Why re-downloading after a fix is free (and how we decide what counts as a fix)

The problem with pay-per-download

Charging per file is honest: you pay when you get value. But 3D printing is iterative. The first print reveals the hole is 0.3 mm tight; paying again for a 0.3 mm change feels wrong, and it pushes people to over-think before downloading. We wanted the opposite: download early, test, fix, download again.

The rule

When you download a design for the first time, one credit is charged and we store a paid snapshot: the program, its hash, the part's volume and bounding box. Every later download of that model is compared with the snapshot:

• Identical program → free. Both STL and 3MF of the same design cost one credit total.
• Small edit → free. The program is at least 65% similar (line by line, ignoring formatting), the volume changed by less than 30%, no bounding-box dimension changed by more than 25%, and the AI did not flag any edit since the snapshot as a major redesign.
• Otherwise → a new credit, and a new snapshot.

Why those thresholds

They come from looking at what "fixing a print" actually looks like: a tolerance, a wall thickness, a longer lip, one more hole. Those edits touch a few lines and barely move the volume. Turning a phone stand into a tablet stand, or a box into a two-part enclosure, rewrites most of the program and changes the volume a lot. The thresholds sit comfortably between the two.

The AI's vote

Every assistant reply carries a change_scope of "minor" or "major". It is a tie-breaker: a request like "start over with a different design" is major even if, by chance, the code looks similar. Tolerance tweaks are always minor.

What you see

The download button tells you before you click: whether this export is covered by your last download or will spend a credit. No surprises. The credits page shows every download with its cost.

What if I disagree with a charge?

Write to us. If the barrier misfired on a small fix, we refund the credit.