ills is 118, which is appropriate for drilling low-carbon steels and cast irons. For harder and tougher metals, such as hardened steel, brass and bronze, larger TAPs (130or 140) give better performance. The helix angle of the flutes of monly used twist drills ranges between 24 and 30 . When drilling copper or soft plastics, higher values for the helix angle are mended (between35 and 45).РFig.4.2 The core drillРCore drills. A core drill consists of the chamfer, body, neck, and shank, as shown in Fig.4.2.This type of drill may have either three or four flutes and an equal number of margins, which ensure superior guidance, thus resulting in high machining accuracy. It can also be seen in Fig.4.2 that a core drill has flat end. The chamfer can have three or four cutting edges, or lips, and the
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