c. The frame may be in the form of a base so that the gage can
be used as a bench micrometer.
d. The frame may have a wooden handle and may be of extra-heavy
construction for use in a steel mill for gaging hot sheet metal.
3. The spindle and anvil may vary in design to accommodate special
physical requirements. For example:
a. The spindle and anvil may be chamfered so that the gage can
slide on and off the work easily, as when gaging hot metal.
b. The ball-shaped anvil is convenient in measuring the
thickness of a pipe section of small diameter.
c. The V-shaped anvil is necessary on the screw thread
micrometer caliper to mesh properly with the screw thread. The spindle of the
screw thread micrometer is cone-shaped. This micrometer measures the pitch
diameter.
d. The interchangeable anvils of various lengths make it
possible to reduce the range of the caliper. A micrometer having a range from 5 to
6 inches can be changed to one having a 4- to 5-, or 3- to 4-inch range by
inserting a special anvil of the proper length.
(b) Inside micrometers. Inside micrometers are used to measure inside
dimensions. 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 up 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 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 micrometer set may also
contain a collar for splitting the inch step between two rods. The collar, which
is 1/2 inch long, extends the rod another half 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, the average have a 1/2-inch screw, and the largest inside micrometers have
only a 1-inch screw.
(2) Mechanics of micrometers.
(a) Design. The micrometer (fig 56) makes use of the relation of the
circular movement of a screw to its axial movement. The amount of axial movement
of a screw per revolution depends on the thread, and is known as the lead. If a
circular nut on a screw has its circumference divided into 25 equal spaces, and if
the nut advances axially 1/40 inch for each revolution, then if it is turned one
division, or 1/25 of a revolution, it will move axially 1/25 x 1/40, or 1/1000 of
an inch. In the micrometer the nut is stationary and the screw moves forward
axially a distance proportional to the amount it is turned. The screw on a
micrometer has 40 threads to the inch, and the thimble has its circumference
divided into 25 parts, so 1 division on the thimble represents an advancement of
1/1000 of an inch axially.
(b) Construction.
1. The steel frame is U-shaped, one end of which holds the
stationary anvil. The stationary anvil is a hardened button either pressed or
screwed into the frame.
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