In the vernier instrument shown in Fig. 1 the reading is as

Reading a Vernier Protractor.-To read any vernier protractor, read off directly the number of whole degrees between the zero of the scale and the zero of the vernier scale. Add to this reading the number of minutes shown by the number

under that line of the vernier scale which coincides with a line of the scale.

In the vernier protractor shown in Fig. 2, there are 52 whole degrees between the zeros of the scale and the vernier scale. The line numbered 45 of the vernier scale coincides with a line of the scale. Hence, the reading of the protractor is 52° 45'.

Setting Vernier Protractor.-The operation of setting the protractor is the converse of that of reading it. For example, if it were required to set the instrument to 52° 45', the operation

would be as follows; Move the true scale to the right until¦ the 52° mark is opposite the zero of the vernier; then, looking at the vernier scale, move the true scale carefully to the right until the line numbered 45 on the vernier scale coincides with a line of the true scale.

Reading Micrometer Calipers.-The hub, or stationary barrel, of the micrometer is divided into 10 equal divisions, numbered 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 each representing or .1 in. Each division is subdivided into four divisions, each of which equals 1880.

0-25

or .025, in. The

show the 25 divisions. To read the micrometer, look for the

last figure that is exposed on the barrel. first decimal figure is 2 (since .2 in.).

=

If it is 2, then the

To the right of this

figure place the number indicated by the number of whole subdivisions between the last exposed figure and the sleeve. Each of these subdivisions equals .025 in., and if there are three subdivisions exposed, 75 must be placed to the right of the first decimal, or 2, giving .275. Next add to the third decimal figure, the number of divisions on the sleeve from 0 to the line which coincides with the horizontal line c on the barrel. If there are four divisions, add 4 to the decimal already obtained, giving .279 in. as the final measurement. A little practice makes the entire mental operation the work of a

moment.

EXAMPLE.-State the reading of the micrometer shown in

Fig. 4.

There is one

SOLUTION.-The last figure that is exposed is 1. whole space between this figure and the end of the sleeve. Since there is one whole space between this figure and the end of the sleeve, annex .025. The line at the end of the fifth space of the sleeve coincides with the

zero line on the barrel, hence add .005 in., making the reading .1+.025+.005=.130 in.

6

Ans.

0-26

Reading Vernier Micrometers.-Vernier micrometers read to ten-thousandths of an inch by having ten horizontal graduations on the sleeve d, as shown in Fig. 5. To read the vernier micrometer, first read to three decimal places as directed in connection with the plain micrometer, Fig. 4. To obtain the fourth decimal figure, note which line of the vernier scale coincides with a line on the sleeve. Count the number of spaces from the zero line of the vernier scale to this line, count

ing on the vernier scale. This number is the fourth decimal figure of the reading.

EXAMPLE.-Read the vernier micrometer shown in Fig. 5. SOLUTION.-Applying the instructions already given for the plain micrometer, the reading to three decimal places is .5 +.025+.018=.543 in. The line at the end of the second space of the vernier scale coincides with a line of the sleeve. Hence, the fourth decimal figure is 2, and the complete reading of the instrument is .5432 in. Ans.

Precision Indicators.-The universal test indicator shown in Fig. 6 will indicate any lack of trueness of inside, outside, or surface work. It can be clamped to a holder in the tool post for lathe work, or to the spindle of a surface gauge for surface work. One of the working points, a or a', both equally distant from the fulcrum pin o, is brought into contact with the work, and the amount of variations

on the work is indicated by the pointer b on the scale cin thousandths of an inch.

FIG. 6

FIG. 7

Cut Meter.-The cut meter, illustrated in Fig. 7 is used to measure the surface speed of a body, in feet per minute, without either using a watch to time the operation or making calculations. The instrument is grasped by the handle a and the wheel b is held against the face of the work whose surface speed is to

be found.

The surface speed, in feet per minute, may then be read directly from the graduated scale c. It is necessary to hold the instrument in such a position that the shaft d is parallel to the face of the work.

Sine Bar.

The sine bar, shown in Fig. 8, is useful for accurately determining the angle of finished surfaces, locating work

FIG. 8

at any required angle, testing gauges, and for miscellaneous purposes in laying out or testing work where angles are involved. The bar a made of steel, hardened and ground so that its edges are parallel with the center line of the plugs b which are located at the ends of the bar. The plugs b are of the same diameter and are a whole number of inches between centers.

To determine any given angle by the use of the sine bar, first find the difference in height of the two studs, divide this difference by the distance between the centers of the two studs and the result will be the sine of the angle. The angle will be found by referring to the table of sines.

The difference in height of the two plugs can be measured by the inside micrometer as shown.

Taper Gauge.-In Fig. 9, a and b are two straightedges that are ground and lapped as nearly true as possible. They are so mounted in a suitable frame that they can readily be shifted and