A micrometer can provide an accuracy of at best 0.01 mm or 10 μm. Placing a vernier scale on the micrometer permits us to take readings up to the next decimal place. In other words, one can accurately measure up to 1 μm or 0.001 mm, which is an excellent proposition for any precision workmanship. As illustrated in Fig. 2, in addition to the barrel and thimble scales, a vernier scale is provided next to the barrel scale. Divisions on this vernier scale have to be read in conjunction with the barrel scale to provide the next level of discrimination in readings. The vernier scale consists of a number of equally spaced lines, which are numbered from 0 to 5 or 10, depending on the scale.
Fig.1 Vernier Micrometer
The principle of measurement of a vernier micrometer is very similar to that of a vernier calliper. If a division on the thimble is exactly coinciding with the reference line (line marked 0 in the vernier scale in Fig. 2) of the vernier scale, the reading is taken in a way similar to an ordinary micrometer explained earlier. However, if none of the divisions on the thimble coincide with the reference line, we need to examine which division on the thimble coincides with one of the divisions on the vernier scale. Hence, an additional step is involved in the calculation since the vernier reading should be taken into account.
Fig.2 Vernier micrometer with readings
Refer to Fig. 2, which shows a sample reading. In this case, the thimble has crossed the 12.5 mm mark on the barrel scale. None of the divisions on the thimble coincides with the zeroth line on the vernier scale, that is, the reference line on the barrel. However, the reference line is between the 24th and 25th divisions on the thimble. Suppose the thimble has 50 divisions, and five divisions on the vernier scale correspond to six divisions on the thimble, we can calculate the least count of the instrument as follows.
If one complete rotation of the thimble moves it by 0.5 mm on the barrel scale, the least count of the micrometer scale is 0.5/50 = 0.01 mm. Since five divisions on the vernier scale correspond to six divisions on the thimble, the least count of the vernier scale is equal to 0.01/5 = 0.002 mm. In Fig. 2, the fourth division on the vernier scale is coinciding with a division on the thimble. Therefore, the reading is 12.5 + 24 (0.01) + 4 (0.002) = 12.748 mm.
Guidelines for Use of Micrometers:
1. Before placing the micrometer on the job being measured, bring it near the desired opening. Do this by rolling the thimble along the hand but not by twirling. Hold the micrometer firmly with one hand, and use the feel of the hand to ensure that the axis of the micrometer is perpendicular to the reference plane of the job. Close the micrometer using the ratchet stop until it disengages with a click.
2. Even though a micrometer can be locked in position by tightening the clamp ring (locknut) and used as a snap gauge for inspection purposes, it is not basically designed for this role. Locking the spindle movement and forcing the measuring faces over the job result in sliding friction, which accelerates wear on the contact surfaces as well as on the micrometer screw.
3. The locknut is a memory device. It retains the reading so that it can be read in a convenient position. However, avoid tightening the locknut when the spindle is withdrawn. Doing so will injure the clamping mechanism.
4. It is not wise to buy a micrometer that does not have a controlled force feature. Excessive force while closing the measuring faces over the job will result in rapid wear and tear of the instrument. A ratchet stops acts as an overriding clutch that holds the gauging force at the same amount for each measurement regardless of the differences in manual application of force.
5. While measuring the diameter of a cylindrical part, rock the cylinder to find the maximum opening that provides the desired feel.
6. Do not expect the micrometer to guarantee reliable measurement if it is (a) dirty; (b) poorly lubricated; (c) poorly adjusted; or (d) closed too rapidly.
7. At the end of each day, the micrometer should be wiped clean, visually inspected, oiled, and replaced in its case to await the next use.