Why .CATPart is unreadable outside Dassault’s tools
CATIA V5 stores geometry in its own CGM kernel format inside a proprietary container; the specification is not published, and the readers that exist are CATIA itself, Dassault’s 3DEXPERIENCE platform and a handful of commercial translators licensed from Dassault or built by reverse engineering at a cost no free web tool can bear. None of them runs in a browser. Any page offering direct CATPart conversion is either uploading your file to a server with such a translator, or opening only the visualisation (CGR) tessellation, which is not your model.
CADverter reads STEP, IGES and BREP exactly, plus meshes and drawings, and does not read CATIA’s native files. The export below takes under a minute and produces a file every other tool also reads.
Set the STEP options once, under Tools → Options
CATIA keeps the STEP export settings out of the Save dialog. Open Tools → Options → General → Compatibility and select the STEP tab. Set the Application Protocol — AP214 for colours and layers and general compatibility, AP203 for an old receiver, AP242 in recent releases when PMI or the current standard is required. Under the export section, leave Curves and points and Show only geometry unticked unless you need reference geometry in the STEP; tick the option to export product structure as an assembly when you will be exporting a CATProduct. Click OK. The settings persist, so this is a one-time step.
Exporting a part or a product
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Open the .CATPart, or the .CATProduct for an assembly.
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Choose File → Save As.
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In Save as type select stp (listed as stp or STEP depending on the release). Give the file a name ending in .stp or .step.
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Click Save. CATIA writes the STEP using the protocol and options set above. A report window may list any elements that could not be translated; read it — a face it skipped is a face that will be missing.
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Drop the .stp on this page. The model opens in 3D with its product tree, and Convert offers STL, 3MF, OBJ, PLY, GLB, USDZ, IGES and BREP; Make drawing produces a dimensioned DXF, SVG or PDF sheet.
Products: keeping the assembly tree
Saving a CATProduct as STEP writes one file containing every instance with its position and the product structure, provided the assembly-export option under the STEP tab is on. Deactivated components are skipped. CADverter shows the tree in its parts panel, where components can be Ctrl+clicked in and out of a conversion or written one file per part into a ZIP. If a supplier needs a single merged body instead, generate a CATPart from the product in CATIA first (Tools → Generate CATPart from Product) and export that.
Millimetres in, millimetres declared
CATIA models in millimetres by default and writes that unit into the STEP header, so a part arrives here at the size it was modelled and the viewer’s bounding-box readout agrees with CATIA’s measure tool. STL and OBJ cannot record a unit; CADverter writes them in millimetres and says so. 3MF, STEP, IGES and DXF carry the unit explicitly.
CATIA’s own STL export, and when this page is still the better route
Honest counter-advice: CATIA V5 can save a CATPart as .stl directly (via File → Save As with the STL type, in workbenches and licences that include it), with the tessellation controlled by the 3D accuracy settings under Tools → Options → Display → Performance. If you are at the CATIA seat and need only STL, use it. Export STEP and convert here when the recipient has no CATIA, when several output formats are wanted from one export, when a 2D drawing sheet is needed without a Drafting licence, or when the conversion must happen on a machine with nothing installed.
Reading the export report, then the model
CATIA’s STEP report lists untranslated elements; an empty report is what you want. Then drop the file here and look: component count in the parts panel, bounding-box size in millimetres, curved faces shading smoothly. Surfaces that arrive as loose faces rather than a solid indicate an open body in CATIA — check with the Join and Close Surface tools in Generative Shape Design and re-export. The check is free: nothing leaves your machine during any of it.
CATIA export questions
Can CADverter open .CATPart or .CATProduct files?
No — there is no reader that can ship in a browser. Export STEP from CATIA as described; the settings persist, so after the first time it is File → Save As → stp.
I only have the .CATPart and no CATIA — what now?
Ask the author for a STEP export; it takes them a minute. FreeCAD and other free tools cannot read CATPart either. Some commercial viewers (paid) can export STEP from it.
Does the STEP include colours from CATIA?
Under AP214 or AP242, yes — graphic properties are written. CADverter shows them and carries them into OBJ, 3MF, GLB and its own STEP output.
What about CATIA V6 / 3DEXPERIENCE?
The export lives under a different menu but is the same idea: export the physical product as STEP AP214 or AP242, then drop it here.