Hole making

Hole-making is a class of machining operations that are specifically used to cut a hole into a workpiece. Machining, a material removal process, creates features on a part by cutting away the unwanted material and requires a machine, workpiece, fixture, and cutting tool. Hole-making can be performed on a variety of machines, including general machining equipment such as CNC milling machines or CNC turning machines. Specialized equipment also exists for hole-making, such as drill presses or tapping machines. The workpiece is a piece of pre-shaped material that is secured to the fixture, which itself is attached to a platform inside the machine. The cutting tool is a cylindrical tool with sharp teeth that is secured inside a piece called a collet, which is then attached to the spindle, which rotates the tool at high speeds. By feeding the rotating tool into the workpiece, material is cut away in the form of small chips to create the desired feature.

Hole-making operations are typically performed amongst many other operations in the machining of a part. However, hole-making may be performed as a secondary machining process for an existing part, such as a casting or forging. This can be done to add features that were too costly to form during the primary process or to improve the tolerance or surface finish of existing holes.

Hole Parameters

In machining, a hole is a cylindrical feature that is cut from the workpiece by a rotating cutting tool that enters the workpiece axially. The hole will have the same diameter of the cutting tool and match the geometry (which may include a pointed end). Non-cylindrical features, or pockets, can also be machined, but they require end milling operations not hole-making operations. While all machined holes have the same basic form they can still differ in many ways to best suit a given application. A machined hole can be characterized by several different parameters or features which will determine the hole-making operation and tool that is required.

Diameter

Holes can be machined in a wide variety of diameters, determined by the selected tool. The cutting tools used for hole-making are available in standard sizes that can be as small as 0.0019 inches and as large as 3 inches. Several standards exist including fractional sizes, letter sizes, number sizes, and metric sizes. A custom tool can be created to machine a non-standard diameter, but it is more cost effective to use the closest standard sized tool.

Tolerance

In any machining operation, the precision of a cut can be affected by several factors, including the sharpness of the tool, any vibration of the tool, or the build up of chips of material. The specified tolerance of a hole will determine the method of hole-making used, as some methods are suited for tight-tolerance holes.

Depth

A machined hole may extend to a point within the workpiece, known as a blind hole, or it may extend completely through the workpiece, known as a through hole. A blind hole may have a flat bottom, but typically ends in a point due to the pointed end of the tool. When specifying the depth of a hole, one may reference the depth to the point or the depth to the end of the full diameter portion of the hole. The total depth of the hole is limited by the length of the cutting tool.

Recessed top

A common feature of machined holes is to recess the top of the hole into the workpiece. This is typically done to accommodate the head of a fastener and allow it to sit flush with the workpiece surface. Two types of recessed holes are a counterbore, which has a cylindrical recess, and a countersink, which has a cone-shaped recess.

Threads

Threaded holes are machined to accommodate a threaded fastener and are typically specified by their outer diameter and pitch. The pitch is a measure of the spacing between threads and may be expressed in the English standard, as the number of threads per inch (TPI), or in the metric standard, as the distance in millimeters (mm) between threads.

Hole-making operations

Several hole-making operations exist, each using a different type of cutting tool and forming a different type of hole.

Drilling

A drill bit enters the workpiece axially and cuts a blind hole or a through hole with a diameter equal to that of the tool. A drill bit is a multi-point tool and typically has a pointed end. A twist drill is the most commonly used, but other types of drill bits, such as a center drill, spot drill, or tap drill can be used to start a hole that will be completed by another operation

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.

Drilling operation

• 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.

Reaming

A reamer enters the workpiece axially and enlarges an existing hole to the diameter of the tool. A reamer is a multi-point tool that has many flutes, which may be straight or in a helix. Reaming removes a minimal amount of material and is often performed after drilling to obtain both a more accurate diameter and a smoother internal finish.

Reaming operation

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.

Tapping

A tap enters the workpiece axially and cuts internal threads into an existing hole. The existing hole is typically drilled by the required tap drill size that will accommodate the desired tap. The tap is selected based on the major diameter and pitch of the threaded hole. Threads may be cut to a specified depth inside the hole (bottom tap) or the complete depth of a through hole (through tap).

Tap Size chart

Boring

A boring tool enters the workpiece axially and cuts along the internal surface of an existing hole to enlarge the diameter or obtain more precise dimensions. The boring tool is a single-point cutting tool, which can be set to cut the desired diameter by using an adjustable boring head.

Boring operation 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.

Counterboring

A counterbore tool enters the workpiece axially and enlarges the top portion of an existing hole to the diameter of the tool. Counterboring is often performed after drilling to provide space for the head of a fastener, such as a bolt, to sit flush with the workpiece surface. The counterboring tool has a pilot on the end to guide it straight into the existing hole.

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.

Counterboring operation

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).

Countersinking

A countersink tool enters the workpiece axially and enlarges the top portion of an existing hole to a cone-shaped opening. Countersinking is often performed after drilling to provide space for the head of a fastener, such as a screw, to sit flush with the workpiece surface. Common included angles for a countersink include 60, 82, 90, 100, 118, and 120 degrees. 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.