End-Measuring Instrument

End-Measuring Instrument


any of a certain class of instruments that are used to measure or lay out inside and outside dimensions. An end-measuring instrument consists of two measuring surfaces, which are usually called jaws. A dimension is determined between the two measuring surfaces. One of the surfaces constitutes the base of a single unit that includes a rule, or a graduated bar; the other surface is on a head that slides along the rule. The rule is divided into millimeters, and a vernier scale is installed or engraved on the sliding head. To increase their reliability, end-measuring instruments are made of materials that have a high abrasion resistance and are not subject to corrosion, such as hardened steels. In addition, the measuring surfaces are chrome-plated or are reinforced by a hard alloy. Plastic end-measuring instruments are also used.

In the USSR, several types of end-measuring instruments with readings in 0.05 or 0.1 mm are manufactured in several standard sizes. The manufacture of end-measuring instruments with readings in 0.02 mm has been discontinued. Depending on their purpose and design features, end-measuring instruments are classified as vernier calipers, height gauges, height-and-depth gauges (seeDEPTH GAUGE), or gear-tooth calipers.

The vernier caliper (Figure 1) is the most widely used end-measuring instrument. The first vernier calipers appeared in the late 18th century in London, although wooden caliper rules without verniers were used as early as the 17th century. Three types of vernier calipers are manufactured in eight standard sizes; the types differ in design and in the number of measuring jaws. For measurements of up to 400 mm, the jaws may be extended from the zero reading. For larger dimensions, the zero point of a measurement does not coincide with the zero mark.

Vernier calipers with an instrument range extending from 0 to 125 or 150 mm and with readings in 0.1 mm have two sets of jaws (Figure 1 ,a) as well as a rule depth gauge for measuring such dimensions as the depth to a shoulder or the depth of a groove or a hole. The lower jaws are used to measure outside dimensions; the upper jaws, inside dimensions.

Vernier calipers with an instrument range extending up to 250 or 160 mm and with readings in 0.1 or 0.05 mm (Figure l,b) also have two sets of jaws. However, the lower jaws are used for both outside and inside measurements, while the upper jaws are employed to lay out or measure outside dimensions inside narrow shallow grooves and holes. The upper jaws may be used to scribe parallel lines, circles, and other elements of the shape of a part to be machined (seeLAYOUT TOOL). In such vernier calipers, the jaws for measuring inside dimensions have cylindrical surfaces. The size of the jaws when closed is usually 10 mm; the jaw size is stamped on a surface that is not used for measurement and is added to the reading when inside dimensions are measured.

Vernier calipers with an instrument range extending to an upper limit of 400 to 2,000 mm have a single set of jaws that is similar in design to the lower jaws of the caliper shown in Figure 1 ,b. The last two types of vernier calipers have a micrometer feed, which is used mainly in layout work, for the more precise determination of a dimension. The readings on such calipers are in 0.1 mm.

In contrast to a vernier caliper, a height gauge (Figure 2) has a base rather than a fixed jaw. Measurements are made from the bottom of the base, which corresponds to the zero reading on the scale. The sliding head is equipped with a holder rather than a movable jaw. For layout work, a stylus or a scriber is attached to the holder; for measurement, a special set of measuring jaws or a bracket for fastening a meter—such as a dial indicator—is attached to the holder. Height gauges are usually employed in layout work on plates. In such work, the gauge is mounted on the plate together with the workpiece to be laid out or measured. On a workpiece to be laid out, lines are scribed with a scriber when the height gauge is moved over the surface of the plate. Height gauges are manufactured in six standard sizes with readings in

Figure 1. Vernier calipers: (1) graduated bar, (2) sliding head, (3) vernier, (4) upper jaws, (5) lower jaws, (6) rule depth gauge, (7) micrometer feed, (8) setscrews

Figure 2. A height gauge: (1) graduated bar, (2) base, (3) scriber

0.05 mm for an instrument range extending to an upper limit of 400 mm or with readings in 0.1 mm for instrument ranges extending to an upper limit of 400 to 2,000 mm.

A gear-tooth caliper, which is a combination of a height-and-depth gauge and a vernier caliper, is designed to measure the thickness of gear teeth. The vertical rule of a gear-tooth caliper is used to determine the depth of a tooth from the tooth’s tip, where the tooth thickness is measured. The horizontal caliper is used for the direct measurement of the tooth thickness. Gear-tooth calipers with readings in 0.05 mm are manufactured in two standard sizes for the measurement of the tooth thickness of gears with a modulus of up to 36 mm. Owing to the rapid abrasion of their tips and to their relatively low accuracy, gear-tooth calipers are being replaced to an increasing extent by displacement tooth gauges and tangent tooth gauges.