MICROMETER CALIPER
The chief mechanical principle embodied in the construction of a micrometer is that of a screw free to move in a fixed nut. The spindle of the micrometer is attached to the thimble at the top point, and extends downward through the inside of the sleeve; the thimble extending downward on the outside of the sleeve. The part of the spindle, which is concealed within the sleeve and thimble, is threaded to fit a nut in the frame of the micrometer. The pitch of the screw threads on the concealed part of the spindle are 40 to the inch. Therefore one complete revolution of the spindle draws it back 1/40, or .025, of an inch.
The sleeve is marked with 40 lines to the inch, corresponding to the number of threads on the spindle. When the spindle is down against the anvil, the beveled edge of the thimble coincides with the lone 0 on the sleeve, and the 0 line on the thimble coincides with the horizontal line on the sleeve. By turning the knurled thimble with the thumb and finger until the 0 line on the thimble again agrees with the horizontal line on the sleeve, the distance between the anvil and the bottom point of the spindle will be 1/40, or .025 of an inch, and the beveled edge of the thimble will coincide with the second vertical line on the sleeve. Each vertical line on the sleeve indicates a distance of 1/40, or .025 of an inch. Every fourth line on the sleeve is made longer than the others, and is numbered 0, 1, 2, 3, 4, etc., up to 0 or the capacity of the micrometer. Each numbered line indicates a distance of four times 1/40 of an inch, or 1/10.
The beveled edge of the thimble is marked in twenty-five divisions, and every fifth line is numbered from 0 to 25. Turning the spindle from one of these marks to the next indicates that the spindle has been moved 1/25 of .025, or one-thousandth of an inch.
Hold the frame stationary and revolve the thimble with the thumb and finger. The spindle, being attached to the thimble, revolves with it, and moves through the nut in the frame, approaching or receding from the anvil. The measurement of the opening between the anvil and the spindle is shown by the lines and figures on the sleeve and the thimble.
To read the micrometer, place the object to be measured on the anvil, turning the thimble up or down until it touches the object lightly. Multiply the amount of vertical divisions visible on the sleeve by 25 and add the number of divisions on the bevel of the thimble from 0 to the line which coincides with the horizontal line on the sleeve.
For example, if there are 5 divisions visible on the sleeve and six lines showing on the thimble, multiply 5 by 25, and add 6. Total .131 of an inch.