Countersinking Guidelines

Countersinking is a vital process in sheet metal fabrication, particularly when creating components that require a smooth surface with no protruding screw heads or fasteners. This technique involves enlarging the top portion of a pre-drilled hole, creating a tapered seat that allows flat-head screws or bolts to sit flush with or beneath the surface of the sheet metal.

Countersink Sizes

Fabworks gives you the ability to upload STEP files with your countersinks pre-drawn. This reduces the amount of time you will spend configuring your order. When taking this approach, you can pick from any diameter of countersink as long as it's one of the following angles: 82°, 90°, 100°, or 120°.

Please keep in mind that we hold a tolerance of -.000"/+.015" on all countersinks, which ensures that your bolts will sit sub flush with the surface of the sheet metal. When powder coating your parts, we recommend accounting for the ~.004" of added thickness if a sub flush fit is critical.

Configurable Countersinks

In some cases you might want to add a countersink to a preexisting model. You can use our hole operations tool to add one of the following countersinks to your parts.

Countersinks in Thin Sheet

A countersink is a cone that narrows down to the through hole. In thin sheet, the cone can run out of material before it gets there. It cuts through the back of the part, and the hole on the bottom face ends up larger than the through hole.

Pick one of our countersinks, or switch to Custom size to enter your own. Then pick a sheet thickness. The amber dimension is the hole on the bottom face, and the dashed lines show the through hole.

Calculate It Yourself

You need the countersink diameter (D), the through hole (d), the sheet thickness (t), and the countersink angle.

1. Find how deep the cone is: (D − d) ÷ (2 × tan(angle ÷ 2))
2. If your sheet is at least that thick, the bottom hole is the through hole.
3. If it is thinner, the bottom hole is D − 2 × t × tan(angle ÷ 2)

For tan(angle ÷ 2), use 0.869 for 82°, 1.000 for 90°, 1.192 for 100°, or 1.732 for 120°.

For example, our #10 countersink is 0.411" across over a 0.201" through hole at 82°. The cone is (0.411 − 0.201) ÷ (2 × 0.869) = 0.121" deep. In 0.100" sheet, the bottom hole is 0.411 − 2 × 0.100 × 0.869 = 0.237".

Flat Head vs. Undercut Screws

A flat head screw has a cone-shaped head that sits flush in a countersink. Inch screws have 82° heads, and metric screws have 90° heads. Our countersinks are sized for socket flat head screws, like a #10-32 or M5 flat head cap screw, so those sit flush.

An undercut flat head screw has a shorter head for thin sheet. When a standard head is taller than the sheet is thick, its tip sticks out the back and pushes on the part underneath. An undercut head stays inside the sheet.

The calculator tells you when you need an undercut screw and draws one. Inch undercut screws have 82° heads and fit our standard countersinks. Metric undercut screws have 100° heads, so they need a 100° countersink. Switch to Custom size and pick 100°.