(1) Types of rules.
(a) Steel rules. Steel rules (fig 41) are available from a fraction of
an inch in length up to 4 feet or more, but in machine shops the 6-inch pocket rule
is the one most commonly used. There are also several standard systems of
graduations. In the English system, rules are graduated in 10ths, 20ths, 50ths,
and 100ths; 12ths, 24ths, and 48ths; 14ths and 28ths; and 16ths, 32ds, and 64ths of
an inch. In the Metric system, rules are graduated in millimeters and one-half
millimeters. Some steel rules have four scales, two on each side (one graduated in
32ds and the other in 64ths), with the scales on the reverse side running in the
opposite directions. There are rules made that have both an inch scale and
millimeter scale, which makes this type rule adaptable to work involving both
systems of measure. Another feature on some rules is a scale etched across the end
of the rule which facilitates measurement in restricted places.
1. Flexible rule. A flexible rule (fig 41) is made of thin,
tempered spring steel which permits it to be bent over a rounded surface.
Types of steel rules.