Carr Lane Mfg. is an industry-leading provider of counterbores for drill bushings and many other vital manufacturing products. Since 1952, Carr Lane Mfg. has provided innovative engineering products and solutions to a large variety of industries. Counterbores are an essential feature in precise and high-quality mechanical assemblies, providing both functional and aesthetic benefits. They are often used when assembling components in machinery, jigs, and fixtures where the screw or bolt heads need to be concealed or protected from wear and tear. However, each assembly is different, making it essential to find the correct counterbore size to meet your specific requirements.
What is a Counterbore?
Counterbores are cylindrical flat-bottomed holes that enlarge another coaxial hole. A counterbore can also refer to the tool used to create these holes (given the term counterbore cutter). The counterbore cutter or specialized drill bit creates the larger-diameter portion of the hole, often for applications where a smooth finish and a precise fit are critical. The hole is typically used when a fastener is required to sit flush with or below the level of a workpiece's surface. It is also used to create a perpendicular surface for a fastener head on a surface that isn’t.
The counterbore enlarges an existing hole and has a flat bottom, ensuring that the head of the fastener sits evenly and securely while maintaining a common centerline with the initial hole. This allows fasteners to be flush with or slightly below the surface, which is crucial for mechanical assemblies requiring a smooth, uninterrupted surface.
What is the Counterboring Process?
The primary goal of counterboring is to enlarge an existing hole to create a flat-bottom, cylindrical recess that allows a screw or bolt head to sit flush with or below the surface of the workpiece. This process creates a cylindrical cavity with a larger diameter at the head of a pre-drilled conical hole. The pre-drilled hole is meant to contain the fastener and prevent the two workpieces 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 procedure most commonly uses a flat-bottomed-head screw to hold the object together. The screw will have a head that is below the surface of the piece being fastened. There are two cutting radii on the tool. One radius creates the counterbore hole, and the other radius creates the recessed cavity inside the material.
The fastener can be placed anywhere between flush with the surface of the material and below the surface so that the head will not interfere with the design.
Steps in the Counterboring Process
- Preparation: The process begins by choosing a counterbore cutter or bit that matches the diameter and depth requirements for the fastener head. Then, secure the workpiece in a vise or clamp it to a workbench to prevent movement during drilling.
- Drill the Pilot Hole: Mark the location of the hole on the workpiece with a center punch. You can then use a standard drill bit to create the pilot hole to the required diameter and depth. This hole should match the diameter of the fastener shank.
- Counterboring: Once the pilot hole is created, position the counterbore cutter so that the pilot tip aligns with the pilot hole. Begin drilling at a slow speed to ensure accuracy and control, gradually increasing the speed as the tool starts to cut into the material. Drill to the required depth for the counterbore, ensuring the tool creates a flat-bottomed, cylindrical recess.
- Finishing: Use a depth gauge or calipers to verify that the counterbore depth matches the required specifications, removing any debris or burrs from the hole. You can also use sandpaper to ensure a smooth finish.
- Test Fit: Place the screw or bolt into the counterbored hole to ensure it sits flush with or below the surface of the workpiece. If the fastener does not fit correctly, recheck the measurements and adjust the counterbore depth or diameter as needed.
How is a Counterbore Measured?
A counterbore is measured by several dimensions that ensure it fits the intended fastener and meets the requirements of the assembly. This can include:
- Counterbore Diameter (D1): This is the diameter of the larger, flat-bottomed portion of the counterbore where the head of the screw or bolt will sit. It must be large enough to accommodate the head of the fastener.
- Counterbore Depth (L1): This is the depth of the larger-diameter portion of the hole. It needs to be deep enough to allow the head of the fastener to sit flush with or below the surface of the workpiece.
- Pilot Hole Diameter (D2): This is the diameter of the smaller, coaxial hole that continues from the bottom of the counterbore and through the workpiece. It is usually the same diameter as the shank of the screw or bolt.
- Total Depth (L2): This includes the depth of the pilot hole plus the depth of the counterbore. This measurement ensures that the fastener will fit properly through the entire thickness of the workpiece.
The counterbore can be measured with different types of gauges, the simplest being a caliper. Other types of measuring equipment, such as pocket comparators, gage pins, and depth micrometers, are also used in measurement.
Counterbore vs. Countersink and Spotface: What’s the Difference?
While they are similar, counterboring, countersinking, and spotfacing are different machining processes used to create specific types of holes in a workpiece to accommodate fasteners or to achieve a particular surface finish. A countersink has an angled bottom, whereas a counterbore has a flat one. This angled bottom on countersinks can vary slightly. While counterbores are typically used for cap screws, socket head screws, and other fasteners with cylindrical heads, countersinks are often used for flat-head screws and bolts.
In addition, a counterbore and spotface have many similarities. The former is used to recess a fastener while the latter is used to create a flat surface so a fastener can be used. In essence, a spotface is a shallow counterbore. Spotfaces ensure that the surface around the hole is smooth and perpendicular to the hole axis, providing a consistent surface for washers and bolt heads. This hole is commonly used for creating flat seating surfaces on rough or uneven workpieces.
Which Type of Counterbore Do I Need?
There are several types to choose from, depending on the type and length of your drill bushings. The charts below illustrate counterbore compatibility with Carr Lane Mfg. drill bushings of various sizes.