Disk-and-Square Method of Determining Angles
The method shown in [Fig. 15] for determining angles for setting up work on a milling machine or planer, possesses several advantages. No expensive tools are required, the method can be applied quickly, and the results obtained are quite accurate enough for any but the most exacting requirements. As will be seen, an ordinary combination square is used in connection with a disk, the head of the square being set at different points on the blade according to the angle that is desired. Theoretically, a one-inch disk could be used for all angles from about 6 degrees up to a right angle, but in practice it is more convenient and accurate to employ larger disks for the larger angles.
The only inaccuracy resulting from this method is due to setting the square at the nearest “scale fraction” instead of at the exact point determined by calculation. This error is very small, however, and is negligible in practically all cases. The dimension x required for any desired angle a can be found by multiplying the radius of the disk, by the cotangent of one-half the desired angle, and adding to this product the radius of the disk.
Example: The square blade is to be set to an angle of 15 degrees 10 minutes, using a 2-inch disk. At what distance x ([see Fig. 15]) should the head of the square be set?
Cot 7 degrees 35 minutes = 7.5113,
and 7.5113 × 1 + 1 = 8.5113 inches.
By setting the square to 8½ inches “full,” the blade would be set very close to the required angle of 15 degrees 10 minutes.