Nowhere it is stated that LSQ routine by default shall be used to find feature's global linear size. LSQ entities are essential to find local sizes of a feature and that is all. "LSQ is default, but not for what is officially called "direct global linear size" - that is my point. Just to stir the pot regarding the LSQ default algorithm: LSQ it is default for the actual local sizes
LINE PROFILE PER ASME Y14.5 1994 ISO
Join new LI group, "GD&T and GPS World" (), to discuss and learn more about differences between the two most commonly used standards for dimensioning and tolerancing - ASME Y14.5 and ISO GPS. Additionally, ISO 2692 offers an interesting note that application of the envelope requirement in conjuction with position or orientation tolerances at MMR or LMR reduces the technical and economical advantages of the maximum and least material requirement concepts, which is yet another subtle but potentially significant difference compared to Y14.5. This basically means that the concept of bonus tolerance, so commonly used in ASME world for position tolerancing at MMC or LMC, is not really applicable for position tolerancing at MMR or LMR in ISO at all. In ISO GPS, however, as defined in ISO 2692:2014 "Maximum material requirement (MMR), least material requirement (LMR) and reciprocity requirement (RPR)", only the surface interpretation exists. 3 - Axis and surface interpretations of position tolerance at MMC per ASME Y14.5. ASME Y14.5 states that this type of position tolerances, when applied to features of size like cylindrical holes or pins, may be interpreted two ways: 1) in terms of axis of the feature or 2) in terms of surface of the feature. So if the axis of the actual hole is produced at or very close to the position tolerance limits (condition fully acceptable per Y14.5), then the inevitable EML straightness error may cause the EML to end up outside of the position tolerance zone and rejection of the part per ISO rules.įinally, the third difference - interpretation of position tolerances applied at MMC/MMR (Maximum Material Condition/Maximum Material Requirement) or LMC/LMR (Least Material Condition/Least Material Requirement). This is nothing but a consequence of what was said earlier - position tolerance in Y14.5, as a location control of perfectly straight UAME axis, does not impose any limits on the shape of the hole, as opposed to position tolerance in ISO, which, apart from dealing with location of the EML, inherently controls its straightness.
2 - Interpretation of position tolerance at RFS per ASME Y14.5 and ISO.Īs can be easily noticed in the figure, the UAME axis falls inside the position tolerance zone but the EML does not. The EML is a line formed by the centers of individual cross sections of the hole made along its depth, and because the hole can never be produced perfectly straight, the EML is never a straight line too (full EML definition can be found in ISO 17450-3:2016 "General concepts - toleranced features").įig. The UAME axis is the axis of the unrelated maximum cylinder inscribed within the hole and by definition is always a straight line. The lower portion illustrates a section through an as-produced hole together with its UAME axis and the EML both superimposed on the position tolerance zone. The upper portion of the figure shows a drawing of a part with a cylindrical hole controlled with a position tolerance at RFS. In ISO GPS the controlled element is the Extracted Median Line (EML is the acronym that will be used throughout this article) or Extracted Median Plane or Extracted Center correspondingly.įigure 2 explains this in more detail. According to Y14.5, this type of position tolerance controls location of the axis or center plane or center point of the Unrelated Actual Mating Envelope (UAME) of the feature, depending on whether the feature is a cylinder, a set of two parallel opposed surfaces or a sphere. In ASME this is called an RFS scenario ISO does not have a corresponding acronym. The second major difference is about interpretation of position tolerance applied to a feature of size regardless of the feature actual size value.
Another positive is that technically ISO 1660:2017 "Profile tolerancing" standard allows surface profile tolerances to be applied to nominally flat faces, hence if surface profile tolerance was used in Figure 1, it would be legal in ISO too. Luckily, regardless of the type of control used - position in ISO or profile of a surface in ASME - the two callouts will in the end define the same tolerance zone for the considered feature, which is two parallel planes 0.1 apart perfectly located at basic/theoretically exact distance from datum plane A. 1 - Position tolerance in ISO applied to nominally flat surface.