A countersink is a conical opening; a counterbore is a larger cylindrical recess with a flat seating bottom. Select the feature to match the fastener and joint design.
| Feature | Countersink | Counterbore |
|---|---|---|
| Basic shape | Cone around the through hole | Cylindrical recess above a smaller hole |
| Common purpose | Seat a matching flat-head fastener | Recess a suitable cylindrical head or washer |
| Key dimensions | Opening diameter and included angle | Recess diameter and depth |
| Depth relationship | Depends on angle and hole diameters | Specified independently of diameter |
| Common confusion | Chamfer size is not full head geometry | Spotface is usually a seating cleanup feature |
Countersink depth example
For a 10 mm opening over a 6 mm hole with a 90° included angle, the ideal axial cone depth is (10 − 6)/2 = 2 mm. An 82° included angle produces a deeper cone for the same diameters. The fastener head angle must match the intended seat; equal opening diameters do not make the seats equivalent.
A countersink tool may have a truncated point, so machine Z travel from the physical tip is not identical to the finished cone depth. Use the tool geometry and an appropriate inspection method.
Counterbore checks
For a 5 mm-thick plate with a 3 mm-deep counterbore, only 2 mm of nominal material remains below the recess before other features are considered. That arithmetic does not establish joint strength. Check bearing area, head fillet clearance, tool corner radius and edge distance against the design.
A socket-head screw may need access for the driver as well as clearance around its head. A washer changes the required recess dimensions. Specify whether a protruding or flush head is intended rather than relying on appearance alone.
What to put on the drawing
Identify the through hole, recess diameter, depth or included angle, tolerances and any surface requirement. Resolve the drawing standard’s symbols and depth convention. Do not replace one feature with the other merely because the fastener can be pushed below the surface; the seating and load paths differ.
Read the seating geometry in section
For a countersink, changing the included angle changes depth even when the two diameters remain fixed. With a 10 mm opening and 6 mm through hole, depth = (10−6)/[2 tan(angle/2)]. The ideal depth is 2.000 mm at 90° and approximately 2.301 mm at 82°. A screw head with the wrong angle may touch at an edge rather than seat as intended.
A counterbore does not have that cone relationship. Its diameter is chosen around the specified head or washer, and its depth is dimensioned independently. Check clearance for the screw’s under-head fillet and the cutter’s corner radius; nominal head diameter alone is insufficient.
Drawing review before machining
- Identify the through-hole size and tolerance.
- Specify the recess diameter and either included angle or cylindrical depth.
- Check head projection, driver access, minimum remaining section and edge distance.
- Define inspection from the finished surface, including any coating.
Use the countersink calculator for ideal cone geometry. Do not interpret its depth as tool-tip Z travel without accounting for the actual cutter tip. Fastener product drawings, not these schematic proportions, establish the required seating dimensions.
Content updated October 8, 2026. Sources and numerical examples are explained above; examples are not production test results. Editorial method · Report a correction.
References and verification
Original explanations and worked examples by MfgWorkbench. Sources below support the stated concepts or selected reference data; they do not certify every example or a production application.
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