Page:Practical Treatise on Milling and Milling Machines.djvu/70

64 Position of the Table in Cutting Spirals. The change gears having been selected, the next step in cutting spirals is to determine the position at which the table must be placed to bring the spiral in line with the cutter as the work is being milled.

The correct position of the table is indicated by the angle shown at A, Fig. 13, and this angle, as may be noticed from that figure, has the same number of degrees as the angle B, which is temred the angle of the spiral, and is formed by the intersection of teh spiral and a line parallel with the axis of the piece being milled. The reason the angles A and B are alike, is that their corresponding sides are perpendicular to each other.

Fig. 14

The angle of the spiral depends upon the lead of the spiral and the diameter of the piece to be milled. The greater the lead of a spiral of any given diameter, the smaller the angle, and the greater the diameter of any spiral with a given lead, the greater the spiral angle.

If the angle wanted is not found in the tables on pages 226 to 228, it can be ascertained in two ways, graphically or more conveniently, by a simple calculation and reference to the tables on pages 309 to 317. In determining it graphically, a right-angle triangle is drawn to scale.