Unit 9 Dimensioning 177 Copyright Goodheart-Willcox Co., Inc. contains 360°. One degree contains 60′ (minutes). One minute contains 60″ (seconds). For an angular dimension, the dimension line is a curve with its center at the vertex of the dimensioned angle, Figure 9-7. With respect to dimensioned values, there are many factors involved. Companies with an inter- national presence and product line have adopted SI (metric) linear units, while other companies con- tinue to use US customary linear units and work with decimal inch units. In certain industries, such as cabinetmaking, construction, or structural steel, dimensions may be given in feet, inches, and fractions of an inch. According to standards, on drawings where all dimensions are either in inches or millimeters, individual identification of the units is not required, but the drawing should contain a general note such as UNLESS OTHERWISE SPECIFIED, ALL DIMENSIONS ARE IN INCHES. In a drawing that contains primarily metric values, inch values should be followed by the abbreviation IN. Likewise, the “mm” symbol should follow a milli- meter value on a drawing primarily done in inches. There are a few other recommendations in the Y14.5 standard that apply to dimensioned values. For millimeter values, the print reader should expect to see a zero preceding the decimal point on dimension values less than one millimeter, such as 0.54, but not on inch values less than one inch, such as .54. Where a millimeter dimension value is a whole number, a decimal point or zero might not be shown, although the rules are different for toleranced values, as discussed in the next unit. Also, the print reader should be aware that a few countries outside the United States use a decimal comma rather than a decimal point. As discussed in the unit on projection methods, be prepared for differences if you find an international print. Dimensioning Rules of Choice and Placement In addition to the mechanics of dimensioning, the drafter or designer must then set about choosing which dimensions to use and then select the best view for those dimensions. In industrial mass pro- duction, parts are dimensioned in such a way that there is only one way to find the location or size of a feature. The elements of choice and placement are the real difficult aspects of dimensioning. Knowing the guidelines for choice and placement may help the print reader know where to look for dimensions on a print. However, the function of the part, the purpose of a feature, or the machining and inspec- tion processes may dictate dimension locations that do not follow these guidelines. In summary, the guidelines for choice and placement are just that— guidelines. In keeping with the global rule “to clearly express the size,” dimensions should be located off of the views, but still central to the drawing. Therefore, you should find most dimensions between the views. Notes and leader lines, how- ever, are likely located around the perimeter of the view arrangement. A feature should be dimensioned in the view that is most descriptive of the feature’s shape. This is known as contour dimensioning. A feature such as a slot or notch has its shape, or contour, shown in one particular view. Closely related to this prin- ciple is a drafter’s rule to avoid dimensioning to a hidden line or the center line of a hidden hole. Figure 9-8 shows two versions of a drawing with contour dimensioning, one properly constructed and one poorly constructed. For objects made of external cylindrical features, the diameter of the cylinder should be dimensioned in the “rectangular” view. See Figure 9-9. Even if a cylindrical feature is milled flat across one or more sides, the diameter of the cylinder is usually given. Goodheart-Willcox Publisher Figure 9-7. For an angular dimension, the dimension line is a curve with its center at the vertex of the dimensioned angle. 34° 101° 69°