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U N I T T H R E E Using Vernier, Dial, and Digital Instruments for Direct Measurements he inspection and measurement of machined parts T require various kinds of measuring tools. Often, the dis- crimination of a rule is sufficient, but in many cases the dis- PRINCIPLE OF THE VERNIER The principle of the vernier may be used to increase the dis- crimination of a rule with a vernier scale is required. This crimination of all graduated scale measuring tools used by a unit explains the types, use, and applications of common machinist. A vernier system consists of a main scale and a vernier dial and digital instruments. vernier scale. The vernier scale is placed adjacent to the main scale so that graduations on both scales can be observed together. OBJECTIVES The spacing of the vernier scale graduations is shorter than the spacing of the main scale graduations. For example, consider a After completing this unit, you should be able to: main scale divided as shown (Figure C-77a). It is desired to fur- ther subdivide each main scale division into 10 parts with the ■ Measure and record dimensions to an accuracy of use of a vernier. The spacing of each vernier scale division is plus or minus .001 in. with a vernier caliper. 1 made 10 of a main scale division shorter than the spacing of a ■ Measure and record dimensions to an accuracy of main scale division. This may sound confusing, but think of it plus or minus .02 mm using a metric vernier caliper. as 10 vernier scale divisions corresponding to 9 main scale divi- ■ Measure and record dimensions using a vernier sions (Figure C-77a). The vernier now permits the main scale to depth gage. 1 1 discriminate to 10 of its major divisions. Therefore, 10 is known as the least count of the vernier. The vernier functions in the following manner. Assume USE OF THE VERNIER that the zero line on the vernier scale is placed as shown (Fig- ure C-77b). The reading on the main scale is 2 plus a fraction For many years the vernier has been used to divide the units of a division. You want to know the amount of the fraction in which a measuring tool measures into smaller increments, over 2 to the nearest tenth, or least count, of the vernier. As you permitting high discrimination measurement. Although the inspect the alignment of the vernier scale and the main scale vernier is highly reliable and accurate, the nature of its func- lines, you note that they move closer together until one line on tion makes it somewhat difficult to read, thus requiring more the vernier scale coincides with a line on the main scale. This time and skill on the part of the machinist. Modern precision is the coincident line of the vernier and indicates the fraction manufacturing techniques and the wide application of digi- in tenths that must be added to the main scale reading. The tal microelectronics developed in recent years have yielded vernier is coincident at the sixth line. Since the least count of many types of high-discrimination measuring instruments 1 6 the vernier is 10, the zero vernier line is 10 past 2 on the main that demonstrate both ease of use and excellent reliability. scale. Therefore, the main scale reading is 2.6 (Figure C-77b). Thus the use of the mechanical vernier has declined. As digi- tal electronics replace more mechanical measurement, use of the vernier may in time disappear completely. However, DISCRIMINATION AND APPLICATIONS vernier measuring equipment is still much used in both OF VERNIER INSTRUMENTS school and industrial shops, and it is likely that vernier appli- cations will be around for some time to come. The theory Vernier instruments used for linear measure in the inch sys- 1 and applications of the vernier as a device for subdividing tem discriminate to .001 in. (1000). Metric verniers generally 1 increments of measurement are discussed in this unit. discriminate to 50 mm. 116
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