Calibrating a caliper involves more than simply closing the jaws, checking zero, and measuring a single known standard. A caliper can produce different errors at different positions along its measuring range and when using its different measuring surfaces. A useful calibration procedure must therefore be designed to find these potential errors.
The procedures described here are based primarily on ASME B89.1.14, the American National Standard covering digital, dial, and vernier calipers.
Checking the Outside Measuring Jaws
The outside jaws are checked using calibrated reference standards, such as gage blocks or a caliper checker. Measurements are made at several points throughout the range of the caliper rather than at only one dimension.
ASME B89.1.14 requires a minimum of three to five test points, depending on the measuring range, and the tests must cover at least 90 percent of the caliper’s range.
The location of the reference standard between the jaws is also varied. Some measurements should be made close to the beam, while others should be made near the tips of the jaws. This is important because wear, damage, or errors in the geometry of the jaws can cause the caliper to produce different results depending on where an object contacts the measuring faces.
Checking the Other Measuring Functions
A typical caliper is actually several measuring instruments combined into one. In addition to outside measurements, many calipers can make inside, depth, and step measurements.
These functions must also be checked.
Inside measurements may be checked using a ring gage, caliper checker, or an appropriate arrangement of gage blocks and accessories. Depth and step measurements can be checked using gage blocks on a surface plate.
This is important because a caliper can perform correctly when using the outside jaws and still have an unacceptable error when making an inside or depth measurement.
Special Considerations for Inside Measurements
The inside jaws of many calipers have narrow, crossed measuring faces. Their geometry can introduce additional error when measuring small holes.
For calipers of this type with measuring ranges up to 300 mm (12 in.), ASME B89.1.14 calls for an additional check using a 5 mm (0.2 in.) ring gage. This test helps detect errors that might not appear when checking a larger internal diameter.
Selecting Calibration Points
The dimensions chosen for calibration should not be arbitrary. Test points should be selected so that the calibration has a reasonable chance of finding errors throughout the instrument’s range.
This is especially important with dial calipers. Errors in the rack-and-pinion mechanism can sometimes repeat as the pointer travels around the dial. Calibration points should therefore be chosen so that the pointer appears at different angular positions on the dial rather than repeatedly testing it at approximately the same position.
Used calipers should also be inspected carefully for wear or damage to the measuring faces.
Calibration as a Search for Errors
The central idea behind caliper calibration is that calibration should be a systematic search for errors.
Checking a six-inch caliper against a one-inch gage block may tell us how the caliper performs at that particular dimension, using that particular portion of the jaws. It does not tell us how the caliper performs at five inches, near the tips of the jaws, when measuring a hole, or when using the depth rod.
A meaningful calibration procedure deliberately tests these different conditions.
The video below from the Mitutoyo Institute of Metrology demonstrates this approach and explains how the requirements of ASME B89.1.14 are applied when calibrating a caliper.
Reference
Mitutoyo Institute of Metrology, Caliper Calibration – How to Calibrate a Caliper, Metrology Training Lab, Mitutoyo America Corporation.
The calibration procedure discussed in the video is based on ASME B89.1.14, Calipers.