Holemaking operations: Drilling Performance

Drilling holes is the most common of all machining processes, and the tool most commonly applied to make them is the twist drill. Twist drills remove metal from holes and reduce the metal to chips in a fast, simple and economical process. Nearly 75 percent of the metal removed by machining is drilled. Drilling is often the starting point for other operations, such as counterboring, spotfacing, countersinking, reaming and boring.

All images: Pamela Tallman

Making starter holes with a combination drill (A), a NC spotting drill (C) and a twist drill (B and D).

• Center drilling uses a short, rigid drill to make a cone-shaped starting hole for twist drills or lathe centers. Using a center drill ensures more accurate hole placement on the spindle axis than starting a twist drill on a flat work surface or center punch mark. Because twist drills flex and wobble when starting a hole, there is no certainty they will begin drilling on the spindle axis. There are two center drill designs: the NC spotting and centering drill and the combination drill and countersink. Both work well for starting holes, but only the combination drill makes properly shaped holes for lathe centers. Spotting drills are available with point angles of 60°, 82°, 90°, 118° and 120°. Combination drills, which have 60° point angles, drill a matching hole for lathe centers. They also drill an additional pocket for lubricant so the center does not burn. When using a spotting drill for a subsequent carbide twist drill, the spotting drill should always have a flatter angle than the carbide drill point so the twist drill’s chisel edge makes contact with the work first—not the drill edges. For example, apply a 120° spotting drill for a 118° carbide twist drill.

• Counterboring produces a larger, square-bottomed hole in the upper portion of an existing hole and provides space and seating for a bolt or cap screw head below a workpiece’s surface. The cylindrical guide, or integral pilot, on the end of the counterbore ensures the enlarged diameter is concentric with the original hole.

When counterboring, a counterbore’s pilot end enters an existing hole (left) and aligns the cutter, and increases the diameter of the existing hole (center).