What is Counterboring?

Counterboring creates a cylindrical recess around a hole, widening it to a limited depth. Counterboring is commonly used to allow socket head cap screws sit to below a part’s surface. Surfaces that are rough or curved can also be counterbored to allow a fastener head to flush with the surface.
A counterbore hole is usually used when the head of a fastener, such as a hex head or socket head capscrew, is required to be flush with or below the level of a workpiece's surface.
For a spotface, material is removed from a surface to make it flat and smooth, usually for a fastener or a bearing. Spotfacing is usually required on workpieces that are forged or cast. A tool referred to as a counterbore is typically used to cut the spotface, although an endmill may also be used. Only enough material is removed to make the surface flat.
A counterbore is also used to create a perpendicular surface for a fastener head on a non-perpendicular surface. If this is not feasible then a self-aligning nut may be required.
Standards exist for the sizes of counterbores, especially for fastener head seating areas. These standards can vary between corporations and between standards organizations.
Counterboring prepares the pipe ends by machining so that the joint seals tightly and can be welded easily.
With a drill, you can do different types of things with metals and non-metals such as wood and plastic. Such things include drilling a hole, boring or making the hole bigger in diameter and countersinking and/or a counterboring to allow a screw, bolt or fastener sit flush with the workpiece.
While countersinking creates a conical cavity that matches the angled shape of the underside of a flathead screw, counterboring creates a flat-bottom cavity. This allows the head of the screw or bolt, often used with a washer, and its flat underside to rest totally within the counterbore. Often, a deeply set flathead screw will usually have a cavity that is about the same size as the head of the screw. However, the cavity meant for a counterbore is somewhat larger than the head. This allows room not only for a washer, but for the driving tool as well, such as a socket wrench.
Manufacturers often hide the fasteners used in the joining process. One of the methods they use for doing so is a counterbore. In practice, the word counterbore is used interchangeably for the process, for the specific tool and for the cavity itself. The process makes a specially shaped cavity that hides not only the entire length of the shaft of the fastener, but the head of the fastener as well. As explained earlier, counterboring differs from countersinking principally in the shape of the cavity it is cylindrical rather than conical as for countersinking.
The process of counterboring creates a cylindrical cavity with a larger diameter at the head of a pre-drilled hole. The pre-drilled hole is meant to contain the fastener and to prevent the two work pieces from separating when their two flat surfaces are joined. The larger cavity matches at least the width and depth of the head of the fastener. This cavity is called the counterbore.
The counterbore tool is typically a specialized drill bit that creates the hole for the body of the fastener and the larger sized cavity for its head, both at the same time. Usually, the fastener chosen is a flat-bottomed screw such as a round head or a pan head screw. The counterbore tool typically has two cutting radii- one to create the pre-drilled hole, and the other for creating the recessed cavity in the workpiece.
By using a counterbore, the manufacturer has an opportunity for hiding the fastener completely during the joining process. The fastener can be made to lie either flush with or below the surface of the workpiece such that the head does not hinder the design. While working with wood, increasing the depth of the counterbore beyond the thickness of the fastener head means there is room for a wooden plug to be fixed in the extra space. That completely conceals the fastener and provides a continuous wood surface.
Sometimes, for ease of assembly, it may be necessary to tighten or loosen the fastener. Usually, a socket wrench is used for the purpose, which means enough space must be available around the screw head to insert the socket wrench and allow it to grip the head of the fastener. For this, the counterbore cavity is drilled to a diameter larger than that necessary to accommodate the socket wrench.