PRECISION MEASURING AND GAGING - OD1642 - LESSON 1/TASK 2
VERNIER BEVEL PROTRACTOR.
on the main scale, the reading is in exact degrees, as shown in figure 49,
view A, on the following page. When the zero of the vernier scale does not
exactly coincide with a graduation on the main scale, check the graduations
on the vernier and locate one that lines up with a graduation on the main
(b) Once this graduation is located it will indicate the number of
12ths of a degree in units of 5 minutes that is to be added to the whole
degree reading. Example: Figure 49, view B, shows the zero on the vernier
between 12 and 13ƒ on the main scale. Counting to the right from the 0 on
the main scale, the 0 on the vernier has moved 12 whole degrees. Reading in
the same direction (to the right), note that the 10th line of the vernier
scale coincides exactly with a line on the main scale. The tenth line of
the vernier indicates 50 minutes (50"), since each line indicates 5 minutes.
Now add 50 minutes to the 12 and the final reading is 12 and 50 minutes.
Since the spaces, both on the main scale and the vernier scale, are numbered
both to the right and to the left from zero, any angle can be measured. The
readings can be taken either to the right or to the left, according to the
direction in which the zero on the main scale is moved.