A dial caliper is a precision instrument used to measure the dimensions of an object with high accuracy, typically down to one-thousandth of an inch (0.001″) or two-hundredths of a millimeter (0.02 mm). This tool provides a significant advantage over a standard ruler by offering a much finer resolution for outside diameter, inside diameter, and depth measurements. Unlike its digital counterpart, the dial caliper operates mechanically, relying on a rack and pinion system to display the fine measurement on a circular gauge, meaning it never requires a battery. This precision makes the dial caliper a standard tool in machining, engineering, and quality control applications.
Essential Components of the Dial Caliper
The caliper’s body, known as the main scale or beam, is the fixed portion that provides the larger part of the measurement reading. This beam is engraved with a graduated scale, usually marked in inches or millimeters, which acts as the primary reference. The slider is the movable component that travels along the main scale, carrying the dial indicator and the measuring jaws.
The dial itself is a circular gauge with a rotating needle, mechanically linked to the slider’s movement via a gear-toothed rack on the beam. This dial provides the fine measurement, with each full rotation typically representing one-tenth of an inch (0.100″) on inch-based calipers. The caliper features two main sets of jaws: the larger, lower jaws for measuring outside dimensions and the smaller, upper jaws for measuring inside dimensions. A thin depth rod extends from the end of the beam to measure the depth of holes or recesses.
Caliper Setup and Zeroing
Before any measurement is taken, the caliper must be properly prepared to ensure the reading is accurate. First, thoroughly clean the measuring faces of the jaws and the workpiece to remove any dust, oil, or debris that could interfere with the reading. Even a small particle can introduce a significant error in a thousandth-of-an-inch measurement.
Next, gently close the jaws completely using the thumbwheel. The needle on the dial must point exactly to the zero mark when the jaws are fully closed to confirm the zero setting. If the needle is slightly off zero, the bezel—the rotating ring around the dial face—must be adjusted. Loosen the bezel clamp screw, rotate the bezel until the zero mark aligns perfectly with the needle, and then retighten the clamp to lock the zero position.
Step-by-Step Measurement Reading
Reading a dial caliper involves combining the coarse reading from the main scale with the fine reading from the dial indicator. After positioning the jaws snugly around the object, determine the whole inches and tenths of an inch from the main scale. The reference edge of the slider indicates the measurement on the fixed beam, which is typically divided into increments of one-tenth of an inch (0.100″).
For example, if the reference edge has moved past the 1-inch mark and past the second tenth-of-an-inch line (0.200″), the coarse reading is 1.2 inches. This value represents 1.200 inches. Note the last tenth-of-an-inch line that is fully visible before the reference edge.
The next step is to read the fine measurement from the circular dial, which provides the thousandths of an inch. On a standard inch caliper, the dial is divided into 100 increments, where each line represents one-thousandth of an inch (0.001″). The needle’s position on the dial indicates the final two digits of the measurement.
If the needle points to the 35 mark on the dial, this represents 35 thousandths of an inch, or 0.035″. The final measurement is calculated by adding the main scale reading and the dial reading together. In this example, 1.200 inches plus 0.035 inches results in a total measurement of 1.235 inches.
Metric dial calipers follow the same principle. The main scale is marked in millimeters, and the dial typically reads in hundredths of a millimeter (0.01 mm). The main scale provides the whole millimeter reading, and the dial provides the fractional part. The two values are added together to obtain the final measurement in millimeters.
Tips for Accurate Caliper Use
Apply consistent and light pressure when closing the jaws onto the workpiece to prevent distortion of the object or the caliper itself. Applying excessive force, often called “springing” the jaws, results in a smaller, inaccurate measurement. Use the thumbwheel to gently bring the jaws into contact with the object, ensuring the caliper is held perpendicular to the surface being measured.
Once the measurement is set, the locking screw can be engaged to hold the slider in place. This is useful for transferring the measurement or for checking against a specification. Engaging the screw prevents the reading from accidentally changing when the caliper is removed from the part. Proper maintenance is also important for the tool’s longevity and accuracy, including storing the caliper in its case and keeping the rack and pinion mechanism clean from debris.