LO 5-3 Identify and describe common instruments and techniques to make linear precision measurements

Sliding Calipers

After simple rulers, various types of calipers are the frequently used measuring tools to make linear measurements of outside, inside, and depth dimensions. Sliding-type calipers range in size from 6–12" in length (15–30 cm) and have markings on graduated scales with either SI or USCS scales. The most common type of caliper that engine technicians use is a sliding caliper, which has two pairs of jaws attached to a frame or bar resembling a steel ruler FIGURE 5. While calipers can measure either inside or outside dimensions, the most versatile have a pair of jaws to measure both inside and outside dimensions, and they have a depth bar attached to the movable section that extends from the end of the caliper bar FIGURE 6. Jaw tips have knifelike edges shaped to make inside or internal measurements more precise on threaded fasteners or round objects. Only one each of the pairs of jaws is movable and slides along the graduated scale marked or stamped into the bar. The opposite jaw is fixed and does not move. A knurled roller, operated with a thumb, moves the sliding jaws along the bar. This allows the caliper to be operated with only one hand. A locking screw on the sliding jaw also allows the operator to lock the movable jaw after taking a measurement.

FIGURE 05-5: A sliding vernier-type caliper with a dial indicator instead of a vernier scale.

FIGURE 05-6: Note the sliding caliper measurement methods for determining inside, outside, and depth dimensions.

To use a sliding caliper, the movable jaw is adjusted to allow its tips to be placed inside or over the work piece FIGURE 7. The thumb roller is turned until the moveable jaw contacts the work with moderate force. The locking screw is tightened before removing the caliper from the work piece to read the measurement. A similar procedure is used to take a depth measurement. If sliding calipers are used to take depth measurements, the bar rests squarely on the upper part of the work surface, such as a ledge around a hole. The depth bar is lowered until it contacts the lower work surface requiring measurement and the adjustment is locked FIGURE 8.

FIGURE 05-7: Using the tips of the sliding caliper jaws to measure inside and outside dimensions.

FIGURE 05-8: Using a sliding caliper to measure the protrusion of a replaceable valve guide from the machined surface of the cylinder head.

To read the dimension, technicians must first carefully note which main scale on the caliper bar is used to measure inside, outside, and depth dimensions. The bar may have the letters “IN” and “OUT” on the upper or lower side of the main scales stamped into the caliper bar or use two separate pointers on the movable jaw for each main scale. The edge of the movable jaw or other marker indicates a dimension on the main scale of the ruler-like bar.