Measuring calipers have been around for nearly 150 years. In that time they have evolved into full-featured digital measuring systems. But it is a testament to the simplicity and versatility of the original design that the evolution has been so slow and that so few changes have been required to produce the instruments we are familiar with today.
Conceptually, the dimensional measurement process is quite simple. You get a drawing for a part-an electronic computer-aided design (CAD) file, blueprint, napkin doodle or whatever-which indicates certain critical part dimensions and tolerances: a particular diameter, for instance, must be 2.2370 inches ±0.0002 inch. All the manufacturer needs to do is to machine the parts to that dimension, then measure them to document that they are within the specified tolerance. What could be easier than that?
There are many ways to measure an outside diameter (OD), including scales, micrometers, calipers, coordinate measuring machines (CMMs), optical beams and vision systems. The choice depends on many factors including tolerance, run size, operator skill, part size and, of course, budget. An easy, fast, accurate and economical way of making such measurements is with an adjustable snap gage.
There are many applications-aside from those where custom gaging is the only possible solution-where custom tailoring a gage for an application can speed things up, improve accuracy, incorporate multiple measurements, generate sophisticated analysis or otherwise add value to the process.
Air gaging was the breakthrough measuring technology of the 1940s and 1950s. Introduced decades before electronic gaging was widely accepted, it provided the first high performance-50 microinch or better-gaging on the shop floor. Today, air gaging is just as important and actually growing in popularity.
Calipers are extremely versatile tools for making a wide range of distance measurements, including both outside diameters (OD) and inside diameters (ID). While micrometers are more accurate, they have a limited measurement range, typically several inches. Calipers can span from 2 inches to 4 feet, depending on the length of the scale. External measurements are made by closing the jaws over the piece to be measured, while internal measurements are made by opening up the inside diameter contacts. Depth and other measurements can be made with a depth rod built into the instrument’s beam.
Circles are the most frequently produced machined form. Generated by many different processes-including turning, milling, centerless grinding, boring, reaming and drilling-there are, correspondingly, a wide variety of gaging methods used to measure inside and outside diameters. At the low end, a hole could be measured with a scale or a fixed go/no-go gage. At the other extreme, any number of precision measuring machines, including coordinate measuring machines (CMMs) and optical or vision machines are available. However, in production environments, most inside diameters and outside diameters (ID/ODs) can be accurately measured using one of several varieties of comparator gage.
