Machinery's Handbook, 31st Edition
Mechanical Erosion Processes 1345 thicker metals in excess of 12 in. (305 mm) thick can be cut slowly. Supersonic water jets pull in metered, abrasive particles just after the orifice, and this solution travels down the mixing tube to the nozzle outlet. The abrasive material often is garnet or similar material, with a particle size selected according to the nozzle opening, workpiece, and process. Grit sizes commonly used range from 50 mesh to 220 mesh (higher numbers indicate finer particles) in the Tyler Mesh Sizes (see Meshes, Sieves, and Screens , beginning on page 1068). An 80-mesh sieve size (0.007 in. or 0.177 mm) is most commonly used. The orifice and mixing tube combination determines the abrasive water jet stream diameter, which can be as small as 0.015 in. (0.38 mm), though 0.03–0.04 in. (0.76–1 mm) is more common. Nozzle mixing tubes wear rapidly with use; this can affect jet size and location. Pure Water Jet Machining: Use of this process is limited to low-strength materials such as foam, fabric, and some plastics. The food industry often uses pure water jet cutting because it can be run as a sanitary process. As the jet contains no particles, very small jet sizes are possible. Typical orifices range from 0.003 in. to 0.015 in. (0.08 mm to 0.38 mm). Ultrasonic Machining (USM).— Also referred to as ultrasonic vibration machining, this abrasive method makes cuts in material or changes its surface texture using a transducer that vibrates a tool at low amplitude (25 to 100 microns) and high frequency (15 to 30 kHz) and in a slurry containing fine abrasive particles. The vibrations force the particles against the mate - rial surface, removing material to the desired depth. For further information, see page 1260. Abrasive Flow Machining (AFM).— Often used for deburring, smoothing, or polishing an interior part surface, but also applicable to final shaping, breakthrough, and drilling holes, this process forces a viscous fluid, containing abrasive material, through a workpiece. Magnetic Abrasive Finishing (MAF).— This finishing process can be used to create a high surface quality, even on freeform parts. It involves adding micron-sized iron particu- lates to a flexible magnetic abrasive brush that can easily deform, based on the magnetic field strength, matching and honing a part, regardless of the complexity of the shape. Electro-Thermal Processes These processes use melting, burning, or vaporization to remove material from a workpiece. Included are electrical discharge machining (EDM), laser beam machining (LBM), plasma arc cutting (PAM), and electron beam machining (EBM). The heat of these cutting methods can cause distortion, discoloration, changes in surface hardness or composition, and introduction of residual stresses in the workpiece. To specify and evaluate quality and tolerances of cuts made with oxyfuel flame systems, plasma arc systems, and laser cutting systems, refer to ISO 9013, “Classification of ther - mal cuts – Geometrical product specification and quality tolerances.” Below are the most commonly used electro-thermal processes. Additional information on some of these pro- cesses is included in the ELECTRICAL DISCHARGE MACHINING on page 1469 and the WELDING section, starting on page 1575. Electrical Discharge Machining (EDM).— This process vaporizes conductive mate- rials in contact with an electrode in a dielectric fluid bath. Both through-cutting wire systems and three-dimensional die-sinking systems are widely available and highly ac- curate. Refer to Table 1 for some typical performance characteristics of EDM processes. For detailed information, see page 1471. Laser Beam Machining (LBM).— Lasers can cut both metals and non-metals through a process of vaporization, fusion (melt and blow), burning, or thermal stress cracking. Shal- low engraving, blind features, and through cuts are all possible. Lasers can cut delicate parts, since there is no contact; however, laser processes generally use gas jets to create a reactive environment or eject material, and that may produce some pressure on the part in the area of the cut. Material, geometry, and type of cut determine the best type of laser to use. CO 2 (gas) laser systems and Nd:YAG (rod crystal, solid state) laser systems have been used in
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