Lesson 1/Learning Event 1
c. A hollow barrel extends from this bearing.
There is a micrometer
scale on the side of the barrel which is graduated in tenths of an
These are in turn divided into subdivisions of 0.025 inch.
The end of the barrel supports the nut which engages the screw
(adjustment nut). This unit is slotted, and its outer surface has a
tapered thread and a nut. These make it possible to adjust, within
limits, the diameter of the slotted nut to compensate for wear.
d. A thimble is attached to the screw. The thimble is a sleeve that
fits over the barrel. The front edge of the thimble carries a scale
broken down into 25 parts.
This scale indicates parts of a
revolution, while the scale on the barrel indicates the number of
The thimble is connected to the adjustment screw
through a sleeve that permits it to be slipped in relation to the
screw to allow for 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
e. Sometimes there is a ratchet on top of the thimble cap. 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.
ratchet eliminates the effects of differences in personal touch and
thus reduces the possibility of error due to differences in
measuring pressure. Not all micrometers have ratchets.
f. Some micrometers have a clamp ring or locknut located in the center
of the spindle bearing. This clamping device makes it possible to
preserve a setting by locking the spindle in any position desired.
The following rules should be followed to keep micrometers in good condition
and to preserve their accuracy:
a. Never store a micrometer with its anvil and spindle closed. Leave a
small gap between the anvil and spindle when storing a micrometer
since flat surfaces which have been pressed together for any length
of time tend to corrode.