Digital calipers are one of the most common hand tools used on the shop floor. In a manufacturing plant, under a quality control system, these tools must be checked and calibrated regularly.

Past articles have discussed the pros and cons of doing gauge calibrations internally or by an external calibration facility. Both have their cost and advantages/disadvantages. However, calipers are measuring instruments with fairly loose performance tolerances and might be a candidate for doing in-house calibrations if you have the tools and facilities in place.

The following information covers digital calipers but can also be used for dial or even Vernier calipers (if those are still in use). Because most calipers have more than outside diameter (OD) jaws, we can also check the tool’s inside diameter (ID) jaws and depth-measuring rod, should it have one. Running through a calibration process for a caliper involves comparing its readings to numerous standards over its measuring range, the most common being 6 in./150 mm. However, with the right standards, any length of caliper can be checked.

Because a caliper is a length-measuring device, gauge blocks are the go-to standard. But there are special kits available that provide different versions of length standards for checking the caliper. When checking any measuring device, it’s a good idea to check the full range of the instrument, say 25%, 50%, 75%, and 100% of the measuring range. But for versions of the tool that have a short-range indicator, such as a dial caliper, you probably want to make some short-range checks that cover the range of the indicator face.

The tools needed to do a caliper calibration include:
• Gauge blocks with accessories or other length standards specifically designed for calipers. Of course, they must be under control themselves with a current calibration certification.
• A ring gauge to check the condition of the ID jaws, or a set of rings to check the performance of the ID jaws over the measuring range
• A gauge pin to check the inside diameter jaws for parallelism
• If you have the luxury of a universal length measuring machine, there are accessories available to help calibrate calipers.

Next, it comes down to determining a procedure, thinking about sources of errors to establish an uncertainty budget, and finding ways to reduce these errors to obtain a budget that meets the calibration requirements.

The procedure includes the list of tools required to begin the calibration and defining the process for doing the calibration, such as:
• Clean the jaws (ID and OD), rack, and depth rod of the caliper.
• Clean gauge blocks, master ring, and pin gauge.
• Zero and rezero the caliper to set zero and check repeat.
• Check parallelism of the OD jaws with the pin gauge.
• Check and record the calibration reading of the caliper against the gauge blocks.
• Check and record the calibration reading of the caliper against the master rings.
• Check and record the calibration reading of the depth rod against a gauge block on a surface plate.
• Compare all readings against the tolerances set for the caliper.

The results from the calibration can be stored in a document that contains all the information required by your policy to verify the performance of the caliper, information about the caliper itself, who performed the check, the condition under which the checks were made, the traceability of the standards, and the uncertainty budget established.

Some of the typical errors seen when establishing the uncertainty budget include: the uncertainty of the standards; human errors, such as inconsistent measuring force; environmental conditions; dirt; temperature and vibration; and influences from the caliper itself, such as wear.

To minimize these sources of errors, take care during the setup of the process to:
• Inspect the caliper well to ensure it’s clean and has no nicks, burrs, or other foreign material.
• Ensure the tool isn’t bent or bowed.
• Wear gloves to reduce heat and dirt contamination.
• Take readings at various places on the jaw at each calibration point, looking for variations.
• Recheck zero often.
• Orient the gauge blocks consistently.
• Ensure that a locking mechanism is loose and the tool moves freely.

A lot of this process is similar to the calibration process for micrometers, depth gauges, and other mechanical gauge tools. Knowing the process, your capabilities, and being able to quantify them is what’s most important. If you can do this, then you may be able to start small by doing some basic calibrations on your own.