Fig. 28. Micrometer Surface and Height Gage
Fig. 28 shows a form of surface gage that has proved very handy, and which can be used also as a height gage for measuring distances from shoulders to the base. If accurately made it is equal, and often preferable, to the vernier or slide caliper now so generally used with an attachment to the sliding jaw. One of its advantages over the vernier is the readiness with which the graduations are discerned, and it is as easy to manipulate as the ordinary micrometer. The part B, which forms the main body of the instrument, is made of tool steel, and one end is fitted into the base where it is held in position by the screw D. The remainder is milled to a thickness of ⅛ inch and has graduations of 0.025 inch for a distance of three inches. The screw A is the most essential part of the tool, and its construction requires great accuracy. Its diameter is ½ inch, and it is cut with 20 threads per inch. In the upper end of the screw is driven the ball H for the sake of giving a neat appearance. The top of the thread is turned off 0.010 inch to allow the scriber F to slide freely on the screw. The barrel I is used for raising and lowering the slide, but instead of having the graduations placed directly upon it, they are made upon the sleeve C, which fits over a shoulder on the barrel. This allows more easy means of adjustment than would be possible were the graduations placed on the barrel itself. The sleeve is graduated with fifty divisions each equaling a movement of the scriber of 0.001 inch. This sleeve may be turned by means of a small spanner wrench so as to bring the zero line into correct position to compensate for wear. A knurled locking nut is also provided for holding the scriber in any fixed position. The scriber itself is hardened and lapped to a finished surface, the tail end being slotted and provided with two screws to compensate for wear. On the scriber is placed the zero mark which shows at a glance the measurement that is being taken. The block K is three inches in height, and by using this block and placing the gage on its top, the range of the gage is increased to six inches. The screw E is used for fastening the gage to the top of the block. The center of the block is drilled out and slots cut through the sides in order to make it light and neat in appearance.[[15]]
Micrometer of from One- to Five-inch Capacity
Fig. 29. Micrometer of from One- to Five-inch Capacity
Fig. 29 shows a very simple and light five-inch micrometer that can be quickly set to exact position from one to five inches. The round beam is graduated by a series of angular grooves, 1 inch apart, which are of such a form and depth that the clamping fingers at the end of part A spring in, allowing one inch adjustment of the beam to be quickly and positively made. The sleeve K is of tool steel, being counterbored from the forward end for all but one-half inch of its length. For this half inch it is threaded on the inside and acts as a micrometer nut. The outside of the same end is threaded to receive the adjusting nut F, and two slots are cut in the sleeve, at 90 degrees with the graduations. These slots, by a movement of the nut F, provide a means for compensating for wear. The bushing E is hardened and lapped, and fitted tightly in the forward counterbore of this sleeve, where it acts as a guide for the front end of the micrometer screw. The barrel J is the same as that of a regular micrometer, and is graduated in 0.025 inch divisions.
The most essential part of the tool is the threaded screw I, over the end of which fits the barrel J. The end is tapped out to receive the speeder H, which serves to hold the barrel in position. The thread is 5/16 inch in diameter, with 40 threads per inch, while the unthreaded part is hardened, ground and lapped. To adjust the instrument, loosen the speeder H and turn the barrel until the proper adjustment is obtained; lock the barrel by again tightening the speeder. The beam C has a ¼-inch hole drilled throughout its entire length in order to make it light. Small 90-degree grooves are cut into it at intervals of 1 inch, and a ⅛-inch slot is milled through one side to within 1¼ inch of the forward end. The back end of part A forms a spring-tempered split chuck, which grips the beam and holds A in position, while the exterior is threaded to receive the knurled cap B by which the chuck is tightened firmly to the beam. From the front end, toward the split chuck, the body is counterbored ⅝ inch and the bushing D driven in tight. This bushing has a key G fitted into it, which slides in the slot of the beam and prevents the arm from turning. The projecting arm is bored and tapped to receive the sleeve K. This gage must be carefully and accurately made to be of value.[[16]]
Inside Micrometer for Setting Calipers
Fig. 30. Method of Setting Calipers from Inside Micrometers