140 Machine Trades Print Reading Copyright Goodheart-Willcox Co., Inc. Extra material around a hole is required to ream an accurate size hole to a fine finish. The basic material allowance is .0156″ (0.3968 mm) for diameters under 1″ (25.4 mm) and .0313″ (0.7938 mm) or more for diameters greater than 1″ (25.4 mm). PRO TIP Counterbored Hole Counterboring is the process of enlarging a previously formed hole cylindrically to a specific diameter and depth. Counterboring produces a flat bottom surface. Its purpose is to provide a recessed hole for fitting fas- tener heads or seating bearings and pins. The specifications for a counterbored hole appear on two lines. See Figure 9‑9. The first line specifies information for the small hole. The information is given in the following order: the diameter symbol, the size of the small hole diameter, the depth symbol, and the depth size. The depth symbol and depth size are only included if required. The second line of the counterbore specification gives information for the counterbored hole. The information is given in the following order: the counterbore symbol, the diame- ter symbol, the size of the counterbore diameter, the depth symbol, and the depth size. Countersunk Hole Countersinking is the process of enlarging the entrance of a hole conically to recess the head of a fastener, such as a flat head screw. A countersunk hole has a specified angle to match the angle of the fastener’s head. The typical angle used for fasteners is 82°. However, 100° is common for fasteners on thin materials to provide additional surface area for fastening. The specifications for a countersunk hole appear on two lines. See Figure 9‑10. The first line spec- ifies information for the small hole in the follow- ing order: the diameter symbol, the size of the small hole diameter, the depth symbol, and the depth size (the depth symbol and depth size are only included if required). This Means this Goodheart-Willcox Publisher Figure 9-7. Representation of a bored hole. Hole is ÿrst drilled to a ∅.562, then ÿnished with a reamer to ∅.578 This Means this Goodheart-Willcox Publisher Figure 9-8. Older drawings included specifications for reamed holes. Means this Counterbore symbol This Goodheart-Willcox Publisher Figure 9-9. Specifications for a counterbored hole.