- You can flatten a model to 2D by using the Scale tool set to Differential scaling.
- You can also use this trick to flatten some drafting work that you want to make sure is FLAT.
- In your model draw a line that is "very long" in the z-axis (I usually draw it in the z direction
from the origin)
- Select your model AND the line, then choose Scale and choose Differential scaling.

Follow the procedure that appears in the ViaCAD (or whatever app) tool window:
-- You'll pick an anchor point (I usually use the origin).
-- Then you'll pick the 'start' point (I usually pick the extreme end of the "very long line".
-- Then choose the 'end' point (usually another point along the "very long line" that's
some ways closer to the 'anchor' point. It doesn't really matter where you put it.

Nothing will seem to have happened:
- You need to set the scaling axis values in the tool window.
- You can press the Tab key to cycle through axes or click in them and set a value.

- If you're trying to flatten your model, leave the x-axis and y-axis alone (set to 1.0),
but put a very small number in the z-axis, like 0.00000001. seriously.
Hit Enter/Return.

:: To check your work, choose the extremely small line segment that's left after you
squashed the model. You'll know you got it, because the Inspector window will report
that you selected something, even if you can't see it. (This is why I draw the "very long line"
from the origin.) Check its measurements in Object Properties, which may report zero length.

Your model is now pretty flat. You can do it again, if you want to be crazy certain.