An STL contains triangles, not the original sketch dimensions or design history. A limited change may be possible, but preserving the designer's intention requires more than making the new measurement look right. Record what changes, what must remain fixed and how you will measure both after export.
This guide uses a controlled digital example. It does not report a test in a named CAD application, a hole-editing operation, a slicer or a physical print.
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.
Keep an untouched source and fixed references
Work on a copy. Record the intended unit, three overall dimensions and an independent reference that must remain unchanged, such as a centre distance. For an unfamiliar mesh, first open and check the STL. A plausible picture is not a dimensional reference.
Describe one change precisely: feature, current measurement, required measurement and direction. A hole needs a diameter, centre position, direction through the part and surrounding wall. An outside dimension needs a moving end face and a fixed reference face. “Four millimetres longer” does not identify either.
If the changed feature mates with another part, distinguish design clearance from manufacturing tolerance. If you need editable CAD afterwards, understand why converting STL to STEP does not restore design history.
Choose a change you can bound and check
A local movement of a flat face, or a simple hole in a generous flat area, is easier to verify than a change through fillets, ribs or thin walls. Stop when connected features move together, references are missing or damaged and disconnected surfaces obscure the intended shape. More automatic repair does not supply missing design information.
Worked file example: move one end face
The stored example begins with a closed ASCII STL block measuring 40 × 20 × 10 mm. A script moved only vertices on the X = 40 end face to X = 44. Reading every vertex again gave 44 × 20 × 10 mm. The opposite face, Y and Z remained unchanged.
| Check | Before | After | Interpretation |
|---|---|---|---|
| X length | 40 mm | 44 mm | Intended change |
| Y width | 20 mm | 20 mm | Preserved |
| Z height | 10 mm | 10 mm | Preserved |
| Surface | Closed practice block | Parsed again | Digital check only |
Compare the input STL and output STL. The recorded file checks found no open edges, non-manifold edges or winding differences in either file. That supports this bounded flat-face change. It proves neither a hole edit nor physical fit or manufacturability.
Reopen the export and measure unchanged features too
Measure the complete bounding box, count disconnected shells and inspect open edges. Look for skewed or doubled neighbouring faces. A requested dimension can be correct while another part of the model has deformed. Keep before-and-after measurements under the same revision record.
Treat a hole as position and diameter
For a hole, record the reference face, two centre distances, diameter and direction. Inspect both openings and the connecting wall. A hole that looks circular can still have the wrong position or effective diameter because of its faceted mesh.
A hole request that is not ready to edit
This editorial example describes a proposed change, not an operation performed:
| Field | Supplied information | Decision |
|---|---|---|
| Existing hole | Approximately 6 mm; measurement method unknown | Do not edit yet |
| Required diameter | 8.0 mm | Requirement known |
| Centre from left face | 12.0 mm | First reference known |
| Centre from bottom face | Missing | Blocking information |
| Nearby fillet | May intersect the hole wall | Assess remodelling |
| Features to preserve | Outside size and fillet | Preservation not demonstrated |
The correct decision is to stop. Obtain the second centre distance and the purpose of the fillet, or rebuild from dimensions. Diameter alone is insufficient. Threads, snap fits, thin walls and linked dimensions also require an assessment beyond this simple example. This mesh check is not approval for structural, electrical, medical or other critical parts.
Hand over the controlled revision or request remodelling
When the intended change and fixed features are verified, send that revision to the 3D printing service, naming the changed dimension and mating part. A digital measurement still needs the agreed physical fit check.
When references are missing or the change crosses several features, send the original STL, a marked sketch and known measurements for 3D modelling. Ask for that specific change and an editable deliverable if needed, rather than another repair of the whole mesh.