Countersink Angles and Their Uses

If you want to achieve professional-looking flush joints without protruding screws on your woodworking or metalworking project, a countersink bit can be your best friend. But do you know that the countersink angle you choose can impact the strength and stability of the joint?

In this guide, I’ll explain the common countersink angles and their uses to help you choose the correct bit for your work.

What is the Angle of a Countersink?

The UTS standard countersink angle for inch screws is 82°, and the metric countersink angle is 90°.

The countersink angle is the included angle between the inclined/cutting edges of a countersink. In layperson’s terms, this means the angle at which the workpiece must be cut to create sufficient space to install a screw such that the head is flush with the surface instead of protruding.

You can measure the angle on the countersink bit (countersunk tool) using a bevel protractor or visual measuring equipment such as profile projector.
Protruding fasteners could create safety hazards by getting caught on materials during operation, interfere with aerodynamics in the case of aerospace and aviation applications, or create a displeasing aesthetic. Countersinking and counterboring are two common techniques used in applications where the fastener head mustn’t protrude from the workpiece.

Common Countersink Angles & Their Uses

While the 82° and 90° are two most common angles, countersinking can be done at various angles, each with its own purpose. Here is a brief overview of the most common countersink angles.

60° Angle

The 60° countersink angle is generally used to countersink flathead screws with a matching angle. Here, the load bearing on the fastener is concentrated in a smaller area. This localized loading is beneficial in applications where the fastener is likely to experience high stresses and where a precise fit is necessary.

The angle of a lathe center is also 60-degrees. Normally, you will use a center drill to create the grove for the center to seat when you want to perform turning between centers. In case you do not have a center drill handy, you could drill a small hole and chamfer it with a 60° countersunk.

82° Angle

The 82° countersink is commonly used for inch screws used in the United States and Canada, complying with Unified Thread Standard (UTS), UNC, UNF, and UNS standards. This also matches the ASME B18.3 standard.

This is going to be your main bit for most of your woodworking screws.

90° Angle

The 90-degree countersink angle matches the ISO and DIN 7991 metric standards and is commonly used to match these screws. This angle is often used to create bevels and chamfers on workpiece edges since it creates an aesthetically pleasing, clean finish.

Outside North America, most countries have adapted to the metric system, and hence the 90° countersink is the most common one.

100° Angle

The 100° degree countersink is used in aerospace applications matching the AN 426 standard. Solid rivets can be easily countersunk, creating a neat, flush fit. This countersink angle is also typical when using fasteners with thin heads, like rivets and some types of screws. The fastener head won’t be damaged during installation when using this angle.

When working with materials prone to cracking and chipping, the 100° countersink offers a wide enough angle to distribute the load evenly, thus alleviating point loads on the more delicate material. It also tapers the drill hole more gradually, further alleviating stresses. Examples include aluminum and brass. If a sharper countersink angle is used with these softer materials, the material could tear or crack.

Fasteners using the 100° countersink also match the British empirical inch screws, in line with BA, BSF, BSW standards, and Unified National Aviation fasteners.

110° Angle

The 110° countersink angle isn’t as common as the others. These specialized applications are mainly found in aircraft building and sometimes when sinking holes in plastic, wood, and non-ferrous materials. Here, a wider angle prevents damage to the material and minimizes the risk of creating splinters.

118° Angle

The 118° countersink angle is also not so common but used in metalworking applications, which also matches the point angle of a standard twist drill bit.

You ensure a clean, precise cut when drilling a hole after creating a chamfer at 118° angle. This is especially true when working with harder materials, like steel or cast iron, where you might enlarge a drilled hole using a drill bit with a larger diameter.

120° Angle

The 120° countersink angle is commonly used in harder materials like titanium and hardened steel. This wider angle protects the workpiece by preventing crack formation, and minimizing the risk of material breakage, as would be the case with sharper countersink angles.

This countersink angle is typical in aerospace applications and uses pop or pulled rivets.

Final Thoughts

Countersink angles vary widely across different applications. It is best to match the countersink angle to the task at hand, ensuring that it fits with the fasteners used and the workpiece material. Sharper countersink angles are better for applications where point loads are acceptable. The fastener must fit securely and be positioned precisely. Wider angles are better suited to applications where thin materials are used, the fastener heads are thin, and the material is prone to cracking, tearing, or breaking.

A sharper countersink angle is also better suited to thicker material. An example for illustration: Imagine two fasteners had the same shaft diameter and head diameter but differing countersink angles. The one with the sharper countersink angle would require greater depth to taper from the head’s external diameter to the shaft diameter. In comparison, the one with the wider angle wouldn’t require as much depth.

Reference:
• https://en.wikipedia.org/wiki/Unified_Thread_Standard
• https://www.asme.org/codes-standards/find-codes-standards/b1-1-unified-inch-screw-threads-un-unr-thread-form
• https://www.efunda.com/designstandards/screws/screwunc.cfm