machine.

• It is made up of heavy cast iron to absorbs impact load and vibrations during cutting stocks.

Saddle:

• The saddle is mounted directly on the top of the base.

• It moves on antifriction ball bearings on hardened ways.

• The column supporting the wheel head is mounted on the saddle and it can he moved up and down and swivelled to either side.

• The saddle also provides the means for moving the work forward and backward.

Table:

• The table rests and moves on a top base which is mounted over the saddle.

• The top of the base contains the gears and 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 work table can be swivelled which enables the operator to grind tapers.

Headstock and tailstock:

• The headstock and tailstock are mounted on either side of the table similar to those on a cylindrical grinder.

• The workpiece is positioned between the centres and driven exactly as in a cylindrical grinder.

Wheel head:

• The wheel head is mounted on a column on the back of the machine.

Grinding wheel:

• Three different types of grinding wheels are extensively used in cutter grinding. They are;

(i) The "straight" or disc-shaped wheel

(ii) The cup type in either the straight or flaring form

(m) The dish type.

Centreless grinding machines:

• This grinding machine is a production machine in which out side diameter of the workpiece is ground.

• The workpiece is not held between centres but by a work support blade.

• It is rotated by means of a regulating wheel and ground by the grinding wheel.

• The principal elements of an external centreless grinder are shown, Figure shows the grinding wheel, regulating or back up wheel, and the work rest. Both wheels are rotated in the same direction.

• The work rest is located between the wheels. The work is placed upon the work rest, and the latter, together with the regulating wheel, is fed forward, forcing the work against the grinding wheel.

• The axial movement of the work past the grinding wheel is obtained by tilting the regulating wheel at a slight angle from horizontal. An angular adjustment of 0 to 8 or 10 degrees is provided in the machine for this purpose.

Through grinding:

• The regulating wheel revolving at a much lower surface speed than grinding wheel controls the rotation and longitudinal motion of the workpiece.

• The regulating wheel is kept slightly inclined to the axis of the grinding wheel and the workpiece is fed longitudinally as shown in figure.

Infeed grinding:

• Which is similar to plunge grinding or form grinding.

• The regulating wheel is drawn back so that workpieces may be placed on the work-rest blade. Then it is moved in to feed the work ,against the grinding wheel.

• This method is useful to grind shoulders, and formed surfaces.

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.

Advantages of centerless grinding:

• As a true floating condition exists during the grinding process, less metal needs to be removed.

• The workpiece being supported throughout its entire length as grinding takes place, there is no tendency for chatter or deflection of the work and small, fragile or slender workpieces can be grounded easily.

• The process is continuous and adapted for production work.

• No centre holes, no chucking or mounting of the work on mandrels or other holding devices are required.

• The size of the work is easily controlled.

• A low order of skill is needed in the operation of the machine.

Disdvantages of centerless grinding:

• Work having multiple diameter is not easily handled.

• In hollow work, there is no certainly that the outside diameter will be concentric with the inside diameter.

• This process is not suitable for large size workpiece.

• This process is useful only for large volume production.

Grinding wheel:

• A grinding wheel is a rotating abrasive wheel used to ground the workpiece.

• Grinding wheel is made up of crushed hard particles [abrasives] and embedded in a matrix called the bond.

• The sharp edges of the abrasive particles act as acting edge to ground the workpiece.

Abrasives:

• An abrasive 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 fracture to be useful in manufacturing grinding wheels.

• Abrasives may he classified in two principal groups;

(i) Natural

(ii) Artificial or manufactured.

Natural:

• The natural abrasives include sandstone or solid quartz, emery, corundum, and diamond.

• Sandstone or solid quartz is one of the natural abrasive stones from which grindstones are shaped.

• The quartz cutting agent is relatively shaft that materials harder than quartz cannot be abraded or ground rapidly.

• Emery is a natural aluminium oxide.

• It contains from 55 to 65 percent alumina, the remaining consist of iron oxide and other impurities.

• Coruntlum is a natural aluminium oxide, It also contains from 75 to 95 percent aluminium oxide, the remaining consists of impurities.

• Both emery and corundum have a greater hardness and better abrasive action than quartz.

• Diamonds of less than gem quality are crushed to produce abrasive grains for making grinding wheels to grind cemented carbide tools and make lapping compound.

• As a result of the impurities in and lack of uniformity of these natural abrasives, only a very a small percentage of grinding wheels are produced from natural abrasives.

Artificial:

• Artificial or manufactured abrasives include chiefly;

(i) Silicon carbide, and

(ii) Aluminium oxide.

Silicon Carbide:

• Silicon Carbide [SiC] abrasive is manufactured from 56 parts of silicon sand, 34 parts of powdered coke, 2 parts of salt, and 12 parts of saw dust in a long, rectangular electric furnace of the resistance type that is built up of loose brickwork.

• Sand furnishes silicon, coke furnishes carbon, sawdust makes the charge porous, and salt helps to fuse it.

• The abrasive wheels are denoted by ‘S’.

• There are two types of silicon carbide abrasives.

• Green grit which contains at least 97 percent silicon carbide, and black grit which contains at least 95 percent ofsilicon carbide.

• This form is harder but weaker than the latter.

• Silicon carbide follows the diamond in order of hardness, but it is not as tough as aluminium oxide.

• It is used for grinding materials of low tensile strength such as cemented carbides, stone and ceramic materials, gray cast iron, brass, bronze, copper, aluminium, vulcanized rubber, etc.

Aluminium oxide:

• Aluminium oxides are manufactured by heating minerals of bauxite.

• The hydrated aluminium oxide clay containing silica, iron oxide, titanium oxide, etc., mixed with ground coke and iron borings in a arc-type electric furnace.

• Aluminium oxide is tough and not easily fractured, so it is better adopted to grinding materials of high tensile strength, such as steels, argon steels, high speed steels, annealed malleable iron, wrought iron, tough bronzes.

• Grinding wheels are denoted by letter A.

Bond and bond materials:

• A bond is an adhesive substance that is employed t0 hold abrasive grains together in the form of sharpening stones or grinding wheels.

• Commonly used bond materials for different bonds are,

• These bonds may be used with either silicon carbide or aluminium oxide.

Vitrified bonding process:

• Vitrified wheels are made by bonding clay and melted to a glass like consistency with abrasive grains.

• The clay and abrasive grains are thoroughly mixed together with sufficient water to make the mixture uniform.

• The fluid mixture is then poured into moulds and allowed to dry. When it has dried to a point where it can be handled, the material is cut trimmed to more perfect size and shape.

• It is then heated or burned in a kiln in much the same manner as brick or tile is burnt.

• When the burning proceeds, the clay vitrifies; that is, it fuse and forms a porcelain, or glasslike substance that surrounds and connects the abrasive grains.

• The high temperature employed in this process tends to anneal the abrasives to some extent.

• Vitrified bond gives a whee] good strength as well as porosity to allow high stock removal with cool cutting.

• Disadvantages of vitrified bonded wheels are their sensitivity to impact and their low bending strength.

• About 75 per cent of the wheels now manufactured are made with this bond.

• A vitrified bonded wheel is denoted by the letter V.

Silicate bonding process:

• Silicate wheels are made by mixing abrasive grains with silicate of soda or water glass.

• The mixture is packed into moulds and allowed to dry.

• The moulded shapes are then baked in a furnace at a temperature of 260°C for several days.

• The silicate bond releases the abrasive grains more readily than the vitrified bond, the abrasive grains are not annealed as in the vitrified process, and silicate wheels are waterproof.

• These characteristics make silicate wheels valuable for grinding edged tools and other operations where heat must he held to a minimum with or without the aid of a coolant.

• A silicate bonded wheel is denoted by the letter ‘S’.

Shellac bonding process:

• Shellac bonded wheels are also known as elastic bonded wheels.

• In this process, the abrasive and shellac are mixed in heated containers and then rolled or pressed in heated moulds.

• Later the shapes are hacked it few hours at the temperature of approximately 150°C.

• The elasticity of this bond is greater than in other types and it has considerable strength.

• It is not intended for heavy duty.

• Shellac bond is cool cutting on hardened steel and thin sections, and is used for finishing chilled iron, cast iron and steel rolls, hardened steel cams and aluminum pistons, and in very thin sections, for abrasive cutting of machines.

• A shellac bonded wheel is denoted by the letter 'F'.

Resinoid bonding process:

• Resinoid wheels are produced by mixing abrasive grains with synthetic resins and other compounds.

• Then the mixture is placed in moulds and heated at about 200°C.

• At this temperature, the resin sets to hold the abrasive grains in whee] form.

• Wheels bonded with synthetic resin, such as Bakelite and Redmanol, are used for purposes which require a strong, free high speed wheel.

• They can remove stock very rapidly.

• They are useful for precision grinding cams, and rolls requiring high finish.

• A resinoid bonded wheel is denoted by the letter B.

Rubber bonding process:

• Rubber bonded wheels are prepared by mixing abrasive grains with pure rubber and sulphur.

• Then the mixture is rolled into sheets, and wheels are punched out of the sheets on a punch press.

• Following that, the wheels are vulcanized.

• Rubber bonded wheels are more resilient, less heat resistant, and more dense than resinoid bonded wheels.

• They are used where good finish is primary require.

• They are strong and tough enough to make extremely thin wheels.

• A rubber bonded wheel is denoted by the letter R.

Oxy-chloride bonding process:

• This process consists of mixing abrasive grains with oxide and chloride of magnesium. The mixing of bond and abrasive is performed in the same way as for vitrified bonded wheel.

• Oxy-chloride bonds are employed in making wheels and wheels segments for use in disc-grinding operations.

• Oxy-chloride bonds are employed in making wheels and wheels segments for use in disc-grinding operations.

• The bond ensures a cool cutting action.

• So grinding is best done dry.

• An oxy-chloride bonded wheel is denoted by the letter 'O'.

Grits:

• The grain or grit number indicates in a general way the size of the abrasive grains used in making a wheel, or the size of the cutting teeth, since grinding is a true cutting operation.

• Grain size is denoted by a number indicating the number of meshes per linear inch [25.4 mm] of the screen through which the grains pass when they are graded after crushing.

• The following list shows the of grain ranging from very coarse to very fine, it includes all the ordinary grain sizes commonly used in the manufacture of grinding wheels:

• In case grinding wheels are manufactured from special grain combinations, the grinding wheel manufacturer may use an additional symbol appended to the standard grain size number.

Example:

(i) 36 - Normal standard.

(ii) 36.5 - Special grain combination.

• The size of abrasive grain required in a grinding wheel depends on the amount of material to he removed, the finish desired, and the hardness of the material being ground.

• In general, coarse wheels are used for fast removal of materials.

• Fine grained wheels are used for soft, ductile materials but generally a fine grain should he used to grind hard, brittle materials.

Grade:

• The term 'grade' as applied to a grinding wheel refers to the tenacity or hardness with which the bond holds the cutting points or abrasive grains in a place.

• It dose not refer to the hardness of the abrasive grain.

• The grade shall be indicated in all bonds and process by a letter of the English alphabet, A denoting the softest and Z the hardest grade.

• The term 'soft' or 'hard' refer to the resistance of a bond offers to disruption of the abrasives.

• A wheel from which the abrasive grains can easily be dislodged is called soft, whereas one which holds the grains more securely is called hard.

• The grades are denoted as shown in table below:

• The grade of the grinding wheel depends on the hardness of the material being ground, the arc of the contact, the wheel and work speeds, and the condition of the grinding machine.

• Hard wheels are recommended for soft materials and soft wheels for hard materials.

Structure:

• Abrasive grains arc not packed in the wheel but are distributed through the bond.

• The relative spacing is referred to as the structure and denoted by the number of cutting edges per unit area of wheel face as well as by number and size of void spaces between grains.

• The primary purpose of structure is to provide chip clearance and it may be open or dense.

• The structure commonly used is denoted by numbers as shown in table:

• The structure of a grinding wheel depends on the hardness of the material being ground, the finish required, and the nature of the grinding operation.

• Soft, tough and ductile materials and heavy cuts require an open structure, whereas hard and brittle materials and finishing cuts require a dense structure.

Wheel shapes and factors for wheel selection:

• Grinding wheels are made in many different shapes and sizes to adapt them for use in different types of grinding machines and on different classes of work.

• They fall into the following broad groups;

(i) Straight-side grinding wheels

(ii) Cylinder wheels

(m) Cup wheels

(iv) Dish wheels.

• The shapes of grinding wheels have been standardized so that those commonly use in production and tool room grinding may be designated by a number or name or both.

• The sizes of wheels may be referred to a system of key letters so that their dimensional specifications may be written easily.

Straight wheels:

• No. 1, 5, and 7 are the kind generally used for cylindrical, centreless, and surface grinding operations. Wheels of this form vary greatly in size.

• The diameter and width of face naturally depending upon the class of work for which the wheel is used and the size and power of the grinding machine.

Tapered face straight wheel:

• No. 4 is primarily used for grinding thread, gear teeth, etc.

Cylinder or wheel ring:

• No. 2 is used for producing flat surfaces, the grinding being done with the end face of the wheel.

Cup wheel:

• No. 6 is used for grinding flat surfaces by traversing the work past the end or face of the wheel. Flaring cup whee] No.11 is used for grinding in tool room.

Dish wheel:

• No. 12 is also used for tool room work. The thinness of the wheel permits it grind the surface at narrow places. Saucer wheel No. 13 is generally used for sharpening of circular or band saws. The principal dimensions of a grinding wheel are the outside diameter, bore diameter, and the width.

Segmented wheels:

• Segmented wheels are used chiefly on vertical spindle, rotary, and reciprocating- table surface grinders and way grinders.

• Grinding wheels of the straight wheel type can be supplied with a large variety of face: flat, pointed, concave, convex, etc. These faces are used for grinding special contours and sharpening saws.

Selection of grinding wheels:

• It is customary for grinding wheel manufacturers to provide, through their published literature, information on the selection and use of grinding wheels, but it may not always be possible or convenient for users to take advantage of such consultative service.

• The need for ready to use general guide on grinding wheels has been keenly felt and the Indian Standard (IS: 1249-1958] gives recommendations on the general considerations which should guide the selection of grinding wheels for different applications.

• In selecting a grinding wheel there are four constant factors and four variables;

• Constant factors:

(i) Material to be ground

(ii) Amount of stock to be removed

(iii) Area of contact.

(iv) Type of grinding machine

• Variable factors:

(i) Wheel speed

(ii) Work speed

(iii) Condition of the machine

(iv)Personal factor

The material to be ground:

(i) Aluminium Oxide abrasive is recommended for materials of high tensile strength and silicon carbide for low tensile strength.

(ii) Fine grain is used for hard and brittle materials and coarse grain for soft ductile metals.

(iii) Hard wheel is used for soft materials and soft wheel for hard materials.

(iv) Generally, close spacing is required for hard and brittle materials and wide for soft and ductile.

(v) The class of work usually dictates the bond to be used. Bond selection, of course, can be safely left to the manufacturers, if the class of work for which the wheel is required is clearly stated. However, majority of wheels are manufactured with vitrified bonds.

• Amount of stock to be removed:

(i) This involves accuracy and finish.

(ii) Coarse grain is used for fast cutting and fine grain for fine finish

(iii) Wide spacing for rapid removal and close for fine finish.

• Resinoid, rubber, and shellac bond for high finish.

Maintenance of grinding wheel:

• To improve the cutting capacity.

• To improve the efficiency of grinding wheel.

• To run the grinding wheel in concentric speed with spindle.

• The grinding wheel can be maintained in following ways;

(i) Dressing

(ii) Truing

(iii) Balancing.

Dressing:

• Dressing removes loading and breaks away the glazed surface, so that sharp abrasive particles are again presented to the work.

• This is done with various type of dressers.

• A common type of wheel dresser, known as the star-dresser, is shown below.

• Star-dresser consists of a number of a hardened steel wheels with points on their periphery.

• The dresser is held against the face of the revolving wheel and moved across the face to dress the hole surface.

• This type of dresser is used to dress coarse-grain abrasive wheels used for rough snagging work.

• Another type of wheel dresser consists of a steel tube filled with a bonded abrasive.

• The end of the tube is held against the wheel and moved across the face.

• The grade of abrasive in the dresser may vary for different types of dressing operations.

• Abrasive wheel dressers operates at high speed are frequently used to dress other wheels.

• They are used to dress wheels where a fair degree of finish is desired on the work.

• For precision and high finish grinding, small industrial diamonds, known in the trade as bort, are used.

• The diamond or group of diamonds is mounted in a holder. The diamond should be kept pointed, since only the point can be used for cutting.

• This is done by the holder down at a 15° angle and using a new surface each time the wheel is dressed.

• A good supply of coolant should he used when dressing with a diamond, as overheating can causes the diamond to fracture or drop out of its setting.

• Very light cuts only may be taken with diamond tools.

• This dressing process may use following dressing tools;

(i) Cluster type tool: Used for roughing and commercial finishes.

(ii) Corrugated disc dressers: Made of special alloy steel, used for superior finish.

(iii) Disc dressers: Steel disc with teeth used for medium grade roughing wheels and for cam and crank shaft grinding wheels.

Truing:

• Truing is the process of changing the shape of the grinding wheel as it becomes worn from an original shape, owing to break away from the abrasive and bond.

• This is done to make the wheel true and concentric with the bore, or to change the face contour for form grinding.

• Truing and dressing are done with the same tools, but not for the same purpose.

• The only satisfactory method of truing a wheel is by the use of a diamond tool in a similar manner as explained before.

• In turning a wheel with a diamond, the feed rate must not exceed 0.02 mm, otherwise grooves may be cut into the wheel.

• More popular is form-truing with a crushing roll. In this, a roll, shaped to the desired profile, is forced against the revolving wheel, crushing the corresponding shape into it.

• Rolls may be of two types: Idler in which the wheel drives the roll; and Power driven, in which a small motor drives the roll which, in turn, rotates the wheel by frictional contact.

• Wheels trued by crushing cut faster and run cooler than those trued with a diamond.

• Crushing produces a wheel with many sharp pointed grains, while diamond truing tends to produce many grains with flat surfaces.

Balancing:

• If wheels become out of balance through wear and cannot be balanced by truing or dressing, they should be removed from the machine and discarded.

• Wheels should be tested for balance occasionally and rebalanced if necessary.

• Wheels that are out of balance not only produce poor work but may put under strains on the machine.

• Small wheels may be balanced by milling a short recess on the inside of the flanges and filling with lead.

• Large wheels should he placed on a balancing stand and balanced by moving weights around a recessed flange.

• Now-a-days, grinding wheel mounts are provided with devices to enable balancing to be done whilst the wheel is running and between grinding operations.

END