slop-cad

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Face references ​

When you press Sketch on face, the new sketch has to remember which face it was drawn on, so that editing the part underneath moves the sketch with it. That memory is a face reference, and it is the trickiest thing in the tool.

The problem ​

The solid is rebuilt from scratch on every edit. Its faces are produced fresh each time, in whatever order the modelling step happens to produce them. "Face number 4" is not the same face from one rebuild to the next.

So a face reference cannot be a number. If it were, widening a plate by 1 mm would leave your boss attached to something arbitrary.

What is remembered ​

Three things, none of them a face number:

The role is the durable part. "The top face of feature 1" survives the plate getting wider, getting thicker, or gaining a pocket somewhere else, because after all of those there is still a top face of feature 1.

The stored plane is the fallback. If the role cannot be resolved, the sketch keeps building where it last knew itself to be, so the model does not silently jump.

When it goes stale ​

Three ways, and the timeline names which:

The face it sits on is gone. The feature that created it was deleted, or an edit removed that face entirely.

The face it sits on is no longer flat. Most often because a blend rounded it. A curved face cannot host a sketch.

The feature it sits on has no solid. The parent feature's extrude was removed.

The timeline row turns amber and says which. The sketch keeps building on its last known plane, so your part does not silently move; it just stops tracking.

What you can do about it ​

Re-attach it. A flagged row carries a link button. Switch to the 3D view, pick the face you want the sketch to sit on, then press that button on the amber row. The sketch keeps its geometry, its dimensions and its constraints; only the frame underneath changes. The button is greyed until a face is picked, and hovering it says so.

Two things it will not do. It refuses a face belonging to the flagged sketch itself, or to anything built from it, since that would make the sketch its own ancestor. And if the sketch carried projected reference geometry, that projection comes out on the way across: it named curves in the old host, and those names mean nothing on the new one. Project again once it has landed.

The older options remain:

  • Undo, if the change that broke it was recent.
  • Leave it, if the last known plane is still where you want it. A stale reference is a warning rather than a failure.

Avoiding it ​

  • Blend last. Rounding a face you later want to sketch on is the most common way to break a reference. Add all your features first, then blend.
  • Delete deliberately. Removing a feature cascades to every sketch on its faces, and drops any blend naming those faces.
  • Prefer the durable roles. A sketch on a top or bottom face is more robust than one on a face that only exists because of an earlier cut.

See also ​