In the case of the teeth on dies and chasers, the wear at the corners c d, in [Fig. 266], is the greatest. Now, the tops of the teeth on the tap (a b, in [Fig. 265]) cut the bottom or full diameter of the thread in the nut, while the tops of the teeth (c d, in [Fig. 266]) in the die cut the bottom of the thread on the bolt; hence the rounded corners cut on the work by the tops of the teeth in the one case, meet the more square corners left by the tops of the teeth in the other, and providing that under these circumstances the thread in the nut were of equal diameter to that on the bolt the latter would not enter the former.
Fig. 267.
If the bolt were made of a diameter to enable the nut to wind a close fit upon the bolt, the corners only of the threads would fit, as shown in [Fig. 267], which represents at n a thread in a portion of a nut and at s a portion of a thread upon a tap or bolt, the two threads being magnified and shown slightly apart for clearness of illustration. The corners a, b of the nut are then cut by the corners a b of the tap in [Fig. 265], and the corners c, c, d correspond to those cut by the corners c, d of the die teeth in [Fig. 266]; corners e, f, [Fig. 267], are cut by corners c, d, in [Fig. 266], and corners g, h are cut by corners g, h in [Fig. 266], and it is obvious that the roundness of the corners a, b, c, and d in [Fig. 267] will not permit the tops of the thread on the bolt to meet the bottoms of the thread in the nut, but that the threads will bear at the corners only.
So far, however, we have only considered the wear tending to round off the sharp corners of the teeth, which wear is greater in proportion as the corners are sharp, and less as they are rounded or flattened, and we have to consider the wear as affecting the diameters of the male and female thread at their tops and bottoms respectively.
Now, since the tops of the tap teeth wear the most, the diameter of the thread decreases in depth, while, since the tops of the die teeth wear most, the depth of the thread in the die also decreases. The tops of the tap teeth cut the bottom of the thread in the nut and the tops of the die teeth cut the bottoms of the thread upon the bolt.
Fig. 268.