Machinery's Handbook, 31st Edition
Twist Drills
931
TWIST DRILLS AND COUNTERBORES Twist drills are rotary end-cutting tools having one or more cutting lips and one or more straight or helical flutes for the passage of chips and cutting fluids. Twist drills are made with straight or tapered shanks, but most have straight shanks. All but the smaller sizes are ground with “back taper,” reducing the diameter from the point toward the shank, to prevent binding in the hole when the drill is worn. Straight-Shank Drills: Straight-shank drills have cylindrical shanks that may be of the same or of a different diameter than the body diameter of the drill and may be made with or without driving flats, tang, or grooves. Taper-Shank Drills: Taper-shank drills are preferable to the straight-shank type for drilling medium- and large-size holes. The taper on the shank conforms to one of the tapers in the American Standard (Morse) Series. American National Standard.— American National Standard B94.11M-1993 covers nomenclature, definitions, sizes and tolerances for High-Speed Steel Straight- and Taper- Shank Drills and Combined Drills and Countersinks, Plain and Bell types. It covers both inch and metric sizes. Dimensional tables from the Standard will be found on the following pages. Definitions of Twist Drill Terms.— The following definitions are included in the Standard. Axis: The imaginary straight line that forms the longitudinal center of the drill. Back Taper: A slight decrease in diameter from point to back in the body of the drill. Body: The portion of the drill extending from the shank or neck to the outer corners of the cutting lips. Body Diameter Clearance: That portion of the land that has been cut away so it will not rub against the wall of the hole. Chisel Edge: The edge at the ends of the web that connects the cutting lips. Chisel Edge Angle: The angle included between the chisel edge and the cutting lip as viewed from the end of the drill. Clearance Diameter: The diameter over the cutaway portion of the drill lands. Drill Diameter: The diameter over the margins of the drill measured at the point. Flutes: Helical or straight grooves cut or formed in the body of the drill to provide cutting lips, to permit removal of chips, and to allow cutting fluid to reach the cutting lips. Helix Angle: The angle made by the leading edge of the land with a plane containing the axis of the drill. Land: The peripheral portion of the drill body between adjacent flutes. Land Width: The distance between the leading edge and the heel of the land measured at a right angle to the leading edge. Lips—Two-Flute Drill: The cutting edges extending from the chisel edge to the periphery. Lips—Three- or Four-Flute Drill (Core Drill): The cutting edges extending from the bot tom of the chamfer to the periphery. Lip Relief: The axial relief on the drill point. Lip Relief Angle: The axial relief angle at the outer corner of the lip. It is measured by projection into a plane tangent to the periphery at the outer corner of the lip. (Lip relief angle is usually measured across the margin of the twist drill.) Margin: The cylindrical portion of the land that is not cut away to provide clearance. Neck: The section of reduced diameter between the body and the shank of a drill. Overall Length: The length from the extreme end of the shank to the outer corners of the cutting lips. It does not include the conical-shank end often used on straight-shank drills, nor does it include the conical cutting point used on both straight- and taper-shank drills. (For core drills with an external center on the cutting end, the overall length is the same as for two-flute drills. For core drills with an internal center on the cutting end, the overall length is to the extreme ends of the tool.)
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