Surface Finishing Processes PDF

Surface Finishing Processes 735 to racks within the barrel so that they will not strike against one another. Tumbling is an inexpensive cleaning method. Various shapes of slug materials may be used. Several shapes may be mixed i...

Surface Finishing Processes 735 to racks within the barrel so that they will not strike against one another. Tumbling is an inexpensive cleaning method. Various shapes of slug materials may be used. Several shapes may be mixed in a given load since shapes must be provided which will reach into all sections and corners that must be cleaned. Tumbling usually is done dry, but sometimes it may also be wet. Work piece Foam Sliding zone Rotation Rotation Fig. 17.8. Barrel Tumbling. Fig. 17.9 Barrel Rolling. Tumbling may be employed for any of the following purposes: (a) Removing fins, flashes and scales from parts. (b) Cleaning of forgings, stampings and castings. (c) Deburring. (d) Improving micrometer finish. (e) Finishing high precision work to a high lustre. (f) Forming uniform radii. (g) Finishing gears and threaded parts without damage. (h) Removing paint or plating. After tumbling, the parts must be thoroughly washed and dried by sawdust or infrared lamps and then oiled to prevent the formation of rust. 17.2.7. Barrel Rolling. Barrel Rolling is similar to tumbling except that the barrel is loaded only about 40 to 60% full and its purpose is not to clean but to cut down the surfaces through the use of suitable cutting abrasive,Fig. 17.9. Rolling is done either in open tilted barrels or in close horizontal barrels. The workpieces, abrasive and water or dilute acid solutions are loaded into the barrel to such a height that as barrel turns, the mass will be carried about 3/4 of the distance up the side and then roll over and fall to the bottom. The abrasives most commonly used are: slag, cinders, sharp sand, granite chips, broken chips of glass or carborundum. The abrasives vary in shape from round to triangular and in size from about 5 mm to 25 mm. The rolling action must be such that there is a relative motion between the work and the abrasive particles, since no cutting action will occur unless the sharp edges of the abrasive pass across the surfaces to be finished. Rolling usually is done wet since this gives a faster cutting action. However dry rolling is also done usually with an abrasive mixed with saw dust to brighten small parts. By using the proper abrasive, a wide range of finishes can be obtained. Rolling time varies from 10 minutes for non-ferrous parts to 2 or 3 hours for steel. Since it usually is done as a batch process, it is simple and very economical. The resulting surface is remarkably uniform, but deep scratches must be removed prior to rolling. 736 A Textbook of Production Technology 17.2.8. SUPER-FINISHING Super-finishing is a micro finishing process that produces a controlled surface condition on parts which is not obtainable by any other method. The operation which is also called ‘microstoning’ consists of scrubbing a stone against a surface to produce a fine quality metal finish. The process consists of removing chatter marks and fragmented or smear metal from the surface of a dimensionally finished parts. As much as 0.03 to 0.05 mm of stock can be efficiently removed with some production applications, the process becomes most economical if the metal removal is limited to 0.005 mm. Reciprocation Traverse in necessary pressure on work Holder stone rotation Work Fig. 17.10. Supper Finishing. The method is performed by rapidly reciprocating a fine grit stone with a soft bond and pressing it against a revolving round work-piece, Fig. 17.10. The stone quickly wears to conform to the contour of the work-piece. The work-piece and tool are flooded with a cutting fluid to carry away heat and particles of metal and abrasive. The time needed for super finishing is quite small. Parts may be super-finished to a smoothness of 0.075 m as rapidly as 15 to 50 seconds. However, to obtain a better finish 2 or 3 minutes may be required. Product applications of super-finishing are: computer memory drums, sewing machine parts, automotive cylinders, brake drums, bearings, pistons, piston rods and pins, axles, shafts, clutch plates, tappet bodies, guide pins etc. 17.2.9. Burnishing: Burnishing operation is the process of getting a smoth and shiny subface by contact and rubbing of the surface against the walls of a hard tool [punch and/or die, rollers and balls etc.). It is a finishing and strengthening process. Burnishing is basically a cold surface plastic deformation process. Cold working of surfaces improves the surface finish and induces surface compressive residual stresses, thus improving the fatigue life of the component. Fig. 10.6 shows the sizing operation with the help of a punch or mandrel and a die to get burnished inside and outside surfaces, inside surface of the bush is burnished when the mandrel gets forced through it and the external surface is burnished when the bush is forced through the die. The surface finish of the gear both is also improved by burnishing. A special hardened gear shaped burnishing die subjects the tooth surfaces to a surface rolling action. In this method, the gear is rolled under pressure with hardened accurately formed burnishing gear. In this cold working process, any high points on the tooth surface are plastically deformed to get accurate and finished tooth profile. Some other burnishing methods are discussed below:–– (a) Barrel Burnishing. Results very nearly comparable with those obtained by buffing may often be obtained by barrel burnishing. It is similar to barrel rolling except that instead of using an Surface Finishing Processes 737 abrasive medium, medium balls, shots or round pins are added to the work in the barrel. There is no cutting action in burnishing. Instead, the slug material producing peening and rubbing action on the work rough surface, spreading the minute surface irregularities to an even surface. Burnishing will not ordinarily remove visible scratches or pits, but will produce a smooth, uniform surface and reduce the porosity in surfaces which are to be or have been plated. Parts which are to be barrel burnished, usually first should be rolled with a fine abrasive. Barrel burnishing normally is done wet, using water to which has been added some lubricating or cleaning agents such as soap. The barrel should not be loaded more than half full with work and shot. Since the rubbing action between the work and the shot material is very important, there should be about two volumes of shots to one volume of parts. The ratio should be such that the workpieces do not rub against one another. The speed of rotation of the barrel should be adjusted so that the workpieces are not thrown out of the mass as they reach the top position and roll down the inclined surface. It is usually necessary to use several sizes and shapes of shot material in order to ensure that the material can come in contact with inside corners and other recesses which must be rubbed. Balls from 3 mm to 6 mm diameter, pins, jacks and ball cones are commonly used. Parts which cannot be permitted to bump against one another may be burnished successfully by fastening them in racks inside the barrel. The shot material is then added and burnishing carried out in the usual manner. When proper conditions have been achieved, barrel burnishing is economical and produces surfaces suitable for subsequent painting or plating. (b) Roller/Ball Burnishing:– Flat, cylindrical or conical surfaces (both internal and external) are burnished with hardened steel or cemented carbide rollers or with steel balls mounted in a holder, fig. 17.11. Fillets and grooves are burnished by rollers rounded to a radius, fig. 17.11 (c). Where strengthening is the aim of the treatment, the burnishing pressure is to be increased. However, this condition results in somewhat lower machining accuracy. Hole burnishing is performed with multi- roller tools on drill presses, turret lathes, horizontal borers, unit built machines and automatic lathes. Burnishing raises the hardness of the surface by 20 to 50% and its wear resistance by 1.5 to 2 times. Internal surfaces are also burnished with the help of balls, the process being called as “ball burnishing” or “Ballizing”. Smooth balls or mandrels slightly larger than the bore diameter are pushed through the length of the hole, see fig. 10.6 Typical product applications of roller burnishing include: Hydraulic system components, Seals, valves, spindles and fillets on shafts. 17.2.10. Powder Coating:– In powder coating, a suitable plastic formation in the form of pulverised powder is spread over the surface to be coated. After that, the surface is subjected to heat. The powder changes to plastic state allowing it to flow and fuse into a uniform continuous coating. The method was discussed briefly in chapter 12 under Art. 12.5.3, under the heading “Plastic Coatings”. Both thermo-plastic resins and thermo-setting resins are used. In addition to thermo-plastic resins mentioned in Art. 12.5.3, the other plastics used include: nylon, PVC, and thermo-plastic polyester. Thermo-setting resins include: epoxy, polyester and acrylic. We get thicker coatings with thermo-plastic resins. Thermo-setting resins are mainly used where thin paint like surface coatings are desired. For epoxy resin the baking temperature ranges from about 120°C to 135°C and a baking time of 20 to 30 minutes. It is used both as thick coating (functional end uses) and for this film decorative coating. For thermo-plastic resins, the “fluidised bed” technique is used. Here, a preheated part is immersed into a fhuidised powder bed. The thickness of coating will depend upon the temperature of the heated part and the time it remains immersed in the powder bed. 738 A Textbook of Production Technology Roller Burnished Roller Surface Burnished Surface (a) Flat Surface (b) Conical Surface Roller Ball Job Work Piece (c) Burnishing of Fillet (d) Ball Burnishing Fig. 17.11 Roller Burnishing. Another common method of coating is “Electrostatic fluidised bed” process. Here, a cloud of charged powder particles is created above the powder bed. The components (hot or cold) are covered into the cloud. An electrical field of attraction is established between the powder particles and the components, due to which powder is deposited over the surface of the components. The resins used in this process include: epoxy, polyester, acrylic, polyethylene and polypropylene. The most widely used method of powder coating is “electrostatic spraying”. The equipment includes: a powder feed unit, powder spray guns, spray booth, electrostatic voltage source and powder recovery unit. Note: The other methods of coating such as metal coating, electro-plating, chrome plating and anodizing etc. have been discussed in detail in chapter 12. 17.2.11. Polishing:- Polishing is a process used to get a smooth and lustrous surface finish on a part. It is done with soft/resilient polishing wheels (made of felt, cloth, wood or coarse calico etc.) coated with abrasive (Al2O3 or diamond) paste or used with fluid carrying abrasives. The process is based on the simultaneous action of the tool ( polishing wheel) and the surface active agents of the pastes. It reduces surface roughness to 0.032-0.012 μm Ra. Unlike lapping, polishing does not improve machining accuracy. Surface Finishing Processes 739 Polishing operations may be classified as : roughing, dry fining and finishing. Roughing and dry fining operations are done with dry wheels, the grain size of abrasive being 20 to 80 for roughing and 90 to 120 for dry fining. In finish polishing, the grain size is fine (150) and we use oil, tallow or beeswax for making pastes. It gives a fine finish. Parts with irregular shape, sharp corners, deep recesses and sharp projections are difficult to polish. PROBLEMS 1. What do you understand by surface irregularities? 2. How is surface roughness evaluated? 3. What are the units of surface roughness? 4. Write on the following surface finishing processes:– (a) Diamond turning and boring. (b) Grinding 5. Define Lapping process. 6. How is lapping done? 7. Write the materials of laps. 8. Name the abrasives used in lapping process. 9. List the functions of lapping process. 10. Write briefly on: (a) Hand lapping of flat surfaces. (b) Hand lapping of external cylindrical work. 11. With the help of a neat diagram, explain the process of Mechanical lapping. 12. List the product applications of lapping process. 13. Define honing process. 14. With the help of a neat diagram, explain the honing process. 15. Write the product applications of honing process. 16. Define Buffing operation. 17. With the help of neat diagrams, discuss:– (a) Barrel tumbling process. (b) Barrel rolling process. 18. What is the difference between barrel tumbling process and barrel rolling process. 19. With the help of a neat diagram, explain the super-finishing process. 20. List the product applications of super-finishing process. 21. Define Burnishing process. 22. Write briefly on Barrel burnishing process. 23. With the help of neat diagrams, explain the Roller/Ball burnishing processes. 24. List the product applications of roller burnishing process. 25. Write a short note on: Powder Coating. 26. What is "Electrostatic Fluidised Bed" process of powder coating? 27. What is "Electrostatic Spraying"? 28. What is "Polishing" operation? 29. Name the materials used for polishing wheels. 30. How is the polishing operation classified? 31. Give the limitations of polishing operations. 32. What is the level of surface roughness achieved after polishing operation? 33. Does polishing improve machining accuracy?  APPENDIX—I A Representation of Welds of Drawing IS : 813-1961 Table I-1 Form of weld Appropriate Sectional symbol representation Fillet Square butt Single V butt Double V butt Single U butt Double U butt Single bevel butt Double bevel butt Single J butt Double J butt Stud Bead edge or seal Sealing run Spot Seam (Continuous) Stitch, Seam 740 Appendix I Table I.2 741 B. Classification of carbide tips according to their range of application (IS : 2428 – 1964) Designat Increasing direction of Range of application ion the characteristic of Indentifi Carbide Cutting Material to be machined Machining conditions cation tip colour P01 Steel, steel casting Precision turning and find boring Cutting speed : high. Feed : Low P 10 Steel, steel casting Turning, threading and milling Cutting speed : high. Feed : low or medium Steel, steel casting, malleale cast iron Turning, milling. Cutting speed and feed : medium planing : with P 20 Resistance to wear forming long chips low feed rate Cutting speed Toughness Steel, steel casting, malleable cast iron Turning, planning,milling. Cutting speed : medium to low. Feed : feed P 30 forming long chips medium to high even if operating conditions are unfavourable Steel, steel casting with stand Turning, planing, shaping. Cutting speed : low, Feed : high. Rake inclusions or shrinkage cavities angle : high for machining under favourable conditions and P40 work on automatic machines Steel, steel castings of medium or low Turning, planning, shaping. Cutting speed : low. Feed : high Rake P50 tensile strength with sand inclusions or angle large for machining under unfavourable conditions and work shrinkage cavities on automatic machines Steel, steel casting, manganese steel, Turning. Cutting speed : mediuum to high. Feed : low to medium M 10 grey cast iron, alloyed cast iron Steel, steel casting, austenitic steel, Turning, milling, Cutting speed : medium Feed : medium M 20 manganese steel, grey cast iron, Resistance to wear sphrodised cast iron and malleable cast Cutting speed Toughness iron feed Steel, steel casting, austenitic steel, Turning, milling, planing. Cutting speed : medium Feed : medium M 30 grey cast iron, heat resisting alloys or high Free cutting steel, low tensile strength Turning, profile turning, parting off especially in automatic M 40 steel, brass and light alloy machines Very hard grey cast iron, chilled Turning, precision turning and boring, milling, scraping casting of hardness up to 60 HRC. K 01 Alluminium alloys with high silicon content, hardened steel, plastics of abrasive type, hard board and cermics Grey cast iron of hardness more than Turning, milling, boring, reaming, broaching, scraping 220 HB, malleable cast iron forming short chips, tempered steel, aluminium K 10 Resistance to wear alloys containing silicon, copper Cutting speed Toughness alloys, plastics, glass, hard rubber, feed hard cardboard, porcelain, stone Grey cast iron of hardness up to 220 Turning, milling, planing, reaming, broaching HB, non-ferrous metals, such as K 20 copper, brass, aluminium, laminated wood of abrasive type Soft grey cast iron, low tensile strength Turning, planing, shaping, milling. rakeangle large. even under K 30 steel laminated wood unfavourable conditions Soft or hard natural wood, nonferrous Turning, milling, planing, shaping. Rakeangle : large, even metals under unfavourable maching conditions K 40 742 A Textbook of Production Technology C. Super-Alloys : Super-alloys are those materials which, under stressed conditions, can with stand high temperatures (600 to 1100°C). Thus, these materials do not loose their strength at high temperature. There are three types of Super-alloys : 1. Iron-Based 2. Ni-Based 3. Co-Based All the three have Cr as one of major constituents, Its main function are : it aids in Carbide formation and also imparts Corrosion-Resistance properties to the alloys. Super-alloys have major fields of application in Aero-gas turbines and Nuclear Reactors, Ni-based super alloys are widely used in the manufacture of turbine disks for turbo-jet engine applications. The nominal composi- tions of these super-alloys are given in a table I.3. D. Specifications of Manufacturing Equipment : 1. Foundry : (a) Moulding Machines : Pressure, Mould box size, Pressing cylinder  (b) Die-Casting Machines : Job weight, Tonnage (Locking Force). (c) Centrifugal Casting Machines :– (i) Bush Casting Machines : Bush × Bush length.. (ii) Water-pipe " " : Pipe . (iii) Canal-pipe " " : Pipe  2. Metal Forming Equipment : (a) Forging and Stamping Machines : Forging Machines (Heading and Upsetting) : Job  Swaging M achines : Tube  Roll Forming Machines : Tonnage Stamping Machines : : Plate Thickness (b) Bending and Forming Machines : (i) Bending Rolls (Pipes/Bars/Shapes/Angles) : Plate thickness (ii) Rotary head and Ram type Bending Machines (Pipes/Bars/Shapes/Angles) : Section size (iii) Folding and Bending/Angles) : Bend length (iv) Flattening and Straightening Machines : Plate thickness (v) Press Brakes : Tonnage (c) Punching and Shearing Machines : (i) Punching Machines : Plate thickness (ii) Guillotine Shearing Machines : Plate thickness (iii) Shearing and Cropping Machines : Section size (iv) Rotary Shears : Plate thickness (v) Combination Punching, Shearing, Notching and Cropping Machines : Plate thickness (vi) Gang slitting Machines (vii) Nibbling Machines : Plate Thickns. (vii) Slab, Ingot and Billet Shearing Machines : Job size (d) Drawing Machines : (i) Wire and Metal Ribbon drawing Machines : Wire  (ii) Tube and Bar drawing Machines : Tube  / Bar  (e) Other Metal Forming Machines : (i) Spring manufacturing machines : Wire  (ii) Chain making machines : Chain size (iii) Container making machines : Job size (iv) Wire/Rope/Cable making machines : Job size  Table I.3: Super-Alloys Appendix I Alloy Nominal Composition, % C Mn Cr Ni Mo Ti Al W Fe Co Iron-Based A—286 0.08 1.35 15 26 1.25 2 — — 54 — V–57 " 0.25 15 25.5 " 3 — — 64 — 19–9DL 0.30 1.10 19 9 " 0.3 — — 68 — 16–25–6 0.06 1.35 16 25 6 — — — 51 — Ni-Based Hastelloy R-235 0.10 0.25 16 61.25 5.5 2.5 2 — 10 1.9 Inconel X-750 0.04 0.5 15 73 — 2.4 0.6 — 0.6 0.4 Udimet–500 0.09 — 19 52 4 3 2.8 — 2 17 Waspaloy 0.06 0.5 19.5 57.25 4.2 3 1.2 — 1 13.5 Co-Based J-1570 0.2 — 20 28 — 4 — 7 2 39 S-816 0.38 — 20 20 4 — — 4 4 44 743 744 A Textbook of Production Technology (v) Nails/Rivets making machines : Job size (vi) Needles and Pins making machines : Wire  (vii) Special nut and bolt forming machines : Thread  3. Joining and Assembly Equipment : (i) Gas welding machines : Tip size (ii) Electric arc welding machines : kVA (iii) Plasma arc welding machines : kVA (iv) Resistance welding machines : kVA (v) Friction welding machines : kVA (vi) Ultrasonic Welding machines : W (vii) Electron-Beam welding machines : kW (viii) Laser-Beam welding machines : W (ix) Brazing and Soldering machines : kVA 4. Machine Tools : (a) Lathes (i) Copying lathes; Multi-tool and production lathes; crankshaf lathes; Relieving lathes; : Swing  wheel and Axle lathes; Flow turning lathes and spinning lathes. (ii) Roll turning lathes : Swing , Job weight (iii) Diamond turning lathes : Swing  (iv) Facing and Boring lathes : Face plate (work table)  or Swing  (v) Capstan lathes, Turret lathes : Swing  (vi) Vertical turning and Boring mills : Swing  (vii) Automats, Single spindle, Bar type, Horizontal, Sliding head (Swiss type) and others : Bar  (viii) Automats, Multi-spindle, Bar type (Hor., Vert.) : Bar  (ix) Automats, Chucking type (Single-spindle/ Multi-spindle; Hor./Vert.) : Chucking  (b) Planner : Plate edge Planner : Tool travel (c) Rifling Machines : : Stroke (d) Drilling Machines : (i) Deep hole (including gun drilling) : Drill , Drill length (ii) Facing and Centering : Job  (e)Milling Machines : (i) Pantograph milling machines : Table size (ii) Plano-milling : Table width (f) Grinding Machines : (i) Slideway grinders : Table size (ii) Tool and cutter grinders : Wheel  (iii) Grinders, Drill point sharpening : Drill  (iv) Grinders, Tap sharpening : Tap  (v) Grinders, Broach sharpening : Broach , Broach length Appendix I 745 (vi) Grinders, Circular saw sharpening : Saw  (vii) Grinders, Band saw blade sharpening : Module, Hob  (viii) Grinders, Hob sharpening : Module, Hob  (ix) Spline grinders : Job length (x) Jig grinders : Bore  (xi) Abrasiver band grinders : Band width (xii) Centre grinders : Job , Job length (xiii) Bearing raceway grinders (exit., int.) : Job  (xiv) Grinders (Brake drum, Piston) : Job  (xv) Polygon grinders : Job  (xvi) Duplex grinders : Wheel  (xvii) Grinders (Form, Profile, Valve seat, Bench, Wing frame : Wheel  Note :- The main specifications of the conventional and more commonly used Manufactur- ing machines/equipment and Machine tools, have been given at appropriate places in the text. 746 A Textbook of Production Technology APPENDIX—II Machining Variables and Related Relations : As already discussed under Art 6.6, the three machining elements of any machining process are : feed, f (mm/rev.), depth of cut, d (mm), and cutting speed, V (m/min.). 1. Lathes. Refer to Art. 6.6 and Fig. 6.13.  DN V , m / min... (1) 1000 Area of Uncut Chip : The cross-sectional area, Ac, of the layer of the work material being removed is AC = width of chip × thickness of uncut chip. =b×t Now if Cs = side cutting edge angle, then d b Cos Cs and t = f Cos Cs  b × t = f.d mm2  Metal removal rate, (MRR) is given as MRR  .D. Ac.N mm 3 / min  MRR = 1000 Ac.V mm3/min.... (From Equation.1) = 1000 fdV, mm3/min Machining Time : The machining time is given as, L Tm  min, per pass (cut) fN where L = Total length of travel of the tool or of the workpiece = (Length of surface to be machined) + (Tool approach) + (Tool overtravel) Tool approach = Distance a tool is fed from the time it touches the workpiece, until it is cutting to the full depth. Tool overtravel = Distance a tool is fed, while it is not cutting. It is the distance over which the tool idles, before it enters and after it leaves the cut. Approach of most of the single point cutting tools is negligible. Refer to Fig. II.1. L=l+x+y where x = Tool approach = d tan Cs or d Cot  mm y = Tool overtravel = 1 to 2 mm   Tool approach angle = 90 – Cs For facing operation, L = D/2 + x + y, mm Now, Total machining time = Tm × i Total machining allowance where i = number of passes or cuts  Material removed per cut Di  D f D f – Di Now, total machining allowance  , for turning and = , for boring 2 2 where Di =initial diameter of the workpiece Df = finished diameter of the workpiece Appendix-II 747 L I d y x  Feed Fig. II.1. Turning Operation. Material removed per cut = Depth of cut, Depth of cut is half the difference between the work diameter at the start of the cut and the diameter of machined surface obtained after the cut. Total machining allowance  d i In practice, the depth of cut per pass is not constant. For roughing operations, it is much greater than for the finishing operations. 2. Shapers, Planers, Slotters : Refer to art. 8.2.4., MRR = f.d.L.N, mm3/min Here, L = Length of the workpiece, mm N = Number of the complete strokes /min. f = feed, mm/stroke Bx y Machining Time, Tm  , min. f N B = Width of the machined surface x = Side approach of the tool = d cot  y = Side over travel of the tool = 2 to 3 mm. Another Method of Calculating the Machining Time B⎛ L L ⎞ Tm  ⎜  ⎟ , min f ⎝ Vw  1000 Vr  1000 ⎟⎠ ⎜ where L = Length of stroke (for Shaper) or length of Table travel (for Planer), mm = Length of workpiece + Approach + Overtravel Vw = working stroke speed or cutting speed, m/min Vr = Return stroke speed, m/min; See Fig. 8.38 B = Width of job, mm f = Cross-feed per full stroke of table (For Shaper) and of tool head (for Planer), mm. Vw can be determined as, L  N  (1  K ) Vw = Cutting speed  , m / min. 1000 Vw is ‘V’ as under Art. 8.2.4. where N = Number of full (double) strokes/min, that is, working plus return strokes