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Chapter 1 abrasive machining & finishing operation
The document discusses various abrasive machining processes, emphasizing grinding operations that use small abrasive grains to achieve precise material removal and fine surface finishes on hard materials. It outlines the types of grinding wheels, their bonds, grades, and structures, as well as the different grinding methods including cylindrical, surface, and centerless grinding. Additionally, the document touches on related processes such as honing and lapping, along with factors affecting grinding wheel wear and cutting temperatures.
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Chapter 1 abrasive machining & finishing operation
- 1. Abrasive Machining &Finishing Operation Prof. S. S. Petkar, AMGOI
- 2. Introduction • Abrasive processesutilise very small abrasive grains to remove material to provide good finish on metallic parts. • Grinding is a process carried out with a grinding wheel made up of abrasive grains for removing very fine quantities of material from w/p. • The required size of abrasive grains are mixed with bonding material and then pressed into disc a disc shape of given diameter & thickness. • This can be compared to a milling process with an infinite number of cutting edges. Prof. S.S.Petkar
- 3. Grinding is aprocess used for….???? • Machining materials which are too hard for other machining processes such as tool and die steels and hardened steel materials. • Close dimensional accuracy of the order of 0.3 to 0.5 micro meter • High degree of surface smoothness such as Ra=0.15 to 1.25 micro meter Prof. S.S.Petkar
- 4. Characteristics of VariousAbrasive Processes Process Particle mounting Features Grinding Bonded Wheels, for finishing. Low material remove rate Creep feed grinding Bonded open soft Wheels, Slow feed, large depth of cut Snagging Bonded, belted High material remove rate, roughing to clean and deburr castings & forgings Honing Bonded Stone contain fine abrasives for hole finishing Lapping Free For super finishing Prof. S.S.Petkar
- 5. Abrasives • Grains arebasically spherical in shape with large sharp point which act as cutting point • All grains are of random orientations and rake angle can vary • The depth of cut taken by each grain is very small • Cutting speed large, chips produced are very small and red hot • Specific cutting energy= 50 J/mm3 Prof. S.S.Petkar
- 6. Grinding wheel selection& Designation • Abrasive types :- These are hard materials with adequate toughness so that they will be able to act as cutting edges for long time. They have Friability. 1. Aluminium oxide (Al2O3) 2. Silicon Carbide (SiC) 3. Cubic boron Nitride (CBN) 4. Diamond Prof. S.S.Petkar
- 7. Grinding Wheel Types Prof.S.S.Petkar Shapes Of grinding Wheels
- 8. Prof. S.S.Petkar Various Facesof wheels form for the straight wheels type1
- 9. Surface Finish Grain SizeSurface Finish , 46 0.8 54 0.5 to 0.8 60 0.4 to 0.6 80 0.2 to 0.4 Prof. S.S.Petkar
- 10. Bond • The functionof bond is to keep abrasive grains together under the action of grinding forces. • Types of Bonds 1. Vitrified 2. Silicate 3. Synthetic resin 4. Rubber 5. Shellac 6. metal Prof. S.S.Petkar
- 11. Vitrified 1. Bond isclay, mixed with fluxes such as feldspar. 2. This bond develops strength. 3. Bond is strong, rigid and not affected by fluids 4. But, this bond is brittle hence sensitive to impacts. 5. It is also called Ceramic bonds Silicate • This is sodium silicate (Na2SiO3) or water glass and hardens when heated. • Not strong as vitrified • It can be used in operations that generates low heat • It is affected by dampness but less sensitive to shocks and is relatively less used. Prof. S.S.Petkar
- 12. Synthetic Resin orResinoid • These bonding materials are such as phenol formaldehyde. • This bond has good strength and more elastic than vitrified • This is not heat and chemical resistant • It is generally for rough grinding, parting off and high speed grinding (50 to 65 m/s) • It can also be used for fine finishing. Rubber • It is most flexible • Made up of natural or synthetic rubber • The strength is developed by vulcanisation. • This has high strength and less porous. • Bond is affected by dampness and alkaline solutions • Used for cutting off wheels, regulating wheels in centreless grinding and for polishing wheels Prof. S.S.Petkar
- 13. Shellac • This isrelatively less used bond • Used generally for getting very high finish. • Typical applications are rolls, cutlery and cam shaft finishing Metal • Used to manufacture of diamond and CBN wheels • Made up of cu alloys or Al alloys • The choice of metal depends upon the required strength, rigidity and dimensional stability. • The periphery of wheel up to a small depth of order of 5mm or less contains abrasive grit • Powder metallurgy technique is used to make the abrasive periphery Prof. S.S.Petkar
- 14. Wheel Grade • Itis called hardness of wheel. This designates force holding the grains. • Grade depends on bond, structure of wheel and amount of abrasive grains. • Harder wheels holds the abrasive grains till the grinding force increases to a great extent Very soft Medium Very hard ABCDEFGHIJKLMNOPQRSTUVWXYZ Prof. S.S.Petkar LETTER GRADE
- 15. Grinding Wheel StandardMarking System Prof. S.S.Petkar
- 16. Wheel hardness fordifferent work materials Work Material Wheel Hardness Cylindrical Surface Internal Deburring Steel up to 80 kg/mm2 L,M,N K,L K,L O,P,Q,R Steel up to 140 kg/mm2 K J,K J Steel more than 140 kg/mm2 J I,J I Light Alloys J, I,K I Cast Iron K J J Bronze, Brass, Copper L,M J,K J Prof. S.S.Petkar
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- 18. Wheel grade andStructure • The structure represents the grain spacing • It can be open or dense. • The spacing between the grains allow for chips to collect as shown in fig. • This helps avoiding the loading of wheel. • Open structures are used for high stock removal and consequently produce rough finish. • Dense structures are used for precision forms and profiling grinding. Prof. S.S.Petkar
- 19. Dressing and Truing •A loaded grinding wheel can be cleaned and sharpened by means of a process called Dressing • Dressing can be done by using small steel disc, silicon carbide abrasive disc. Prof. S.S.Petkar
- 20. Prof. S.S.Petkar 1. Thestick is applied directly on the wheel. 2. A free rotating wheel mounted on the table 3. The wheel will be crushing the grinding wheel Surface thus providing an improved control of the dressed surface characteristics 4. A simple dressing done by small steel disk, which are free to rotate at he end of disk. 5. disc when contact to wheel face, will sharpen the wheel
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- 22. • A diamondused for truing is set in a closely fitting hole at the end of a short steel bar and is brazed. • The grinding wheel is rotated at its normal speed and a small depth of 0.025mm is given while the dressing tool across the face of the grinding wheel, in an automatic feeds. • The cross feed rates are controlled depending upon the required surface. • Slow feed rates are used for generating fine finishes while faste feeds are used for free cutting. Prof. S.S.Petkar
- 23. Grinding Machines •Grinding operations are classified into 1. Cylindrical grinding 2. Surface grinding 3. Centreless grinding Grinding machines used for precise work to produce parts with close tolerance Typical grinding operations are in figure <<<< Prof. S.S.Petkar
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- 25. Cylindrical Grinding 1. Thismachine is used to produce external cylindrical surface 2. Both the work and wheel will rotate in anticlockwise direction 3. The work is held between work centres is rotated at low speed as compared to wheel speed. 4. Very fine finishes are obtained with this grinding. Possible to get accuracies to within 0.25 micrometre with extreme care. 5. Transverse feed of the w/p, past the grinding wheel, is provided by using hydraulic arrangement. Prof. S.S.Petkar
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- 27. Horizontal spindle &rotating table • In this machine grinding wheel cuts on its periphery. • Feed is accomplished by moving the work mounted on table, up into the wheel • Since table and work both are rotating , the surface pattern is a series of intersecting arcs. • This machine is used for round, flat parts. Vertical Spindle and Rotating Table • The compete machining surface is covered by the grinding wheel face. • They are suitable for production grinding of large flat surface. In this machine both work and wheel rotate and feed each other. • Side and face of wheel does the grinding • It a versatile machine and surface pattern is a series of intersecting arcs • It is used to grind production parts and very large parts. Prof. S.S.Petkar
- 28. Horizontal Spindle &Reciprocating Table • The table is moved by hydraulic power • Wheel head is given a cross feed motion at the end of each table motion • Wheel should overtravel the workpiece at the both ends to prevent the wheel also removing the metal at the same work spot during reversal • Used to maintain high accuracy and fine surface finishes that imparts. • The grinding wheel traverses in a straight pattern that results in superfinish and high precision. Vertical Spindle & Reciprocating Table • Wheel is cylindrical and cuts on its side than periphery • The work is fed by reciprocating motion of the table • Diameter of wheel is wider than work • No traverse feed is required • Used for high production machine tools removing large amounts such as 10mm in a single pass Prof. S.S.Petkar
- 29. Centreless Grinding Prof.S.S.Petkar Through Feed In feed End Feed
- 30. • It makesto possible to grind cylindrical w/p without actually fixing the w/p using a centre or a chuck. (Through feed) • It contains 1 large wheel, small regulating wheel. w/p is held on work rest table • The centre of w/p is slightly above the centre of grinding wheel. • w/p is supported by blade & held against regulating Wheel by grinding force. • As result the w/p rotates at the same speed as that of regulating wheel. • Regulating wheel is made up with rubber or resinoid bonded with wide face. • In Infeed , grinding is done by plunge feeding so that any form could be made • w/p will be loaded into machine while rest blade & wheel are withdrawn. • In end feed only tapered w/p can be machined Prof. S.S.Petkar
- 31. Advantages • No needfor maintaining centres and centre hole • w/p can be loaded & unloaded from machine rapidly • Continuous grinding through feed grinding • Backing up of w/p by regulating wheel and rest blade eliminates deflection of w/p • Minimum wear • w/p may often be loaded into the machine by the automatic feeding devices • Less grinding allowances required, because out of roundness is corrected across the diameter rather than radius. Limitations • Set up time is large • Process is useful only for large volume production. • Sometimes it may be necessary to have a special equipment and additional setup time for special profiles • The process is not suitable for large workpiece size Prof. S.S.Petkar
- 32. Internal centreless grinding Prof.S.S.Petkar
- 33. • In thisw/p needs to be supported by 2 support rolls. • Ground hole will be concentric with outside dia of work • Process is capable of straight grinding, cylindrical or tapered hole. These holes can be blind, interrupted, through or even with shoulder. • 1st support roll is mounted below the work to support it. • 2nd support roll is pressure roll and holds the work in contact with other two. • This roll moves in and out to allow for loading and unloading the machines. • 3rd roller is regulating roller that drives the work & control its speed and motion. • The grinding wheel remains in a fixed position & work traverses past the grinding wheel. Prof. S.S.Petkar
- 34. Grinding Operations 1) CreepFeed Grinding Prof. S.S.Petkar
- 35. • In thisprocess entire depth of cut is completed in one pass only using very small in feed rates • High depth of cut order of 1 to 30 mm with low work speed of 1 to 0.025 m/min • In this process idle time is reduced. (stopping and wheel/table reversal) • The cutting force and power required increases in this grinding process. But has favourable G ratio. • It is necessary to continuously dress the grinding wheel. Prof. S.S.Petkar
- 36. Creep feed grindingwith dressing Prof. S.S.Petkar 1. Grinding wheel speed are low 18 to 30 m/s. 2. Feed rates are low 0.005 mm / pass. 3. Oil based fluids are used at low speed. 4. Volume of fluid is much as high heat generated in process. 5. If diamond dresser used then possible for Continuous dressing with high rate of removal
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- 38. • It islow abrading process using low abrasive sticks for removing stock from metallic and non metallic surface. • It is rarely used on external cylindrical or flat surfaces but widely used for internal grinding. • This is final operation of all types of grinding to correct the errors. • Characteristics :- 1) Correction of geometry accuracy – out of roundness, taper, axial distortion 2) Dimensional accuracy • Abrasive grains are bonded in the form of sticks & sticks are presented to the work so that their full cutting forces are in contact with work. Prof. S.S.Petkar
- 39. • For cylindricalsurface, grains are given in a combination of 2 motions, rotation & reciprocation. • These grains put more pressure on high spot. • After crest removed the bore is made straight. Uniform surface finish is obtained • Here pressure and temperature are not concentrated t any point. • Therefore less surface damage as compared to other machining process • All materials can be honed depends upon hardness Prof. S.S.Petkar
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- 41. • It isa finishing operation done with loose abrasive grains. This process gives • Extreme accuracy • Correction of shape • Refinement of surface finish • Close fit between mating surface. • Service life of component can be increased by this process. • Silicon carbide and aluminium oxide abrasives are used. • It is done by charging a lap (soft mtrl.) with abrasive particles & rubbing it over the w/p surface with slight pressure manually or special designed machines. • Pressure is applied on lap & is moved with loose abrasives, which removes the metal from w/p till lap is confirmed. • Lap materials are CI, Bronze, soft steels, brass. • Lapping speed is 100 to 250m/min • Special lubricant called as Vehicles are used in this process Prof. S.S.Petkar
- 42. Grinding wheel wear •The abrasive grains which make up the entire geometry of wheel act as independent small cutting tools. • A common attributing factor to wheel wear is grain fracture, which can be an advantage. • A portion of each of the individual grains on the wheel surface breaks apart and leaves the remaining grain bonded to the wheel. • The fractured grain is left with newly exposed sharp edges which attribute the self-sharpening characteristic of grinding wheels and cutting tools in general. Prof. S.S.Petkar
- 43. Effect of cuttingtemperature • The lifespan of the grinding wheel and final surface properties of the workpiece are directly affected by the operating cutting temperature. • Heat generated during grinding penetrates the grinding wheel and the workpiece which can cause dimensional errors due to thermal expansion • Several adverse effects of a high cutting temperature are as follows: • Tempering • Burning • Thermal cracks • Residual stresses Prof. S.S.Petkar
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