Manufacturing Process
Countersinking
沉孔 / 锪孔
Machining a conical recess so a flat-head screw or rivet sits flush with the part surface before final assembly.
Process Steps
- Part positioned on fixture
- Countersink tool machines cone
- Depth and angle gauged
- Deburr and clean
What is countersinking?
Direct answer: countersinking cuts a conical recess around a hole so that the head of a flat-head screw or rivet finishes flush with the surface of the part. The cone angle has to match the fastener: 82° for inch flat-head screws, 90° for metric, 100° and 120° for rivets and aerospace fasteners.
Countersink vs counterbore — which do I need?
| Countersink | Counterbore | |
|---|---|---|
| Shape | Conical seat | Cylindrical flat-bottomed recess |
| Fastener | Flat-head / countersunk screw, rivet | Socket-head (cap) screw, washer, bolt head |
| Result | Head sits flush with the surface | Head sits below the surface in a flat pocket |
| Made by | Countersink tool, or formed in the die for high volume | End mill or counterbore tool |
| Critical dimension | Cone angle and depth to the seating surface | Pocket diameter, depth and flatness of the floor |
The confusion between them is the most common fastener question in sheet-metal design: a countersink is a cone, a counterbore is a flat pocket, and the screw you have chosen decides which one the drawing needs.
What the drawing should specify
| Feature | What to specify | Typical tolerance |
|---|---|---|
| Cone angle | 82°, 90°, 100° or 120° to match the fastener | ±1° |
| Screw head diameter | Nominal head size plus clearance, so the head does not stand proud | — |
| Depth | Depth to the seating surface, or the resulting head protrusion | ±0.05–0.10 mm |
| Concentricity to the hole | Coaxial to the drilled or pierced hole | 0.05–0.10 mm |
| Edge condition | Burr-free; no interrupted cut at breakout | Visual or Ra 3.2 µm |
How the cone is actually produced
- Machined after drilling — a piloted countersink tool follows the existing hole. The pilot keeps the cone concentric and stops chatter. This is the normal route for low and medium volume.
- Formed in the stamping die — a coining punch presses the cone into the part in the same stroke as the hole. This is the cheaper route at volume, but the cone has to be formed from the sheet thickness, so the head sits in a shallow seat rather than a deep machined one.
- On a CNC machine — spot-drill, drill, then countersink in one setup, which is what keeps concentricity tight on a machined part.
Common problems
- Chatter. A countersink without a pilot, run too fast on thin sheet, produces a faceted cone. Fixed with a piloted tool, lower speed and steady feed.
- Oversize or bell-mouthed cone. Usually tool runout or too much axial force on thin material.
- Breakout on thin sheet. The cone breaks through the far side and leaves a knife edge. Fixed by thickening the area, forming the seat instead of machining it, or reducing the head size.
- Burr ringing inside the hole. Removed by a light deburring pass — important before plating, because plating builds up on a burr edge.
Frequently asked
What is the difference between a countersink and a counterbore?
A countersink is a conical seat for a flat-head screw; a counterbore is a flat-bottomed cylindrical recess for a socket-head screw or a bolt head with a washer. Countersink = cone, counterbore = pocket.
What angle should I specify for a countersink?
Match the fastener: 82° for inch flat-head screws, 90° for metric flat-head screws, and 100° or 120° for rivets and aerospace fasteners.
Can a stamped part have countersinks?
Yes. At high volume the cone can be coined in the die in the same stroke as the hole, which avoids a separate machining operation. On thin material the seat is shallower than a machined countersink, so the head sits slightly proud unless the drawing allows for it.
Why does my countersink break through the back of the sheet?
The cone is deeper than the material. Options are to thicken the area with a formed boss, coin the seat instead of machining it, use a smaller head, or accept a machined counterbore. A DFM review flags this before tooling.
Related: deburring · tapping · piercing · metal machining · deep drawing