The most essential part of this tool is the threaded screw _B_, which acts as a rack, and the worm-wheel, solid with the dial _C_. The upper end of the screw forms a split chuck which grips the measuring rods, while the part marked _R_ is flatted off, and against this portion bears a threaded sleeve _G_, which acts as a key to keep the screw in position. This sleeve is threaded, both inside and outside, and screws into the body of the tool, while the binding screw _F_ fits into it and binds against a small piece of copper, marked _H_, which in turn holds the screw in position. The thread on _B_ is 0.245 inch in diameter and is cut with 40 threads per inch. The worm-wheel which meshes into this screw is solid with the dial, as shown at _C_. It is 0.18 inch in diameter, and requires great accuracy in cutting; it is not hobbed, but the teeth, of which there are twenty, are milled with a circular cutter of the same diameter as the screw _B_ plus 0.002 inch. The little studs, marked _EE_, on the dial and on the body _K_, hold the coiled spring in position. Very great accuracy must be attained when locating the holes in _K_ that are to receive the screw and dial _B_ and _C_. The screw marked _J_ fits into the dial, where it serves as a bearing and also holds the dial in position. The knurled cap _D_ tightens the split chuck in order to hold the measuring rod firmly.[11] Indicator for Accuracy of Lead-screws [Illustration: Fig. 23. Indicator for Accuracy of Lead-screws] All of the tools that have been described require an accurately cut screw, and, as very few lathes are capable of producing this, it may be well to illustrate an indicator for testing the accuracy of the lead-screw, and to explain the method by which it is used. This instrument is shown in Fig. 23, where it is applied to a test screw _K_. It consists of a body _A_ on one end of which is a projection _L_ serving as the upper bearing for the pivoted lever _D_. This lever swings about a small steel pivot which can be adjusted by the screw _E_. The rear end of the lever is forked, and between the prongs is passed a thread making a double turn about the pivot _F_ that carries the pointer _J_. Any movement of this lever will, therefore, cause this pointer to revolve about the dial _C_. This dial has 20 divisions, each indicating one-half thousandth of an inch movement of the front end of the lever, so that a total revolution of the pointer about the dial would indicate a movement of the front end of the lever of 0.020 inch. The screws _I_ serve to hold the dial in place on the body of the indicator, while the spring _M_ keeps the pointer normally at the zero mark. The indicator is held in the toolpost by the arm _G_, which can be set at any angle and firmly clamped by the screw _H_.