Four different types of drilling tools are used in metalcutting applications: solid HSS (high speed steel) twist drills, solid carbide fluted drills, spade drills with inserted HSS and carbide blades, and carbide indexable insert drills. Each type of these drills has advantages and performance benefits. However, this chapter describes drilling only with indexable insert drills.
Drilling tools with carbide indexable inserts can produce holes from a solid at faster rates and lower costs than HSS twist drills and spade drills in many applications. About 60% of all drilling applications in the metalcutting industry are for short holes having depths of up to four times the drill diameters (4:1), and many of these holes can be produced with indexable drills (Ref. 25, p. 1).
Indexable insert drills have been in use for over twenty years. During this time, the following major improvements were implemented to indexable drills (Ref. 26, p. 42):
Modifications in the holder flute design to maximize chip flushing
Higher wear-resistant and stronger cemented carbide inserts
New carbide insert coatings and more efficient insert geometries
A typical drill has two asymmetrically-placed indexable inserts with overlap: the inboard insert cuts through the central portion of a hole from a solid, and the outboard insert does peripheral cutting (Fig. 15). Normally, the inboard insert is made of a tougher carbide grade to better withstand the greater pressure at the center of the drill. The outboard insert is made of a more wear-resistant carbide grade to better withstand...
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