Measuring with spring calipers: Testing & Quality Assurance

Spring calipers have been around for a hundred years and, like micrometers, still have a place in the metrology arsenal. It’s my understanding that some in the machine shop world don’t know what these measurement tools can do, so this is a primer. The drawing shows a part that I make repeatedly, and, yes, the last revision to this drawing took place in 1985, which kind of dates me. The part has internal and external snap ring grooves and an internal cavity for steel balls. The diameters of these three features can be measured with spring calipers. I like to measure parts in process, and that’s what I’ll talk about here.

The drawing shows a part that I make repeatedly. Image courtesy of B. Taylor

Blade micrometers are available that can measure the external groove with high accuracy, but this part doesn’t need that accuracy, and I find spring calipers to be quicker. For the internal features, there are more expensive tools that can be used, but I don’t see their advantage.

Let’s look at what spring calipers are. They take some skill to get good, consistent measurements, but demonstrating skill is what machinists do. These tools have a pair of legs held on a journal by a compression spring at one end. The included angle between the legs is adjustable by means of a threaded rod and adjusting nut. The ends of the legs away from the journal perform the measurement. There are two types: one for external measurements and another for internal measurements. External calipers have blade ends facing inward that can fit in external snap ring grooves. Internal calipers have small-diameter ends that are bent outward so they can fit in internal grooves. Like micrometers, spring calipers are available in a range of sizes.