2. Hook rule. A hook rule (fig 41) has a sliding hook which
facilitates measuring from a shoulder, particularly if the end of the rule is
hidden so that it cannot be lined up with the shoulder. The sliding hook is also
convenient in setting calipers and dividers.
3. Short rules and holders. Short rules with a holder (fig 41) are
available for measuring in a recess or in a restricted area.
4. Flexible fillet rule. A flexible fillet rule (fig 41) is used to
span fillets and corner fills which are frequently in the way when measuring
flanges, shoulders. and so forth.
(b) Folding rules. Folding rules (fig 42) are obtainable in wood or
metal, having 4 to 12 folds, from 2 to 6 feet long total length. These rules
cannot be relied on for extremely accurate measurements because a certain amount of
play develops at the joints after they have been used for a time.
(2) Using the rule.
(a) Select proper scale. When using a rule to check a dimension, the
proper graduated scale should be used to control the reading of the dimension (A,
fig 43). If the work being measured lines up between two graduations on the scale
as shown in B and C, figure 43, and it is not possible to read this dimension to a
1/16 on a 1/16-inch scale, a 1/32-inch scale should be used, and if it is still
impossible to read a dimension to a 1/32, a 1/64-inch scale should be used.