Surface Finishing Processes PDF

Surface Finishing Processes 729 Table 17.1. Typical Ranges of Surface Finish, Ra,  m Process Average applications Turning, Boring...

Surface Finishing Processes 729 Table 17.1. Typical Ranges of Surface Finish, Ra,  m Process Average applications Turning, Boring 0.4 to 6.3 Reaming 0.8 to 3.2 Shaping, Planing 1.6 to 12.5 Drilling 1.6 to 6.3 Milling 0.8 to 6.3 Broaching 0.8 to 3.2 Grinding 0.1 to 1.6 Roller burnishing 0.2 to 0.8 Lapping 0.05 to 0.4 Honing 0.1 to 0.8 Buffing 0.05 to 0.5 Super finishing 0.05 to 0.2 17.2. SURFACE FINISHING PROCESSES The various methods used for finishing the surfaces of the parts are discussed below: 17.2.1 Diamond Turning and Boring. It is customary to find, where the application demands it, light alloys, bronzes and tin alloys, bearing metal, being turned or bored using diamond tools with a geometric control of about 0.0125 mm or below and with surface rougness measurement of between 0.075 and 0.125  m (AA). Fine finishing by the diamond tools is more or less confined to those materials which do not include hard or abrasive particles in their make up which would chip or damage the stone, and which cut cleanly with a definite chip. Materials which come away in an abrasive powder rapidly erode the cutting edge and thus make the use of diamond tools expensive and uncertain. The machines employed for fine turning and boring must obviously be in perfect condition, with all ways and guides straight and true and all bearings and spindles running in perfect truth with minimum clearance and no vibration. 17.2.2. Grinding. Grinding is the general method of finishing steel but it requires a high degree of skill to repeat continuously a restricted grade of surface finish to fine geometric tolerances. In works concerned with the production of hardened components with tolerances of between 0.05 mm and 0.025 mm, finishes in the range of 0.2 to 0.3  m (AA) are commonly produced and it has to be noted that in producing finishes finer than the above figures, the danger of surface burning and cracking is greatly increased. Machine employed for fine griding must be in first class condition with all ways and guides straight and true and particular attention must be paid to the balancing of wheels. Wheel surface preparation is of utmost importance. Adequate flow of chip and granule free coolant is essential as it has been found that the cleanliness of the coolant has a direct bearing on the quality of the surface produced. The choice of wheel grit, speeds coolants will vary with the material being cut and the finish demanded. Whilst grinding is convenient and a reasonable economic method of producing fine surface, it brings in its train a number of pitfalls. An inevitable result of grinding is the formation by the high temperature generated at the work and wheel contact line, of an extremely thin layer of decarburised material which has a considerable effect on the initial efficiency of a surface as a bearing and has to be removed in many cases before really satisfactory bearing conditions are attained. A further effect of high temperature is burning of the surface and the formation of grinding cracks which are large enough to affect the fatigue strength of the 730 A Textbook of Production Technology component. Chatter is also evident on ground cylindrical components, particularly on those which have been centreless ground. 17.2.3. Lapping: Lapping is a surface finishing process used on flat or cylindrical surfaces (mainly external). Lapping is the abrading of a surface by means of a lap (which is made of a material softer than the material to be lapped), which has been charged with the fine abrasive particles. When the lap and the work surface are rubbed together with the fine abrasive particles between them, these particles become embedded in the softer lap. It then becomes a holder for the hard abrasive. As a charged lap is rubbed against a hard surface, the hard particles in the surface of the lap remove small amounts of materials from the harder surface. Thus it is the abrasive which does the cutting and the soft lap is not worn away, because the abrasive particles become embedded in its surface, instead of moving across it. In lapping, the abrasive is usually carried between the lap and the work in some sort of a vehicle. The vehicle or lubricant controls to some extent the cutting action and prevents scoring the work and caking of the abrasive. Some of the vehicles used include: kerosene plus a small amount of machine oil, greases, fine sperm oil for fine job, olive oil, lard oil, spindle oil, and soapy water. Naphtha is used to clean the laps. Laps may be made of almost any material soft enough to receive and retain the abrasive grains. They may be made of soft cast iron, wood, leather, brass, copper, lead or soft steel. The most common lap is fine grain cast iron. Copper is used rather often and is the common materials for lapping diamonds. For lapping hardened metals for metrolographic examination, cloth laps are used. For steel surfaces, artificial corrundum is used as an abrasive for preparatory lapping and again used in a final state for finishing. Silicon carbide gives good results on cast iron and alumina for finest lapping. For lapping small components, diamond dust or boron carbide in the finest grain size give good results. The other abrasives used are: rough (Ferric oxide Fe2O3), green rough (chromium oxide Cr2O3) and crocus powder. The abrasive particles are from 120 grit up to the finest powdered sizes. In nearly all cases, a paraffin lubricant is used. The exception being for soft materials when a soluble oil or water lubricant is used. In lapping, the material removal is usually less than 0.025 mm, although rough lapping may remove as much as 0.075 mm and finish lapping as little as 0.0025 mm. Commercial lapping operations can produce parts to limits of 0.000625 mm. Since it is such a slow metal removal process, it is used only to remove scratch marks left by grinding orhoning or to obtain very flat or smooth surfaces such as required on gauge blocks or liquid tight seals where high pressures are involved. Materials of almost any hardness may be lapped. However, it is difficult to lap soft materials since the abrasives tend to become embedded. Thus, lapping is done: (i) to produce geometrically true surface. (ii) to correct minor imperfections is shape. (iii) to obtain fine dimensional accuracy to provide a very close fit between the contact surfaces. (iv) to secure a fine surface finish. Lapping Methods:– Lapping may be done by hand or mechanically with the help of special lapping machines. (a) Hand lapping for flat work : Here, the lap is a flat similar to a surface plate. Grooves are usually cut across the surface of a lap to collect the excessive abrasive and chips. For finishing of the work surface, either the lap or the workpiece is held by one hand and the irregular rotary motion of the other by the second hand, enables the abrading of the two surfaces in contact. The Surface Finishing Processes 731 work is turned frequently to obtain uniform cutting action. The method is used for lapping: press work dies, dies and metallic moulds for castings etc., surface plate, engine valve and valve seat, gauge blocks and piston rings etc. Piston rings are customarily made parallel and to high precision by hand lapping. Manual lapping is used to bring gauge blocks to their final stage of dimensional accuracy and parallelism, a finish of 0.025 to 0.050  m and a tolerance as small as ± 0.000025 mm. (b) Hand lapping for external Cylindrical work (Ring Lapping) : An external lap for external workpieces (round) is shown in Fig. 17.3. It is split by a saw cut and can be closed in by tightening one or more screws. The diameter of the hole is made the same as that of the piece to be lapped, and the hole is, of course bored before the saw cut is made. Internal laps are made to expand. The ring lap is reciprocated over the work piece surface. The method is usually used for stepped plug gauges or gauges made in small quantities. (c) Machine Lapping : Mechanical lapping is a high production process, for example gudgeon pins25 mm diameter and 75 mm long are lapped at the rate of 500 pieces per hour, removing 0.05 to 0.075 mm Fig. 17.3 External Lap. of material with a limit of accuracy of roundness, straightness and size within 0.025 mm. Mechanical lapping machines are of vertical construction with the work holder mounted on the lower table which is given an oscillating motion. The upper lap is stationary and floating, while the lower one revolves at about 60 rev/min. Several types of lapping machines are available for lapping round surfaces. A special type of centreless lapping machine is made for lapping small parts such as piston pins, ball bearing races etc. A general purpose machine for lapping both cylindrical and flat surfaces is shown in Fig. 17.4 A number of workpieces are placed between the upper and the lower lap, whose surfaces have previously been lapped flat. The workpieces are placed in slots in a work holder so that their axes X-X are not quite radial. The shape of the slots will depend upon that of the workpieces. The two laps are rotated and the work holder is given as oscillation of about 25 mm amplitude. A stream of vehicle in which fine abrasive flour is suspended is fed to the centre of the laps and flows outwards and the workpieces are thus gradually lapped to size. During machine lapping a pressure of 0.007 to 0.02 N/ Fig. 17.4. Machine Lapping. mm2 for soft materials and upto 732 A Textbook of Production Technology 0.07 N/mm2 for hard materials is satisfactory. Mechanical lapping machines can be used for lapping: (i) External cylindrical surfaces, and (ii) flat surfaces. The following are examples of work done by lapping: aircraft piston pins, automotive wrist pins, diesel engine injector-pump parts and spray nozzles, plug gauges, certain dies and moulds, gauge blocks, refrigerator-compressor parts, oil-burner parts, micro-meter spindles, roller bearings, taper rollers, worm and worm gears, crankshafts, camshafts, ball bearing race-ways etc. Lapping has become a common production process with the demand for hardened surfaces having only a few micrometers of surface finish. However, because, it is such a slow method of metal removal, it is obviously relatively expensive and is not economically justified unless operating requirements make such surface finishes absolutely necessary. Lapping and polishing differ in the following manner-Polishing is meant to produce a shiny surface whereas a lapped surface does not usually have a bright shiny appearance. Lapping definitely removes metal from lapped surface, whereas, polishing as a rule does not remove any appreciable amount of metal. Lapping improves the geometrical shape of the body, whereas, polishing does not. Lapping is essentially a cutting process, while polishing consists of producing a kind of plastic flow of the surface crystals so that the high spots are made to fill the low spots. As written above, lapping is normally adopted for external surfaces. However, it can be used for internal surfaces also. 17.2.4. Honing. Honing is a grinding or abrading process. In it, a very little material is removed. This process is used primarily to remove the grinding or tool marks left on the surface by previous operations. The cutting action is obtained from abrasive sticks (aluminium oxide or silicon carbide) mounted in a mandrel or fixture. A floating action between the work and the tool prevails so that any pressure exerted on the tool is exerted and transmitted equally on all sides. The honing Fig. 17.5 Vertical spindle Honing Machine. Surface Finishing Processes 733 tool is given a slow reciprocating motion as it rotates, having resultant honing speeds from 15 to 60 mpm. This action results in rapid removal of stock and at the same time, the generation of a straight and round surface. Defects such as slight eccentricity, a wavy surface, or a slight taper caused by previous operations can be corrected by this process. Parts honed for finish remove only 0.025 mm or less. However, when certain inaccuracies must be corrected, amounts upto 0.50 mm represent usually practice. Collants are essential to the operation of this process to flush way small chips and keep temperatures uniform. Sulphurised mineral base or lard oil mixed with kerosene is generally used. Paraffin is also used. Most honing is done on internal surfaces, or holes, such as automobile cylinders. There are a few applications of honing to external surfaces. Parts can be of any shape, but the surface must be cylindrical. Practically any material can be honed. Soft materials which cannot be lapped, can be honed because of the use of bonded abrasive. Hard and soft cast iron, steel, carbides, bronze, aluminium, brass and silver, as well as glass, ceramics and some plastic can be honed. Honing machines are similar in general construction to vertical drilling machines Fig. 17.5 but the spindle reciprocation is usually by hydraulic means. The rotary motion may be from a hydraulic motor or by gearing. The speed ratio of two motions affects the work finish and may be varied throughout the operation or for different materials. For cast iron, the speed ranges from 60 to 150 mpm for rotation; with 15 to 21 mpm for reciprocation. The corresponding speeds for steel are : 45 to 60 mpm for rotation and 12 mpm for reciprocation. The reciprocation motion distributes the wear over the whole length of the sticks and keeps the bore cylindrical. Semi-automatic honing machines used in the finishing of automobile cylinder bores are of vertical type. Both single and multi spindle machines are used for this operation. The abrasive stones are mounted on a honing tool. In order to expand the abrasive stones outward to fit the hole to be honed, the conical wedge is moved relative to the honing tool. A general arrangement of a vertical honing machine is shown in fig. 17.6. The abrasive sticks (upto 8 in number) are expanded while honing takes place, if required, by micrometer controlled, mechanical or hydraulic, means. The honing tool will follow the axis of the original hole, therefore, the honing tool or fixture must be free to float. This is done by using universal joints as shown in the figure. Due to this, the honing tool becomes self centring and it is not necessary to line up the hole and hone axes precisely. Vertical machines have been designed for work upto 500 mm diameter. Horizontal honing machines are used only for honing Universal joints large, long gun barrels and similar work. The workpiece is held on the left and the tool is rotated and reciprocated by Micrometer the head on the right end of the bed of the machine. adjustment Machines of this type are made with strokes of upto 22.5 m and hone holes as large as about 1 m in diameter. Honing stick All honing gives a smooth finish with a characteristic cross hatch appearance. The depth of these hone marks can be controlled by variations in pressures, speed and type of abrasive used. Accurate dimensions can be maintained by Fig. 17.6 Vertical Honing Machine. the use of automatic size controlled devices in connection with honing. Typical applications of honing are finishing of automobile engine cylinders, bearings, gun barrels, ring gauges, piston pins. Shafts and flange faces. 734 A Textbook of Production Technology The grit size of abrasive material used in abrasive sticks is 80 to 180 for primary honing and 300 to 500 for secondary honing. Surface finish of the order of 0.05  m R can be obtained by honing. 17.2.5. Buffing. Buffing is a polishing operation in which the workpiece is brought in contact with a revolving cloth buffing wheel, that usually has been charged with a very fine abrasive Fig. 17.7. The polishing action in buffing is very closely related to lapping in that when a polishing medium such as ‘rouge’ is used, the cloth buffing wheel becomes a carrying vehicle for the fine abrasives. In (this action the abrasive removes amounts of Fig. 17.7 Buffing. metal from the workpiece, thus eliminating the scratch marks and producing a very smooth surface. When softer metals are buffed, particularly without the use of an abrasive, there is some indication that a small amount of metal flow may occur which helps to reduce the high spots and produce a high polish. Buffing wheels are made of discs of linen, cotton, broad cloth and canvass. They are made more or less firm by the amount of stitching used to fasten the layers of the cloth together. Buffing wheels for very soft polishing or which can be used to polish into interior corners may have no stitching, the cloth layers being kept in position by the centrifugal force resulting from the rotation of the wheel. Buffing wheel speeds are in the range of 32.5 to 40 m/s. Various types of buffing rouges are available. Most of them being primarily ferric oxide in some soft type of binder. Buffing should be used only to remove very fine scratches or to remove oxide or similar coatings which may be on the work surface. It ordinarily, is done manually, the work being held against the rotating wheel. This procedure is apt to be relative expensive because of the labour cost. There are semi-automatic buffing machines available consisting of a series of individually driven buffing wheel which can be adjusted to the desired position so as to buff different portions of the workpiece. The workpieces are held in fixtures on a rotating circular worktable so as to move past the buffing wheels. If the workpieces are not too complex in shape, very satisfactory results can be achieved with such equipment and the buffing cost will be low. Product applications of buffing process which produces mirror-like finish are : objects used on mobile homes, automobiles, motor-cycles, boats, bicycles, sporting items, tools, store fixtures, commercial and residential hardware and household utensils and appliances. 17.2.6. Barrel Tumbling. Barrel tumbling is the process of revolving workpiece in a barrel with abrasive and water for the purpose of producing a high lustre or for the purpose of removing burrs. A typical tumbling barrel is eight sided, lined with wood and about 1.8 m long and 1.2 m in diameter. It rotates at about 24 rpm. The barrel may have one to six compartments. The time involved may be from 1 to 4 hours depending upon the job. Parts are packed into the barrel or drum until it is nearly full, together with slugs; stars or jacks or some abrassive such as sand, granite chips or aluminium oxide pallets. The barrel is then rotated. The movement of the parts as they tumble and roll over one another and the accompanying impingement of the slug or abrassive against the parts produce a fine cutting action, Fig. 17.8. Delicate parts should not shift loosely during tumbling and in some cases the parts must be attached