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drill,tool used to create a hole, usually in some hard substance, by its rotary or hammering action. Many different tools make up the drill family. The awl is a pointed instrument used for piercing small holes. In its early form it was a thorn or a tool of bone or chipped flint; many prehistoric awls of flint have been found. The gimlet, which consists of a cross handle holding a metal shaft with a screw point, is used for boring small holes. Its principle and that of a similar tool, the auger, were known and used in ancient times, but the tools were much improved in the Middle Ages. The auger has a cross handle containing a shaft that usually ends with a central screw-shaped point; the point acts to pull two knife edges about it into the material to do the actual cutting. Spiral channels extend part way up the shaft to allow chips to be removed from the hole. The term auger is also applied to various augerlike tools. More elaborate types of drills are composed of two main parts: a replaceable device that does the actual cutting, called a bit, and a second device that drives it. Both devices may be referred to as drills. Two common types of rotary bits are the auger and the twist bit. Hand-powered rotary driving devices include the hand drill, which has a crank that transmits turning power to the bit through a gear and pinion, and the bit brace, which is a bow-shaped device that is rotated to turn a bit. Motor-powered rotary driving devices include the drill press and the portable electric drill, both used at home or in industry for cutting holes in such materials as metal, wood, and plastic. The core drill cuts an annular hole through minerals. For a small hole the drill bit may contain diamonds, which bore a hole by their abrasive action when they are rotated. A rotary oil well drill uses a bit containing either rolling cutters with hard teeth or a fixed, chisel cutting edge. The percussive types of drill force a bit to move forward by a hammering action that chips away material, instead of cutting or abrading it. For example, an air percussion drill chips through rock with a cross-shaped bit driven by a piston; the similar churn drill is lifted and dropped by cable. One special kind of drill, the fusion drill, creates a hole by melting or flaking minerals with an oxyacetylene torch. An agricultural implement for planting certain seeds or for placing fertilizer in the soil is called a drill. Small drills for gardens are pushed by hand; large drills for field work are drawn by horses or tractors. The agricultural drill is named for its special uses, e.g., the grass drill and the grain drill, or for its construction, as the disk drill and the hoe drill. The fertilizer drill is commonly combined with the seed drill.
drill,monkey: see mandrillmandrill,
large monkey, Mandrillus sphinx, of central W Africa, related to the baboons. Mandrills are found in forests, while baboons live in open country. The fur of the mandrill is mostly dark brown, but the bare areas—face and buttocks—are patterned in
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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.