The barrel is graduated in fifty divisions, each division equaling 0.001 inch. On the inside of the barrel is a 45-degree bearing which rides on the cone M, the cone being held stationary on the end of the body. Thus it will be seen that both front and back ends of the micrometer screw are carried in cone bearings, which give a very small point of contact, thereby causing but little friction and preventing any danger of gumming up so as to run hard. The sliding jaw C is made of tool steel, hardened, ground and lapped, and combined with it is the micrometer nut which is drawn to a spring temper. This nut is split and adjusted by two screws to compensate for wear. On this jaw are the two zero marks that tell at a glance the outside or inside measurements taken. The screw and washer, marked H and I, go onto the end of the micrometer screw and take up the end play. To make a neat appearance, the cap E is placed in the forward counterbored hole, being held in place by a tight fit. The adjustment of the tool is accomplished by loosening the speeder G and turning the barrel on the screw; when the adjustment is made, the speeder is again tightened down and the barrel locked.[[10]]
Micrometer Depth Gage
The depth gage, shown in Fig. 22, has a ½-inch movement of the rod, and may be used with rods of any desired length. These have small 45-degree-on-a-side grooves cut into them at intervals of ½ inch. A small spiral spring, marked I, gives the rod a constant downward pressure, so that, when taking a measurement, the base of the tool is placed on the piece of work, and the rod always finds the bottom of the hole; then, by tightening the knurled screw F the rod is clamped in position and the tool may be picked up and its measurement read from the dial. The graduations on this instrument are similar to those of the vernier caliper, only they are much plainer, as a half-inch movement of the rod turns the dial one complete revolution. The figures on the dial denote tenths of an inch, and those on the body of the tool thousandths; each graduation on the dial is therefore equal to 0.010, so that to show the depth of a hole to be 0.373 the dial would be revolved around so that the seventh division beyond the 3 mark would be near to 0, and then by looking from the 0 mark toward the left, the third graduation on the body and one on the dial would be in line, thus denoting 0.373.
Fig. 22. Micrometer Depth Gage
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
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.