Design intent and a feature tree that survives

Why models break when you edit them, and the referencing habits that stop it happening.

Every model answers a question you did not know you were asked

Two people model the same 60 × 40 × 6 mm plate with a hole 10 mm from one corner. Both look identical on screen. Then the plate needs to be 80 mm long.

In the first model the hole moves with the corner it was measured from, because that is what it was attached to. In the second, the hole was dimensioned from the opposite edge and now sits in the wrong place — or the sketch was drawn on a face that no longer exists and the whole thing turns red.

The difference is design intent: whether the model encodes what is supposed to stay true. Not the shape — the reasons. Where does the hole actually belong, relative to what? Which two thicknesses must always match? What is the datum this part measures itself from?

The edit test

There is one reliable way to know whether you captured intent. Take the dimension most likely to change and change it — not by 0.5 mm, by something violent. Double the length. Halve the wall thickness. Then look.

Nothing broke and everything moved sensibly. The intent is in there. Undo and carry on.
Nothing broke but something is now in a stupid place. The model is robust but the intent is wrong. Fix the reference: that hole should have been dimensioned from the other edge, or centred.
Features turned red. Something downstream was attached to geometry that stopped existing. This is the one worth understanding.

Why features go red: the fragile reference

When you sketch on the face of a solid, or dimension to an edge, the model stores a reference to that specific face or edge. Those are outputs of an earlier feature. Change the earlier feature enough — split a face in two, round the edge away, reorder something — and the reference points at nothing.

This is why experienced modellers are fussy about what they attach things to. In rough order of trustworthiness:

Origin planes and the origin point. They cannot be deleted, renamed, or rebuilt away. Free, permanent, always there.
Your own construction planes and sketches. You made them deliberately, they are early in the tree, and they exist to be referenced.
Large flat faces created early and unlikely to change. Acceptable, in moderation.
Edges, fillet faces, and anything produced by the feature you are about to edit. Avoid. This is where the red comes from.

Four habits that do most of the work

One sketch, one job. A single sketch containing the outline, the holes, and the slots is a single point of failure. Smaller sketches rebuild faster and fail independently.
Name features as you make them. Cable slot beats Extrude 14. You are writing for the version of yourself who opens this in six months with no memory of it.
Model the shape before the decoration. Structure first, then holes, then fillets and chamfers. The tree reads like a description of the part.
Sketch symmetric things symmetrically. If both sides must match, use a mirror or a symmetric constraint. Two hand-placed copies will diverge.

Hands on

Break a bracket on purpose, then make it unbreakable

Done when: The bracket survives doubling its length, with every hole still where it belongs.

On your bracket, add a second 5 mm hole by sketching on the top face and dimensioning it 10 mm from the nearest edge. This is the fragile way, done deliberately.
Now change the plate length from 60 mm to 120 mm. Note exactly what happens to that hole — it either stays glued to one end or fails outright.
Undo the length change. Delete the sketch and redo it, but this time start it on the Top origin plane and dimension the hole from the origin point, not from an edge.
Change the length to 120 mm again. The hole now holds its position relative to the datum you chose, which is what you meant all along.
Rename every feature in the list to say what it is. Then change the length one more time and read the tree top to bottom — it should read like instructions a person could follow.
Save a version (Versions and history → Create version) and call it v1 robust. Onshape keeps it forever and you can branch from it.

Check your recall

Answer from memory — no scrolling back.

1. What is design intent, in practical terms?
2. Which reference is the most durable to attach a sketch to?
3. A feature turns red after you edit an earlier dimension. Why?
4. What does the edit test tell you that inspection cannot?