Copyright Goodheart-Willcox Co., Inc. 137 Holes UNIT 9 Learning Objectives After studying this unit, you will be able to: ■ ■ Identify machining processes for holes, including drilling, boring, reaming, counterboring, countersinking, counterdrilling, and spotfacing. ■ ■■ Recognize symbols used for specifying holes. ■■ Evaluate specifications for holes. ■ Determine dimensions for holes. Key Terms blind hole bolt circle boring counterboring counterdrilling countersinking drill drilling reamer reaming spotfacing through hole Vholes, arious machining operations make holes, enlarge finish holes, and prepare surfaces around holes. These operations include drilling, boring, ream- ing, counterboring, countersinking, counterdrilling, and spotfacing. The ASME Y14.5 standard specifies practices for noting holes on a drawing. A circle on a drawing can represent a hole, but it also can represent a surface. The size of a hole is defined by its diameter and depth. Hole dimensions are speci- fied using a leader line, a diameter symbol, and a value. THRU placed after the value indicates the hole goes through the part. See Figure 9‑1. The depth of a hole is noted by the depth symbol followed by its value. The depth measurement always follows the diameter, as shown in Figure 9‑2. The depth symbol is not required when a drawing clearly shows a hole going through the part. Multiple holes are specified by noting the number of holes followed by the letter X. The number and the X are placed before the diameter dimension. For exam- ple, nine holes with the same diameter are specified as shown in Figure 9‑3. Note that there is no space between the 9 and the X. When a space precedes the X, the specification has a different meaning. The letter X means “by” when a space appears before and after the letter. The letter X is used in this manner when specify- ing a countersink, as explained later in this unit. Other ways of dimensioning holes include speci- fying by fractions, abbreviations, machine processes, and drill or tool sizes. See the Reference Section for Diameter symbol Hole size (diameter) Indicates the hole goes through the part Leader points to the feature it represents Circle indicates a hole Center point Goodheart-Willcox Publisher Figure 9-1. Current ASME Y14.5 standard for specifying a hole.