Parametric models · practical guides
Change a named dimension, not every coordinate
An arbitrary uploaded STL does not retain its original variables or feature history. PurCad does not claim a general local mesh-edit operation.
Prefer editable source
For a controlled change, start with SCAD or another source model that names the required dimensions. Change one parameter, rebuild, and compare the feature you meant to preserve.
Know what scaling cannot preserve
Whole-mesh scaling changes bores, slots, walls, and rounded edges together. A downloaded STL may need manual mesh work or a remade source model; neither is represented here as an automatic PurCad capability.
A measured edit: longer plate, unchanged bore
The brief is to lengthen a 30 × 20 × 4 mm plate to 40 mm, keeping the 6 mm bore centred 10 mm from both datum edges. Two source builds produce 30 × 20 × 4 and 40 × 20 × 4 mm bounds. Only length_mm changes; the bore and its centre remain independent. This is a deterministic source edit. The linked chainring case shows an existing native CAD example; this edit does not claim a new AI generation or a printed fit.
Reproduce a missing-helper error and fix it
Compile main.scad alone: the referenced parts/spacer.scad is missing and no successful mesh is produced. Then add the helper at exactly that relative path and compile again. The verified result is a 14 × 14 × 8 mm spacer. Upload the main source and helper separately in the SCAD workspace. The files below let you reproduce the dependency problem without an AI edit.
Questions this page answers
How can I lengthen a part without enlarging its holes?
Use a source model with separate length and hole parameters, then rebuild and measure both. Whole-mesh scaling cannot preserve holes.
Can I change an outer diameter while keeping the bore fixed?
Yes in a source model that defines outer diameter and bore independently. Rebuild and inspect the resulting geometry.
Can I widen only a slot in a downloaded STL?
Not through a general PurCad STL feature editor: that capability is not claimed. Seek editable source or use a separately verified mesh-edit workflow.
How can I change just one arm of a U-shaped bracket?
Use source geometry where each arm is independently defined, or remodel the feature. An imported STL does not reliably expose that construction history.
Why did adding a rounded edge change the measured height?
Some rounding operations add geometry beyond the original envelope. Re-measure the rebuilt bounds and adjust the source construction rather than assuming the prior height remains fixed.