Cutting angles on top molding of bookcase

[J D Thomas]

Hello all,

I've been away from this forum for an eternity, but now I have a question I haven't figured out. Probably all due to abysmal math skills.

I am nearing the final stages of building a tall bookcase for my living room, and I want to add custom crown molding across the top and two sides. This is not normal crown molding, but one I am doing a simple custom mill job to match the casework I did many years ago elsewhere in the house. I am looking to have the molding at a 45° angle on the top and sides, and for the life of me, I cannot figure out how to cut the angled miter joints at the outside corners to achieve a nice tight miter. Any help would be greatly appreciated.

Attached is a screenshot of one corner from SketchUp.

505886

[lou Brava]

From what I see it looks a 1/2 lap joint I wouldn't really call that a miter joint. When I do crown I don't use math I use the bottom bevel as a reference and cut the parts upside down & backwards if that makes sense. But that's only for true outside miters & coped inside. What you drawn looks like a very difficult cut and will require a table saw & some hand saw & chisel work. If I was doing that I would cut the slot in side boards only and slip the face board into it. And it would about 100% trial & error.
Good luck & when you done post up pic & how you did it, sorry I can't be more help.

[Jim Morgan]

Tilt the blade to 30 degrees, cut the miter at 35.5 degrees. See https://jansson.us/jcompound.html#nsidedbox

[Jamie Buxton]

How wide is your molding? If it less than 4” or so, don’t cut it laying down flat on the table. Cutting flat on the table is a compound angle cut, and math is required. Instead, you set the molding up at the molding angle, and the saw blade is not beveled over. It is a nice 90 degrees. You mark and cut without math.

[Mick Simon]

Here's a little book I always recommend to my students. It's packed with information including how to cut your molding depending on your cutting method - table saw, miter saw, etc.
https://www.amazon.com/Handy-Shop-Reference-Figures-Woodworker/dp/1440354804/ref=sr_1_1?crid=2UNZQ30FSIW0O&keywords=the+handy+shop+reference+book&qid=1691846677&sprefix=the+handy+shop%2Caps%2C183&sr=8-1