Differences Between the ASME and ISO Standards for GD&T

Differences between the ASME and ISO standards for GD&T Nomenclature differences ASME Basic Dimension Feature Control Frame Reference Dimension True Position

ISO Theoretically Exact Dimension Tolerance Frame Auxilliary Dimension Theoretically Exact Position

Other Differences: 1. In ASME the rule #1 controls Size and Form simultaneously. In ISO, Form is independent of Size and is additive to size. An encircled E symbol in ISO invokes the envelope principle in ISO. If the envelope principle is invoked, the Size tolerance encompasses the Form tolerance as well. The 2009 edition of ASME has introduced the principle if Independence for Features governed by Form. 2. General Tolerances are not covered under ASME. General tolerances can be applied in ISO drawings by putting a note ISO-2768. When ISO-2768 is referred on the drawing, general tolerances are invoked on linear and angular dimensions. 3. ASME does not specially define non-rigid parts as separate entities. A Fmodifier is used to show that the geometry should be checked under Free State. ISO shows non-rigid parts by putting a note stating ISO-10579-NR in the title block. 4. Under ASME, tolerance zones are always Normal to the geometry. Under ISO, you can define tolerance zones that are at another angle to the geometry. 5. ASME defaults to third-angle projection. ISO defaults to first-angle projection 6. Under ASME datum feature leader cannot point towards axes and Center planes. Under ISO the datum feature leader can point to an axis or a Center plane. 7. Under ASME, datum precedence has to be strictly maintained. Under ISO, ambiguous order of datums is allowed, so long as the order is not important. 8. Under ASME, composite position tolerances are interpreted as PLTZF and FRTZF. Under ISO, the two segments of a composite position tolerance frame are interpreted as separate tolerances. 9. Under ASME, Concentricity and Symmetry are always applied RFS. Under ISO they can be applied at MMC, LMC or RFS. 10. ASME is a single standard. ISO has collection of standards addressing various Geometric tolerances