R Radius
Radius · Indicadores dimensionales · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “R12” states a radius requirement.
What controls
A dimension defining the distance from the center of an arc to its surface. On its own it only limits the size of the arc within its tolerance: it does not require the arc to be tangent to the surfaces it joins, nor does it fix where its center is, nor does it control its form beyond what the general tolerance imposes. All of that, when needed, is achieved with a profile frame or with the controlled radius variant. The value is always measured as a radius, not as a diameter.
How to verify
Radius gauges for a quick check on the shop floor, an optical comparator for small, accessible radii, and coordinate measurement when the radius is large or part of a toleranced profile. With the gauge you have to look against the light: what you are judging is whether there is light between the gauge and the part, not whether it looks like it fits. When the drawing requires tangency or form, the gauge is not enough and the full profile has to be evaluated.
Common mistakes when interpreting it
- Assuming a dimensioned radius requires the arc to be tangent to the faces it joins: that is required by the controlled radius variant or a profile frame.
- Mistaking the value for a diameter and machining twice or half what is required.
- Judging the gauge fit by eye instead of checking for light showing through.
How to get it in the workshop / if it comes out of tolerance
A dimensioned radius in an inside corner dictates the tool: the end mill cannot have a larger radius than required, and if the drawing gives a maximum radius, not much smaller either without leaving an undercut. On outside corners, the radius is usually produced by interpolation and the problem shows up at the blend with the adjacent faces. Check whether the radius is a minimum, a maximum or has a bilateral tolerance, because it changes the tool choice.
A radius that doesn't match the gauge even though the value is correct: there is usually a step at the blend with the adjacent faces, not a size error; review the finishing strategy. When milling inside corners, a radius smaller than required forces a tool change. And if the radius comes out irregular, look at chatter and wear before the program.
Related symbols
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