featuretree: Verified Recovery of Editable CAD Feature Trees from STEP, for Open CAD Systems

Abstract

Importing a STEP file into a parametric CAD system yields a single frozen solid: the design history is gone, and no pad depth or hole diameter can be edited. Recovering that history is a well-studied problem — the acceptance criterion used here was stated as a definition by Gupta and Nau in 1995, and commercial implementations have shipped since roughly 1998. It is nonetheless absent from every open-source CAD system and from every callable library we could find: the capability exists only inside proprietary seats costing thousands of dollars per year, with no API. This paper describes featuretree, an MIT-licensed, training-free recogniser that recovers an editable feature tree from an exact boundary representation, gates every recovery by re-executing the tree and comparing the result against the input, and emits into FreeCAD and Onshape — two systems with no import-time feature recognition at all. On the eleven-part NIST MBE PMI conformance corpus it returns VERIFIED for 3 parts, PARTIAL for 5, and explicitly refuses 3, producing no silently incorrect trees; all three VERIFIED recoveries exceed 99.68% IoU against the input. We also report a negative result about the acceptance criterion itself: a scalar volume comparison lets over-cut and uncut material cancel, understating the true geometric discrepancy by 1.56× on one part, which a two-sided IoU test eliminates. No methodological novelty is claimed. The contribution is an available, honest implementation and a measured account of where it fails. A full prior-art scan is included in the software repository.