Unit 9 Holes 143 Copyright Goodheart-Willcox Co., Inc. ■ ■ Counterboring is the process of enlarging a previously formed hole cylindrically to a specific diameter and depth. Its purpose is to provide a recessed hole for fitting fastener or seating bearings and pins. ■heads ■ Countersinking is a process of drilling a conical-shaped hole to a specified depth to allow a fastener’s head to sit at or below the part’s surface. A countersunk hole has a specified angle to match the angle of the head. ■fastener’s ■ Counterdrilling is the process of drilling a conical-shaped hole to a specified depth that allows a fastener’s head to sit at or below a part’s surface. A counterdrilled hole is similar in appearance to a counterbored hole, except bottom of the hole has a conical shape. ■the ■ Spotfacing is the process of providing a smooth, flat surface around a hole. A rough surface, such as a casting, is spotfaced to accommodate the seating of a washer or bolt head. SUMMARY ■ ■ A circle on a drawing can represent a hole, but it also can represent a surface. The size of a is defined by its diameter and depth. ■hole ■ Multiple holes are specified by noting the number of holes followed by the letter X. There must be no space between the number and the X. The letter X means “by” when there is a before and after it. ■space ■ Drilling is the most common machining operation. Drilling is the process of cutting a in or through a surface with a drill. ■hole ■ A through hole is a hole that passes completely through the part or material. A blind hole is a hole drilled to a specific depth that does not go through the part or material. ■completely ■ Boring is the process of enlarging a hole to a close tolerance and fine finish. It produces a straight, hole more accurately than by drilling. ■round ■ Reaming is the process of finishing an existing hole to a specific size and required finish. It is an operation required for precision fits.