Source: Reid Supply

Unlike countersinks, which have a conical cavity matching the angled underside of the flathead screw, counterbore holes create a flat-bottom cavity, allowing fastener heads and their washers to lie flush or under the face of the part.

Cap screw counterbore holes are often made so that they conceal an entire fastener, deep enough for a dowel, which conceals the counterbore and the fastener, and large enough for a hex socket to fit onto the socket head cap screw until fully tight.

Importance of Counterbore Holes

By using counterbores, machine designers and manufacturers have the opportunity to hide all the fasteners during the joining process and thus improve upon the design, functionality, and aesthetics of a particular machine or component.

Besides aesthetics and design, counterbore holes also solve the issues of light clearances, in which a protruding fastener could interfere with other parts or components.

The Process of Creating Counterbore Holes

The process of counterboring creates a cylindrical cavity whose diameter is larger than that of the pre-drilled hole using a counterboring tool, typically a specialized carbide or high-speed steel (HSS) cutting tool that creates both the hole and the larger-sized cavity for the fastener head.

The counterbore resembles a typical drill bit, but it has two radii; one creates the pilot hole, also known as pilot counterbores, while the other creates a recessed hole, called a spotface, into the workpiece. Counterbore tools are typically used on end mills, lathes, and CNC machining. Hand counterboring is typically done by using various jigs to ensure precision.

The cutting bits of the counterboring tools are typically mounted onto straight shanks or drill chucks. More expensive variants of the counterboring tools typically ship in outer diameters only, with interchangeable pilots, allowing a wider range of through-holes to accommodate a wider range of screw sizes.

The counterbore is typically formed by drilling a through-hole in a workpiece using a drill bit and a reamer. If the hole has to be tapped (to make a thread), it’s recommended that you thread the hole before counterboring. This will allow the tap of the counterbore tool, or a counterbore pilot, to correctly center against the pre-drilled hole. Then, when the tool is centered and the cut-off surface of the counterboring tool rests against the surface of the workpiece, you can begin counterboring.

It’s worth noting that the larger cavity must, at the very least, match the depth and width of the fastener head.

Mistakes to Avoid When Creating Counterbore Holes

Machining counterbores requires meticulous measurement, which is often the most common error leading to inaccurate or inconsistent recess depth and diameter across a given run, so always double-check the measurements for diameters and depths. Using the wrong tools or type of material is another common mistake, so make sure to match the counterbore tool with the fastener.

Incorrect speed settings often lead to overheating, which can soften or cause other damage to the tool or workpiece, so make sure to check the specs for cutting different materials. Lastly, misalignment leads to uneven holes, which can compromise the structural integrity of your machine assembly.

Industrial Hole-Making Parts and Components

If you want to learn more about hole-making and counterbores, don’t hesitate to visit our online store or explore our digital catalogs for more information.