The cutting action of a twist drill is very complex. This paper describes an investigation of this complex phenomenon through use of a quick-stop technique to freeze the action. The formation of chips, both along the cutting edges and under the chisel edge, is considered. Chip-flow angles, measured experimentally, are related to effective rake angle along the cutting edges, using a new derivation of the normal rake angle for a twist drill. The effects of point angle, drill design, and workpiece material are considered. The split (crankshaft) point for heavy web drills is investigated and its operation described.

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