of Steel Rules
The six-inch rule is considered to be the most convenient size to carry
around. Many steel rules have a "hook" feature that provides an
accurate "stop" at the end of the rule (Figure 12). This can be
used to set calipers, dividers, etc. and for taking measurements where it
is not possible to be sure that the end of the rule is even with the edge
of the work. Some workers refer to steel rules as "machinists
||Figure 12: Hook Rule:
Automatically aligns the end of the rule with the end of the
|Figure 13: Narrow Rule: Is
used to measure the depth of narrow slots and small diameter holes
where the standard rule is too wide to be used.
||Figure 14: Flexible Rule:
Can be bent to the contour of arcs and curved lengths permitting
measurements impossible to obtain with a rigid rule.
|Figure 15: Narrow Rule with
Holder: Used to measure grooves, recesses, keyways, and short
lengths from shoulders. The rule sections are interchangeable in
the holder and can be set at various angles (sometimes referred to
as a short recess rule with holder).
Decimal Inch Rules
|Many industries use decimal notations, which is simply dividing
the English inch into 10 parts or multiples of 10 parts, such as
40 or 100. A decimal rule is used to provide decimal measurements
Figure 16: Decimal rule in 50ths
Decimal inch dimensions are specified and read as thousandths of an
inch. Decimal rules, however, are not graduated in thousandths, but are
typically graduated into 1 /10", 1/50", or 1 /100" (Figure
16). A typical decimal rule may have 1/50" graduations on one edge
and 1 /100" graduations on the other edge. As the inch is divided
into 10 equal parts, each graduation is 1 /10" or 100/1000" (one
hundred thousandths of an inch). On the edge that is divided into
1/50", each 1 /10" increment is further divided into 5 equal
parts, making the value of each of these divisions .020 (twenty
thousandths) of an inch. The edge that is divided into 1 /100", each
inch is divided into 100 equal parts, with each part equal to 1 /100 or
10/1000 (ten thousandths) of an inch.
Figure 16: 100ths graduation decimal rule
of Decimal Inch Readings:
Distance "A" falls on the second marked graduation. Thus the
reading is 2 /10 or 200/1000" (0.200) inch.
Distance "B" falls on the ninth graduation beyond the .20
graduation and thus can be read only on the 100th,, scale. The reading is
.200" plus .90" or .290". This distance cannot be read on
the .50 rule because the discrimination on this scale is not sufficient.
|Distance "C" falls on the second mark past the
.600" line. This reading is .600" plus.020" or
.620". Since .020 is equal to 1/50 of an inch, it can be read
on either rule.
Distance "D" falls three marks past the first full
inch mark- The reading is 1.00" plus .030" or
|Many products are made in metric dimensions and require workers
to be able to use a metric rule. The typical metric rule has
millimeter (mm) and half-millimeter graduations (Figure 18).
Meters are typically divided into centimeters (1 /100 meter) and
millimeters (1 /1000 m). All metric measures are expressed in mm.
Thus 1.5 meters (m) would be 1500mm.
of Reading Metric Rules
|Distance "A" falls at the 22nd graduation on the mm
rule. The reading is 22 mm.
Distance "B" falls at the 12th graduation on the mm
rule. The reading is 12 mm.
Figure 19: Millimeter rule
with .5 mm graduations
Distance "C" falls at the half-mm mark between 31mm and 32
mm. The reading is 31mm plus one half mm or 31.5mm.
Distance "D" falls one half mm past the 4 cm graduation.
Since 4 cm are equal to 40mm, the distance is 40.5mm.