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).
• Spotfacing mills a flat area around an existing hole to make a flat seating surface for a bolt or washer and is usually necessary on castings and parts with sloping surfaces. Failure to spotface an uneven workpiece puts excessive stress on the bolt. On flat work, use a spotfacing bit or counterbore in a drill press, but on a sloping surface, because of the interrupted cut, use an endmill in a milling machine. Spotfacing cutters differ from counterbores because they cut larger diameter holes and sustain greater cutting forces, but they work the same way. Large spotfacing cutters have separate centers, allowing the same cutter to work in different hole diameters. On sloped surfaces, spotfacing is performed before drilling. This provides a flat surface on which to start the drill and makes it easier to follow-in straight. Remember to use a counterbore or milling cutter slightly larger than the washer, cap screw or bolt head diameter specified on the print.
Countersinking uses a cone-shaped cutting tool to chamfer or bevel the edges of an existing hole.
• Countersinking uses a cone-shaped cutting tool to chamfer or bevel the edges of an existing hole, usually so flathead screws can be seated below the workpiece surface. Countersinks also deburr holes. Countersinking can be done with a hand-held drill or in a drill press, lathe or milling machine.
• Reaming enlarges an existing hole to a precise diameter and improves wall surface finish. It can be performed equally well in a drill press, lathe or mill.
• Boring enlarges the diameter of an existing starter hole with a single-point lathe tool. It can be performed in a drill press, but is more often done in a lathe or mill. Boring offers precise diameter control and perpendicularity. It typically produces smoother walls than drilling. CTE
About the Author: Frank Marlow, P.E., has a background in electronic circuit design, industrial power supplies and electrical safety. He can be e-mailed at orders@MetalArtsPress.com. Marlow’s column is adapted from information in his book, “Machine Shop Essentials: Questions and Answers,” published by the Metal Arts Press, Huntington Beach, Calif.
Related Glossary Terms
- boring
boring
Enlarging a hole that already has been drilled or cored. Generally, it is an operation of truing the previously drilled hole with a single-point, lathe-type tool. Boring is essentially internal turning, in that usually a single-point cutting tool forms the internal shape. Some tools are available with two cutting edges to balance cutting forces.
- center drill
center drill
Drill used to make mounting holes for workpiece to be held between centers. Also used to predrill holes for subsequent drilling operations. See centers.
- center drilling
center drilling
Drilling tapered holes for mounting workpiece between centers. Center-drilled holes also serve as starter holes for drilling larger holes in the same location. See centers; drilling.
- centering
centering
1. Process of locating the center of a workpiece to be mounted on centers. 2. Process of mounting the workpiece concentric to the machine spindle. See centers.
- centers
centers
Cone-shaped pins that support a workpiece by one or two ends during machining. The centers fit into holes drilled in the workpiece ends. Centers that turn with the workpiece are called “live” centers; those that do not are called “dead” centers.
- counterbore
counterbore
Tool, guided by a pilot, that expands a hole to a certain depth.
- counterboring
counterboring
Enlarging one end of a drilled hole. The enlarged hole, which is concentric with the original hole, is flat on the bottom. Counterboring is used primarily to set bolt heads and nuts below the workpiece surface.
- countersink
countersink
Tool that cuts a sloped depression at the top of a hole to permit a screw head or other object to rest flush with the surface of the workpiece.
- countersinking
countersinking
Cutting a beveled edge at the entrance of a hole so a screw head sits flush with the workpiece surface.
- drilling machine ( drill press)
drilling machine ( drill press)
Machine designed to rotate end-cutting tools. Can also be used for reaming, tapping, countersinking, counterboring, spotfacing and boring.
- endmill
endmill
Milling cutter held by its shank that cuts on its periphery and, if so configured, on its free end. Takes a variety of shapes (single- and double-end, roughing, ballnose and cup-end) and sizes (stub, medium, long and extra-long). Also comes with differing numbers of flutes.
- flat ( screw flat)
flat ( screw flat)
Flat surface machined into the shank of a cutting tool for enhanced holding of the tool.
- gang cutting ( milling)
gang cutting ( milling)
Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.
- interrupted cut
interrupted cut
Cutting tool repeatedly enters and exits the work. Subjects tool to shock loading, making tool toughness, impact strength and flexibility vital. Closely associated with milling operations. See shock loading.
- lathe
lathe
Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.
- lathe bit ( lathe tool)
lathe bit ( lathe tool)
Cutting tool for lathes and other turning machines. Normally a single-point cutting tool, square in cross section and ground to a shape suitable for the material and task. Intended for simple metal removal, threading, slotting or other internal or external cutting jobs. Clearance to prevent rubbing is provided by grinding back rake, side rake, end relief and side relief, as well as side- and end-cutting edges.
- milling
milling
Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.
- milling cutter
milling cutter
Loosely, any milling tool. Horizontal cutters take the form of plain milling cutters, plain spiral-tooth cutters, helical cutters, side-milling cutters, staggered-tooth side-milling cutters, facemilling cutters, angular cutters, double-angle cutters, convex and concave form-milling cutters, straddle-sprocket cutters, spur-gear cutters, corner-rounding cutters and slitting saws. Vertical cutters use shank-mounted cutting tools, including endmills, T-slot cutters, Woodruff keyseat cutters and dovetail cutters; these may also be used on horizontal mills. See milling.
- milling machine ( mill)
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
- milling machine ( mill)2
milling machine ( mill)
Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.
- numerical control ( NC)
numerical control ( NC)
Any controlled equipment that allows an operator to program its movement by entering a series of coded numbers and symbols. See CNC, computer numerical control; DNC, direct numerical control.
- spotfacing
spotfacing
Similar to counterboring except that, in spotfacing, material around the original hole is cut. Application example: the recessed area into which a washer fits. See counterboring; countersinking.
- twist drill
twist drill
Most common type of drill, having two or more cutting edges, and having helical grooves adjacent thereto for the passage of chips and for admitting coolant to the cutting edges. Twist drills are used either for originating holes or for enlarging existing holes. Standard twist drills come in fractional sizes from 1¼16" to 11¼2", wire-gage sizes from 1 to 80, letter sizes A to Z and metric sizes.
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