Engineers, machinists and mechanics often need to accurately measure extremely small distances. This used to be accomplished manually with calipers and a ruler; however, these tools limit the accuracy of the measurements. The development of micrometer calipers enable the measurement of distances in extremely small increments. There are three main types of micrometer calipers.
The difference between the older calipers and micrometer calipers lies with the vernier scale, which can take measurements in increments as small as .0001 inch. There are three scales -- they look like lines on a ruler -- that change when the caliper ends are moved. Each scale provides an increasingly small and accurate increment of distance. When the arms of the caliper touch the edges of the object, the readings on each of the scales are added together to get the total distance.
The outside, or external, micrometer caliper is used to measure the outside diameter of an object. The ends of the micrometer are slowly moved together via a screw mechanism in the handle of the caliper. Based on the movement of the screw, very minute adjustments are reflected in the vernier scale. Another type of outside micrometer is the screw thread micrometer, which is used to measure the pitch diameter of screws.
The inside micrometer caliper is used to measure the internal distance of an object, such as the inside of a pipe or cylinder head. The caliper must fit inside the object and there must be room for it to be adjusted. One end of the caliper rests against an inside edge, and the extending arm is screwed outward until it touches the other side. The vernier scale is read in the same way as the outside caliper.
The depth micrometer caliper measures the depth of a hole or slot. To ensure accurate measurements, the caliper base must rest securely on a flat surface. An extending arm, called a spindle, is screwed into the recess until it touches the bottom. The measurement is the distance the end of the arm has moved from the base of the caliper. Interchangeable arms of different lengths are available to increase the measuring range of the depth micrometer.