The caliper is a must-have machinist tool especially you need precise measurement in the machine shop. The common caliper tools including dial calipers, vernier calipers, digital calipers, etc. To avoid getting confused with brake calipers, in this article, we’ll introduce what is a caliper as a measuring tool, and different types of calipers and their uses.
A caliper is a measuring tool or device that consists of two adjustable legs or jaws for measuring the distance between two opposite sides of an object. Calipers are often employed for measuring thicknesses and internal or external diameters inaccessible to a scale. To obtain accurate sizes of a CNC machined part, the calipers need to be adjusted to the required dimension on a ruler or a standard plug or hole gauge. The sense of touch of the operator will affect the measurement accuracy.
Calipers can be used to take measurements of outside size, inside size, depth, and step. Outside calipers measure the thickness and outside diameters, inside calipers measure hole diameters and distances between surfaces. Calipers are often used for precise measurement in scientific labs and engineering schools, the tool is also widely used in many types of disciplines, including medicine, science, mechanical engineering, woodworking, metalworking, and forestry.
Calipers come in a variety of shapes and designs, they have different specifications for accordingly purposes. Let’s take a look at different types of calipers and their uses. (All images are from Google)
Inside calipers are used to measure the internal size of an object, including the internal distance of objects and inner linear dimensions of surfaces featuring grooves or holes in the workshop. There are several types of inside calipers, depending on the design of the two legs of the caliper, some calipers have one leg featuring a manual adjustment and others featuring an adjusting screw.
Outside calipers are used to measure the external size of an object, which can deliver a high level of accuracy and repeatability as long as you understand their limitations and uses. Outside caliper is ideal for measuring large distances and curved surfaces like pipes due to its bow-leg design and the extensibility of the feet.
The vernier caliper is named after the Frenchman Pierre Vernier who invented it, this type of caliper has dual scales on its beam that show both imperial and metric measurements at the upper and lower scale. The working of a vernier caliper relies on the main scale and addition of the vernier scale, which also displays metric and imperial results. The two sets of jaws consist of a larger pair that measures external diameters and a smaller pair for internal diameters. The calipers comprise a calibrated scale with a fixed jaw, and another jaw, with a pointer, that slides along the scale, the distance between the jaws is indicated by the reading. When measuring with the vernier scale, you need to read the fixed scale first and read the one with a more accurate result. The easiest situation is that you can read the position of the pointer directly, for the more complex situations, when the pointer is between two markings, the user can interpolate to improve the precision of the reading with the help of a vernier scale, this is how the vernier caliper works. The vernier, dial and digital calipers are identical in function but vary in the reading method.
The divider caliper sometimes is referred to as compass, used to mark out locations, it has sharp pointers, one leg can be placed in in the dimple created by a center and the other leg pivoted so that it scribes a line on the surface of the object, then forms a circle or arc. The divider caliper can also be used to measure the distance between two points on a map, however, you will need a ruler to measure the distance of the opening of the caliper and convert it to the actual value.
Different from the vernier scale, the dial caliper reads the result (fraction of a millimeter or inch) on a simple dial. On the dial caliper, there is a small, precise rack and pinion drives a pointer on a circular dial, usually, the pointer rotates once every inch, a tenth of an inch, or 1 millimeter, when measuring with a dial caliper, you need to read the whole inches or centimeters from the slide and then add the measurement in the circular dial. The dial is generally designed to be rotary for the measuring of the difference in size between two objects. The slide of a dial caliper can be locked using a lever or screw.
What makes a digital caliper differ from a dial and vernier caliper is that it uses an electronic display to replace the dial or vernier scale, to show the reading as a numeric value. Digital calipers use a linear encoder, most digital calipers can be switched between centimeters or millimeters, and inches. At any point, you can zero the display, the caliper may be able to hold the reading, allowing to keep the dimension reading when it is used in where the display can’t be seen. The digital caliper is a useful measuring tool especially for beginners because of its ease of use. Some digital calipers may offer data output and interface with a dedicated recorder, to record a series of measurements, which saves time and improves reliability.
Oddleg calipers are generally used for marking a line at a certain distance from the edge of a workpiece. The bent leg is used to run along the workpiece edge while the scriber makes its mark at a predetermined distance, this ensures a line parallel to the edge.