Tài liệu 08 grinding & advanced machining processes

ADVANCED MANUFACTURING ENGINEERING 8: Grinding and Advanced Machining Processes December 2016 Machining and finishing processes Grinding machines • Schematic illustration of a horizontal-spindle surface grinder 3 Machining by abrasion • Abrasion: machining allowance is removed in the form of minute and invisible chips by hard, tiny, randomly oriented abrasive grit (bonded or loose) of indefinite number and shape. Voids Wheel speed Bond Abrasives Workpiece Feed 4 Machining by abrasion: Classification Machining by Abrasion Grinding •Surface •Cylindrical Finishing •Honing •Lapping •Superfinishing 5 Grinding machines • Grinding is the removal of metal by a rotating abrasive wheel; the action is similar to that of a milling cutter. Schematic illustration of a horizontal-spindle surface grinder 6 Grinding operations  The wheel is composed of many small grains of abrasive, bonded together, with each grain acting as a miniature cutting tool.  The process produces extremely smooth and accurate finishes. Because only a small amount of material is removed at each pass of the wheel, grinding machines require fine wheel regulation.  The pressure of the wheel against the workpiece can be made very slight, so that grinding can be carried out on fragile materials that cannot be machined by other conventional devices. 7 Grinding operations • Schematic illustrations of surface-grinding operations. • (a) Traverse grinding with a horizontal-spindle surface grinder. • (b) Plunge grinding with a horizontal-spindle surface grinder, producing a groove in the workpiece. • (c) Vertical-spindle rotary-table grinder. 8 Grinding wheel marking • Abrasive materials 1. 2. 3. 4. 5. Aluminum Oxide (Al2O3) Silicon Carbide (SiC) Boron Carbide B4C Boron Nitride (CBN) Diamond • Grain size • • • Coarse: 10, 12, 14, 16, 20, 24 Medium: 30, 36, 46, 56, 60 Fine: 70, 80, 90, 100, 150, 180, 200, 220, 240, 280, 320, 400, 500, 600 9 Grinding wheel marking • Wheel bond • Vitrified (V) • Resinoid (B) • Silicate (S) • Rubber (R) • Shellac (E) • Oxychloride (O) • Metal  Wheel grade  Very soft: Soft: Medium: Hard: Very hard:     C, D, E, F, and G H, I, J, K L, M, N, O P, Q, R, S T, U, V, X, Y, Z Soft grades are, generally, used for machining hard materials  Hard wheels are used for soft materials  10 Wheel Structure • Open structure wheels are used for high material removal rates and consequently produce a rough surface finish. • Dense structures are used for precision form grinding operations. Very compact: Compact: Semi-compact: Porous: Very porous: Extra porous 1, 2 3, 4 5, 6 7, 8 9, 10 11 to 15 Wheel shape 12 Wheel selection The selection of a grinding wheel for a particular operation depends on:• Material to be machined. • Accuracy and surface finish. • Machining variables. • Grinding machine condition. 13 Truing and dressing • Dressing clears the cutting surface of the wheel of any dull grits, and embedded swarf in order to improve the cutting action. • Truing is employed to remove material from the cutting surface to bring the wheel to the required geometric shape. It also restores the cutting action of a worn wheel as in the case of dressing. Grinding wheel Tool holder 3o – 15o Tool holder 30o Grinding wheel 14 Wheel balancing Grinding operations Schematic illustrations of internal-grinding operations external cylindrical grinding 16 Face Surface Grinding with Reciprocating Feed vg vg St e dg vw bw l External Cylindrical Grinding Grinding wheel ag dg, vg St dw, vw Workpiece Cylindrical Plunge Grinding Grinding wheel dg, vg Infeed dw, vw Workpiece Internal Grinding with Longitudinal Feed vw St ag Workpiece l Grinding wheel