Countersink vs Counterbore

A countersink and a counterbore both recess a screw head into your part. The difference is the shape of the pocket. A countersink is a cone, sized for a flat head screw. A counterbore is a flat-bottomed cylinder, sized for a socket head cap screw or another cylindrical head.

In sheet metal, the choice usually comes down to thickness. A counterbore needs the sheet to be thicker than the screw head is tall. A countersink needs less depth, but it has a limit too: in very thin sheet, the cone cuts through the back of the part.

What is a countersink?

A countersink is a cone cut into the top of a hole. A flat head screw has a matching cone under its head, so the head seats against the countersink and finishes flush with or just below the surface.

The angle has to match the screw. Inch flat head screws use 82° heads and metric flat head screws use 90°. Fabworks cuts countersinks at 82°, 90°, 100°, and 120°.

Because the cone centers the screw, countersunk fasteners self-align. That helps parts line up at assembly, but it also means the screw pulls the parts into line with the hole.

For more on angles and when to use them, see the benefits of countersinking.

What is a counterbore?

A counterbore is a larger, flat-bottomed hole cut partway into a smaller through hole. The screw head sits on the flat shoulder, inside the pocket. It is the standard recess for socket head cap screws.

A counterbore has two diameters: the body diameter, which clears the head, and the pilot diameter, which is the through hole for the screw shank. Its depth is set by how far below the surface you want the head.

Key differences

Sheet thickness decides it

For a flush counterbore, the sheet has to be thicker than the screw head is tall. A standard socket head cap screw's head is about as tall as the screw is wide: a #10 head is 0.190" tall and an M5 head is 5 mm tall. Model the counterbore depth to the head height, and the head will sit flush or just below the surface. If the part is powder coated, add about 0.004" for the coating.

A countersink for the same screw is shallower. Our #10 countersink is 0.121" deep. In thinner sheet, the cone runs out of material and the hole on the back of the part grows larger than the through hole. Check your size and thickness with the countersink calculator.

If the sheet is thinner than the cone is deep, the cone cuts all the way through.

In sheet that is thick enough for the countersink but thinner than the socket head is tall, the flat head screw still sits flush. The counterbore is too shallow, so the socket head sticks out above the surface.

Screws for thin sheet

If your sheet is too thin for a standard head, use a screw with a shorter one.

• Undercut flat head screws have a shorter cone, so the countersink is shallower and the head stays inside the sheet. Inch undercut screws have 82° heads and fit our standard countersinks. Metric undercut screws have 100° heads, so they need a 100° countersink.
• Low-head socket head cap screws have a head about two-thirds as tall as a standard one, so the counterbore can be shallower.

When to use a countersink

• A flush, smooth face with nothing to snag on, like enclosure panels and sliding surfaces.
• Thinner material, where a counterbore pocket will not fit.
• Self-aligning assemblies, where the screw should pull the parts into position.

When to use a counterbore

• Thick plate, where there is room for the full head.
• Socket head cap screws, which are easier to drive and take more torque than flat heads.
• Precise hole positions, where you do not want the screw to pull the parts out of alignment.
• Clearance below the surface for a moving part or a mating face.

Ordering countersinks and counterbores

Countersinks can be modeled in your STEP file at any diameter with an 82°, 90°, 100°, or 120° angle, or added to an existing hole in Hole Ops. See the countersinking guidelines.

Counterbores must be modeled in your STEP file. They can't be added from Hole Ops. You can order any depth as long as the body and pilot diameters match one of our supported sizes, and depth is held to +.015" / -.000". See the counterboring guidelines for the full size table.

Summary

Use a countersink for flat head screws and a flush face, especially in thinner sheet. Use a counterbore for socket head cap screws in plate thick enough to hold the head. If you're not sure your material is thick enough, start with the countersink calculator and the counterbore size table.