This section is from the book "Machine Shop Work", by Frederick W. Turner, Oscar E. Perrigo, Howard P. Fairfield. Also available from Amazon: Machine shop work.

A common use of a Vernier is its application to a caliper square, termed a Vernier caliper. Fig. 24 shows a representative tool.

The following text represents the L. S. Starrett instructions for reading their tool:

The scale of the tool is graduated in fortieths, or .025 of an inch, every fourth division, representing a tenth of an inch, being numbered. On the Vernier plate is a space divided into twenty-five parts and numbered 0, 5, 10, 15, 20, 25. The twenty-five divisions on the Vernier occupy the same space as the twenty-four divisions on the scale.

The difference between the width of one of the twenty-five spaces on the Vernier and one of the twenty-four spaces on the scale is, therefore, 1/25 of 1/40, or 1/1000 of an inch. If the Vernier is set so that the 0 line on the Vernier coincides with the 0 line on the scale, the next two lines will not coincide by 1/1000 of an inch; the next lines will be two thousandths apart, and so on.

Fig. 24. Front and Back View of Vernier Caliper Courtesy of L. S. Starrett Company, Athol, Massachusetts.

To read the tool, note how many inches-tenths, or .100, and fortieths, or .025-the 0 mark on the Vernier is from the 0 mark on the scale; then note the number of divisions on the Vernier from 0 to a line which exactly coincides with a line on the scale.

In Fig. 25 the Vernier has been moved to the right one and four-tenths and one-fortieth inches (1.425"), as shown on the scale, and the eleventh line on the Vernier coincides with a line on the scale. Eleven thousandths of an inch are therefore to be added to the reading on the scale, and the total reading is one and four hundred and thirty-six thousandths inches (1.436"), which is the distance the jaws of the tool have been opened.

In making inside measurements, the width of the jaws, as given in the list, is to be added to the apparent readings on the side having the Vernier to allow for the space occupied by the measuring points. No such allowance is necessary when using the back side, without Vernier, as the two lines marked "in" and "out" indicate inside and outside measurements.

1. A micrometer caliper shows a reading of .463; how many times must the thimble be turned to produce a reading of .587? (Assume 40 threads per inch.) Ans. 4 24/25 times

2. What are the readings of the micrometer calipers shown in Figs. 26 and 27? Ans. .039

3. State the readings of the micrometer calipers shown in Figs. 28 and 29.

Ans. .1546

Fig. 25. Enlarged View of Vernier.

Figs. 26 and 27. Positions of Caliper for Example 2.

Figs. 28 and 29. Positions of Caliper for Example 3.

4. Give the readings of the micrometer calipers shown in Figs. 30 and 31. Ans. .7398

5. Ske h the front and back of a micrometer caliper when the reading is .6327.

6. Whas is the reading of the Vernier and scale when in position Fig. 32?

Ans. 6.36

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