Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. PowerPoint to accompany Krar Gill Smid Technology of Machine Tools 6 th Edition Surface Finishing Processes Unit 26

26-2 Objectives Identify and explain the purposes of surface finishing processes Explain the benefits of honing Be familiar with oxide coatings and their purposes

26-3 Surface Finishing Processes Improve appearance and sales value of product Used to resist wear, electrolytic decomposition, and corrosive wear Treatment process (chemical or electrical) produces oxide of original metal on surface Common methods: burnishing, electropolishing, honing, and tumbling

26-4 Burnishing Cold-working process that sizes, finishes and work hardens internal and external metal surfaces of pressure contact of hardened rools Process displaces peaks and valleys of irregular height Burnishing tool incorporates planetary system of tapered rolls evenly spaced by retaining cage

26-5 Burnishing Process Hardened mandrel forces burnishing tool against surface of part –Mandrel tapered inversely to taper of rolls Tool is adjusted slightly larger than part Slight pressure created when tool passes through part –Exceeds yield point of softer part surface Results in mirrorlike, tough, wear and corrosion-resistant surface

26-6 Electropolishing Referred to as reverse plating Uses combination of rectified current and blended chemical electrolyte bath Removes flaws from surface of metal parts Benefits include bright finish, deburring, size control, and microfinish improvement

26-7 Electropolishing Process Power Source - + Tank fabricated from steel and rubber-lined Chemical bath Cathode plate: Lead, Copper, or Stainless steel (negative charge) Rack of titanium, copper, or bronze holds parts (positive charge) 1. Metal part is charged positive and immersed. 2. Current applied and electrolyte allows metal ions to be removed from part. 3.Electrolyte maintains dissolved metals in solution. 4.When process completed, part run through cleaning and drying

26-8 Honing Abrasive finishing operation Small amount of stock removed from internal or external surface of part Improves flatness and surface finish Low speed operation –85 to 300 sf/min or 25 to 95 m/min

26-9 Differences Between Honing and Grinding Honing is low-speed operation –85 to 300 sf/min, or 25 to 95 m/min Grinding is high-speed operation –5000 to 6500 sf/min, or 25 to 33 m/min Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

26-10 Grinding Chips produced are short, hot sparks due to intermittent contact of each abrasive particle on workpiece surface. Grinding tends to cause thermal damage to surface of workpiece, often to depth of up to.002 in. (0.05 mm) or more.

26-11 Honing Gentle, cooler finishing operation, with little or no damage or distortion of the workpiece surface. Continuous cooler chip produced due to continuous contact of abrasive hone with the workpiece.

26-12 Types of Honing Processes Conventional –Cutting pressure applied when mandrel forces stone and guide-shoe surfaces into contact with bore Rotational and reciprocal action of hone causes thousands of small cutting edges on stone to shear minute chips from workpiece Single-stroke –Expandable, diamond-plated abrasive sleeve on tapered arbor expanded to size by calibrated adjusting screw –Faster than convention honing and maintains consistent size

26-13 Various Honing Stones Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Single-stone hones used on small bores less than 3.00 in. Multi-point-contact hones used on production for honing large parts. Four-point-contact hones are capable of fast metal removal

26-14 Machine Requirements For fullest potential of superabrasives (CBN) –Be rigid, no spindle vibration, and sufficient power for high material-removal rates –Have high-spindle speeds—about three times speed for conventional hones and closely controlled increments of stone feed and pressure –Have coolant system capable of supplying adequate volumes of filtered honing fluids

26-15 Honing Guidelines Considerations for selecting proper CBN (Cubic Boron Nitride) hone –Type and hardness of the work material –Amount of material to be removed

26-16 General Rules for any CBN Hone For maximum removal rates, use coarse grit sizes, lower concentration, and narrow stones at high speed. For fine finishes, use fine grit sizes and higher concentrations. For hard materials, use a soft bond; for soft materials, use a hard bond. Always use an adequate supply of coolant to clean the stone, clear the chip, and cool the workpiece.

26-17 Tumbling Used for cleaning, polishing and removing sharp corners and burrs from metal parts Economical since material handled in bulk Various sizes available of tumbling barrels –Material and shape of part to be cleaned –Depth and hardness of scale or rust –Previous finish to be removed

26-18 Selecting the Equipment and Tumbling Media Must choose: proper type of barrel, abrasive, lubricant, and ratio of work to total volume of batch and barrel Surfacing operations range from deburring and polishing to honing and mirror finishing Common abrasives: aluminum oxide, ceramic, plastic, stone chips, crushed corncob, and steel balls

26-19 The Tumbling Process Parts loaded into suitable tumbling barrel and appropriate tumbling media added Water level in loaded barrel bears definite relation to type of finish desired –Burnishing: level just above top line of loading –Honing and light deburring: level 1/3 to halfway below load line –Fast deburring: level 2/3 or even lower below load line

26-20 Black-Oxide Coatings Chemical conversion process produced by reaction of iron in ferrous metal with oxidizing salts present in black oxide Salts include penetrates, catalysts, activators and proprietary additives Result is formation of black iron oxide, magnetite on surface of metal Used for decorative and corrosion prevention

26-21 Black-Oxide Process Can be produced of ferrous metal using –Molten salt bath (600ºF and above) –Cold black solution at room temperature –Hot alkaline aqueous solution (285ºF and 300ºF) Hot alkaline aqueous most commonly used –Produces deep black consistent and uniform finish –Penetrates into metal surface up to 5 to 10 millionths of an inch