+ Submit your article here

What is a Counterbore?

11/8/2013 7:51:09 AM | by Anonymous


A counterbore is usually a cylindrical flat bottomed hole that enlarges another coaxial hole. It can be the tool that is used to create the feature as well. It is typically used when a fastener, such as the socket head cap screw, is needed to sit flush with or below the level of the work piece’s surface.


It is often desirable to hide the fasteners used in the joining process in manufacturing. One way to hide them is to use a counterbore. Counterbores can also refer to the process of creation of the cavity, the specific tool that is used to create the cavity and may be used in the reference to the cavity itself. The cavity is usually designed to hide the entire shaft of a fastener as well as the head of the fastener. Counterboring is very much similar to countersinking. The principle difference would be that counterboring cavity is generally cylindrical in nature whereas countersinking results are usually conical.


Counterboring as a process creates the cylindrical cavity that is of a larger diameter at the mouth of a pre drilled hole. The pre drilled hole is established in order to line up the fasteners. They also help to prevent the work piece from splitting into two when two flat planes are being joined together. This larger cavity is needed to be created at least the depth and width of the head of the intended fastener. They are also usually referred to as the counterbore itself.
When referring counterbores as a form of tool, it is typically specialized drill bit used to create the pre drilled hole and the larger diameter cavity both concurrently. The fastener tends to be a flat bottomed screw, either pan headed or round headed screw. A counterbore tool has two cutting radii, usually the first for the pre drilled hole and the second to create the recessed cavity in the work piece.


Counterbores are usually made with the standard dimensions for a certain size of screw or are produced in a variety of sizes. They need not be related to any particular screw size. In both cases, the tip of the counterbore has a reduced diameter that is referred to as the pilot. This is an essential feature to assure the concentricity between both the counterbore and the hole that is being counterbored.

Counterbores matches the specific screw size typically have integral pilots that fits the diameter of the clearance hole associated with a particular screw size. For counterbores that are not related, they are designed to accept a removable pilot. This allows any given counterbore size to be adapted to a variety of their hole sizes. The pilot does not matter much when running the cutter in a milling set up where its rigidity is assured and the hole center is already achieved by using the XY positioning.

The use of counterbores provides the opportunity for a fastener to be hidden completely during the process of joining. The fastener lies either below or flushes with the surface of the work piece so that it will not hinder the design. 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 during the wood working process. This allows the complete concealing of the fastener and thus providing a single continuous wood surface. On top of that, counterbores can be drilled to larger diameters than it is necessary to accommodate the fastener head in situations that requires clearance for a socket wrench is more desirable for the ease of assembly.

Are you sourcing for a product or service?

Do you need a quotation?