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62 Part II: Metrology
cushion of air generated by a completely self-contained pneumatic system. Dedicated
function programs, along with a keypad and interactive LCD display, are
designed to guide operators smoothly and efficiently through a variety of measurement
operations. The gage can be used for direct data transfer to other data
collection devices.
Micrometers
Part II.A.1
The micrometer caliper illustrated in Figure 7.4 is representative of instruments that
use a precision screw as a basis for measuring. The measuring elements consist of
a fixed anvil and a spindle that moves lengthwise as it is turned.
The thread on the spindle of a typical metric micrometer has a lead of ½ or
0.5 mm so that one complete revolution of the thimble produces a spindle movement
of this amount. The graduated scale on the sleeve of the instrument has
major divisions of 1.0 mm and minor divisions of 0.5 mm. Thus, one revolution
of the spindle causes the beveled edge of the thimble to move through one small
division on the sleeve scale. The periphery of the beveled edge of the thimble
is graduated into 50 equal divisions, each space representing 1/50 of a complete
rotation of the thimble, or a 0.01 mm movement of the spindle. Micrometers with
scales in inch units operate in a similar fashion. Typically, the spindle thread has a
lead of .025 inch and the smallest division on the sleeve represents .025 inch. The
periphery of the beveled edge of the thimble is graduated into 25 equal divisions,
each space representing 1/25 of a complete rotation of the thimble or a spindle
movement of .001 inch.
A reading on a micrometer is made by adding the thimble division that is
aligned with the longitudinal sleeve line to the largest reading exposed on the
sleeve scale. For example, in Figure 7.5 the thimble has exposed the number 10,
representing 10.00 mm, and one small division equal to 0.50 mm. The thimble
division 16 is aligned with the longitudinal sleeve line, indicating that the thimble
has moved 0.16 mm beyond the last small division on the sleeve. Thus the
final reading is obtained by summing the three components, 10.00 + 0.50 + 0.16
= 10.66 mm.
Figure 7.4 A 0–25 mm micrometer caliper.
Courtesy Fred V. Fowler Company, Inc.