PRECISION MEASURING AND GAGING - OD1642 - LESSON 1/TASK 2
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 the 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.
c.
Inside Micrometer.
(1) General. An inside micrometer (figure 42 on the following page)
is used to measure inside diameters or between parallel surfaces. They are
available in sizes ranging from 0.200 inch to about 107 inches.
However,
the average inside micrometer set has a range that extends from 2 to 10
inches. The various steps in covering this range are obtained by means of
extension rods. The minimum dimension that can be checked is determined by
the length of the unit with its shortest anvil in place and the screw set to
zero. It consists of an ordinary micrometer head, except that the outer end
of the sleeve carries a contact point attached to a measuring rod.
The
micrometer set may also contain a collar for splitting the inch step between
the rods. The collar, which is 1/2 inch long, extends the rod another 1/2
inch so that the range of each step can be made to overlap the next. The
range of the micrometer screw itself is very short when compared to its
measuring range. The smallest models have a 1/4 inch screw, and the largest
has only a 1 inch screw.
(2) Extension Rods.
The individual interchangeable extension rods
that are assembled to the micrometer head vary in size by 1 inch. A small
sleeve or bushing, which is 0.500 inch long, is used with these rods in most
inside micrometer sets to provide the complete range of sizes.
Using the
inside micrometer is slightly more difficult than using the outside
micrometer, primarily because there is more chance of not getting the same
"feel" or measurement each time the surface is checked.
(3) Using an Inside Micrometer.
(a) The correct way to measure an inside diameter is to hold the
micrometer in place with one hand and "feel" for the maximum possible
setting of the micrometer by rocking the extension
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