How to Take Accurate Readings Using Micrometer Screw Gauge?

A micrometer screw gauge is used for measuring dimensions smaller than those measured by the vernier calipers. A micrometer screw gauge is a small measuring device which works on the “screw” principle. To more about the different parts and the working principle of the micrometer screw gauge read here.

Having a U shaped metallic frame, a micrometer screw gauge measures even the minutest length with acute precision. In the following article we will learn how to read a micrometer with utmost accuracy.

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Just like a vernier calipers, a micrometer screw gauge also carries two scales - a main scale and an auxiliary scale. The main scale is a millimeter scale graduated to 0.5 mm, whereas the auxiliary scale is divided into 50 equal divisions. The auxiliary scale is on the thimble of the screw gauge and measures hundredth of the measurement. The jaws of the gauge are moved rotating the thimble. The auxiliary scale on the thimble is also known as the vernier rotating scale. Moreover, the thimble is so adjusted that 2 revolutions of the thimble will allow the jaws to move by 1 mm. This means that a single rotating will move the jaws only by 0.50 mm. The main scale lies on the part of the screw gauge known as the “sleeve”.

In order to take the reading using a screw gauge, the object is placed between the jaws which are moved by the thimble. The ratchet knob is used to adjust the object firmly between the jaws. For accurate reading, the thimble should be moved until three clicks are heard from the ratchet. The ratchet ensures accuracy and also prevents the object from getting damaged. The main scale reading is taken by considering that marking on the sleeve which is visible just to the left of the thimble. It is also to note that the 0.5 mm divisions that are provided below the main scale should also be considered while taking the reading. The auxiliary scale reading is taken by observing the marking on the thimble that coincides with the main scale on the sleeve. The auxiliary reading figures will follow the main scale reading figures in the final reading. Let’s take an example to understand this.

Imagine that the scales have come to the positions as shown in the figure below, after the jaws are kept around the object. The lock can be used to assure that readings don’t change due to the movement of the thimble. In the figure, it can be seen that the marking on the main scale which is just to the left of the thimble is 7 mm. However, the half scale division that is visible below the main scale shouldn’t be neglected and thus the reading on the main scale will be 7.5mm. For the auxiliary scale reading, it is noted that the 22^nd division on the thimble scale matches with the main scale. Thus the thimble scale reading would be 0.22 mm. The final reading will be the addition of these two readings i.e. 7.5 + 0.22 = 7.72 mm.

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Let’s take one more example to understand it properly. Suppose the scales came to the positions shown in the figure. The main scale reading would be the marking that is fully visible immediately to the left of the thimble, i.e. 5.5 mm. For the auxiliary reading, the 30^th division of the thimble matches with the main scale and thus its reading will be 0.30 mm. The final reading will be the addition of the readings of both the scale i.e. 5.5 + 0.30 = 5.80 mm.

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