Machinery's Handbook, 31st Edition
1478 ELECTRICAL DISCHARGE MACHINING Compressed air can be used to flush out the graphite dust from blind holes, for instance, but provision must be made for vacuum removal of the dust to avoid hazards to health and problems with wear caused by the hard, sharp-edged particles. Air velocities of at least 500 ft/min (152 m/min) are recommended for flushing, and of 2000 ft/min (610 m/min) in collector ducts to prevent settling out. Fluids can also be used, but small-pore filters are needed to keep the fluid clean. High-strength graphite can be clamped or chucked tightly but care must be taken to avoid crushing. Collets are preferred for turning because of the uniform pressure they apply to the workpiece. Sharp corners on electrodes made from less dense graphite are liable to chip or break away during machining. For conventional machining of graphite, tools of high-quality tungsten carbide or polycrystaline diamond are preferred and must be kept sharp. Recommended cutting speeds for high-speed steel tools are 100–300 ft/min (30–91 m/min), tungsten carbide 500–750 ft/min (152–229 m/min), and polycrystaline diamond, 500–2000 surface ft/min (152–610 m/min). Tools for turning should have positive rake angles and nose radii of 1 ∕ 64 to 1 ∕ 32 inch. Depths of cut of 0.015 to 0.020 inch produce a better finish than light cuts such as 0.005 inch because of the tendency of graphite to chip away rather than flow across the tool face. Low feed rates of 0.005 inch/rev for rough- and 0.001 to 0.003 inch/rev for finish-turning are preferred. Cutting off is best done with a tool having an angle of 20 ° . For bandsawing graphite, standard carbon steel blades can be run at 2100–3100 surface ft/min (640–945 m/min). Use low power feed rates to avoid overloading the teeth and the feed rate should be adjusted until the saw has a very slight speed up at the breakthrough point. Milling operations require rigid machines, short tool extensions, and firm clamp - ing of parts. Milling cutters will chip the exit side of the cut, but chipping can be reduced by use of sharp tools, positive rake angles, and low feed rates to reduce tool pressure. Feed/tooth for two-flute end mills is 0.003 to 0.005 inch (0.076 to 0.13 mm) for roughing and 0.001 to 0.003 inch (0.025 to 0.076 mm) for finishing. Standard high-speed steel drills can be used for drilling holes but will wear rapidly, causing holes that are tapered or undersized, or both. High-spiral, tungsten carbide drills should be used for large numbers of holes over 1 ∕ 16 inch (1.59 mm) diameter, but diamond-tipped drills will last longer. Pecking cycles should be used to clear dust from the holes. Compressed air can be passed through drills with through coolant holes to clear dust. Feed rates for drilling are 0.0015 to 0.002 inch/rev for drills up to 1 ∕ 32 , 0.001 to 0.003 inch/rev for 1 ∕ 32 - to 1 ∕ 8 -inch drills, and 0.002 to 0.005 inch/rev for larger drills. Standard taps without fluid are best used for through holes, and for blind holes, tapping should be completed as far as possible with a taper tap before the bottoming tap is used. For surface grinding of graphite, a medium (60) grade, medium-open structure, vitreous- bond, green-grit, silicon-carbide wheel is most commonly used. The wheel speed should be 5300–6000 surface ft/min(1615–1829 m/min), with traversing feed rates at about 56 ft/min (17.1 m/min). Roughing cuts are taken at 0.005 to 0.010 inch/pass (0.13– 0.25 mm/pass), and finishing cuts at 0.001 to 0.003 inch/pass (0.025–0.076 mm/pass). Surface finishes in the range of 18 to 32 μ inch (0.457–0.813 micron) Ra are normal, and can be improved by longer spark-out times and finer grit wheels, or by lapping. Graphite can be centerless ground using a silicon-carbide, resinoid-bond work wheel and a regulating wheel speed of 195 ft/min (59.4 m/min). Wire EDM, orbital abrading, and ultrasonic machining are also used to shape graphite electrodes. Orbital abrading uses a die containing hard particles to remove graphite, and can produce a fine surface finish. In ultrasonic machining, a water-based abrasive slurry is pumped between the die attached to the ultrasonic transducer and the graphite workpiece on the machine table. Ultrasonic machining is rapid and can reproduce small details down to 0.002 inch (0.05 mm) in size, with surface finishes down to 8 μ in (0.203 micron) Ra. If coolants are used, the graphite should be dried for 1 hour at over 400 ° F (204 ° C) (but not in a microwave oven) to remove liquids before use.
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