PRECISION MEASURING AND GAGING - OD1642 - LESSON 1/TASK 2
(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 of the thread.
(d) The interchangeable anvils of various lengths make it possible
to reduce the range of the micrometer. A micrometer having a range of 5 to
6 inches can be changed to one having a 4 to 5, or a 3 to 4 inch range by
inserting a special anvil of the proper length.
(5) Design of Micrometers. The micrometer 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 1/40th of an inch for
each revolution, each space will represent 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 proportionate to the amount it is turned. The screw on a
micrometer has 40 threads per 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.
(6) Construction.
(a) The steel frame is U-shaped, and one end holds the stationary
anvil. The stationary anvil is a hardened button, either pressed or screwed
into the frame.
(b) The steel spindle is actually the unthreaded part of the
screw.
It is the spindle that advances or retracts to open or close the
open side of the U-frame.
The spindle bearing is a plain bearing and is
part of the frame.
(c) The hollow barrel extends from this bearing, and on the side
of the barrel is the micrometer scale, which is graduated in tenths of an
inch, then divided into subdivisions of 0.025 of an inch. The end of the
barrel supports the nut which engages the screw. This nut is slotted and
its outer surface has a taper thread and a nut which makes it possible to
adjust the diameter of the slotted nut, within limits, to compensate for
wear.
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