The earliest attempts at prediction in three-dimensional machining concentrated on hc. Stabler (1951) suggested that hc should equal the cutting edge inclination angle ls (defined in Figure 2.2 and more rigorously in Section 6.4.2); this is a first approximation. As seen later, it is not well supported by experiment. A better idea, based on geometry and due to
Colwell (1954), is that, in a view normal to Uwork, the chip will flow at right angles to the line AD joining the extremities of the cutting edge engagement (Figure 6.14(a)).