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?