TYPES OF MICROMETER CALIPERS

Figure 2-21.—Common types of micrometers. the point of contact of the two lower caliper legs. In this figure the inside caliper is being adjusted to the size of the outside caliper. As careful measurements with calipers depend on one’s sense of touch, which is spoken of as “feel,” calipers are best held lightly. When you notice a slight drag, the caliper is at the proper setting. CARE Keep calipers clean and lightly oiled, but do not over oil the joint of firm-joint calipers or you may have difficulty in keeping them tight. Do not throw them around or use them for screwdrivers or pry bars. Even a slight force may spring the legs of a caliper so that other measurements made with it are never accurate. Remember, calipers are measuring instruments and must be used only for the purpose for which they are intended. PRECISION MEASURING EQUIPMENT In much wider use by ABEs than even common calipers are the various types of micrometer calipers. As was stated earlier, you can use micrometer calipers to take accurate measurements to the nearest one ten-thousandth of an inch. However, in most applications a measurement to the nearest one-thousandth of an inch is considered acceptable accuracy. These measurements are expressed or written as a decimal (0.0001, 0.001, 0.01), so you must know how to read and write decimals. TYPES OF MICROMETER CALIPERS There are three types of micrometer calipers, commonly called micrometers or simply mikes, used throughout the Navy: the outside micrometer, including the screw thread micrometer; the inside micrometer; and the depth micrometer. (See fig. 2-21.) The outside micrometer is used for measuring outside dimensions, such as the outside diameter of a piece of round stock or the thickness of a piece of flat stock. The screw thread micrometer is used to determine the pitch diameter of screws. The inside micrometer is used to measure the inside diameter of a cylinder or hole. The depth micrometer is used for measuring the depth of a hole or recess. Outside Micrometer The nomenclature of an outside micrometer is illustrated in figure 2-22. The sleeve and thimble scales of a micrometer (fig. 2-23) have been enlarged and laid out for demonstration. To understand these scales, you need to know that the threaded section on the spindle, which revolves, has 40 threads per inch. Therefore, every time the thimble completes a revolution, the spindle advances or recedes 1/40 inch, or 0.025 inch. Note the horizontal line on the sleeve is divided into 40 equal parts per inch. Every fourth graduation is 2-8