The junction of a bolt-thread with the remainder of the stem should be taper, being so formed that the depth of the thread-groove may gradually decrease along about two steps, thus preventing the thread-groove being as deep at its termination as at any other place along its length. This gradual enlargement of the bolt at the bottom or base of the thread renders it less liable to break than if it terminated suddenly ; but a little extra length of thread is requisite, and this is the only objection to the tapering-process. The three screws shown by Figs. 1094, 1095, and 1096 are tapered in this manner; but the two shown in Figs. 1097 and 1098 are nearly or quite parallel from one end to the other.

To facilitate the making of a screw parallel very nearly to its termination, a shallow groove is formed at the proper place, similar to that seen at the termination of the thread in Fig. 1097. Such a groove would suit a connecting-bolt, if the screw were required to be parallel; and it is formed previous to beginning the screw-cutting. The width of the groove is about equal to the step of the thread, and the depth of it is about three-quarters of the intended thread-groove's depth. A groove of this shape is very efficient to avoid tapering the screw, and is suitable for any bolt which is to have a comparative large or coarse thread.

Fig. 1098 represents a joint-bolt, or pivot-bolt. The cylindrical portion situate between the head and the screw is that which is to sustain the rod-boss, lever-boss, or other portion to be connected, and the screw is to hold a nut, or two nuts. While such a bolt is in use no strain is exerted upon the screwed end, and it may therefore be smaller than the intermediate part, and may have also a comparative large groove at the thread-junction, similar to the one shown in the Figure, to facilitate the screw-cutting.