What Do Countersinks Look Like After CNC Machining?

• 1
  Key takeaways
• 2
  What Countersinks Look Like on a Finished Part
• 3
  Angle, Depth, and What They Change
• 4
  Single, Double, and Countersunk Holes
• 5
  How to Judge a Countersink Before Assembly
• 6
  Countersink Forms Side by Side

A countersink reads as a shallow cone cut into the surface. The opening is a circle; the walls slope inward at a fixed angle until they meet the pilot hole. From above you see two concentric circles: the outer rim where the cone meets the face, and the inner circle where the cone meets the drill hole. The gap between them is the conical band that the screw head presses against.

Seen from the side, the profile is a triangle with the top corners cut off. The included angle is the number that matters. An 82° countersink is shallower and wider than a 90° one at the same head diameter. That difference changes how much material you remove and how much bearing surface the head gets.

The surface inside the cone is not polished. On aluminium it usually shows fine helical tool marks running around the cone. On steel it can look matte grey. Both are normal. What you do not want is a torn rim, a step halfway down the wall, or a cone that is visibly off-centre from the pilot hole.

Under magnification, a good countersink shows a continuous contact band roughly 70 to 80 percent of the head's bearing area. A poor one touches only at the top edge or only at the bottom. The screw may still go in. It will not hold torque the way the drawing intended.

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  Clean rimNo raised burr, no rolled-over lip.
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  ConcentricCone axis within 0.05 mm of the pilot hole axis for most work.
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  Even bandContact width stays constant all the way around.

The included angle is set by the fastener, not by the machine. Inch flat-head screws are usually 82°. Metric flat heads are usually 90°. Aerospace rivets and some thin-sheet fasteners use 100° or 120°. If the drawing says 90° and you cut 82°, the head seats on its top edge only. It looks close and it is wrong.

Depth controls flushness. For a flat head to sit flush, the cone must be deep enough that the head's top face lands level with the part surface. Cut 0.10 mm too shallow and the head stands proud. Cut 0.10 mm too deep and the head sinks below the face, leaving a sharp lip around it that catches fingers and paint.

The relationship is simple. Depth equals the head height for a flush fit. If the head is 2.5 mm tall and you want it flush, the cone must be 2.5 mm deep from the surface. Add 0.05 mm if the part will be anodized or painted, because coating builds on the rim and pushes the head up.

There is a limit. As the cone gets deeper, the wall thins and the pilot hole loses material around it. On a 3 mm thick plate with a 90° countersink for an M4 screw, the remaining wall under the head is under 0.5 mm. That is where the part cracks during tightening, not during machining.

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  82°Inch flat-head screws and most US drawings.
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  90°Metric flat heads, general purpose fastening.
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  100° or 120°Thin sheet, rivets, and some aerospace hardware.
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  Flush depthHead height plus 0.05 mm for coating.

A single countersink is one cone, one angle, one purpose. It is what most people picture. A double countersink is two cones stacked on the same axis. The upper cone takes the screw head; the lower cone takes a washer, an O-ring, or a rivet tail. From the side it looks like a stepped funnel.

A countersunk hole is different from a countersink feature. The hole is the clearance bore; the countersink is the cone around it. Drawings that call out only a hole diameter and a depth for the cone are common, and they are a frequent source of argument at inspection. Ask which diameter the depth is measured from.

On curved or angled surfaces, the cone must be cut normal to the local surface, not normal to the machine table. If it is not, the rim is an ellipse instead of a circle, and the head seats on one side. Five-axis work handles this by tilting the tool. Three-axis work needs a fixture that presents the surface flat.

Blind countersinks are cut into a pocket or a bore where the tool cannot reach straight down. They use a back-spotfacing tool or a custom form tool. The result looks the same from the surface. The difference is that the cone is cut from the inside out, and the rim quality depends heavily on tool rigidity.

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  SingleOne cone, one fastener, the default.
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  DoubleTwo stacked cones for a head plus a washer.
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  Normal to surfaceRequired on angled faces to keep the rim round.

Start with a visual check at 5× to 10× magnification. Look for a continuous contact band inside the cone. If the band is broken or sits only at the top, the angle or the depth is off. If the band is polished and even, the tool ran concentric and the depth is close.

Then check concentricity. The cone axis should sit within about 0.05 mm of the pilot hole axis for general work, tighter for anything that sees vibration. A quick test is to drop the matching screw in without force. If it rocks side to side, the cone is off-axis or the angle is wrong.

Check the rim for burrs. A raised lip will stop the head from seating flat and will flake off into the assembly. A rolled-over edge on aluminium usually means the tool was dull or the feed was too high. Both are correctable on the next run.

Finally, measure one countersink per batch with a gauge or an optical comparator. Record the angle and the depth. If the drawing calls out a diameter at the surface, measure that too. Small errors repeat across thousands of parts, and catching them at part one is cheaper than at part five hundred.

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  Contact band70 to 80 percent of the head bearing area.
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  ConcentricityWithin 0.05 mm of the pilot hole for general work.
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  Burr-free rimNo lip that lifts the head or breaks off.