I use 3d sketches regularly, creating geometric wireframes, from which surfaces are created. The 'future' surfaces must be planar. Clearly, any surface that is formed only from a triangle is already planar. But a quadrilateral, or with more edges, isn't inherently planar.
My 'standard' method to guarantee planarity is to sketch the diagonals, and then to make the two diagonals coincident at a point. It works, often with a great struggle to get SW to recognize the coincidence, since it doesn't always see how close the two lines/points actually are unless I drag them together. It's like coercing toddlers to make friends.
Another method I have tried is to close the 'base wireframe', create a plane on three of the wireframe points, draw a 3d-planar quadrilateral on that plane, detach the coincident points from the underlying wireframe, send my planar-3d sketch back in time before the wireframe, and then reattach the 3d wireframe onto the planar quadrilateral. Very involved and difficult to do on several adjoining surfaces.
I've attached a JPG showing roughly the sequence I go through... wireframe, triangular surfaces, quad surfaces, and pic 4 and 5 clearly show the non-planar quad. 6 shows the warped wireframe, 7 shows my diagonals, and 8 shows a failed constraint assignment, before I drag the two lines close enough to mate (not shown).
I need a better way to constrain my 3d wireframe's quadrilateral points to a plane, without an actual plane to place them on.
Is there some kind of virtual 'on-plane' constraint, or some way to create a temporary plane, while in the midst of a 3d sketch? I don't see how '3d-sketch-on-a-plane' can help me, but I'm open to all suggestions.