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.
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.
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.
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 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.
Countersink cutters are cone-shaped tools available in single-flute and multi-flute configurations. Single-flute countersinks work well in softer materials and produce less chatter, while multi-flute versions (3, 5, or 6 flutes) provide a smoother finish in harder metals. Combined drill-and-countersink bits — sometimes called center drills — create both the pilot hole and countersink in a single operation and are commonly used for high-volume production.
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.
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.
Counterbore cutters are flat-bottomed tools with a pilot that fits into the existing hole to ensure concentricity. They're available as solid tools or with interchangeable pilots for different hole sizes. For precision work, indexable counterbore tools allow for fine diameter adjustments.
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.
Power generation and heavy industrial applications primarily utilize counterbores due to their superior load-bearing capabilities. Turbine components, generator housings, and industrial automation equipment require the higher torque capacity of socket head cap screws, typically ranging from 20-100 ft-lbs. The design of counterbores also accommodates lock washers and other fastener retention devices essential in high-vibration environments.
Industrial automation systems demonstrate the practical differences between these hole types. Robotic arm components and guide rail mounting blocks use counterbores for their higher clamping force, while control panels and access covers employ countersinks for flush mounting requirements. Fastener selection typically ranges from #4-40 to 3/8-16 for countersunk flathead screws, while counterbores accommodate 1/4-20 to 3/4-10 socket head cap screws.
Designs often incorporate both features based on specific requirements. Thin-wall applications under 0.250" benefit from countersinks, while nested component mounting and multi-layer stack-ups frequently require the precise depth control offered by counterbores. This combination of features allows for optimal fastener selection in complex assemblies where space constraints dictate design choices.
If you're reading or creating machining blueprints, you'll encounter specific symbols for countersink and counterbore callouts. These are standardized under ASME Y14.5.
A typical countersink callout on a drawing looks like this:
∅.201 THRU ⌵ ∅.370 X 82°
This reads as: drill a 0.201" through hole, then countersink to a 0.370" diameter at an 82° included angle.
The key dimensions in a countersink callout are the pilot hole diameter, the countersink diameter (measured at the surface), and the included angle. Some callouts also specify a countersink depth instead of a diameter, depending on the fastener and the application.
A typical counterbore callout looks like this:
∅.201 THRU ⌴ ∅.375 ↧ .190
This reads as: drill a 0.201" through hole, then counterbore to a 0.375" diameter and 0.190" depth.
The key dimensions are the pilot hole diameter, the counterbore diameter, and the counterbore depth (measured from the part surface to the flat bottom of the recess). Some callouts use the depth symbol (↧) while others simply note the depth value after the diameter.
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 quality requirements.
Reach out to our team to get a quote for custom precision fasteners.
