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a cutting tool for making holes by drilling or for enlarging the diameter of a hole by boring. In metalworking, drills are classified by design and purpose as general-purpose twist drills, drills with one or two cutting lips for producing deep holes, and center drills designed for making center marks.
The twist drill (Figure l) is the most commonly used type of drill. It is a rod with cutting elements (lips) along the body, secondary cutting edges (margins), a chisel edge, and a shank, by means of which the drill is attached to a machine spindle, chuck, or drill head. The body is made with a uniform inverse taper of 0.03 to 0.12 mm per 100 mm of drill length. There are specially designed drills made without a chisel edge, with a special point, or with chip-separating flutes. Standard twist drills range from 0.25 to 80 mm in diameter.
Five different forms of points are used (Figure 2), depending on the properties of the material to be worked, the cutting operation, and the drill material. The principal standardized geometric parameters for twist drills are shown in Figure 3. They include the helix angle ω, the point angle 2Φ, the chisel-edge angle ψ, the clearance angle α, and the rake angle γ. The following specifications have been adopted for the entire range of drill diameters: ω = 18°-30°, 2Φ = 80°-140°, ψ = 47°-55°, and α = 8°–14°. The rake angle γ is determined from the formula
where dp is the diameter of the cutting end at the point for which the angle is determined. The cutting ends of drills are made of high-speed steels and hard alloys or composite materials. Shanks are made of steel grades 45 and 40Kh for cutting ends made of high-speed steel and steel grades KhS, 40Kh, or 45Kh for cutting ends made of hard alloys or composite materials.
D. L. IUDIN
Woodworking uses twist drills with guide centers and rippers, crown saws, and hollow drills with rippers, in addition to twist drills with conical points. Twist drills are the most commonly used type. For twist drills, ω = 22°-30°, 2Φ = 120° for
drilling perpendicular to the grain and 60°-80° for drilling with the grain, and α = 20°-30°. In order to reduce the cutting forces of twist drills equipped with a guide center and rippers, the height of the rippers h is assumed to be no more than the maximum feed. The height h is usually 0.8–2 mm, and the height of the guide center is 3.5–8.5 mm, and the height of the guide center is 3.5–8.5 mm.
Drills are made of tool steel grade Kh6VF high-speed steel grade R6M5. Drills equipped with blades and bits made of hard alloys are used for drilling chipboard, fiberboard, plywood, and other wood materials.
V. S. RYBALKO
REFERENCESGrube, A. E. Derevorezhushchie instrumenty, 3rd ed. Moscow, 1971. See also references under METALCUTTING TOOLS.
a training subject for military personnel and subunits. Drill is designed to instill in personnel a model appearance, precision, quick response, agility, and endurance; it also develops the ability to carry out commands correctly and quickly and to perform drill movements with or without a weapon. Drill is used to train subunits in coordinated actions in various formations and in precombat procedures under combat conditions. It is the basis for successful tactical training of individual soldiers and subunits. It includes individual drill and the drilling and coordination of subunits in unmounted formations and in vehicles. Drill is conducted in special exercises as well as in the everyday life of enlisted personnel and commissioned officers according to the requirements of the Drill Manual.