What You Must Know About Calipers

This primer on various measurement processes shows you how to match different caliper designs with selected applications to prevent measurement errors from creeping in.

For the curious, the word caliper is short for caliper compass, which was a device with curved legs used to measure the caliber, or bore, of cannon and small arms. In today’s machine shop, calipers are very versatile instruments used to make a wide variety of measurements, including ODs and IDs (such as the bore of a cannon).

Frequently confused with micrometers (which are actually a hybrid, more accurate form of caliper with more limited range), calipers consist of a precision beam with an integral scale and two sets of jaws, one fixed, the other sliding.

Calipers can span from two in to four ft, depending on the length of the scale. External measurements are made by closing the jaws over the piece to be measured, and internal measurements by opening up the inside diameter contacts. Depth and other measurements can typically be made with a depth rod built into the instrument’s beam.

BASIC TYPES OF MODERN CALIPERS
Vernier. This caliper was the original design and is still the most rugged. The Vernier scale was invented by Pierre Vernier in 1631. It allows measurements to be read more precisely from an evenly divided scale by the alignment of an etched scale on the Vernier plate with an equally spaced scale running the length of the tool’s handle.

Skillful alignment of the tool and interpretation of the reading is necessary to achieve the measurement tool’s stated accuracy. A typical Vernier caliper can read to a precision, or resolution of 0.02 mm.

Dial. Similar in construction to the Vernier caliper but offering better resolution and easier reading, this caliper replaces the Vernier scale with a dial indicator. The indicator is fixed to the moveable jaw, and engaged with a toothed rack on the body of the unit.

The dial, which is typically balanced (i.e., can move in either plus or minus directions from zero), may be graduated in either inch or metric units.

The dial caliper is a dual-purpose instrument and can make either direct or comparative measurements. To make a comparison, first measure the reference dimension and set the dial indicator to zero. Then measure the dimension to be compared. The indicator will show how much the compared dimension varies from the original (plus or minus).

Digital. The most recent innovation is the digital caliper. The latest designs incorporate numerous electronic features which make the device easier to use, while adding little in the way of cost. These include: easy switching between inch and metric units on the readout; tolerance indications; digital output to electronic data collection systems; zero setting anywhere along the caliper’s range; and retention of the zero setting even when the caliper is turned off.

With no moving parts in the readout, the digital caliper is exceptionally durable, standing up to some of the toughest manufacturing environments. Newer units are even waterproof.