Caliper

A caliper is a precision measuring instrument used to measure the distance between two opposite sides of an object — its outside diameter, inside diameter, or depth. The most common type is the vernier caliper, which reads measurements from a sliding scale.

How it works

A caliper has two jaws: fixed jaws on the main body and sliding jaws on a movable carriage. You open the jaws, place them around (or inside) the object, and close them until they contact the surface. The measurement is then read from the scale on the main beam.

Types of calipers

• Vernier caliper: Uses a vernier scale — a secondary scale that slides along the main scale — to achieve precision of 0.02 mm or 0.001 inches. The vernier principle allows readings between the main scale’s graduations without a magnifying lens.
• Dial caliper: Swaps the vernier scale for a dial indicator that shows the fractional reading as a needle on a circular face. Easier to read than vernier but slightly bulkier.
• Digital caliper: Uses an electronic sensor and LCD display to show the measurement directly. The most convenient type for quick work, though it requires a battery.

What they measure

Most calipers can measure three things:

• Outside dimensions: Using the lower jaws — diameter of a rod, thickness of a plate.
• Inside dimensions: Using the upper jaws — diameter of a bore, width of a slot.
• Depth: Using the depth probe that extends from the end of the beam — depth of a counterbore or recess.

Accuracy and care

A good vernier caliper reads to 0.02 mm (about 0.001 inches). To maintain accuracy, keep the jaws clean, store the caliper in its case, and never use it as a clamp or pry bar. Calipers are measuring instruments, not holding tools.