Multipoint Tool Operations
Milling. Removal rate is high with correct cutter design, speed and feed, machine rigidity, and power. When cutting speeds are high, the heat developed is retained mostly in the chips, with the balance absorbed by the coolant. Speeds are high with cutters of high-speed and cast alloys and very high with sintered carbide cutters. All common types of solid-tooth, high-carbon, or high-speed steel cutters can be employed. High-carbon cutters operating at a maximum edge temperature of 400F are preferred for short-run production. For long runs, high-speed steel or inserted-tooth cutters are used. Speeds of 15,000 fpm (5000 mpm) are not uncommon for carbide cutters. Maximum speeds for high-speed and high-carbon steel cutters are around 5000 and 600 fpm (1650 and 200 mpm), respectively.
Drilling. General-purpose drills with bright finishes are satisfactory for use on aluminum. Better results may be obtained with drills having a high helix angle. Flute areas should be large; the point angle should be 118 (130–140 for deeper holes). Cutting lips should be equal in size. Lip relief angles are between 12 and 20, increasing toward the center to hold the chisel angle between 130 and 145. No set rule can be given for achieving the correct web thickness. Generally, for aluminum, it may be thinner at the point without tool breakage. A 1Xs-Hi drill at 6000 rpm has a peripheral speed of 2000 fpm (680 mpm). For drilling aluminum, machines are available with speeds up to 80,000 rpm. If excessive heat is generated, hold diameter may be reduced even below drill size. With proper drills, feeds, speeds, and lubrication, no heat problem should occur.
For a feed of 0.008 inches per rotation (ipr) and a depth-to-diameter ratio of 41, the thrust value is 170 lb and the torque value is 10 lb-in. for a 1A-Ui drill with alloy 6061- T651. Aluminum alloys can be counterbored, tapped, threaded by cutting or rolling, and broached. Machining fluid should be used copiously.
Grinding. Resin-bounded silicon carbide wheels of medium hardness are used for rough grinding of aluminum. Finish grinding requires softer, vitrified-bonded wheels. Wheel speeds can vary from 5500 to 6000 fpm (1800–2000 mpm). Abrasive belt grinding employs belt speeds from 4600 to 5000 square feet per minute (sfpm). Grain size of silicon carbide abrasive varies from 36 to 80 for rough cuts and from 120 to 180 for finishing cuts. For contact wheel abrasive belt grinding, speeds are 4500–6500 sfpm. Silicon carbide or aluminum oxide belts (24–80 grit) are used for rough cuts.
Sawing, Shearing, Routing, and Arc Cutting Aluminum. Correct tooth contour is most important in circular sawing. The preferred saw blade has an alternate hollow ground side— rake teeth at about 15. Operating speeds are 4000–15,000 fpm (1300–5000 mpm). Lower speeds are recommended for semi-high-speed steel, intermediate speeds for high-speed inserted- tooth steel blades, and high speeds for carbide-tipped blades. Band-sawing speeds should be between 2000 and 5000 fpm (655–1640 mpm). Spring-tempered blades are recommended for sheet and soft blades with hardened teeth for plate. Tooth pitch should not
exceed material thickness: four to five teeth to the inch for spring tempered, six to eight teeth to the inch for flexible backed. Contour sawing is readily carried out. Lubricant should be applied to the back of the blade. Shearing of sheet may be done on guillotine shears. The clearance between blades is generally 10–12% of sheet thickness down to 5–6% for light gauge soft alloy sheet. Hold-down pads, shear beds, and tables should be covered to prevent marring. Routing can also be used with 0.188–0.50 in. (4.8–12.7 mm) material routed at feeds of 10–30 inches per minute (ipm) [25–75 centimeters per minute (cmpm)]. Heat-treated
plates up to 3 in. (7.6 cm) in thickness material can be routed at feeds up to 10 ipm (25 cmpm). Chipless machining of aluminum can be carried out using shear spinning rotary swaging, internal swaging, thread rolling, and flame cutting.
J. G. Kaufman
Kaufman Associates
Columbus, Ohio
Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright 2006 by John Wiley & Sons, Inc.
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