Straight Flute and Twist Drills

Abstract

This chapter discusses classification, geometry, and design of straight flute and twist drills. It argues that the design, manufacturing, and implementation practices of drills are lagging behind the achievements in the tool materials, powerful high-speed-spindles rigid machines, and high-pressure MWF (coolant) supply. Although the wide availability CAD design tool and CNC precision grinding machines make it possible to reproduce any drill geometry, have not many new drill designs become available recently. The chapter points out that the prime objective of the drilling system is an increase in the drill penetration rate, i.e., in drilling productivity as the prime source for potential cost savings. As the major problem is in understanding particularities of drill geometry and its components, this chapter walks the reader from simple concepts starting from the basic terminology in drill design and geometry to the most complicated concepts in the field, keeping the context to the simplest possible fashion and providing practical examples. It provides an overview of important results concerning drill geometry and synthesizes the most relevant findings in the field with the practice of tool design. Geometry of Single-point Turning Tools and DrillsChisel edge angle -the angle included between the chisel edge and the cutting lip, as viewed from the end of the drill. Clearance -the space provided to eliminate undesirable contact (interference) between the drill and the workpiece. Cutter sweep -the section formed by the tool used to generate the flute in leaving the flute. Cutting tooth -a part of the body bounded by the rake and flank surfaces and by the land. Double margin drill -a drill whose body diameter clearance is produced to leave two margins on each land and is normally made with margins on the leading edge and on the heel of the land. Drill diameter -The diameter over the margins of the drill measured at the periphery corners. Flute length -the length from the periphery corner of the lips to the extreme back end of the flutes. It includes the sweep of the tool used to generate the flutes and, therefore, does not indicate the usable length of flutes. 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.Galling -an adhering deposit of nascent work material on the margin adjacent to the periphery corned of the cutting edge. Hill -the trailing edge of the land. Helix angle -the angle made by the leading edge of the land with the plane containing the axis of the drill. Land -the peripheral portion of the cutting tooth and drill body between adjacent flutes. Land clearance -see preferred term body diameter clearance. Land width -the distance between the leading edge and the heel of the land measured at right angles to the leading edge.