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Inspection Methods - Types of Steel Rules
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Types 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 scales."

Figure 12. Hook Rule: Automatically aligns the end of the rule with the end of the workpiece.
Figure 13. Narrow Rule: Is used to measure the depth of narrow slots and small diameter holes where the standard rule is to wide too 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).

Reading Decimal Inch Rules

Many industries use decimal notations which is simply dividing the English inch into ten parts or multiples of ten parts such as 40 or 100. A decimal rule is used to provide decimal measurements (Figure 16).


Figure 16. Decimal Rule in 50ths graduations.

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.

Examples 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 1 00th,, scale. The reading is .200" plus .09" 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 1.030".

 


Figure 17

Reading Metric Rules

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.


Figure 18.

Examples 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. 

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