“Equal diagonals” screens part of in-plane squareness but does not prove coplanar corners, flat mounting faces or vertical posts. Establish functional datums and separately inspect lengths, diagonal difference, flatness, perpendicularity/parallelism and interface location in final unclamped condition after thermal stabilization.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
LAYERED INSPECTION
1. Small frame: clean a surface plate, establish datum A on three supports, and sweep corners/mounting points with feeler gauge or height gauge. 2. Medium/large frame: a calibrated laser tracker, arm or total station can create coordinates; specify datum fitting and temperature compensation. 3. Diagonals: use identical targets and tension; report D1, D2 and |D1−D2|. This is a process signal, not a GD&T substitute. 4. Interfaces: use a functional fixture representing customer feet, rails or panels to verify real assembly.
TOLERANCE SPECIFICATION
ISO 13920:2023 can cover untoleranced dimensions/geometries of welded constructions, but functional faces need explicit drawing callouts and datums. “Overall ±1 mm” is ambiguous about direction, extent and reference. For a 2 m steel frame, a 10°C change causes about 0.24 mm thermal growth (α≈12×10⁻⁶/K), material to tight tolerance decisions.
PROJECT CHECKLIST
Functional datums; measurement condition; supports; temperature; lengths/diagonals; flatness; perpendicularity/parallelism; interface gauge; uncertainty.