Configurations obtained from continuous-path machining are the result of a series of straight-line, parabolic-span or higher-order curves. The degree to which curved surfaces correspond to their design depends on how many lines or spans are used. Four equal chords in a circle describe a square (Figure 32-C). Six make a hexagon, and, as the number increases, the lines themselves come closer to a perfect circle. The number of lines needed is determined by a maximum tolerance allowed between the design of the curved section and the actual chord programmed. This is the dimension T. The program for a parabolic span control unit requires enough spans for any deviation to stay within an acceptable tolerance. For a tolerance of T = 0.001 in., how long should the span be for a curve with a 5-in. radius.? Assume that the arc is part of a circle. What is the span angle here, in degrees?

l Pt. 4 Pt. 3 ; Y axis \ Pt5tOrigin Pt. 2 Pt. 1 End milling cutter 1 diameter X axis FIGURE 32-B Chord or span Arc Arc Span Span angle FIGURE 32-C