How external calipers work?

It requires slightly different techniques to measure an outside dimension and an inside dimension. The external caliper is designed specially for taking external dimensions. It doesn’t matter that if the part to be measured is flat or round , however, how the instrument is held relative to the part is very important. Use the hand-held instruments to take measurements relies heavily on the “feel�?the operator has for performing the operation. The instrument must be closed on the part throughly, but not forced. Besides, the instrument must be held as square and perpendicular to the part as possible. You can only be achieved with practice, patience, and experience to learning the correct feel or touch of using a hand-held measuring instrument.

In general, the part should be stationary. For instance, fixed in a machine or vice, or at least set on a table or surface plate. This aids are used to keep the instrument positioned correctly to the part. If you can't do this, please hold the instrument in the dominant hand (right hand if the operator is right handed, left hand if the operator is left handed) and hold the part in the other hand. This is necessary for the operator to add dexterity of the dominant hand to operate the jaws of the caliper (and the fine adjustment, when applicable).
The sliding jaw of the caliper is moved toward the part until it contacts it. Most of the calipers are equipped with a fine adjustment if they lack dial or electronic capabilities. Once the sliding jaw and fixed jaw contact the part, tighten the screw on the top of the fine adjustment. Only when you begin to move the sliding jaw toward the part with the fine adjustment screw while simultaneously making sure to align then instrument so that the flat of the jaws is completely contacting the part squarely. Once the jaw will no longer move forward with modest pressure from the adjusting screw, stop and take the reading. Never force a measurement instrument if necessary, clamp the sliding jaw in place with its clamping screw to assure that the caliper remains in position while taking the reading. This eliminates the possibility of twisting the instrument slightly while removing it from the part and consequently forcing the jaws apart slightly resulting in an inaccurate measurement.