PRECISION MEASURING AND GAGING - OD1642 - LESSON 1/TASK 2
(d) The thimble is attached to the screw and is a sleeve that fits
over the barrel. The front edge of the thimble carries a scale divided into
25 parts. This scale indicates parts of a revolution, where the scale on
the barrel indicates the number of revolutions. The thimble is connected to
the screw through a sleeve that permits it to be slipped in relation to the
screw for the purpose of adjustment.
The inner sleeve is sweated to the
screw.
The outer sleeve is clamped to the inner one by the thimble cap.
Loosening the cap makes it possible to slip one in relation to the other.
(e) On top of the thimble cap there may be a ratchet. This device
consists of an overriding clutch held together by a spring in such a way
that when the spindle is brought up against the work, the clutch will slip
when the correct measuring pressure is reached. The purpose of the ratchet
is to eliminate any difference in personal touch, and so reduce the
possibility of error due to a difference in measuring pressure.
Not all
micrometers have ratchets.
(f) A clamp ring or locknut is located in the center of the
spindle bearing on those micrometers equipped with it. This clamping makes
it possible to lock the spindle in any position to preserve a setting.
(7) Use of Micrometers.
(a) Reading a Standard Micrometer.
1 Reading a micrometer (figure 37 on the following page) is
only a matter of reading the micrometer scale or counting the revolutions of
the thimble and adding to this any fraction of a revolution. The micrometer
screw has 40 threads per inch. This means that one complete revolution of
the micrometer screw (1), view A, moves the spindle (2) away from or toward
the anvil (3) exactly 1/40 or 0.025 inch.
2 The lines on the barrel, view B, (4) conform to the pitch
of the micrometer screw (1), each line indicating 0.025 of an inch, and each
fourth line being numbered 1, 2, 3, and so forth.
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