Machinery's Handbook, 31st Edition
Cutter Grinding 901 purpose cutters over 3 inches (76.2 mm) in diameter should be provided with a clearance angle of 4 to 5 degrees. The land width is usually 1 ⁄ 64 , 1 ⁄ 32 , and 1 ⁄ 16 inch (0.4, 0.8, and 1.6 mm), respectively, for small, medium, and large cutters. The primary clearance or relief angle for best results varies according to the material being milled, as follows: low-carbon, high-carbon, and alloy steels, 3 to 5 degrees; cast iron and medium and hard bronze, 4 to 7 degrees; brass, soft bronze, aluminum, magnesium, plastics, etc., 10 to 12 degrees. When milling cutters are resharpened, it is customary to grind a secondary clearance angle of 3 to 5 degrees behind the primary clearance angle to reduce the land width to its original value and thus avoid interference with the surface to be milled. The following is a general formula for plain milling cutters, face mills, and form relieved cutters, which gives the clearance angle C , in degrees, necessitated by the feed per revolu tion F , in inches; the width of land L , in inches; the depth of cut d, in inches (mm); the cutter diameter D , in inches; and the Brinell Hardness Number B of the work being cut: C DB 45860 1.5 L D F d D d π = + − a ^ h k Rake Angles for Milling Cutters.— In peripheral milling cutters, the rake angle is gener ally defined as the angle in degrees that the tooth face deviates from a radial line to the cutting edge. In face milling cutters, the teeth are inclined with respect to both the radial and axial lines. These angles are called radial and axial rake, respectively. The radial and axial rake angles may be positive, zero, or negative. Positive rake angles should be used whenever possible for all types of high-speed steel milling cutters. For sintered carbide-tipped cutters, zero and negative rake angles are fre quently employed to provide more material back of the cutting edge to resist shock loads. Rake Angles for High-Speed Steel Cutters: Positive rake angles of 10 to 15 degrees are satisfactory for milling steels of various compositions with plain milling cutters. For softer materials, such as magnesium and aluminum alloys, the rake angle may be 25 degrees or more. Metal slitting saws for cutting alloy steel usually have rake angles from 5 to 10 degrees, whereas zero and sometimes negative rake angles are used for saws to cut copper and other soft nonferrous metals to reduce the tendency to “hog in.” Form relieved cutters usually have rake angles of 0, 5, or 10 degrees. Commercial face milling cutters usually have 10 degrees positive radial and axial rake angles for general use in milling cast iron, forged and alloy steel, brass, and bronze. For milling castings and forgings of magnesium and free-cutting aluminum and their alloys, the rake angles may be increased to 25 degrees positive or more, depending on the operating conditions; a smaller rake angle is used for abrasive or difficult to machine aluminum alloys. Cast Nonferrous Tool Material Milling Cutters: Positive rake angles are generally pro vided on milling cutters using cast nonferrous tool materials, although negative rake angles may be used advantageously for some operations such as those where shock loads are encountered or where it is necessary to eliminate vibration when milling thin sections. Sintered Carbide Milling Cutters: Peripheral milling cutters, such as slab mills, slotting cutters, saws, etc., tipped with sintered carbide, generally have negative radial rake angles of 5 degrees for soft low-carbon steel and 10 degrees or more for alloy steels. Positive axial rake angles of 5 and 10 degrees, respectively, may be provided, and for slotting saws and cutters, 0 degree axial rake may be used. On soft materials, such as free-cutting aluminum alloys, positive rake angles of 10 to so degrees are used. For milling abrasive or difficult to machine aluminum alloys, small positive or even negative rake angles are used. Eccentric-Type Radial Relief.— When the radial relief angles on peripheral teeth of mill ing cutters are ground with a disc-type grinding wheel in the conventional manner, the ground surfaces on the lands are slightly concave, conforming approximately to the ra- dius of the wheel. A flat land is produced when the radial relief angle is ground with a cup wheel. Another entirely different method of grinding the radial angle is by the eccentric method, which produces a slightly convex surface on the land. If the radial relief angle at
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