GD&T Symbols Explained (Geometric Tolerances)
The 14 geometric tolerance symbols from straightness to runout, datum and material condition modifiers (MMC/LMC), tolerance zone pictures and how to read a feature control frame, with search
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A reference for the 14 geometric tolerance symbols of GD&T: a drawing of each tolerance zone, the datum and material condition rules, a feature control frame that explains itself in words, and a bonus tolerance calculator.
How to use
- On "Symbols", type a name in "Search symbols" or pick a category, then open a symbol to see its tolerance zone, what it applies to and a drawing example.
- On "Read a feature control frame", choose the "Characteristic", "Tolerance value", Ø, material condition and up to three datums; the frame is drawn and read out in a sentence.
- On "Bonus tolerance", enter the size limits of a hole or shaft and the tolerance with Ⓜ or Ⓛ to see the allowed tolerance at each size and the virtual condition.
- On "Datums and modifiers", read how datums and the 3-2-1 frame work and what Ⓜ, Ⓛ, Ⓔ, Ⓟ, Ⓕ, Ⓣ, Ⓤ, CZ and Ø mean.
How it works
- Form tolerances never use a datum; orientation, location and runout always do; profile may use one.
- Bonus = |actual size − MMC size| under Ⓜ (distance from LMC under Ⓛ); virtual condition = MMC size − tolerance for a hole, + tolerance for a shaft.
- Concentricity and symmetry were removed from ASME Y14.5-2018 but remain in ISO 1101. Symbols are drawn by the page so they look the same in every browser.
- Summarised for learning; the standard named on the drawing takes precedence.
Questions and answers
- What are the 14 GD&T symbols?
- Form: straightness, flatness, circularity, cylindricity. Profile: line, surface. Orientation: angularity, perpendicularity, parallelism. Location: position, concentricity, symmetry. Runout: circular, total.
- How do I read Ø0.1 Ⓜ A B C?
- The axis must lie inside a cylinder 0.1 in diameter at its true position from datums A, B and C at maximum material condition; a larger hole gains bonus tolerance.
- What is the difference between MMC and LMC?
- MMC is the size with the most material (smallest hole, largest shaft) and protects assembly; LMC is the opposite and protects a minimum wall.
- Which datum is primary?
- The first letter in the frame. It removes three degrees of freedom, the secondary two and the tertiary one, so the order changes the measurement.