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Benedict O'Neill on Muck Rack

Benedict O'Neill

As seen in: MSN, MSN Ireland, MSN UK, MSN South Africa, Planet Football, VoxelMatters, The Set Pieces, Wevolver
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Articles

GD&T Explained: Symbols, Datums, and Tolerances

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By Benedict O'Neill
| Wevolver
GD&T (geometric dimensioning and tolerancing) replaces ambiguous plus/minus coordinate dimensions with tolerance zones tied to a datum reference frame, so design intent passes clearly to manufacturing, then to inspection. ASME Y14.5-2018 (R2024) is the current US standard. It removed concentricity and symmetry, leaving 12 active geometric characteristic symbols rather than the 14 that most references still list.
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True Position in GD&T: Formula, Tolerance Zones, and MMC

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By Benedict O'Neill
| Wevolver
True position uses a diametral tolerance zone centered on the theoretically exact location set by basic dimensions. The true position formula is TP = 2 × √(Δx² + Δy²). The factor of 2 converts a radial deviation into the diameter of the tolerance zone quoted in the feature control frame. Applying maximum material condition (MMC) to the tolerance value earns bonus tolerance equal to the departure of the actual mating envelope from MMC, which is why MMC callouts reduce scrap on clearance holes.
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H7 Tolerance Explained: ISO 286 Hole Tolerance Charts

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By Benedict O'Neill
| Wevolver
H7 tolerance means the hole is never smaller than nominal and never larger than nominal plus the IT7 grade. The lower deviation is exactly zero, so a 20H7 hole runs 20.000 to 20.021 mm. The tolerance band widens with size, from 10 micrometers below 3 mm to 57 micrometers in the 315 to 400 mm range. H7 is the preferred hole in the hole-basis system. The fit is altered by changing the shaft letter, not the hole: g6 for sliding, h6 for location, and so on.
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