How to use a Caliper

A caliper is a precision measuring instrument used to dimension an object with high accuracy. These can be found in a variety of lengths, imperial or metric, and either with a vernier scale, dial, or in digital forms. For this article, I will be using a dial caliper as an example. A caliper can take four types of measurements which includes the outside jaws, inside jaws, depth probe, and step measurement. Each type of caliper will have their own advantages and disadvantages. A vernier caliper is the most basic type of caliper, it does take the longer and require more skill to read, but has minimal moving parts and there is no need for battery maintenance. A caliper equipped with a dial indicator, on the other hand, is easier to read, there is no battery maintenance, but has more moving parts. A digital caliper does have electronics which require battery upkeep but is by far the easiest to read.

Outside jaws – Used to measure the width, length, thickness, or diameter of an object.

Inside jaws – Jaws used to measure the inside width, length or diameter of a hole or slot, however, it can be limited for smaller holes due to the thickness of the jaws.

Depth probe – The probe can be used to measure the depth, thickness, length, or elevation differences of various parts. When measuring within a hole or slot, this can also be limited due to the thickness of the probe.

Step measurement – Used to check the thickness, length, or difference in surface elevation. Just like the other measuring methods, this can be limited in applications due to the size of the jaws.

Outside jaws: Used to measure the outside diameter, length, width, or thickness of an object.

Inside jaws: Used to measure the inside width of a slot, groove, or hole or the inside diameter of a bore.

Depth probe: The depth probe or rod is used to measure a groove, slot, step, or hole.

Beam: Or sometimes referred to as a bar is the frame of the caliper that is stationary and houses the depth probe.

Scale: Located on the frame to determine the measurement, in this case, how many inches and tenth decimal place.

Reference edge: Assists in determining where the measurement sits on the scale.

Dial indicator: Determines the hundredth and thousandth decimal places. The face can be rotated, in order to zero the dial.

Thumbwheel: A fine adjustment to achieve accuracy when setting up or adjusting the feel or drag when measuring an object.

Thumb grip: A grip for quick or large adjustments.

Track: A geared track where the dial indicator runs on and needs to be clean at all times.

Thumb screw lock: Used to hold the caliper measurement in place.

Before using a caliper, always ensure the measuring surfaces of the object and the caliper are clean and free of any contaminants which may affect the accuracy of your readings. The accuracy of the caliper can always be checked using a known reference such as a gauge block. A dial caliper will need to be zeroed, therefore close the jaws and rotate the dial indicator face accordingly so the needle is at zero.

When using the inner or outer jaws, you will want to use light to medium pressure when in contact with the workpiece's surface. This must be kept consistent with all the parts you are measuring. If you are using the depth probe, extend the probe out, insert onto the object you are measuring, and then allow the beam of the caliper to rest on the outer surface. When using the step measurement surface, rest the movable surface on the higher surface, then slide the stationary portion of the caliper on the lower surface.

Using the appropriate measuring method for the task at hand, take the measurement and lock the caliper using the thumb screw. Locating the reference edge, first, we need to determine it's location in accordance with the scale on the beam. Our first increment is the full inch measurement (red arrow), giving us a reading of 0.XXX" currently. Determining the first decimal place, the reference edge has passed the seventh increment (green arrow), updating our reading to 0.7XX". And finally the last reading on from the dial indicator for the second and third decimal place (yellow arrow), finalizing our reading with 0.724".

Considering this is a precision measuring instrument, it is important that we take care of this tool. The caliper should be kept clean at all times, free of any foreign contaminants and stored in a dry place where it cannot be affected by moisture. A very light solvent can be used for cleaning, care must also be taken around the numbering scale as some can wipe away easily depending on the quality. A light lubricating oil specific for measuring instruments can be used on the sliding portions in order to ensure smooth operation. If the dial indicator's lens becomes scratched, a polishing compound that is typically used on vehicle paint can help remove these scratches and restore the clarity of the lens.