HOW TO READ A MICROMETER CALIPER

By George Leonard Herter, Edited by Lisa M. McMahon

The illustration below shows Herter's adjustable Rod Builder's Micrometer with a rod strip being measured. The micrometer reading is 290 thousandths of an inch. The figures 1 and 2 are completely visible. The distance between each of these numbers represents 100 thousandths of an inch. Each marked interval between these numbers represents 25 thousandths of an inch. The numbers on the thimble (E) represents thousandths. Numbers 1 and 2 are completely visible (200 thousandths). But the number 3 is not completely visible so only the three marks that are completely visible between the numbers 2 and 3 are counted. This gives us three markings on the barrel (D) (75 thousandths) and 15 thousandths on the thimble (E) which totals the reading to 290 thousandths  of an inch.

In fine bamboo rod building, it is necessary to use a micrometer to measure your bamboo rod strips for absolute accuracy. Bamboo rod strips are always measured from the apex of the strip to the center of the enamel side, as the illustration shows. The thickness of the enamel and under-ena-mel are not counted in measuring, so if they are left on the strip they should be deducted.

It is not difficult to learn to read a micrometer. In fact, if you follow these instructions and do not get impatient, you can learn to read a micrometer in ten minutes.

The paragraphs below describe the various parts of the micrometer, how it works and how to read it. In using a micrometer to measure remember to never turn the thimble more than enough to barely make the spindle touch the object to be measured. The spindle (C) is attached to the thimble (E) on the inside. The spindle is threaded to fit a nut in the frame (A), where it passes through the frame. The frame is held stationary, the thimble (E) is revolved by the thumb and the finger, and the spindle (C), being attached to the thimble, revolves with it and moves  through the  nut in the frame, approaching or receding from the anvil (B). The article to be measured is placed between the anvil (B) and the spindle (C). The measurement of the opening between the anvil and the spindle is shown by the lines and figures on the barrel (D) and the thimble (E).

The pitch of the screw threads on the concealed part of the spindle is 40 to an inch. One complete revolution of the spindle therefore moves it longitudinally one fortieth (or twenty-five thousands) of an inch. The barrel (D) is marked with 40 lines to the inch, corresponding to the number of threads on the spindle. When the caliper is closed, the beveled edge of the thimble coincides with the line marked O on the barrel, and the O line on the thimble agrees with the horizontal line on the barrel. Open the caliper by revolving the thimble one full revolution or until the O line on the thimble again coincides with the horizontal line on the barrel; the distance between the anvil (B) and the spindle (C) is then 1/40 or (.025) of an inch, and the beveled edge of the thimble will coincide with the second vertical line on the barrel. Each vertical line on the barrel indicates a distance of 25 thousandths of an inch. Every fourth line is made longer than the others, and is numbered 0, 1, 2, 3, etc. Each numbered line indicates a distance of four times 1/40 of an inch, one-tenth of an inch or one hundred thousandths.

The beveled edge of the thimble is marked in twenty-five divisions, and every fifth line is numbered, from 0 to 25. Rotating the thimble from one of these marks to the next moves the spindle longitudinally 1/25 of twenty-five thousands, or one thousandth of an inch. Rotating it two divisions indicates two thousandths, etc. Twenty-five divisions will indicate a complete revolution, .025 or 1/40 of an inch.

To read the caliper, therefore, multiply the number of vertical divisions completely not partially visible on the barrel by 25, and add to this figure the number of divisions on the bevel of the thimble, from 0 to the line which coincides with the horizontal line on the sleeve, thus you have the reading of the micrometer.