Geometric Dimension & Tolerancing

A: Primary datum selection is the tripwire. A painted external face as datum A allows compliant inspection while breaking assembly alignment; functional mounting pads belong at the top of the datum hierarchy.

A: 0.07 mm is enough to break marginal tolerances. Aluminum expands roughly 23 µm/m·K, so over 0.3 m and 10 K the math lands right there, and many CMMs won't fix it unless told.

A: 0.02 mm is the scrap driver. Position already controls orientation and location of the bore axis; adding tight flatness on the face often adds cost without protecting any failure mode.

A: 0.60 mm is the gating value. The 0.20 mm size bonus adds directly to the Ø0.40 mm positional tolerance at MMC, and that's what the functional gage must allow.

A: Bonus tolerance is the lever. With fixed pins, moving to MMC gives extra positional freedom as holes grow, usually improving fit without touching the nominal geometry.

A: Rule #1 is the fault line. ASME ties perfect form at MMC to size, while ISO doesn't unless stated; mixing them makes acceptance arguments collapse during audit.

A: 0.03 mm is the leak boundary. Exceeding it allows local gaps that defeat O‑ring compression, especially under pressure and thermal cycling.

A: 0.5 mm does triple duty. Profile wraps form, orientation, and location into one zone relative to the datums, which is why it's favored for skins.

A: Datum order is the trigger. Changing precedence can shift the entire datum reference frame, altering fit and load paths even when the numbers look friendlier.