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Countersink vs. Counterbore Holes in Machining

Countersink vs. Counterbore Holes

Machined parts can have holes of all different shapes and sizes. Each type of hole has a name that describes the shape of it. Two popular types of machined holes are countersunk holes and counterbored holes.

Countersink vs. Counterbore Holes

The difference between countersink and counterbore holes is the shape of the top of the hole. Countersink holes have a cone-shape, while counterbore holes have a cylindrical shape.

Countersink vs Counterbore Hole
Countersink vs Counterbore Hole

Both types of holes are designed to hide the head of a fastener, making it flush with the part when it’s assembled. The different type of hole is designed to be used with different fasteners.

Fasteners that have an angled head require a countersunk hole to be flush. A fastener with a flat head needs a counterbored hole.

Countersunk holes

A countersunk hole is a cone-shaped hole that is drilled into a material. A cone-shaped cutting tool is used to machine the chamfer at the desired angle, and then the rest of the hole is drilled with a regular drill bit.

Countersink holes are often used in woodworking or when working with softer materials. The angle can be adjusted, but the 3 most common are 60°, 82°, and 90°. ISO standard screws often have a 90° angle. In the aerospace industry, countersunk fasteners usually have an angle of 100°

Countersink Hole Angles
Countersink Hole Angles

Advantages and disadvantages

Countersink holes are used to hide the screw head by making it flush with the metal. This protects the screws and increases their lifetime. This also improves the visual appearance, and can make the part or assembly more aerodynamic.

Drilling countersink holes does add to the machining cost compared to drilling a regular thru hole.

Counterbore holes

A counterbore hole is a type of machined hole created to seat a fastener flush with, or below, the surface of a part. Counterbore holes are typically shallower than a countersink hole and have straight sides rather than angled sides.

Washers and lock washers can also be used in counterbored holes to secure the fastener.

Advantages and disadvantages

A counterbore usually has more holding strength compared to a countersink hole because the force applied by the socket cap screw head is parallel to the axis. The force applied by the screw or bolt is distributed evenly over a larger surface area. This is not the case with a countersunk hole, which has tapered sides.

Countersink vs. Counterbore: Applications

Countersinks are typically used for wood and metal screws, while counterbore holes are used for larger fasteners such as lag bolts, and socket head fasteners.

Countersink and counterbore holes are often used in applications with moving parts, and for assemblies with limited space. Because they provide more space for lock washers, counterbores are used in heavy-duty applications including machinery, construction, and automotive applications.

Callout symbols

The callout symbol on a machining blueprint for a countersink hole is “⌵” and the callout symbol for a counterbore hole is “⌴”. The symbols are designed to match the top shape of each hole. 

Countersink and counterbore sizes are expressed in terms of their diameter, depth, and angle of countersink. The diameter is the width of the countersink or counterbore, while the depth is the distance from the surface of the material being drilled down to the top of the pilot hole.

Spex offers a large selection of custom precision machined parts for whatever your project needs. We are an ISO 9001:2015 certified company, and our team specializes in precision machining and supply chain efficiency.

Our machined components are available in an array of different materials and finishes and are manufactured to meet all the highest quantity needs.

Reach out to our team to get a quote for custom precision fasteners. 

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