What is a Counterbore?

In manufacturing, it is often desirable to hide the fasteners used in the joining process. One way to do so is to use a counterbore. Counterbore can mean the process of creating the cavity, the specific tool used to create the cavity, or may be used in reference to the cavity itself. The cavity is typically designed to hide the entire shaft of the fastener, as well as the head of the fastener. Counterboring is similar to countersinking — the principle difference is that a counterboring cavity is cylindrical in nature rather than the conical results achieved with countersinking.

As a process, counterboring creates a cylindrical cavity of larger diameter at the mouth of a pre-drilled hole. The pre-drilled hole is established to line up the fastener and prevent the workpiece from splitting when two flat planes are joined together. This larger cavity is created to be at least the width and depth of the head of the intended fastener, and is referred to as the counterbore.

When counterbore refers to a tool, it typically is a specialized drill bit that is used to create the pre-drilled hole and the larger diameter cavity at the same time. The fastener is usually a flat-bottomed screw, such as a panhead or round head screw. A counterbore tool generally has two cutting radii — one for the pre-drilled hole and one that creates the recessed cavity in the workpiece.

Use of counterbores provides the opportunity for a fastener to be completely hidden during the joining process. The fastener lies either below or flush with the surface of the workpiece so that it does not hinder the design. In woodworking, an increased counterbore depth beyond the thickness of the fastener head can leave room for a wood plug to be fixed into the remaining space, thus completely concealing the fastener and providing one continuous wood surface. Counterbores can also be drilled to larger diameters than is necessary to accommodate the fastener head in situations where clearance for a socket wrench is desirable for ease of assembly.