MP Notes Module 3 Finishing Processes PDF

15-11-2024 Abrasive Machining Dr. Shailendra Kumar Professor Dept. of Mechanical Engineering...

15-11-2024 Abrasive Machining Dr. Shailendra Kumar Professor Dept. of Mechanical Engineering SVNIT, Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Abrasive Machining Processes Table 1: Parameters for surface finish Almost all surfaces are rough. It means that most parts of a surface are not flat but form either a peak or a valley. The typical amplitude between the peaks and valleys for engineering surfaces is about one micrometre. A surface is said to have a good finish when the heights of peaks and valleys on it are less. The roughness (or finish) is characterized by various parameters as shown in Table 1. Different manufacturing processes result in different roughness parameters which are listed in Table 2. From Table 2, we can clearly see that why grinding, honing, lapping, etc. are called as surface finishing processes. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 1  15-11-2024 Surface parameters obtained by different machining processes Abrasive Machining Processes Roughness Average - Ra in microns Process 25 12.5 6.3 3.2 1.6 0.8 0.4 0.2 0.1 0.05 0.025 1. Grinding Planing / 2. Honing shaping Drilling 3. Lapping Reaming 4. Buffing / Polishing Milling Turning / 5. Abrasive jet machining Boring Grinding 6. De burring (shot blasting, vibratory and barrel finishing) Honing Polishing Lapping Super finishing 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Introduction Grinding is the process of removing metal in the form of minute chips by the action of irregularly shaped abrasive particles. The abrasive grains usually cut with a zero to negative rake angle and produce a large number of short, small, curly or wavy chips. Grinding 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 2  15-11-2024 Classification Classification.. Grinding is classified according to process, purpose and procedure in the following ways: 4. Abrasive belt grinding: For flats, curves or cylinders Based on Purpose: 5. Manual grinding: Work piece or grinder held by hand 1. Non-precision grinding: 6. Special grinding processes: Cam shaft / crank shaft grinders, gear grinders - Done primarily to remove stock that can not be taken off as conveniently by other methods. The accuracy and surface finish are of secondary importance. 2. Precision grinding: - For producing good surface finishes and accurate dimensions. Based on the process & procedure: 1. Surface grinding: For flat surfaces 2. Cylindrical grinding: Centered or Center less – for cylindrical surfaces 3. Form grinding: Grinding shapes like gears, threads, etc. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Grinding Machines Grinding machines fall into five categories: 1. Surface grinders 2. Cylindrical grinders 3. Centerless grinders 4. Internal grinders 5. Special types of grinders. Surface Grinding Surface grinders are used to produce flat, angular, and irregular surfaces. In the surface grinding process, the grinding wheel revolves on a spindle and the workpiece, mounted on either a reciprocating or rotary table, is brought into contact with the grinding wheel. Surface grinding process A typical hand operated surface grinder is shown in Figure. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 3  15-11-2024 Surface Grinding Four types of Figure: (a) Typical standard surface grinder. (b) Schematic illustration of the surface grinders commonly used in basic components and motions of a surface grinder. industry: (a) horizontal spindle/reciprocating table, (b) horizontal spindle/rotary table, (c) vertical spindle/reciprocating table, (d) vertical spindle/rotary table 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Schematic illustration of the basic components and motions of a Cylindrical Grinding cylindrical grinder Cylindrical grinding is the process of grinding the outside surfaces of a cylinder. These surfaces may be straight, tapered or contoured. Cylindrical grinding operations resemble lathe turning operations. They replace the lathe when the workpiece is hardened or when extreme accuracy and superior finish are required. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 4  15-11-2024 Centerless Grinding.. Centerless Grinding Operating principle of a centerless grinder Centerless grinding machines eliminate the need to have center holes for the work or to use work-holding devices. In centerless grinding, the workpiece rests on a workrest blade and is backed up by a second wheel, called the regulating wheel. The rotation of the grinding wheel pushes the workpiece down on the workrest blade and against the regulating wheel. The regulating wheel, usually made of a rubber bonded abrasive, rotates in the same direction as the grinding wheel and controls the longitudinal feed of the work when set at a slight angle. By changing this angle and the speed of the wheel, the workpiece feed rate can be changed. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Internal Grinding Special Grinding Processes Special types of grinders are grinding machines made for specific of work and operations. 1. Tool and Cutter Grinders: - These grinding machines are designed to sharpen milling cutters, reamers, taps, and other machine tool cutters. - The general purpose cutter grinder is the most popular and versatile tool grinding machine. Various attachments are available for sharpening most types of cutting tools. 2. Jig Grinding Machines: - Jig grinders were developed to locate and accurately grind tapered or Internal grinders are used to accurately finish straight, tapered, or formed straight holes. holes. The w/p is held by a work holding device, usually a chuck or collet, - Jig grinders are equipped with a high speed vertical spindle for holding and revolved by a motorized headstock. A separate motor head in the same and driving the grinding wheel. direction as the w/p revolves the grinding wheel. It can be fed in and out of the work and also adjusted for depth of cut. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 5  15-11-2024 Special Grinding Processes.. Machining parameters in surface grinding 3. Thread Grinding Machines: Two types of feeds are possible in this process. 1. Cross feed: parallel to width of work piece – Required when width of - These are special grinders that resemble the cylindrical grinder. They work piece is larger than that of grinding wheel. must have a precision lead screw to produce the correct pitch, or lead, 2. In feed – Perpendicular to surface – to set depth of cut on a threaded part. - Thread grinding machines also have a means of dressing or truing the Cutting speed (V) cutting periphery of the grinding wheel so that it will produce a precise thread form on the part. V =  Dg Ng / 1000 (m/min), 4. Crankshaft Grinder Where, Dg = Diameter of grinding wheel (mm) Ng = Rotational speed of wheel (rpm) 5. Piston Grinder 6. Cam Grinder Metal removal rate, 7. Tool-post Grinder (Lathe Grinder) Z (mm3/min) = fc d Ft Where, d = depth of cut (mm) fc = cross feed (mm / stroke) Ft = Table speed (mm/ min) 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Machining Time : Abrasives tm = [h w /(fr fc )] tst + ts Abrasives used in grinding should be pure and have uniform physical Where, properties of hardness, toughness, and resistance to fracture. h = Thickness of layer to be removed, Two types of abrasives are used in grinding wheels: w = width of work piece 1. Natural, and fr = In feed or depth of cut, fc = cross feed 2. Manufactured / Artificial. tst = time for one stroke of table, Natural Abrasives: ts = spark out time (Time required for grinding without any change in feed Sandstone or solid quartz, Emery, Corundum, and Diamond. until a little sparking is noticed) Except for diamonds, manufactured abrasives have almost totally replaced natural abrasive materials. Even natural diamonds have been replaced in some cases by synthetic diamonds. Manufactured abrasives: Aluminum oxide, Silicon carbide, Cubic boron nitride, and Diamond. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 6  15-11-2024 Most grinding wheels are made of silicon carbide or aluminum oxide, both of Abrasive Grain Size: which are artificial (manufactured) abrasives. This is an important parameter in determining surface finish and material Silicon carbide is extremely hard but brittle. removal rate. Aluminum oxide is slightly softer but is tougher than silicon carbide. It dulls Abrasive grains are selected according to the mesh of a sieve through which more quickly, but it does not fracture easily therefore it is better suited for they are sorted. grinding materials of relatively high tensile strength. For example- Grain number 40 indicates that the abrasive grain passes through a sieve having approximately 40 meshes to the linear inch. A grinding wheel is designated coarse, medium, or fine according to the size of the individual abrasive grains making up the wheel. Small grit sizes produce better finishes, larger grit sizes permit larger material removal rates. Harder materials need smaller grain sizes to cut effectively, while softer materials require larger grit size. Grain sizes used in grinding ranges between 10 - 600, 10 is very coarse, 600 is very fine. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Bonding Material:  Bonds used in grinding wheels are - vitrified,  The abrasive particles in a grinding wheel are held in place by the bonding - silicate, agent. - shellac,  The percentage of bond in the wheel determines, to a great extent, the - resinoid, "hardness" or "grade" of the wheel. The greater the percentage and strength of - rubber. the bond, the harder the grinding wheel will be. - Metal bonds  "Hard" wheels retain the cutting grains longer, while "soft" wheels release the - Electroplated bond grains quickly. - Brazed bond  If a grinding wheel is "too hard" for the job, it will glaze because the bond Vitrified: prevents dulled abrasive particles from being released so new grains can be exposed for cutting. A vast majority of grinding wheels have a vitrified bond.  Besides controlling hardness and holding the abrasive, the bond also provides Vitrified bonded wheels are unaffected by heat or cold and are made in a the proper safety factor at running speed. It holds the wheel together while greater range of hardness than any other bond. centrifugal force is trying to tear it apart. They adapt to practically all types of grinding with one notable exception: if the wheel is not thick enough, it does not withstand side pressure as in the case of thin cutoff wheels. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 7  15-11-2024 Rubber: Metal bond: In rubber-bonded wheels, pure rubber is mixed with sulfur.  Metal bond is extensively used with superabrasive wheels. It is extremely flexible at operating speeds and permits the manufacture of grinding wheels as thin as 0.006 inch for slitting nibs.  Extremely high toughness of metal bonded wheels makes these very effective in those applications where form accuracy as well as large stock Most abrasive cutoff machine wheels have a rubber bond. removal is desired. Silicate: Silicate bond releases the abrasive grains more readily than vitrified bond. Electroplated bond: Silicate bonded wheels are well suited for grinding where heat must be kept This bond allows large (30-40%) crystal exposure above the bond without to a minimum, such as grinding edged cutting tools. need of any truing or dressing. It is not suited for heavy-duty grinding. This bond is specially used for making small diameter wheel, form wheel Thin cutoff wheels are sometimes made with a shellac bond because it and thin superabrasive wheels. provides fast cool cutting. Presently it is the only bond for making wheels for abrasive milling and ultra Resinoid: high speed grinding. Resinoid bond is strong and flexible. It is widely used in snagging wheels (for grinding irregularities from rough castings). It is also used in cutoff wheels. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Brazed bond: Grades of Hardness This is relatively a recent development, allows crystal exposure as high 60- The grade of a grinding wheel designates the hardness of the bonded 80%. material. In addition grit spacing can be precisely controlled. A soft wheel is one on which the cutting particles break away rapidly while a hard wheel is one on which the bond successfully opposes this breaking This bond is particularly suitable for very high material removal either with away of the abrasive grain. diamond or CBN wheel. Most wheels are graded according to hardness by a letter system. The bond strength is much greater than provided by electroplated bond. Most manufacturers of grinding abrasive wheels use a letter code ranging This bond is expected to replace electroplated bond in many applications. from A (very soft) to Z (very hard). - Vitrified and silicate bonds usually range from very soft to very hard, - shellac and resinoid bonds usually range from very soft to hard, and - rubber bonds are limited to the medium to hard range.  The grade of hardness should be selected as carefully as the grain size. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 8  15-11-2024 GRINDING WHEEL ABRASIVE A grinding abrasive wheel that is too soft will wear away too rapidly, the abrasive grain will be discarded from the wheel before its useful life is realized. On the other hand, if the wheel is too hard for the job, the abrasive particles will become dull because the bond will not release the abrasive grain, and the wheel's efficiency will be impaired. The actual hardness of the grinding wheel is equally dependent on grade of hardness and spacing of the grains or structure. If the grain and bond materials in each of these are alike in size and hardness, the wheel with the wider spacing will be softer than the wheel with the closer grain spacing. Bond strength of a grinding wheel is not wholly dependent upon the grade of hardness but depends equally on the structure of the wheel, that is, the spacing of the grain or its density. The structure or spacing is measured in number of grains per cubic inch of wheel volume. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat GRINDING WHEELS Grinding Wheel -Components Specification of Grinding Wheel Selection of Grinding Wheel 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 9  15-11-2024 Typical structure of a grinding wheel Specification of Grinding Wheel A grinding wheel requires two types of specification - (1) Geometrical specification, and (2) Compositional specification 1. Geometrical specification:  This is decided by the type of grinding machine and the grinding operation to be performed in the workpiece.  This specification mainly includes wheel diameter, width and depth of rim and the bore diameter.  The wheel diameter, for example can be as high as 400 mm in high efficiency grinding or as small as less than 1mm in internal grinding. Similarly, width of the wheel may be less than an mm in dicing and slicing applications. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Standard wheel configuration for conventional grinding wheels Standard wheel configuration for superabrasive wheel 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 10  15-11-2024 2. Compositional specification: Marking system for conventional grinding wheel : Conventional abrasive grinding wheels are specified encompassing the The standard marking system for conventional abrasive wheel can be as following parameters - follows: 1. Type of grit material 2. Grit size 3. Bond strength of the wheel, commonly known as wheel hardness Where, 4. Structure of the wheel denoting the porosity i.e. the amount of inter grit spacing  The number ‘51’ is manufacturer’s identification number indicating exact kind of abrasive used. 5. Type of bond material  The letter ‘A’ denotes that the type of abrasive is aluminium oxide. In 6. other than these parameters, the wheel manufacturer may add their own case of silicon carbide the letter ‘C’ is used. identification code prefixing or suffixing (or both) the standard code.  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. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat  The letter ‘K’ denotes the hardness of the wheel, which means the Marking system for superabrasive grinding wheel : amount of force required to pull out a single bonded abrasive grit by Marking system for superabrasive grinding wheel is somewhat bond fracture. The letter symbol can range between ‘A’ and ‘Z’, ‘A’ different as illustrated below 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 high porosity and ‘20’ indicating low porosity.  The letter code ‘V’ means that the bond material used is vitrified. where The codes for other bond materials used in conventional abrasive The letter ‘R’ is manufacture’s code indicating the exact type of wheels are B (resinoid), BF (resinoid reinforced), E(shellac), superabrasive used. O(oxychloride), R(rubber), RF (rubber reinforced), S(silicate) The letter ‘D’ denotes that the type of abrasive is diamond. In case  The number ‘05’ is a wheel manufacturer’s identifier. of cBN the letter ‘B’ is used. The number ‘120’ specifies the average grain size in inch mesh. However, a two number designation (e.g. 120/140) is utilized for controlling the size of superabrasive grit. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11  15-11-2024 Like conventional abrasive wheel, the letter ‘N’ denotes the hardness Glazing and Loading in wheels of the wheel. However, resin and metal bonded wheels are produced with almost no porosity and effective grade of the wheel is obtained Glazing of the wheel is a condition in which the face or cutting edge takes a by modifying the bond formulation. glass-like appearance. It means the cutting points of the abrasives have become dull and worn down to the bond. The number ‘100’ is known as concentration number indicating the amount of abrasive contained in the wheel. The number ‘100’ Continued work with a wheel that glazes increases the smoothness of the wheel corresponds to an abrasive content of 4.4 carats/cm3. For diamond face and decreases its cutting capacity. grit, ‘100’ concentration is 25% by volume. For cBN the Glazing takes place when a wheel is too hard or revolves at too fast a speed. corresponding volumetric concentration is 24%. The wheel may also become “loaded”. The cutting face of a loaded wheel has The letter ‘M’ denotes that the type of bond is metallic. The other particles of the metal being ground adhering to it, the openings or pores of the types of bonds used in superabrasive wheels are resin, vitrified or wheel face having been filled up with metal, thus preventing the wheel from cutting freely. metal bond, which make a composite structure with the grit material. However, another type of superabrasive wheel with both diamond Loading may be caused by grinding a soft material, or by using a wheel of too and cBN is also manufactured where a single layer of superabrasive hard a bond and running it too slowly. It may also be caused by taking cuts that are too deep and by not using the right cutting fluid. grits are bonded on a metal perform by a galvanic metal layer or a brazed metal layer. The remedy for loading is to increase the speed of the wheel or use a softer wheel. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Dressing & Truing Dressing and truing conventional grinding wheels are two separate and distinct operations, although they may sometimes be done with the same Dressing : tool. The tools used for conventional grinding wheel dressing include the It is a process used to clean and restore a dulled or loaded grinding wheel- following: cutting surface to its original sharpness. 1. Mechanical dresser: In dressing, dulled abrasive grains and excess bonding material are - commonly called star dressers, are held against the wheel while it is removed. In addition, dressing is used to customize wheel face, so that it running. The picking action of the points of the star shaped wheels in will give desired grinding results. the tool remove dull grains, bond and other bits of swarf. Truing: - Star dressers are used for relatively coarse-grained conventional wheels, generally in off-hand grinding jobs, where grinding accuracy is It is the process of removing material from the face of the wheel so that the not the main consideration. resultant cutting surface runs absolutely true. 2. Dressing sticks: This is very important in precision grinding, because an out truth wheel will - used for offhand dressing of smaller conventional wheels, especially produce objectionable chatter marks on the work piece. cup and saucer shapes. A new wheel should always be trued before being put to work. Also it is a - some of these sticks are made of an extremely hard abrasive called good idea to true the wheel if it is being remounted on a machine. boron carbide. Other dressing sticks contain coarse Crystolon or Alundum grains in a hard vitrified bond. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 12  15-11-2024 3. Diamond dressing tools: Grinding Ratio (G) - utilize the unsurpassed hardness of a diamond point to clean and restore the wheel grinding face. - Although single point diamond tools were once the only products G = Volume of material removed available for this kind of dressing, the increasing scarcity of diamonds has led to the development of multi-point diamond tools. Volume of wheel wear  Vary greatly (2-200 or higher) depending on the type of wheel, grinding fluid, and process parameters  Higher forces decrease the grinding ratio 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Abrasives –workpiece material Compatibility Mechanics of Grinding: Affinity of an abrasive grain to the workpiece material is important Chip Formation by Abrasive Grain: The less the reactivity of the two materials, the less wear and dulling of the grains occur during grinding The following combinations are generally recommended –  Aluminum oxide: for carbon steels, ferrous alloys, and alloy steels  Silicon carbide: for cast iron, nonferrous metals, and hard and brittle materials, such as carbides, ceramics, marble, and glass  Diamond: for cemented carbide, ceramics and some hardened steels  Grinding chip being produced  Schematic illustration of chip formation by  CBN: for steels and cast irons at 50 HRC or above and for high- by a single abrasive grain. an abrasive grain. temperature superalloys.  Note the large negative rake angle of the grain, the small shear angle 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 13  15-11-2024 The following are factors that differentiate the action of a single grain from of a single-point cutting tool: Negative rake angle V/s Positive rake angle 1) The individual grain has an irregular geometry and is spaced randomly along the periphery of the wheel. Positive rake angle Negative rake angle 2) The average rake angle of the grains is highly negative, typically [-1.05 rad (-60o)]; consequently, the shear angles are very low. 3) The grains in the periphery of a grinding wheel have different radial position 4) The cutting speeds of grinding wheels are very high, typically, on the order of 30m/s 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Application Guidelines  For hardened tool steels and certain aerospace alloys, use  Cubic boron nitride as the abrasive  To optimize surface finish, select  Small grit size and dense wheel structure  For hard abrasive materials (e.g., ceramics, cemented carbides, and  Use higher wheel speeds (v) and lower work speeds (vw) glass) use  Smaller depths of cut (d) and larger wheel diameters (D) will also help  Diamond as the abrasive  To maximize material removal rate, select  For soft metals, use  Large grit size  Large grit size and harder grade wheel  More open wheel structure  Vitrified bond  For hard metals, use  Small grit size and softer grade wheel  For steel and most cast irons, use  Aluminum oxide as the abrasive  For most nonferrous metals, use  Silicon carbide as the abrasive 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 14  15-11-2024 Lapping is a final abrasive finishing operation that produces extreme dimensional accuracy, corrects minor imperfections of shape, refines surface finish, and produces close fit between mating surfaces. Very thin layers of metal (0.005 to 0.01 mm) are removed in lapping. Abrasive powders such as emery, corundum, iron oxide, chromium oxide etc. mixed with oil or special pastes with some carrier are used in lapping. Lapping Most lapping is done by means of lapping shoes or quills, called laps, that are rubbed against the w/p. The face of a lap is charged with abrasive particles. Laps are generally made of soft cast iron, brass, copper, lead or soft steel. Laps may be operated by hand or m/c, the motion being rotary or reciprocating. Lapping, though it is an abrasive process, differs from grinding or honing because it uses a ‘loose’ abrasive instead of bonded abrasives like grinding wheels. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat These abrasives are often purchased ‘ready mixed’ in a ‘vehicle’ often made with an oil-soap or grease base. These vehicles hold the abrasive in suspension before and during use. Typical grit sizes (Grit number) between 300 (medium) to 600 (very fine) Applications: optical lenses, metallic bearing surfaces, finishing of round work such as precision plug gages, to tolerances of 0.0005 in. to 0.00002 in. Scheme of Lapping Process The lapping process in lens-making 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 15  15-11-2024 Lapping…  Technical parameters affecting lapping process are – 1. Pressure 2. Grain size of abrasives 3. Concentration of abrasives in vehicle 4. Lapping speed To maximize the MRR, an optimum lapping pressure and abrasive concentration in the vehicle must be chosen. (a) Schematic illustration of the lapping process. (b) Production lapping on flat surfaces. (c) Production lapping on cylindrical surfaces. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Pressure in the range of p1-p2 gives the best values for MRR and roughness It appears that grain size corresponding to permissible surface roughness and of the lapped surface. maximum MRR may be different. Primary consideration is made on the permissible surface roughness in selecting abrasive grain size. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 16  15-11-2024 Lapping… Advantages: There is no warping since the parts are not clamped and very little heat is generated. No burrs are created. In fact, the process removes light burrs. Any size, diameter, and thickness from a few thousandths thick up to any height the machine will handle can be lapped. Limitations:  Lapping is still something of an art. There are so many variables that starting a new job requires experience and skill. Even though there are general recommendations and assistance from manufacturers, and past experience Honing is useful, trial and error may still be needed to get the optimal results. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Honing is a low-velocity abrading process. Material removal is Honing stones are made from common abrasive and bonding materials, accomplished at lower cutting speeds than in grinding. Therefore, heat and often impregnated with sulphur, resin, or wax to improve cutting action and pressure are minimized, resulting in excellent size and geometry control. lengthen tool life. The most common application of honing is on internal cylindrical surfaces. Materials honed range from plastics, silver, aluminium, brass, and cast iron to The cutting action is obtained using abrasive sticks mounted on a metal hard steel and cemented carbides. mandrel. It is primarily used to correct some out of roundness, taper, tool marks, and axial distortion. Grit number (grain size) range between 30 (medium) and 600 (very fine) (the smaller grain size, the larger grit number) Surface finishes of 0.12 µm (5 µ-in) or better Creates a characteristic cross-hatched surface that retains lubrication Since the work is fixed in such a way as to allow floating, and no clamping or chucking, there is no distortion. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 17  15-11-2024 Schematic illustration of a honing tool used to improve the surface finish of bored or ground holes The honing process: (a) the honing tool used for internal bore surface, and (b) cross-hatched surface pattern created by the action of the honing tool 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Fig.: Components of an internal hone Superfinishing Superfinishing is an operation using bonded abrasive stones in a particular way to produce an extremely high quality of surface finish in conjunction with an almost complete absence of defects in the surface layer. A very thin layer of metal (0.005 to 0.02 mm) is removed in superfinishing. This operation may be applied for external and internal surfaces of parts made of steel, cast iron and non-ferrous alloys, which have been previously ground or turned. It is most frequently used to obtain very fine surface finish. In this process, a very fine grit (grain size 400 to 600) abrasive stick is retained in a suitable holder and applied to the surface of the w/p with a light spring pressure. The stick is given a feeding and oscillating motion, and the w/p is rotated or reciprocated according to the requirements of the shape being superfinished. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 18  15-11-2024  Differences with honing:  Shorter strokes  Higher frequencies  Lower pressures between tool and surface  Smaller grit sizes Special general purpose m/c tools are available for superfinishing. Other types of ordinary m/cs, in particular, lathes, are sometimes employed for this purpose. Superfinishing on an external cylindrical surface 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat Schematic illustration of the superfinishing process for a cylindrical part. (a) Cylindrical microhoning (b) Centerless microhoning. 11/15/2024 1:56 PM S. K. Budhwar, MED, SVNIT Surat 19