An inclined thread is one which is not situate square to the length of the bolt. Generally speaking, all vee-threads should be square to their respective rods and bolts, if not, their nuts will not enter upon the screws both ways; the nut will not advance unless it is placed so that the thread inclines in the same direction as that of the bolt-thread, supposing that both nut-thread and bolt-thread are inclined, and that they properly fit each other.

If a vee-thread in a nut is properly situated, its base is exactly beneath its summit, whenever the nut is held with the axis of the hole horizonal. Consequently a line which is right-angular to the length of the hole or axis, and which equally divides the thread at its base, will also equally divide it at its summit. A knowledge of this fact enables us to make gauges, whereby the relative positions of threads in their nuts can be ascertained quite near enough for practical purposes.

A gauge for ascertaining whether a thread is right-angular to the length of the screw, is termed a dentin, and represented by Fig. 1060. The tool is a straightedge having a tooth extending from one end, and solid with the remainder or straight-edge portion. The tooth is shaped to fit a vee-thread's groove, and the entire tooth, during the final adjustment, is made to extend exactly right-angular to the length of the straight edge. This position is obtained by a careful lining, after the tool is filed straight and smooth, but previous to shaping the tooth. The lines first made for this purpose, are three, which are straight, parallel with each other, and square to the length of the tool. In Fig. 1064 these three lines are shown, the Figure being a comparative large sketch of a dentin which has a gap opposite the tooth. The middle line of the three is the one first marked, and is scribed with an el-square and scriber. A springy divider is next employed to scribe a small circle at each end of this primary line, the diameter being equal to the intended width of the tooth, and gap, if a gap is desired. The two outer lines are now to be marked, and by moans of them, the angular lines showing the shape of the tooth, are marked; after which the shaping proceeds, and is conducted with regard to the lines, and completed by fitting to the thread-groove of a Whitworth-hob, supposing that such a thread is desired.

Fig. 1061 indicates a dentin in use. The tool is placed with its tooth in a thread-groove of a nut, so that the tooth shall be fairly situate in the groove. While it is thus held, the observer can plainly see whether the length of the straight portion is parallel with the hole or screw; if so, the thread is not inclined to any great extent, but is situated as it should be. But if the length of the dentin is seen to be inclined to the length of the hole, while the tooth is exactly placed in the groove, the thread must be also inclined, instead of being situate square to the length of the hole.

A dentin can be used to ascertain the condition of a thread after it is finished; and can be used for a thread during its formation in a nut attached to the lathe-chuck. If the tool is applied to a nut already formed, and the thread is found to be inclined, the operator can observe to which face of the nut the thread leans, and can accordingly incline the bolt-thread to fit the nut, in which case a good fit will be secured, but the nut will not enter either way first upon the bolt.

When the dentin is used during the screw-cutting of a nut, it is applied to the partly formed thread, and to the screw-tool. For this purpose, the end of the dentin is shaped like Fig. 1064, having both tooth and gap. The gap is employed at the fixing of the stock in the tool-holder, at which time the gap is put into contact with the tooth of the screw-tool, and the length of the dentin observed, to see if it is parallel with the length of the hole to be screwed, and if not, the stock in the tool-holder of the slide-rest is shifted, until the tooth of the cutter is in the proper position, when the stock may be firmly fastened. The tooth of the dentin is not used until the thread is partly produced, at which time the tooth will enter the thread-groove and indicate the inclination, if any exists; but it will be found that if the screw-tool is accurately placed in the slide-rest, no application of the dentin to the nut will be necessary.