When installing common crown molding on a 90 degree corner (with a 38 degree spring angle, i.e., molding that is rotated 38 degrees down from vertical), one can set their saw to miter at 31.62 degrees and bevel at 33.86. Many saws have stops at these angles; search online for “crown molding calculator” and you can confirm these numbers.
I’m trying to model this process in SketchUp. Using the protractor tool, I can measure the 31.62d angle on one face of a rectangle, the 33.86d angle on a perpendicular face, then move edges to match. However, when I try to match up two such “boards”, they don’t align properly - the miter angle is right, but the bevel angle is wrong. Depending on how you align, the pieces either intersect or leave a gap.
However, if the bevel is set to 38.2d, things match perfectly. (I found this angle by making “crown molding” using the follow-along tool, then measuring angles.)
My question: What’s going on? Why do I need to bevel at an angle that doesn’t match up with online calculators and my personal experience?
I expect your problem has to do with how you are measuring to lay out the bevel.
Try this to get the correct angle.
Place your molding profile in the orientation it would used. From it draw an L-shaped path to represent the two walls. Use Follow Me to extrude the profile around the corner. Then separate the molding pieces. You can measure the angles which will be correct.
Maybe a skimpy representation of a miter (mitre) saw would be useful. After forming the miter as @DaveR shows you can lay the moulding on it’s back and do some measuring.
Also, the wood butcher in me has to point out that I’d try very hard to avoid mitering any inside corner.
I feel they should always be coped. I guess the exception would be a very large moulding.
I like the skimpy miter saw approach; I made one, too. When I mitered/beveled my blade according to the various charts, I get the right angles. I didn’t realize that you can’t just measure the bevel angle on a face of a rectangle; when you also miter, that angle changes.
This wasn’t about how to make crown molding, but rather about showing how it is made, so now I’m happy and understand it all better. Thanks to both.