Figure 58.
Using outside micrometer.
4. For measuring very large diameters, micrometer calipers are made
in various sizes up to 168 inches. Figure 59 shows a pulley being checked with a
micrometer whose range has been reduced by a special anvil which has been screwed
into the frame. A set of different length anvils permits the use of this
micrometer over a wide range of sizes; yet the spindle only moves 1 inch. This
micrometer has been lightened in weight by the I-section construction and by boring
holes in the frame.
(d) Using inside micrometer. The normal procedure in using an inside
micrometer is to set it across a diameter or between the inside surfaces, remove
it, and then read the dimension. For this reason, the thimble on an inside
micrometer is much stiffer than on a micrometer caliper-it holds the dimension
well. It is good practice to verify the reading of an inside micrometer by
measuring it with a micrometer caliper.
1. Figure 60 shows an inside micrometer with extension rod being
used to check the diameter of a bored hole. Note the arrows which indicate the
direction the operator is feeling for the largest dimension horizontally and the
smallest dimension vertically. Inside micrometers have spherical contact points
which require more practice to "feel" the full diametral measurement. One contact
point is generally held in a fixed position and the other rocked in different
directions to be sure the tool is spanning the true diameter of a hole or the
correct width of a slot.
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