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.

Reading in thousandths. As stated, the micrometer screw has usually forty threads per inch and the thimble has twenty-five divisions on its circumference. The barrel is divided to correspond to the pitch of the screw with each fourth division numbered. In reading the indicated measurement, first note the highest number visible on the barrel and call it hundreds of thousands-in Fig. 21 it is 400 thousandths or .400; then read the short divisions on the barrel, calling the first division 25 thousandths, or .025; the second, 50 thousandths, or .050; and the third, 75 thousandths, or .075. In Fig. 21 the third division is the last one visible. Now read the number indicated on the thimble, that is, the number that has passed the line running lengthwise. In the figure it is 16; or 16 1/2 if the reading is to be finer than thousandths. Add this reading to the readings of the short divisions, thus: 75+ 161/2 = 91 1/2; this is .091 1/2. Adding the .400 to this we get .491 1/2.This means that the distance from the anvil to the measuring point is 4915/10000 of an inch, or .4915 inch. If the micrometer caliper is a good one, we may be sure the distance is between .491 inch and .492 inch.

Fig. 22. Inside Micrometers Courtesy of L. S. Starrett Company, Athol, Massachusetts.

In reading measurements finer than thousandths, use is made of a Vernier. The following description tells how to read a ten-thousandths micrometer: As applied to a micrometer, the Vernier consists of ten divisions on the sleeve which occupy the same space as nine divisions on the thimble. The difference of width of one of the ten spaces on the sleeve and one of the nine spaces on the thimble is one-tenth of a space on the thimble. In Fig. 23 at B, the third line from the zero on the thimble coincides with the first line on the sleeve. In opening the tool by turning the thimble to the left, each space on the thimble represents an opening of the tool equal to one-thousandth of an inch. If the thimble be turned so the lines marked 5 and 2 coincide, the tool will have been opened two-tenths of one-thousandth, or 2 ten-thousandths. At C the thimble has been turned until line 10 matches with line 7 on the sleeve. The tool has therefore been opened 7 ten-thousandths. Therefore, first note the thousandths as in reading the ordinary micrometer, then observe the line on the sleeve which matches with a line on the thimble. If it is the second line, marked 1, add one ten-thousandth to the previous reading; if the third line, marked 2, add 2 ten-thousandths, etc.

A

B

Fig. 23. Diagrams Showing How to Read Micrometer Caliper

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