How does an internal caliper work?

Generally speaking, the internal caliper works in the same manner as the external caliper. The indirect—measurement internal caliper is a reverse of the external caliper, with the contact measuring points turned out instead of in. In contrast, the direct-measurement vernier caliper with a completely different type of jaw from its external measuring cousin.
A combination internal-external dial vernier caliper does not use a fine adjustment feature, it is necessary to clamp the sliding jaw directly while holding the caliper in position . The dial indicator replaces the vernier scale for reading the finest graduation of one-thousandth inch. Note that the internal caliper jaws (on the top of the bar in this particular caliper) are actually “blades�?that pass each other to arrive at a distance of zero.
Each of these blades has a near edge where the instrument will contact the part for internal dimensions. This feature is used for internal calipers for a very logical reason. For instance, consider measuring between two flat surfaces for an internal dimension. In this case, the shape of the caliper jaw would be immaterial. Now consider measuring the diameter of the hole with the internal caliper. If the jaws were anything other than a knife edge or point, the width of the jaw blade would contact the internal diameter at a tangent point and give an inaccurate measurement.
Knife edge internal caliper jaws alone will not assure accurate measurement of an internal surface. As a matter of fact, a great deal of the use of hand-held measuration instruments relies on the ability of the operator to “feel�?the correct positioning of the instrument two flat internal surfaces, then it is imperative that the instrument be held exactly perpendicular to these surfaces to avoid measurements which are too large. By moving the instrument back and forth slightly while applying minimal the instrument back and forth slightly while applying minimal pressure outward. The operator will be able to feel where the shortest distance is being measured. On the contrary, if the operator is measuring the diameter of a hole, then you have to make sure that the jaws contact the hole at the greatest possible distance to avoid taking a measurement that is too short.
Given all of this necessity for human finesse when taking internal measurements with the internal caliper, it may seem this is not the ideal instrument for making these sorts of measurements. This may or may not be true, depending on two factors: the time allotted for taking the measurement, and the degree of accuracy necessary from the measurement. The internal caliper is a relatively accurate instrument, down to one one-thousandth of an inch or two one-hundredths of a millimeter, and can be employed with great speed by an experienced operator with a developed touch. However, if greater accuracy is required, and repeated checking of an identical hole is required, another instrument is probably a better choice.
There are specialized instruments used for taking internal diametrical measurements, and there vary greatly depending on factors such as available time, degree of required accuracy, and number of repetitions. Internal micrometers are more accurate,and custom gauges are as accurate (or more so ) and can be employed with greater speed if it the number of holes to be employed with greater speed if the number of holes to be measured is sufficient to justify making a gauge. We will cover these other internal measuring devices later in the book.