138 Machine Trades Print Reading Copyright Goodheart-Willcox Co., Inc. more information about drill sizes. The ASME Y14.5 standard does not recommend those formats for dimensioning, but they are still used. The Y14.5 stan- dard recommends using symbols instead of words for specifying dimensions. See Figure 9‑4. When specific manufacturing processes are required, the standard recommends noting them with a separate note on the drawing or in a related document. Drilled Hole Drilling is the most common machining operation. Drilling is the process of cutting a hole in or through a surface with a drill. A drill is a cylindrical tool with a sharpened point and edges used to cut a specific size hole. The diameter of the drill determines the size of the hole it makes. The chuck of a rotary machine, such as a hand drill, drill press, lathe, or milling machine, holds the drill. As the machine spins the chuck, the operator moves the drill toward the part’s surface while it rotates and cuts a hole in the material. There are two classifications of drilled holes— through holes and blind holes. A through hole is a hole that passes completely through the part or material. Through holes appear on a drawing as hidden lines that extend to the outside surfaces, as shown in the front view of Figure 9‑5A. A blind hole is a hole drilled to a specific depth that does not go completely through the part or material. A drill will produce a conical point at the bottom of the hole, as shown in Figure 9‑5B. Typical dimensions for drilled holes require information for the quantity, size, and depth of the holes. If no depth is given for a hole, it is assumed to be a through hole. A circular centerline, known as a bolt circle or circle of centers, is used to locate holes in a circular pattern. In specific situations, equal division of centerlines, an equal spacing note, or angular dimensioning specifies Indicates number of holes Diameter symbol Hole size (diameter) Goodheart-Willcox Publisher Figure 9-3. Multiple holes of the same size specified once on a drawing. A B C Goodheart-Willcox Publisher Figure 9-4. Different ways to dimension the same sized hole. A & B—A front or side view is needed because the top view does not indicate the depth of the hole. C—Only one view is needed because the top view indicates the depth of the hole. Hole Leader Diameter symbol Depth symbol Hole size (diameter) Hole depth Goodheart-Willcox Publisher Figure 9-2. Dimension of a hole with a specified depth.