Let it be supposed then that the points of the teeth of a tap have worn off to a depth of the 1⁄2000th part of an inch, which they will by the time they become sufficiently dulled to require resharpening, and that the teeth of a die have become reduced by wear by the same amount, and the result will be the production of threads such as shown in [Fig. 268], in which the diameter of the bolt is supposed to be an inch, and the proper thread depth 1⁄10th inch. Now, the diameter at the root of the thread on the bolt will be .802 inch in consequence of the wear, but the smallest diameter of the nut thread is .800 inch, and hence too small to admit the male or bolt thread. Again, the full diameter of the bolt thread is 1 inch, whereas the full diameter of the nut thread is but .998 inch, or, again, too small to admit the bolt thread. As a result, it is found in practice that any standard form of thread that makes no allowance for wear, cannot be rigidly adhered to, or if it is adhered to, the tap must be made when new above the standard diameter, causing the thread to be an easy fit, which fit will become closer as the thread-cutting tools wear, until finally it becomes too tight altogether. The fit, however, becomes too tight at the top and bottom, where it is not required, instead of at the sides, where it should occur. When this is the case, the nuts will soon wear loose because of their small amount of bearing area.
Fig. 269.
Fig. 270.
Fig. 271.