A counterbore is a hole-making tool that enlarges a portion of an existing hole to create a flat-bottomed recess for a screw, bolt, or nut head. This allows fasteners to sit flush or slightly below the workpiece surface, which provides a cleaner, more secure assembly.
Counterbore Basics: Types, Uses & Machining Applications
Get the lowdown on counterbores, the tool that makes precision recessing and clean assemblies possible.
What Is a Counterbore?
Unlike a countersink, which produces a conical recess for tapered fastener heads, a counterbore creates a cylindrical seat ideal for flat or fillister head screws. By maintaining the original hole’s center alignment, counterbores ensure accuracy and repeatability in both manual and CNC machining. They are widely used in metalworking, woodworking, and precision manufacturing to create clean, accurate seats without altering the hole’s center position.
Types of Counterbores
Counterbores are available with either solid or interchangeable pilots, and many are engineered to produce precision recesses for specific fastener types.
Solid Pilot Counterbore
• The pilot is permanently attached to the cutting end of the counterbore.
• Ensures precise alignment and produces highly accurate, flat-bottomed holes.
• Ideal for applications where the pilot size will not change.
• Commonly used in production environments for consistent, repeatable results in metal, wood, or plastic.
Interchangeable Pilot Counterbore
• No fixed pilot; accepts a separate pilot that slides through a pilot hole in the counterbore.
• Offers greater flexibility, as one counterbore body can be used for multiple hole diameters by swapping pilots.
• Useful for shops that work with varied fastener sizes and need to minimize tool inventory.
• Also available as sets.
Reverse Counterbore
• Used when machining access is only possible from the opposite side of the workpiece.
• Requires an arbor that passes through the hole, with the cutter attached from the back.
Counterbore Pilot
• Keeps the counterbore perfectly aligned within the existing hole.
• Ensures the finished recess is the correct size for the fastener head.
• Helps produce a clean, flat-bottomed seat for nuts, bolts, or screws.
Counterbore Uses
Counterbores are used in applications where a fastener head needs to sit flush or below the surface, or when a perfectly flat seating surface is required. Common uses include:
Socket Head Cap Screws: Machine a flat-bottom recess so the screw head sits flush or below the surface.
Machine Screws: Create a precise seat for consistent fastening strength and alignment.
Fillister Head Screws: Enlarge a hole to fit the rounded head without interference.
Spot Facing: Produce a smooth, flat surface around a hole for full contact with a bolt head, washer, or nut.