PRINCIPLE OF THE VERNIER

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PRINCIPLE OF THE VERNIER Powered By Docstoc
					                                           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.

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