Parametric models · practical guides
Turn a physical requirement into a checkable geometry brief
Use direct measurements, stated assumptions, and a controlled revision. A photo helps orientation, but it cannot prove a missing dimension or physical fit.
Capture interfaces before shape
Record mating diameters, hole centers, edge offsets, thicknesses, load direction, and which faces must clear other parts. Mark an origin and note uncertain dimensions instead of guessing them.
Request and verify one controlled change
Keep the editable source, compare geometry with the measurement record, and use a small test where possible. A generated model or a viewport preview is not proof that a printed replacement will fit or carry load.
Describe functional geometry precisely
State critical dimensions, tolerances to test, interfaces that must not move, intended material limits, and the evidence you can supply. Do not ask an AI tool to infer unknown measurements from a photo.
Record the mating interface before making a replacement
Use one datum for the outline, hole centres, connector openings and mating faces. Record required and measured values separately. After a small test print, change one named source parameter, rebuild and measure again. The downloadable record leaves physical results blank: no installed-fit or print-success result has been invented.
Download the measurement worksheet (CSV)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.
Questions this page answers
What measurements do I need to reproduce a broken part?
Measure the mating interfaces first: hole centers, diameters, thicknesses, edge offsets, clearances, and load direction. Record uncertain values explicitly.
Can a photo help if I also provide a known dimension?
A photo can document shape and orientation when paired with a known scale, but direct measurements remain the authority for functional interfaces.
How can I revise a replacement after a test print?
Keep an editable source and a measurement record, then change one named parameter and compare the rebuilt geometry before another test.
Which source file should I keep for the next adjustment?
Keep the editable source, the successful STL export, and the measurement record together. The STL alone does not preserve the original parameters.
How do I describe a part so critical dimensions are preserved?
Name each critical interface, provide a datum, state what must remain fixed, and separate confirmed measurements from assumptions.
Can an AI CAD tool make functional geometry rather than an artistic mesh?
A functional result needs explicit dimensions and verification against the stated interfaces. PurCad does not represent an unverified generated model as a successful physical part.
What should I provide besides a photo or drawing?
Provide measured dimensions, hole centers, thicknesses, clearances, intended use, and which dimensions cannot change. Include the source or prior part if available.
How do I check that the generated part follows the request?
Compare its geometry and source parameters with the measurement record, then test the relevant interface. A visual preview alone is not sufficient for fit or strength.