178 Print Reading for Industry Copyright Goodheart-Willcox Co., Inc. Older standards called for the diameter symbol to be omitted if there is a “round” view. However, current standards request the symbol to be used in all cases. Leader lines should only be used to point to a circle if it is a hole. Holes located around a bolt circle, or “circle of centers,” can be located in several ways. Some common methods are shown in Figure 9-10. A drawing should never be “over dimen- sioned.” There should be one way and one way only for a print reader to determine a feature’s exact location or size. Sometimes a reference dimension is given to assist in analyzing the design or drawing. However, these dimensions are not to be used for manufacturing or quality control. Reference dimen- sions are discussed later in this unit. An angled surface can be dimensioned by two offset measurements or by an angular value and an offset measurement, Figure 9-11. Sometimes an angular surface, by the nature of the angle, cre- ates an edge that does not need a size dimension. If a print seems to be missing a dimension, check to see if this condition exists. Again, hopefully the drawing is dimensioned with the function and purpose of the angled surface in consideration. Manufacturing or inspection methods often dictate a particular choice of dimensions. In some cases, of course, the angled feature may not be critical. Unit 13, Geometric Dimensioning and Tolerancing, dis- cusses various ways that geometric features, such as angled features, can be controlled with more pre- cision and intent. Chain dimensioning, wherein dimensions are linked together end-to-end, can result in the accu- mulation of tolerances, if applied. To prevent this accumulation, features can be dimensioned from a common datum surface or feature, Figure 9-12. This Goodheart-Willcox Publisher Figure 9-8. Whenever possible, dimensions should be placed in the view that shows the shape of the feature being dimensioned. Also, try to avoid dimensioning to a hidden line. This is not always possible. .420 .210 .285 .788 .488 .143 .285 .570 .570 Very poor! Much better R.488 .375 .788 .285 .143 .420 .210 .285 Goodheart-Willcox Publisher Figure 9-9. External cylindrical features should be dimensioned in the rectangular view. 2.000 1.500 .750