Unit 9 Dimensioning 179 Copyright Goodheart-Willcox Co., Inc. is called baseline dimensioning. Tolerancing and datum references are covered in detail in Unit 10. Features with rounded ends provide many choices for dimensioning. See Figure 9-13 for exam- ples. The “correct” choice is often based on the accuracy required or the machining process used to create the feature. Parts with rounded ends are often not dimensioned overall. In this case, a radius is given and a center location is also required to determine overall measurements. The rounded ends of internal slots are often dimensioned as the distance across the slot, instead of the radius at the end of the slot. If the slot will be machined with a cutting bit, then the diameter of the bit is the more valuable dimension. Another dimensioning rule is that center axes on corner arcs are usually self-locating. The loca- tion of the arc’s center axis is based on the radius of the arc and the edges to which the curved surface is tangent. However, if a hole is located at the center of the Goodheart-Willcox Publisher Figure 9-10. Some common methods of locating holes on a bolt circle are shown here. 1.857 5X .375 EQUALLY SPACED 1.857 29° 54° 3X .375 Goodheart-Willcox Publisher Figure 9-11. An angled surface can be dimensioned by two offset measurements (left) or by an angular value and an offset measurement (right). .943 .750 1.500 38° Goodheart-Willcox Publisher Figure 9-12. Features are dimensioned from a common datum surface or feature to prevent the accumulation of tolerances. 1.750 1.250 .750 .250 1.125 .938 .563 .313