Get Approved Sheet Metal’s Proprietary Formula for Designing Countersinks (Updated for 2024)

Last updated on March 18th, 2024 at 01:49 pm

Do you need support designing countersinks into your custom fabrication?

Countersinks are conical holes cut or formed into precision sheet metal parts that allow hardware to sit flush with the material. Here’s everything you need to know about designing countersinks, including Approved Sheet Metal’s proprietary formula for getting countersinks right every time.

Important Countersink Measurements

There are three dimensions to understand when it comes to countersinks:

• Major: the larger diameter at the top of the opening, where the screw head will sit
• Minor: the smaller diameter at the bottom of the opening
• Angle: the angle of the screw or rivet head

The standard angles used in countersink designs are:

• 82° for screws in American manufacturing
• 90° for ISO 9001:2015 parts
• 100° for fasteners in the aerospace industry
• 120° for rivets

Different Types of Countersinks

When applying countersinks to a part, there are two main approaches:

1. Machined countersinks

Machined or "cut" countersinks are made using a drill press, end mill, or Bridgeport. First, a laser cuts the through-hole to the exact size of the minor dimension. Then, a drill bit spins into the material and makes the conical hole required to fit the hardware. As the drill bit comes down, it self-locates onto the hole and adheres to the programmed measurements.

Cutting countersinks is excellent for prototypes and low volume orders (less than 50) as machinists have to cut each countersink separately. While the setup is quick and no tooling is required, it’s only possible to make two countersinks per minute using this method.

2. Formed countersinks 

Formed countersinks are made using a CNC punch press or turret press for the entire operation—no laser cutting or manual machining is necessary.

First, a hole is punched through the material resulting in a slightly bigger minor measurement. Using a process called "coining," a die, shaped like the screw used in the end product, is then punched through. The resulting pressure moves material downward toward the bottom opening of the hole. This material is repositioned at the bottom opening, reducing the diameter of the minor to achieve its correct size. The screw is punched in at this step.

Hardware Determines Countersink Angles

It’s important to select hardware for your project before you start thinking about countersinks, as the hardware's type, size, and material will determine your next steps. Make sure your fastener isn't going to collide with the mating part below it.

These factors will dictate the angles and measurements necessary for your minor and major.

You'll know the optimal angle based on the screw you select, but keep material thickness in mind. For thin materials, choose a wider angle and consider undercut hardware for added security.

Our Proprietary SOLIDWORKS sheet metal Formula for Correct Countersink Measurements

If you've read our blog before, you probably know that we are SOLIDWORKS® enthusiasts here at Approved Sheet Metal. But keep in mind that SOLIDWORKS® won't always check your work, and you might design a countersink that isn’t easily manufacturable.

For example, if you pick out a material thickness that your minor dimension can't get to, or you have a 120° diameter going into a vertical edge, SOLIDWORKS® won't flag it for being impossible to make.

Our team can plug your design into SOLIDWORKS® and figure out the measurements for you using our proprietary formula. If you want to do it yourself, use our formula that increases the minor diameter to ensure a 0.005" flat edge so you can get measurements that will work with your part.

Our Countersink Formula: Pre-Punch Hole Size = B – [(B – C) x .75]

In our formula, "B" is the largest opening at the top of the countersink, and "C" is the desired opening at the bottom.

Not paying close attention to the diameters could result in a burr that prevents mating parts from sitting flat next to one another.

Countersinks are necessary in sheet metal fabrication, and designing them doesn't have to be challenging. If you would like a precision sheet metal expert to guide you through the process of creating countersinks, please get in touch with us!

And if you're ready to start your next sheet metal fabrication project, we'd love to show you what we can do. Request a quote to get started today.