Engineering-grade geometry analysis.

Grid-accelerated ray casting measures wall thickness across the whole mesh rather than relying on a bounding-box approximation. A spatial grid buckets triangles so nearby-surface queries stay fast even on dense meshes, then rays are cast inward along local surface normals to find the opposing wall and measure true local thickness at that point, not a global average.

Overhang angle comes directly from the angle between each face's normal and the vertical build axis. Undercut detection for moulding and casting is a different, more fundamental check: a grid of rays is cast along the mould's pull direction, and the number of surface crossings is counted per ray. A simple, pull-moldable shape crosses exactly twice; a re-entrant tunnel, hook, or pocket crosses more, which is how genuine undercuts are distinguished from merely shallow draft.

Every connected surface component in the mesh is labeled via a flood-fill over edge adjacency. A fully sealed interior shell, one with no boundary edges connecting it to the outside, is a trapped cavity: resin poured in has no way out. This is real mesh topology analysis, not a heuristic borrowed from FDM.

Tool access flags sharp, unrounded internal corners a round end mill physically cannot reproduce: it always leaves at least its own radius behind. Deep-pocket detection groups upward-facing floor regions, then compares each region's own depth against the surrounding rim height and its own footprint's narrowest span, flagging aspect ratios that would need a long, chatter-prone tool.

Sheet thickness is estimated via the same ray-cast sampling as wall-thickness detection, then curved transition regions between flat facets are located and their local bend radius measured by point sampling. Individual bend-edge hits are clustered into full bend lines via union-find on edge adjacency, which is what lets flange-length and bend-relief checks reason about a bend as a whole line rather than isolated points.

A multi-solid STEP import is split into its real, independent solids by the CAD kernel itself, not a mesh heuristic. Every pair of parts is first compared by axis-aligned bounding box, a cheap, purely local check, before the expensive step ever runs; only pairs whose boxes genuinely overlap get a true boolean intersection via the CAD kernel, reported as an exact overlap volume. This bounding-box pre-filter is what keeps checking practical up to 150 parts instead of a token single-digit cap: a real 30-part grid assembly stays under 20 real boolean checks out of 435 possible pairs.

Supported STEP edits (fillets, hole compensation, countersinking, draft, wall thickness) are performed as real CAD-kernel geometry operations on the actual B-rep solid, then re-tessellated for preview, not a visual mesh overlay pretending to be a fix. Every fix function runs its own independent validity check after the edit and refuses rather than risk producing broken geometry.