Grinding Wheel. FIGURE 9.1 Schematic illustration of a physical model of a grinding wheel, showing its structure and grain wear and fracture patterns.

Transcription

1 Grinding Wheel Grinding wheel Bond Porosity Grain Bond fracture Microcracks Attritious wear Wheel surface Grain fracture FIGURE 9.1 Schematic illustration of a physical model of a grinding wheel, showing its structure and grain wear and fracture patterns. TABLE 9.1 Knoop hardness range for various materials and abrasives. Common glass Titanium nitride 2000 Flint, quartz Titanium carbide Zirconium oxide 1000 Silicon carbide Hardened steels Boron carbide 2800 Tungsten carbide Cubic boron nitride Aluminum oxide Diamond

2 Grinding Wheel Types Grinding face Grinding face Type 1 straight Type 2 cylinder Grinding face Grinding face (c) Type 6 straight cup (d) Type 11 flaring cup Grinding faces Grinding faces (e) Type 27 depressed center (g) Mounted (f) Type 28 depressed center FIGURE 9.2 Some common types of grinding wheels made with conventional abrasives (aluminum oxide and silicon carbide). Note that each wheel has a specific grinding face; grinding on other surfaces is improper and unsafe.

3 Superabrasive Wheels Type 1A1 2A2 1A1RSS (c) 11A2 DW DWSE (d) (e) (f) FIGURE 9.3 Examples of superabrasive wheel configurations. The rim consists of superabrasives and the wheel itself (core) is generally made of metal or composites. Note that the basic numbering of wheel types (such as 1, 2, and 11) is the same as that shown in Fig The bonding materials for the superabrasives are:, (d), and (e) resinoid, metal, or vitrified; metal; (c) vitrified; and (f) resinoid.

4 Grinding Wheel Marking System Example: 51 A 36 L 5 V 23 Prefix Abrasive Abrasive Grade Structure Bond type grain size type Manufacturer!s record Manufacturer!s symbol (indicating exact type of abrasive) (use optional) A Aluminium oxide C Silicon carbide Coarse Medium Fine Very fine Soft Medium Hard A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Grade scale Dense Open etc. (Use optional) Manufacturer!s private marking (to identify wheel) (use optional) B Resinoid BF Resinoid reinforced E Shellac O Oxychloride R Rubber RF Rubber reinforced S Silicate V Vitrified FIGURE 9.4 Standard marking system for aluminum-oxide and silicon-carbide bonded abrasives.

5 Diamond and cbn Marking System Example: M D 100 P 100 B 1/8 Prefix Abrasive type Grit size Grade Diamond concentration Bond Bond modification Diamond depth (in.) Manufacturer!s symbol (to indicate type of diamond) B Cubic boron nitride D Diamond A (soft) to Z (hard) 25 (low) (high) B Resinoid M Metal V Vitrified 1/16 1/8 1/4 Absence of depth symbol indicates solid diamond A letter or numeral or combination (used here will indicate a variation from standard bond) FIGURE 9.5 Standard marking system for diamond and cubic-boron-nitride bonded abrasives.

6 Abrasive Grains Grain Chip Abrasive grain Chip V Wear flat 10 m v FIGURE 9.6 The grinding surface of an abrasive wheel (A46-J8V), showing grains, porosity, wear flats on grains (see also Fig. 9.7b), and metal chips from the workpiece adhering to the grains. Note the random distribution and shape of the abrasive grains. FIGURE 9.7 Grinding chip being produced by a single abrasive grain. Note the large negative rake angle of the grain. Source: After M.E. Merchant. Schematic illustration of chip formation by an abrasive grain. Note the negative rake angle, the small shear angle, and the wear flat on the grain.

7 Grinding Variables v l t V D Grinding wheel Grains FIGURE 9.8 Basic variables in surface grinding. In actual grinding operations, the wheel depth of cut, d, and contact length, l, are much smaller than the wheel diameter, D. The dimension t is called the grain depth of cut. d Chip length, external grinding l = Dd 1 + (D/D w ) Chip length, internal grinding l = Dd 1 (D/D w ) Chip length, surface grinding 4v d t = VCr D

8 Grinding Parameters Chip Ridges Groove FIGURE 9.9 Chip formation and plowing (plastic deformation without chip removal) of the workpiece surface by an abrasive grain. Process Variable Conventional Grinding Creep-Feed Grinding Buffing Polishing Wheel speed (m/min) Work speed (m/min) Feed (mm/pass) TABLE 9.2 Typical ranges of speeds and feeds for abrasive processes.

9 Specific Energy in Grinding Specific Energy Material Hardness W-s/mm 3 hp-min/in 3 Aluminum 150 HB Cast iron (class 40) 215 HB Low-carbon steel (1020) 110 HB Titanium alloy 300 HB Tool steel (T15) 67 HRC TABLE 9.3 Approximate Specific-Energy Requirements for Surface Grinding. Temperature rise: ) 1/2 Temperature rise D 1/4 d 3/4 ( V v

10 Residual Stresses Residual stress (psi x 10 3 ) Compression Tension mm ft/min (20 m/s) 3000 (15) 2000 (10) Depth below surface (in.) MPa Residual stress (psi x 10 3 ) Compression Tension mm Soluble oil (1:20) Highly sulfurized oil 5% KNO 2 solution Depth below surface (in.) MPa FIGURE 9.10 Residual stresses developed on the workpiece surface in grinding tungsten: effect of wheel speed and effect of type of grinding fluid. Tensile residual stresses on a surface are detrimental to the fatigue life of ground components. The variables in grinding can be controlled to minimize residual stresses, a process known as low-stress grinding. Source: After N. Zlatin.

11 Dressing Single-point dressing diamond for dressing forms up to 608 on both sides of the grinding wheel 60 Fixed-angle swivelling dresser to dress forms up to 908 on both sides of the grinding wheel Rotary dressing unit for dressing hard grinding wheels or for high-volume production Grinding wheel Precision radius dresser for single- and twin-track bearing production Formed diamond roll dressing for high-volume production Silicon carbide or diamond dressing wheel for dressing either diamond or cbn grinding wheels Dressing tool Dressing tool Grinding face Diamond dressing tool Grinding wheel FIGURE 9.11 Methods of grinding wheel dressing. Shaping the grinding face of a wheel by dressing it with computer-controlled shaping features. Note that the diamond dressing tool is normal to the wheel surface at point of contact. Source: OKUMA America Corporation. Manufacturing Processes for Engineering Materials, 5th ed.

12 Surface Grinding Wheel Horizontal-spindle surface grinder: Traverse grinding Wheel Horizontal-spindle surface grinder: Plunge grinding s Rotary table (c) Wheel Work table FIGURE 9.12 Schematic illustrations of surface-grinding operations. Traverse grinding with a horizontal-spindle surface grinder. Plunge grinding with a horizontal-spindle surface grinder, producing a groove in the workpiece. (c) Vertical-spindle rotary-table grinder (also known as the Blanchard-type grinder). Wheel guard Worktable Saddle Feed Wheel head Column Bed FIGURE 9.12 Schematic illustration of a horizontal-spindle surface grinder.

13 Thread and Internal Grinding Grinding wheel FIGURE 9.14 Threads produced by traverse and plunge grinding. Wheel Wheel Wheel Traverse grinding Plunge grinding (c) Profile grinding FIGURE 9.15 Schematic illustrations of internal-grinding operations.

14 Centerless Grinding Through-feed grinding Plunge grinding Grinding wheel Feed! Grinding wheel End stop Work-rest blade Regulating wheel Regulating wheel Internal centerless grinding Pressure roll Regulating wheel Grinder shaft (revolves clockwise) Support roll FIGURE 9.16 (a-c) Schematic illustrations of centerless-grinding operations. (d) A computernumerical-control centerless grinding machine. Source: Cincinnati Milacron, Inc. (c) (d)

15 Creep-Feed Grinding d = 1 6 mm Low work speed, v (c) FIGURE 9.17 Schematic illustration of the creep-feed grinding process. Note the large wheel depth of cut. A groove produced on a flat surface in one pass by creep-feed grinding using a shaped wheel. Groove depth can be on the order of a few mm. (c) An example of creep-feed grinding with a shaped wheel. Source: Courtesy of Blohm, Inc. and Society of Manufacturing Engineers.

16 Finishing Operations Make coat Backing Abrasive grains Size coat FIGURE 9.18 Schematic illustration of the structure of a coated abrasive. Sandpaper, developed in the 16th century, and emery cloth are common examples of coated abrasives. Spindle Stone Nonabrading bronze guide FIGURE 9.19 Schematic illustration of a honing tool to improve the surface finish of bored or ground holes. Oscillation (traverse if necessary) Motor Stone Rotation Holder Stone FIGURE 9.20 Schematic illustration of the superfinishing process for a cylindrical part: cylindrical microhoning; centerless microhoning. Rolls

17 Lapping Lap position and pressure control Upper lap Lap Abrasive Before Workholding plate After Guide rail s Machine pan (c) Lower lap FIGURE 9.21 Schematic illustration of the lapping process. Production lapping on flat surfaces. (c) Production lapping on cylindrical surfaces.

18 Chemical-Mechanical Polishing carrier Abrasive slurry Polishing pad carrier (disk) Polishing table Polishing table Side view Top view FIGURE 9.22 Schematic illustration of the chemical-mechanical polishing process. This process is widely used in the manufacture of silicon wafers and integrated circuits, where it is known as chemical-mechanical planarization. Additional carriers and more disks per carrier also are possible.

19 Polishing Using Magnetic Fields Drive shaft Guide ring S-pole N-pole Magnetic fluid and abrasive grains Ceramic balls (workpiece) Float Permanent magnets N S N S N S N S N S N S Magnetic fluid FIGURE 9.23 Schematic illustration of the use of magnetic fields to polish balls and rollers: magnetic float polishing of ceramic balls and magnetic-field-assisted polishing of rollers. Source: After R. Komanduri, M. Doc, and M. Fox.

20 Ultrasonic Machining Power supply Transducer Glass-graphite epoxy composite Glass 1.2 mm (0.048 in.) Tool Abrasive slurry 50 mm (2 in.) diameter Slots mm ( in.) Holes 0.4 mm (0.016 in.) diameter (c) FIGURE 9.24 Schematic illustration of the ultrasonic-machining process; material is removed through microchipping and erosion. and (c) Typical examples of cavities produced by ultrasonic machining. Note the dimensions of cut and the types of workpiece materials. Contact force: Contact time: t o 5r ( co ) 1/5 F ave = 2mv t o c o v

21 Process Characteristics Process Parameters and Typical Material Removal Rate or Cutting Speed Chemical machining (CM) Electrochemical machining (ECM) Electrochemical grinding (ECG) Electrical-discharge machining (EDM) Wire EDM Laser-beam machining (LBM) Electron-beam machining (EBM) Water-jet machining (WJM) Abrasive water-jet machining (AWJM) Abrasive-jet machining (AJM) Shallow removal (up to 12 mm) on large flat or curved surfaces; blanking of thin sheets; low tooling and equipment cost; suitable for low production runs. Complex shapes with deep cavities; highest rate of material removal; expensive tooling and equipment; high power consumption; medium to high production quantity. Cutting off and sharpening hard materials, such as tungsten-carbide tools; also used as a honing process; higher material removal rate than grinding. Shaping and cutting complex parts made of hard materials; some surface damage may result; also used for grinding and cutting; versatile; expensive tooling and equipment. Contour cutting of flat or curved surfaces; expensive equipment. Cutting and hole making on thin materials; heataffected zone; does not require a vacuum; expensive equipment; consumes much energy; extreme caution required in use. Cutting and hole making on thin materials; very small holes and slots; heat-affected zone; requires a vacuum; expensive equipment. Cutting all types of nonmetallic materials to 25 mm (1 in.) and greater in thickness; suitable for contour cutting of flexible materials; no thermal damage; environmentally safe process. Single or multilayer cutting of metallic and nonmetallic materials. Cutting, slotting, deburring, flash removal, etching, and cleaning of metallic and nonmetallic materials; tends to round off sharp edges; some hazard because of airborne particulates mm/min V: 5-25 dc; A: mm/min, depending on current density. A: 1-3 A/mm 2 ; typically 1500 mm 3 /min per 1000 A. V: ; A: ; typically 300 mm 3 /min. Varies with workpiece material and its thickness m/min. 1-2 mm 3 /min Varies considerably with workpiece material. Up to 7.5 m/min. Varies considerably with workpiece material. Advanced Machining Processes TABLE 9.4 General characteristics of advanced machining processes.

22 Chemical Milling 4 mm (before machining) 2 mm (after machining) Chemically machined area Section FIGURE 9.25 Missile skin-panel section contoured by chemical milling to improve the stiffness-toweight ratio of the part. Weight reduction of space launch vehicles by chemical milling of aluminumalloy plates. These panels are chemically milled after the plates have first been formed into shape, such as by roll forming or stretch forming. Source: ASM International.

23 Chemical Machining Tank Chemical reagent Maskant Agitator Heating Cooling coils Undercut 3rd Steps 2nd 1st Edge of maskant Material removed Depth FIGURE 9.26 Schematic illustration of the chemical machining process. Note that no forces are involved in this process. Stages in producing a profiled cavity by chemical machining.

24 Roughness and Tolerance Capabilities µin (c) (d) FIGURE 9.27 Surface roughness and dimensional tolerance capabilities of various machining processes. Note the wide range within each process. (See also Fig ) Source: Machining Data Handbook, 3rd ed., Used by permission of Metcut Research Associates, Inc Surface Roughness, R a (µm) Note: Depends on state of starting surface. Titanium alloys are generally rougher than nickel alloys. (c) High current density areas. (d) Low current density areas. MECHANICAL Abrasive-flow machining Low-stress grinding Ultrasonic machining ELECTRICAL Electrochemical deburring Electrochemical grinding Electrochemical milling (frontal) Electrochemical milling (side wall) Electrochemical polishing Shaped tube electrolytic machining THERMAL Electron-beam machining Electrical-discharge grinding Electrical-discharge machining (finishing) Electrical-discharge machining (roughing) Laser-beam machining Plasma-beam machining CHEMICAL Chemical machining Photochemical machining Electropolishing CONVENTIONAL MACHINING Turning Surface grinding ±0.001 in Tolerance, ±mm x 10-3 Average application (normally anticipated values) Less frequent application (unusual or precision conditions) Rare (special operating conditions)

25 Chemical Blanking FIGURE 9.28 Typical parts made by chemical blanking; note the fine detail. Source: Courtesy of Buckabee-Mears St. Paul.

26 Electrochemical Machining Insulating coating DC power supply (+) (-) Pump for circulating electrolyte Tool Tank Electrolyte 65 mm 75 mm 140 mm 14 holes Forging Machined workpiece 112 mm 86 mm Telescoping cover Insulating layer Feed Electrolyte Copper electrode Ram Electrode carrier (c) FIGURE 9.29 Schematic illustration of the electrochemical-machining process. This process is the reverse of electroplating, described in Section FIGURE 9.30 Typical parts made by electrochemical machining. Turbine blade made of a nickel alloy, 360 HB; the part on the right is the shaped electrode. Source: ASM International. Thin slots on a 4340-steel rollerbearing cage. (c) Integral airfoils on a compressor disk.

27 Electrochemical Grinding Electrolyte from pump Electrode (grinding wheel) Spindle Insulating abrasive particles Work table (1) Insulating bushing Electrical connection (2) DC power supply 1 in (3.1 mm) 8 1 in. (0.4 mm) 64 Inconel in. (0.5 mm) FIGURE 9.31 Schematic illustration of the electrochemical grinding process. Thin slot produced on a round nickel-alloy tube by this process.

28 Electrical Discharge Machining Rectifier Current control Servo control Movable electrode Power supply (+) (-) Tank Worn electrode Spark Melted workpiece Dielectric fluid FIGURE 9.32 Schematic illustration of the electrical-discharge-machining process.

29 EDM Examples 1.5 mm dia. 8 holes, 0.17 mm Electrode (c) FIGURE 9.33 Examples of shapes produced by the electrical-discharge machining process, using shaped electrodes. The two round parts in the rear are a set of dies for extruding the aluminum piece shown in front; see also Section 6.4. Source: Courtesy of AGIE USA Ltd. A spiral cavity produced using a shaped rotating electrode. Source: American Machinist. (c) Holes in a fuel-injection nozzle produced by electrical-discharge machining. FIGURE 9.34 Stepped cavities produced with a square electrode by EDM. In this operation, the workpiece moves in the two principal horizontal directions, and its motion is synchronized with the downward movement of the electrode to produce these cavities. Also shown is a round electrode capable of producing round or elliptical cavities. Source: Courtesy of AGIE USA Ltd.

30 Wire EDM Wire Dielectric supply Wire diameter Spark gap Slot (kerf) Wire guides Reel FIGURE 9.35 Schematic illustration of the wire EDM process. As much as 50 hours of machining can be performed with one reel of wire, which is then recycled.

31 Laser Machining Reflective end Flash lamp Laser crystal Partially reflective end Lens Power supply FIGURE 9.36 Schematic illustration of the laser-beam machining process. Cutting sheet metal with a laser beam. Source: Courtesy of Rofin-Sinat, Inc. TABLE 9.5 General applications of lasers in manufacturing. Application Laser Type Cutting Metals PCO 2 ; CWCO 2 ; Nd:YAG; ruby Plastics CWCO 2 Ceramics PCO 2 Drilling Metals PCO 2 ; Nd:YAG; Nd:glass; ruby Plastics Excimer Marking Metals PCO 2 ; Nd:YAG Plastics Excimer Ceramics Excimer Surface treatment (metals) CWCO 2 Welding (metals) PCO 2 ; CWCO 2 ; Nd:YAG; Nd:glass; ruby Note: P=pulsed; CW=continuous wave.

32 Electron-Beam Machining High voltage cable (30 kv, DC) Cathode grid Anode Optical viewing system Valve Electron stream Magnetic lens Viewing port Deflection coils Vacuum chamber Work table High vacuum pump FIGURE 9.37 Schematic illustration of the electron-beam machining process. Unlike LBM, this process requires a vacuum, and hence workpiece size is limited by the chamber size.

33 Water-Jet Machining Fluid supply Accumulator Controls Valve Mixer and filter Pump Intensifier Hydraulic unit Sapphire nozzle Jet Drain Control panel y-axis control x-axis control Abrasive-jet head Collection tank FIGURE 9.38 Schematic illustration of water-jet machining. A computer-controlled water-jet cutting machine. (c) Examples of various nonmetallic parts machined by the water-jet cutting process. Source: Courtesy of OMAX Corporation. (c)

34 Abrasive-Jet Machining Filters Powder supply and mixer Exhaust Pressure regulator Hand holder Hood Nozzle Gas supply Vibrator Foot control valve FIGURE 9.39 Schematic illustration of the abrasive-jet machining process. Examples of parts produced by abrasive-jet machining; the parts are 50 mm (2 in.) thick and are made of 304 stainless steel. Source: Courtesy of OMAX Corporation.

35 Design Considerations Poor Sharp corner Good Radius 0.25 mm (0.010 in) or greater Breakaway chipping Undercut 3 mm (1/8 in) wide or greater Backup plate Coolant hole Best Through hole FIGURE 9.40 Design guidelines for internal features, especially as applied to holes. Guidelines for grinding the internal surfaces of holes. These guidelines generally hold for honing as well. The use of a backing plate for producing high-quality through-holes by ultrasonic machining. Source: After J. Bralla.

36 Economic Considerations µm Machining cost (%) Surface finish, R a (µin.) As-cast, sawed, etc. Rough turn Semifinish turn Finish turn Grind Hone FIGURE 9.41 Increase in the cost of machining and finishing operations as a function of the surface finish required. Note the rapid increase associated with finishing operations.

37 Case Study: Stent Manufacture Proximal and distal markers indicate position of stent on radiograph 8 mm 38 mm ( in.) Guide wire mm (0.014 in.) max 2.5 mm 4.0 mm ( in.) Catheter and balloon used for stent expansion Variable Thickness Strut (VTS TM ) Pattern a b Notes: a mm ( in.) section thickness to provide radiopacity b mm ( in.) thickness for flexibility FIGURE 9.42 The Guidant MULTI-LINK TETRA TM coronary stent system. FIGURE 9.43 Detail of the MULTI-LINK TETRA TM pattern. (c) FIGURE 9.44 Evolution of the stent surface. MULTI-LINK TETRA TM after lasing. Note that a metal slug is still attached. After removal of slug. (c) After electropolishing.