Furthermore, as the tap becomes heated it expands in length, and its pitch increases, hence here is another influence tending to cause the pitches of the nut threads to vary, because although the temperature of the tap when in constant use reaches a limit beyond which, so long as its speed of rotation is constant, it never proceeds; yet, when the tap is taken from the machine to remove the tapped nuts which have collected on its shank, and it is cooled in the oil to prevent it from becoming heated any more than necessary, the pitch as well as the diameter of the tap is reduced nearer to its normal standard.
So far, then, as theoretical correctness, either of pitch or diameter in nut threads, is concerned, it could only be attained (supposing that the errors induced by hardening the tap could be eliminated) by employing the taps at a speed of rotation sufficiently slow to give the oil time to carry off all the heat generated by the cutting process. But this would require a speed so comparatively slow as not to be commercially practicable, unless followed by all manufacturers. Practically, however, it may be considered that if two nuts be tapped by a tap that has become warmed by use, they will be of the same diameter and pitch, and should, therefore, have an equal area and nature of contact with the bolt thread, supposing that the bolt thread itself is of equal and uniform pitch. But the dies which cut the thread upon the bolt also become heated and expanded in pitch. But if the temperature of the dies be the same as that of the tap, the pitches on both the bolt and in the nut will correspond, though neither may be theoretically true to the designated standard.
In some machines for nut tapping the tap is submerged in oil, and thus the error due to variations of temperature is practically eliminated, though even in this case the temperature of the oil will gradually increase, but not sufficiently to be of practical moment.
Let it now be noted that from the hardening process the taps shrink in length and become of finer pitch, while the dies expand and become of coarser pitch, and that this alone precludes the possibility of having the nut threads fit perfectly to those on the bolt. It becomes apparent, then, that only by cutting the threads in the lathe, and with a single-toothed lathe tool that can be ground to correct angle after hardening, can a bolt and nut be theoretically or accurately threaded. Under skilful operation, however, both in the manufacture of the screw-cutting tools and in their operation, a degree of accuracy can be obtained in tapped nuts and die-threaded bolts that is sufficient with a single nut for ordinary uses, but in situations in which the direction of pressure on the nut is periodically reversed, or in which it is subject to shocks or vibrations, the check nut becomes necessary, as before stated.
Fig. 414.
An excellent method of preventing a nut from slackening back of itself is shown in the safety nut in [Fig. 414]; it consists of a second nut having a finer thread than the first one, so that the motion of the first would in unscrewing exceed that of the second, hence the locking is effectually secured.
