Measuring Outside Diameter with Vernier Calipers
Generally speaking, the diameter is the greatest distance between the reference point and the meaured point when you use a vernier caliper to measure the outside diameter.Thus you will possibly find it is easier to measure a diameter than the distance between flat surfaces. And you might have a parallelism mlsalignment, which could lead to a reading longer than the true value (a plus error), however, most of the misalignment errors will be shorter (a minus error). You must hold the caliper to ensure that it is balanced and does not tend to upend when measure an outside diameter. At the same time, you also have to hold the fixed jaw against the edge of the diameter and swing the movable jaw back and forth past center. Then close the caliper slowly until you can just feel the measured point as the caliper passes over it. What is more important is that even the slightest feel implies that some metal is being compressed or sprung unless the part is very thin. If this is the case, it is most probably that the caliper is springing. In this situation, you will only increase the springing of the caliper if you continue to close the measuring instrument. Do keep in mind that heavy gaging pressure lead to wear of the jaws more rapidly, and it also burnishes the part, or even damages the caliper greatly.