Ⓡ Reciprocity
Reciprocity · Modificadores de zona · ISO 1101
Example in plan
How to interpret it
On the drawing, “⌀ 25 H7 with Ⓡ and ⌖ ⌀ 0.05 |A|B|” states a reciprocity requirement with a cylindrical tolerance zone of ⌀ 25 mm, measured from datums A, B in that order of precedence.
What controls
An ISO modifier that only makes sense together with the maximum material requirement Ⓜ (or the least material requirement Ⓛ) and that works in the opposite direction to bonus tolerance: if the geometric tolerance is not fully used, the unused part can be used to enlarge the size tolerance. What stays constant is the virtual condition of the feature, that is, the boundary that guarantees assembly; within that boundary it does not matter how much comes from size and how much from geometry. With Ⓜ alone, the exchange goes one way (unused size becoming geometric tolerance); with Ⓡ added, it also goes the other way. What it does NOT mean is unlimited tolerance: if the feature violates the virtual condition, the part is bad, and size also cannot grow beyond what the geometry has left unused. This is the approach of ISO 2692, and ASME has no equivalent modifier written this way.
How to verify
A functional gauge made to the virtual condition is still the cleanest check: since that boundary does not change, if the gauge goes in, the part conforms to the combined size plus geometry. If measured separately, you have to measure actual size and actual geometric deviation and check that the feature does not violate the virtual condition, rather than judging each characteristic against its own limit: judging them separately rejects parts that reciprocity accepts. On a CMM you have to explicitly enable evaluation with the maximum material requirement and reciprocity; if the software does not support it, do the calculation by hand and document it in the report.
Common mistakes when interpreting it
- Taking it as unconditional permission to go outside the size dimension: the limit is still the virtual condition, and the margin only exists if the geometry was not used up.
- Applying it on its own, without Ⓜ or Ⓛ: without a material requirement to be attached to, reciprocity means nothing.
- Reading the Ⓡ glyph on an ASME drawing as reciprocity: there it stands for Regardless of Feature Boundary (RFB), which is a different concept.
How to get it in the workshop / if it comes out of tolerance
Rare on shop drawings. If Ⓡ appears, check the standard cited on the drawing before releasing the lot. Careful: the same Ⓡ glyph on an ASME drawing is not this — it is «Regardless of Feature Boundary» (RFB), a different concept. Check whether the drawing cites ISO 1101 or ASME Y14.5 before interpreting it. When it really is reciprocity, the shop-floor reading is very practical: a hole that comes out better positioned than required allows you to accept a diameter slightly outside its dimension, and vice versa, as long as the functional gauge goes in.
Before rejecting a part on a size dimension, calculate how much geometric tolerance was left unused: that margin is exactly what Ⓡ gives back to you. If it still fails, attack whatever dominates the actual error (position or orientation of the feature versus diameter), because that is where there is margin to recover. And if the customer does not recognize the criterion at receiving inspection, get the verification method in writing before shipping the lot: with this modifier it is common for supplier and customer to evaluate by different rules.
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