Machinery's Handbook, 31st Edition
MACHINING MAGNESIUM 1257 Machining Magnesium.— Magnesium alloys are readily machined and with relatively low power consumption per cubic inch of metal removed. The usual practice is to employ high cutting speeds with relatively coarse feeds and deep cuts. Exceptionally fine finishes can be obtained so that grinding to improve the finish usually is unnecessary. The horse power normally required in machining magnesium varies from 0.15 to 0.30 hp per cubic inch per minute (0.41-0.82 kW/cm 3 /s). While this value is low, especially in comparison with power required for cast iron and steel, the total amount of power for machining mag nesium usually is high because of the exceptionally rapid rate at which metal is removed. Carbide tools are recommended for maximum efficiency, although high-speed steel fre quently is employed. Tools should be designed so as to dispose of chips readily or without excessive friction, by employing polished chip-bearing surfaces, ample chip spaces, large clearances, and small contact areas. Keen-edged tools should always be used. Speeds and Feeds for Magnesium: Speeds ordinarily range up to 5000 ft/min (1524 m/ min) for rough- and finish-turning, up to 3000 ft/min (915 m/min) for rough-milling, and up to 9000 ft/min (2743 m/min) for finish-milling. The following combinations of speed, feed per revolution, and depth of cut are recommended: Process Speed Feed Depth of Cut ft/min m/min in/rev mm/rev inch mm
300–600 600–1000 1000–1500 1500–2000 2000–5000 up to 900 900–1500
90–185 185–305 305–460 460–610 610–1525 up to 275 275–455 455–915
0.030–0.100 0.020–0.080 0.010–0.060 0.010–0.040 0.010–0.030 0.005–0.025 0.005–0.020 0.005–0.010
0.76–2.5 0.50–2.0 0.25–1.5 0.25–1.0 0.25–0.76 0.13–0.64 0.13–0.51 0.13–0.25
0.5 0.4 0.3 0.2 0.1
12.7
10
Rough Turning
7.6 5.0 3.8
up to 0.5
up to 12.7 up to 9.5
Rough Milling
up to 0.375
1500–3000 up to 5.1 Lathe Tool Angles for Magnesium: The true or actual rake angle resulting from back and side rakes usually varies from 10 to 15 degrees. Back rake varies from 10 to 20, and side rake from 0 to 10 degrees. Reduced back rake may be employed to obtain better chip breakage. The back rake may also be reduced to from 2 to 8 degrees on form tools or other broad tools to prevent chatter. up to 0.2 Parting Tools: For parting tools, the back rake varies from 15 to 20 degrees, the front end relief 8 to 10 degrees, the side relief measured perpendicular to the top face 8 degrees, the side relief measured in the plane of the top face from 3 to 5 degrees. Milling Magnesium: In general, the coarse-tooth type of cutter is recommended. The number of teeth or cutting blades may be one-third to one-half the number normally used; however, the two-blade fly-cutter has proved to be very satisfactory. As a rule, the land relief or primary peripheral clearance is 10 degrees followed by secondary clearance of 20 degrees. The lands should be narrow, the width being about 3 ∕ 64 to 1 ∕ 16 inch (1.19 to 1.59 mm). The rake, which is positive, is about 15 degrees. Drilling Magnesium: If the depth of a hole is less than five times the drill diameter, an ordinary twist drill with highly polished flutes may be used. The included angle of the point may vary from 70 degrees to the usual angle of 118 degrees. The relief angle is about 12 degrees. The drill should be kept sharp and the outer corners rounded to produce a smooth finish and prevent burr formation. For deep hole drilling, use a drill having a helix angle of 40 to 45 degrees with large polished flutes of uniform cross section throughout the drill length to facilitate the flow of chips. A pyramid-shaped “spur” or “pilot point” at the tip of the drill will reduce the “spiraling or run-off.” Drilling speeds vary from 300 to 2000 feet per minute (90–610 m/min) with feeds per revolution ranging from 0.015 to 0.050 inch (0.38–1.27 mm). Reaming Magnesium: Reamers up to 1 inch in diameter should have four flutes; larger sizes, six flutes. These flutes may be either parallel with the axis or have a negative helix
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