The basic caliper is often used for length/diameter measurements, but other caliper styles extend the tool’s advantages to special measurement applications.
Modern Machine Shop, George Schuetz
Convenience is one of the reasons the caliper is often the measurement tool of choice. The basic caliper provides direct size information, has relatively high resolution and is easily adaptable to different measurement applications. Beyond the basics, there are various caliper styles that extend these advantages to special measurement applications.
A caliper consists of two opposing surfaces: a stationary anvil/jaw and a moveable slide. On most calipers, these are made from hardened steel, but optional carbide-tipped or ceramic contact surfaces are available. Calipers also can be equipped with built-in contact tips with unique forms for measuring special part characteristics, or jaws with replaceable and configurable contact points.
If walls could talk. When measuring wall thickness on cylindrical parts, the two flat surfaces of a standard caliper will cause errors because one jaw will measure a chord rather than a line contact. To measure true wall thickness, one of the jaws needs to be cylindrical to enable a line measurement on both the ID and OD of the part.
In the groove. Measuring the OD, ID or the width of a machined groove calls for another type of caliper contact blade. Often, these grooves can be so narrow that neither a standard nor reduced face caliper will fit completely into the groove. Pointed blade contacts are slender and flat. They nest readily into narrow-bottomed grooves. Other configurations funnel down into a narrow point for getting into deep groves. These narrow-tipped jaws fit into small grooves and tracks, easing difficult ID and OD measurements. Excessive pressure on these narrow blades, such as rocking the tool to find the true diameter, can result in premature wear.
Even more grooves. But wait—not all grooves are created the same. There are grooves or recessed IDs and ODs that a typical caliper cannot access. For these applications two special jaw configurations are available, one series for IDs and the other for ODs. The point jaw is ideal for measuring wall thickness, grooves and recesses, while the flat-jaw type is designed for measuring grooves and recesses primarily on ODs.
Getting to the center of things. Calipers with tapered jaw tips are designed to measure center-to-center distances on holes. Often, one of the jaws is adjustable in height to enable measuring holes on offset planes.
Speaking of offset planes. Often, parts are not nice and square but have obstructions that interfere with measurement. Usually the standard reference jaw on a caliper can be adjusted by height. This enables the main scale jaw to slide up and down to facilitate measurements of stepped sections on the part. Now, these hard-to-get dimensions are a piece of cake.
Reaching for it. Knife-edge jaws with extra-long reach are designed for parts that are deeper than a typical caliper can reach. The long-reach jaws are more than 50 percent longer than a standard caliper, making them well suited for deep-hole applications.
Even the most basic hand-measuring tool can be adapted to special-application requirements. Choosing an appropriate style for your application will help you measure faster and more accurately. Each style, however, has unique requirements for care and use. If you’re going to measure with style, make sure you know how to do it properly.