The evils thus pointed out as attending the wear of screw-cutting tools for bolts and nuts, may be overcome by a slight variation in the form of the thread. Thus in Fig. 272, at A is shown a form of thread for the tools to cut internal threads, and at B a form of thread for dies to cut external threads. The sides of the thread are in both cases at the same angle, as say, 60°. The depth of the thread, supposing the angle of the sides to meet in a point, is divided off into 11, or any number of equal divisions. For a tap one of these divisions is taken off, forming a flat top, while at the bottom two of these divisions are taken off, or if desirable, 1-1/2 divisions may be taken off, since the exact amount is not of primary importance. On the external thread cutting tool B, as say a solid die, two divisions are taken off at the largest diameter, and one at the smallest diameter, or, if any other proportion be selected for the tap, the same proportion may be selected for the die, so long as the least is taken off the largest diameter of the tap thread, and of the smallest diameter of the die thread.
The diameter of the tap may still be standard to ring or collar gauge, as in the Franklin Institute thread, the angle at the sides being simply carried in a less distance. In the die the largest diameter of the thread has a flat equal to that on the bottom of the tap, while the smallest diameter has a flat equal to that on the tops of the tap teeth, the width or thickness of the threads remaining the same as in the Franklin Institute thread at each corresponding diameter in its depth.
The effect is to give to the threads on the work a certain amount of clearance at the top and bottom of the thread, leaving the angles just the same as before, and insuring that the contact shall be at the sides, as shown in Fig. 273.
This form of thread retains the valuable features of the Franklin Institute that it can be originated by any one, and that it can be formed with a single-toothed or single-pointed tool. Furthermore, the wear of the threading tools will not impair the diametral fit of the work, while the permissible limit of error in diameter will be increased.
By this means great accuracy in the diameters of the threads is rendered unnecessary, and the wear of the screw-cutting tools at their corners is rendered harmless, nor can any confusion occur, because the tools for external threads cannot be employed upon internal ones. The sides only of the thread will fit, and the whole contact and pressure of the fit will be on those sides only.