⌵ Countersink
Countersink · Indicadores dimensionales · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
⌵ identifies a conical countersink at the entry of the hole, with the angle and diameter given.
What controls
A cone at the entry of a hole to accommodate the head of a flat-head (countersunk) screw or to create a lead-in. It is dimensioned by the included angle of the cone and the major diameter at the surface, or alternatively by depth. The angle is the critical figure: it must match the screw head angle, because a mismatch makes the screw bear on an edge instead of the whole conical surface, so it works loose or marks the part.
How to verify
The major diameter at the surface with a caliper, which is the most sensitive and most practical measurement to control; the angle with an angle gauge, an optical comparator or by coordinate measurement. In production, dedicated countersink gauges are used, or the height of a master ball seated in the cone is checked, which is a fast and repeatable method. The definitive functional check is to fit the screw and see whether it sits flush.
Common mistakes when interpreting it
- Using a tool with a different angle from the one on the drawing because it is the one already in the machine.
- Controlling only depth without checking the major diameter, which is what the screw actually sees.
- Countersinking thin sheet without considering that the cone can take up almost the entire thickness.
How to get it in the workshop / if it comes out of tolerance
Check the angle the drawing calls for and the angle of the tool you are going to use: not all countersunk heads have the same angle, and using the wrong tool leaves the head proud or sunk too deep. In thin sheet, the countersink can take up almost the entire thickness and distort the material: think about the order of operations. Control depth with a positive stop, because a few tenths change the visible diameter noticeably.
Head sticking out: countersink too shallow or major diameter too small; correct the depth stop. Head sunk with a mark around it: countersink too deep. Countersink off-center from the hole: machine it in the same setup. And if the angle does not match the screw's, contact is on an edge and going deeper won't fix it: you have to change the tool.
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