My copy of ISO 1101 is old (1983), the latest revision I think is (2004), I don’t have it. The (1983) revision has two tolerance descriptions as well, (concentricity to a datum point) and (concentricity to a datum axis). The distinction between the point and the axis seems to depend upon whether the datum reference (not to be confused with datum feature) is considered as a circle (2D if you will) or a cylinder (3D). I don’t believe that the distinction rests in whether there is one datum feature as opposed to two as depicted in the illustration. I would assume from your comments that some refinements (specifically ACS) may have been added to the (2004) revision so I cannot comment on that.
I think that the ISO definition of concentricity more closely resembles the (1982 ANSI 14.5) description of concentricity where the “individual cross sectional centers” of the considered feature form must reside within a (cylindrical) diameter tolerance zone.
I have not read the (2004) revision of ISO 1101 but I don’t think that the TC 213 (ISO’s equivalent to our Y14 standards committee) has ever entertained the “median point of diametrically opposed surface elements” that our 1994 ASME standard revisers “unfortunately” saddled us with.
When the ASME definition changed in 1994 I was disappointed because as powerhound stated in CONCENTRICITY VS. THE CMM (CONCENTRICITY IS EVIL) “there is no substitute for concentricity when balance …is a factor” Although the new ASME definition can reasonably control the average center of uniform continuous surfaces with even lobbing i.e. ellipses, etc. it fails to control odd lobbed or interrupted surfaces i.e. odd tooth gears, vane pump rotors, etc.
The 1982 standard cautioned that “determining the average center may be difficult to measure” but that is precisely CMMs do with circular regression equations figuring “best –fit” centers from measured surface points. I know that the ASME committees sought to refine the analysis distinctions between Runout, Position, and Concentricity in concert with Y14.5.1 Mathematical Definition but I think that the effort fell short with concentricity by reducing the interpretation to “median point of diametrically opposed surface elements” never-the-less that is what we have with the ASME standard.
I have consulted on numerous DIN, ISO, BS, and JIS conversions and/or interpretations to American National Standards and I have seen concentricity specified used interchangeably with position where the toleranced features were not coaxial with the datum features, so don’t be surprised if you see that. In fact, the ISO 2692 first edition (1988-12-15) “Geometrical Tolerancing – Maximum material principle” standard that I have shows concentricity specified with MMC modifiers on both the datum and toleranced features.
Paul F. Jackson