I had a request from a friend asking for some help on how to create a 'miter' for a crown mold in SketchUp.
I apply the same technique for compound joinery.
Resolution is low as I threw it together sitting in a cafe with slow internet... I can post a larger file with voiceover doing a valley rafter if there is interest...
Basic steps:
Copy 1 of your moldings off to the side. Leave the originals in place. (be sure they are components). Create a dummy box that is taller than your molding. (you can make it a component if needed) Rotate the box 45 degrees. Move it to the corner to start the miter. Explode the dummy box. Edit the molding component. Triple click to select all geometry in the molding. Right click and select 'Intersect' > with model. Exit the component. Delete the dummy block geometry. Edit the molding component. Delete the faces you do not need.
