⌢ Arc length
Arc length · Indicadores dimensionales · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “⌢ 37.7” states a arc length requirement.
What controls
A symbol placed over a dimension indicating that the value is measured along the curve of the arc, not in a straight line between its ends. The distinction has real consequences: the chord is always shorter than the arc, and the difference grows with the angle subtended. It appears when what matters is the true developed length of the curved surface, as in sheet metal that is going to be bent or rolled, in toothed sectors or in partial perimeter dimensions.
How to verify
With a flexible tape or a string laid over the curved surface and then measured, which is the practical method on large parts. On small parts, coordinate measurement or an optical comparator, calculating the developed length from the measured actual radius and angle. Measuring the chord with a caliper and comparing it with an arc dimension is a plain error, not an approximation.
Common mistakes when interpreting it
- Measuring the chord between the ends and comparing it with an arc dimension.
- Calculating the sheet metal developed length with the chord instead of the curved length.
- Using the arc dimension without knowing the radius, which leaves the geometry undefined.
How to get it in the workshop / if it comes out of tolerance
In sheet metal and bent profiles, the developed length is what has to be cut before forming; using the chord instead leaves the part short. When programming a curved toolpath, check whether the dimension you are using is an arc or a chord before calculating the travel. And remember that the arc is only defined if the radius and the center are also known.
Bent parts that consistently come out short: check whether the developed length was calculated using the chord or whether the stretching of the material during forming was not taken into account. If the measured arc does not match the calculated one, check the actual radius achieved first, because a different radius changes the length even if the angle is correct.
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Other symbols in Dimension indicators
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