Unit 9 Holes 139 Copyright Goodheart-Willcox Co., Inc. the needed information. See Figure 9‑6. A bolt circle is dimensioned with a leader line, a diameter symbol, a value, and the letters BC. Bored Hole Boring is the process of enlarging a hole to a close tol- erance and fine finish. It produces a straight, round hole more accurately than by drilling. A boring bar and single point tool can produce holes in a wide range of diameters. A bored hole is the same as a drilled or reamed hole. The diameter of a bored hole appears in the top view of the part. A tolerance or a specific note may appear after the diameter of the hole to specify the boring process, as shown in Figure 9‑7. Reamed Hole Reaming is the process of finishing an existing hole to a specific size and required finish. Reaming is an oper- ation required for precision fits. It usually follows drill- ing to provide a closer tolerance and smoother finish. A reamed hole is machined using a reamer. A reamer is a straight or helical multi-fluted rotary cutting tool used with a drill press, lathe, or milling machine. A reamer is used to enlarge, smooth, and size a drilled hole by removing a small amount of material. A reamed hole is dimensioned the same as a bored hole. The tolerances and finish specifications desig- nate the machining process. This method allows the machinist to use the proper process that makes the hole to the required specifications. On older drawings, it is common to find a drill and ream size noted for a hole. See Figure 9‑8. Through Hole A Blind Hole B Goodheart-Willcox Publisher Figure 9-5. Two types of holes. A—A through hole goes com- pletely through a part. B—A blind hole is machined to a spec- ified depth. Equal Spacing A Bolt circle Angular Dimensioning B Goodheart-Willcox Publisher Figure 9-6. Specifying holes in circular patterns. A—The equal division of centerlines indicates equal spacing. B—Angles indi- cate the spacing along the bolt circle.