A. Design, Purpose, and Construction
The caliper is a very versatile measuring tool in that this one tool is designed to perform inside, outside, and depth measurements.
There are three basic designs of calipers. Some have a vernier scale much like a micrometer, some have a scale and dial, and others are digital. Most calipers are graduated in one-thousandths of an inch if they are in U.S. measurements and two-hundredths of a millimeter if they are metric. Some are graduated in both U.S. and metric.
The inside jaws are designed to measure the distance between inside surfaces. However, the depth that can be measured is restricted by the length of the jaws. For example, you can measure the diameter of a cylinder bore at the top of the cylinder, but you cannot measure the diameter of that bore at the middle or the bottom of the bore.
The outside jaws are designed to measure distances over and around adjacent surfaces. For example, the diameter of a wrist pin or a crankshaft journal can be measured, using the outside jaws of a caliper.
The caliper can also be used to measure depth. Such things as the depth of a bolt hole or the distance a seal sets below a surface can readily be measured with the caliper.
B. Advantage and Disadvantages of the Caliper
Advantages. One advantage that a caliper has over the micrometer is that calipers are available that will measure a large area; as much as ten feet or more. Another advantage that we’ve already discussed is the fact that three types of measurements can be made with the one tool.
A disadvantage of the caliper is that it will measure only to thousandths vice ten-thousandths of an inch, as a vernier micrometer is capable of.
C. Using the Calipers.
A caliper is used in much the same way as the micrometer. Usually the caliper is set by measuring the work and then comparing that reading with the desired measurement or the caliper is set to the desired measurement on the scale and then the work is checked to see if it is the correct size.
1. Outside measurements are made by placing the work between the jaws of the caliper or placing the caliper jaws around the work and adjusting the caliper. To obtain a correct measurement, the caliper should be placed on the axis of the work and the caliper adjusted just tight enough to make good contact and still be able to slide on the work. Do not overtighten, as this will give an incorrect reading and damage the caliper.
2. Inside measurements are made by placing the inside jaws into the work and adjusting the caliper. Be sure to hold the caliper parallel to the work to obtain a correct reading.
3. To measure the depth of a hole, extend the caliper depth gauge into the hole and seat the caliper beam against the lip of the hole. Be sure to hold the caliper perpendicular to the work to obtain a correct reading.
4. As we mentioned earlier, there are three basic types of calipers, with three methods for reading the measurements.
(a) The U.S. vernier caliper is read by first reading the numbers on the stationary beam, which are graduated into inches, hundred thousandths and twenty-five thousandths of an inch and then adding the number on the vernier scale, which is graduated into thousandths.
(b) The dial type caliper has inches and hundred-thousandths on the beam like the vernier type and the dial is graduated in thousandths. Simply add the beam reading and the dial reading.
(c) The digital type takes any guesswork out of measuring. Simply place the caliper on the work and look at the digital display for the correct reading.
5. A sense of feel must be acquired to use calipers properly. This comes through practice.
d. Care of Calipers.
The caliper is a precision tool and should be handled with care. Don’t throw it around or pile things on it. A caliper should be kept clean and lightly lubricated on the threads, and when stored, a light coat of oil should be applied to keep it from rusting. The caliper requires calibration every six months, which is done by the calibration lab.