ASME Y14.5 vs ISO GPS
The symbols look nearly the same, but the two systems differ in defaults that change how a drawing is read. Check which one your drawing follows before you measure anything.
| Topic | ASME Y14.5 | ISO GPS |
|---|---|---|
| Main standard | ASME Y14.5-2018 | ISO 1101:2017 and the ISO GPS family (ISO 5459 datums, ISO 8015 principles) |
| Number of characteristics | 12 (concentricity and symmetry removed in 2018) | 14 |
| Circularity | Circularity | Roundness |
| Projection | Third angle | First angle (third allowed) |
| Feature control frame | Same layout: symbol, tolerance, datums | Same layout, called a tolerance frame |
| Basic dimension | Basic dimension in a box | Theoretically exact dimension (TED) in a box |
| Size and form | Rule #1: size limits also control form by default | Independency principle by default (ISO 8015): size and form are separate unless the envelope is called for |
The difference that matters most
Under ASME, a feature at its maximum material size must have perfect form (Rule #1, the envelope principle). Under ISO 8015, size and form are independent unless the drawing asks for the envelope. The same drawing can accept different parts depending on which standard applies.
See the full GD&T symbol chart.