STEP viewer that reads the surfaces, not a mesh

ISO 10303, the neutral format CAD packages use to exchange solid models. Where a mesh format lists triangles, a STEP file describes surfaces exactly — this face is a cylinder of radius 3 trimmed by these edges, that one is a spline patch with these control points — plus the units, the assembly structure and the relationships between parts. It is the format you get when someone sends you a part rather than a print, and it is what SolidWorks, Fusion, Inventor, CATIA, NX, Onshape and FreeCAD all read and write.

No. The contents are identical and both open here. The short spelling is a survival from an era of three-character extensions; it stuck in the Windows CAD world while Unix tools kept the longer one. If you have both and they were exported from the same model, they are the same file. There is a separate page for the short extension for the people who search that way.

By being the CAD software, for the small part of the job that a viewer needs. This page ships its own reader: it parses the ISO-10303 entity graph, rebuilds the planes, cylinders, cones, spheres, tori and B-spline patches the file describes, works out where each trimmed edge falls in each surface's own parameter space, and tessellates the result to a tolerance set from the part's size. That is a few thousand lines of arithmetic, and it runs entirely in this tab.

Closely, and knowably so. The reader is checked against Open CASCADE, the kernel FreeCAD is built on, over a set of parts covering every surface type: face counts match exactly and volumes land within a fraction of a percent. That remaining fraction is not error — a tessellation is a polyhedron drawn inside the true surface, so it is always slightly smaller, and it shrinks as the tolerance tightens. Treat the volume here as a good estimate and your CAD package's as the exact figure.

Yes, placed correctly, which is the part that goes wrong quietly elsewhere. A STEP assembly stores each component in its own coordinate system and keeps the placement transforms somewhere else entirely, so a reader that ignores the product structure stacks every part at the origin and produces a picture that looks like a model rather than a mistake. The panel says how many components were found and placed.

Geometry is; presentation is not. Colours, GD&T annotation, PMI, and product data attached for a downstream process are all skipped. Faces that cannot be tessellated are counted and shown rather than dropped in silence, so a part with a surface type this reader does not know says so instead of quietly rendering with a hole in it. Compressed .stpZ archives are not read at all — unzip them first.

Not on this page, which views rather than converts, though the tessellation it produces is exactly what such a conversion would export. Going the other way — mesh to solid — is the genuinely hard direction, because triangles have to be recognised as the surfaces they approximate before anything can be written. That is a different tool and a much harder problem than it sounds.