Precision Measuring Instruments

With a need to make quality products which meet design specified tolerances, a large number of firms, research and development centers, and school and college laboratories use measuring instruments that have high accuracy and precision. The special branch of science that deals with such instruments is known as metrology. Here we will look at the features and use of certain well known precision measuring instruments used for linear measurement.

1. Vernier Caliper: It is an instrument used to measure internal and external dimensions of an object with a great accuracy. It is provided with inward jaws and outward jaws to facilitate the measurement. A screw clamp is provided that can lock the position of the vernier scale, so that the instrument can be moved without disturbing the reading. A manually operated vernier caliper has a main scale (in millimeters or inches) and a sliding vernier scale attached to the movable jaw, as shown in the figure.

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The term "least count" is used to define the accuracy of a vernier caliper. It is the ratio of the smallest division on the main scale to the total number of divisions on vernier scale. So when we measure a dimension of an object using a vernier caliper, the formula for final dimension is,

Final Dimension=Main scale reading + (Vernier scale reading x Least count)

Disadvantage: It is very delicate to choose the vernier scale reading, which is the line on the vernier scale, exactly coinciding to a line on the main scale. Reading accuracy thus depends on the operator’s skill and eyesight.

Nowadays, vernier calipers with digital displays or dial gauges are available. These reduce the complexity of choosing the coincidence of main scale and vernier scale and calculating the final reading as in manually operated vernier caliper. But due to less cost, manually operated vernier calipers are more popular.

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2. Micrometer Screw Gauge: To measure diameter of thin wire or thickness of thin metal sheets with accuracy, micrometer screw gauge is very useful. It has U-shape frame with fixed sleeve. Rotating thimble movement is converted to linear movement of spindle. Main scale and vernier scale are displayed on sleeve and thimble respectively.

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In the case of the micrometer screw gauge, the least count is the ratio of pitch to the number of divisions on the circular scale. Here the distance moved by the spindle per revolution is termed as "pitch." When we put a wire in between the anvil and spindle, we rotate the spindle till it touches the wire. The ratchet will take care of the excessive pressure on the wire. So the final diameter of wire can be given by the formula

Final reading = Linear scale reading + (Coinciding circular scale reading x Least count)

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A digital version of the micrometer screw gauge which gives a direct reading is also available. For special engineering applications like "over-pin diameter of gear" measuring, gear tooth micrometers are provided with ball-anvil spindles which can be changed according to gear profile. Tooth thickness micrometer measures "root tangent of a gear."

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3. Dial Indicators: Measurement of deflection is needed in many industrial applications. Examples of this include measurement of the movement of a car body while testing in bending moments and checking the waviness of a surface of a machined component. In both cases, small linear displacements are to be measured with accuracy. Dial indicators, mounted on magnetic stand provide accurate measurements of deflection for the parts under study. They are also available in an electronic version.

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4. Height Gauge and Surface plate: Accurate height measurement of machined components can be done with a height gauge placed on a surface plate. The working principle of a height gauge is similar to the vernier caliper. It is available in all the three types, i.e. with a conventional vernier scale or dial gauge or a digital display. The surface plate is a thick solid granite plate placed horizontal to the ground. A sharp pointed scriber is provided on the gauge to make a mark on the object. The object to be measured for height is placed on the surface plate. With the help of an adjusting screw, the scriber can be moved up and down.

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Here we discussed the precision measuring instruments for linear measurements widely used in industry according to their features.