⌴ Counterbore
Counterbore · Indicadores dimensionales · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
⌴ identifies a counterbore: a larger, shallow diameter at the entry of the hole, with the diameter and depth given next to the symbol.
What controls
A flat-bottomed cylindrical recess at the entry of a hole to accommodate the head of a screw or a washer so that it sits below the surface. It is dimensioned with the counterbore diameter and its depth, plus the corresponding through hole. What the dimension does not state on its own is how the counterbore must relate to the through hole: if coaxiality matters, a feature control frame is needed to control it.
How to verify
Counterbore diameter with an inside caliper or bore gauge, and depth with a depth gauge resting on the outer surface. Flatness and perpendicularity of the bottom are checked with a dial indicator or by coordinate measurement when the drawing requires them. The relationship to the through hole, if toleranced, is evaluated by coordinate measurement, not with a caliper.
Common mistakes when interpreting it
- Assuming the counterbore will automatically be coaxial with the through hole without anything controlling it.
- Leaving the bottom with the conical shape of the drill point, where the screw head is supposed to bear.
- Measuring depth with the gauge resting on a surface that is not the drawing's reference surface.
How to get it in the workshop / if it comes out of tolerance
Always machine the through hole first and the counterbore afterwards, piloted by it, so they come out concentric; if they are done in different setups, misalignment shows up immediately. The bottom must be flat and perpendicular to the axis, because it is the face the screw head bears against: a conical bottom or one with a leftover drill point makes the screw seat poorly and work loose. Check that the chosen diameter leaves room for the wrench.
Counterbore off-center from the through hole: machine both in the same setup or use a tool piloted by the hole itself. Bottom not flat: use a flat-bottom tool and take a finishing pass; a drill with a conical point is no good for this job. Insufficient depth leaving the head sticking out: check whether the dimension is measured from the as-cast face or the machined face.
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Other symbols in Dimension indicators
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