Definitions of Twist Drill Terms

Twist drill bits Terms quoted from Machinery's Handbook 27th Edition

  • Axis: The imaginary straight line which 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 bottom 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 which 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 it is the same as for two-flute

  • Point: The cutting end of a drill made up of the ends of the lands, the web, and the lips. In form, it resembles a cone, but departs from a true cone to furnish clearance behind the cutting lips.

  • Point Angle: The angle included between the lips projected upon a plane parallel to the drill axis and parallel to the cutting lips.

  • Shank: The part of the drill by which it is held and driven.

  • Tang: The flattened end of a taper shank, intended to fit into a driving slot in the socket.

  • Tang Drive: Two opposite parallel driving flats on the end of a straight shank.

  • Web: The central portion of the body that joins the end of the lands. The end of the web forms the chisel edge on a two-flute drill.

  • Web Thickness: The thickness of the web at the point unless another specific location is

  • indicated.

  • Web Thinning: The operation of reducing the web thickness at the point to reduce drilling thrust.