A STEP model represents curved CAD surfaces differently from an STL. Exporting to STL approximates those surfaces with triangles. Coarse tessellation can make a circle visibly polygonal; very fine tessellation increases file size without proving a better physical print. Compare exports and reopen the result.
Download the English project worksheet. Open it as UTF-8 CSV, replace the guide context with your project inputs and record observations separately from acceptance decisions. Blank cells do not indicate a passed check.
Record the critical curve and its purpose
Keep the STEP source and revision. Record the important diameter, radius or cylindrical fit, units and measurement reference. Without a source dimension you can count triangles but cannot check design intent.
Define a shape criterion too. A display object may need an acceptable silhouette at a specified viewing distance; a functional cylinder may need a cross-section and fit check. The example below does not establish a universal acceptable circle or tolerance.
Distinguish distance from angular deviation
A linear or surface-deviation setting limits distance between mesh and CAD surface. Angular deviation affects changes in facet direction. Names and interpretation vary between exporters, so record the actual application, version and units. A value of 0.05 radians is an angle; 0.005 mm is a distance. Do not copy a setting between applications without checking its meaning.
This process starts from CAD. The reverse process, STL to STEP, does not recreate the original feature history from triangles.
Compare two exports from one unchanged source
Keep object selection, units and orientation fixed so tessellation is the intended variable. Inspect the 20 × 8 mm cylinder STEP, coarse STL and fine STL locally. No downloaded script is required.
The stored trial created a real STEP cylinder with 20 mm diameter and 8 mm height and reopened it with an independent OpenCascade reader. It found one mesh with three CAD faces. The coarse export used 0.4 mm linear and 1.0 radian angular deviation; the fine export used 0.005 mm and 0.05 radians.
| Export | Triangles | Vertex records reread | Overall dimensions |
|---|---|---|---|
| Coarse | 60 | 180 | 20 × 20 × 8 mm |
| Fine | 1,004 | 3,012 | 20 × 20 × 8 mm |
The finer setting created more facets without changing the bounding box. Equal X, Y and Z extremes do not prove equal chord deviation, smoothness, surface finish or physical fit. This was a programmatic test of one cylinder, not a GUI, slicer or physical print test.
Inspect more than the bounding box
Reopen the actual STL. Check dimensions, triangle count and the curved silhouette at both close range and the intended viewing distance. Inspect the critical cross-section. Use the basic STL checks for unit, object and mesh validation.
Choose the lightest export that meets the criteria
For the fixture, both exports pass the three extreme-dimension comparison. If the agreed silhouette criterion rejects the coarse polygon, only the fine candidate remains. Neither receives automatic approval without the criterion. Select the lightest export that passes the relevant dimension and shape checks, not simply the largest file.
Stop when the comparison is unexplained
Stop for changed diameter or height, missing objects, unacceptable faceting, unexpectedly large files or export settings that cannot be reproduced. Confirm object count and units, allowed dimensional difference and the chosen visual or functional criterion before approving the export for the next step.
Send the source with the chosen export
For 3D printing, identify the visible curved face and any diameter that must mate with another part. Keep STEP, STL and export settings under the same revision so an inadequate curve can be exported again without guessing the geometry.
If the design diameter itself must change, request 3D modelling. If the design is right but the physical connection is uncertain, use tolerance and clearance guidance to define a fit trial. More triangles do not fix an incorrectly designed fit.