First, thanks to folks that pointed out potential issues with the previous version of the bookcase design. The key concern so far was around the length of the through tenons. While I’ve seen other cases built this way, I can see it seriously weakening the case sides. So, here is the previous version for comparison first:
I decided to make some changes to address this. First all of the through tenons were made into split tenons. 3″ wide on the main case and 2.5″ wide on the side pods. Between the twin tenons is a 3/8″ long stub tenon that fits into a groove in the case sides.
On the opposite side of the through tenons there is just a wide stub tenon and matching dado in the inner case side. This means less of the sides is removed for the joinery. I’m on the fence about whether 3/8″ is long enough for the stub tenon on the sides without a though tenon. Maybe that should be a half inch or even 5/8″? It’s a balance I guess, between glue surface and side strength. My gut feel is to increase in on the inner sides to a half inch.
I also made the back splashes taller, I like that better than the shorter version. And I removed the through tenons on those parts. I don’t think it added anything visually, and it’s one less visible joint that could show problems.
So, here is the second version. It’s better I think. The back splashes might be a tiny bit too tall, but I could go either way. I’m concerned about the strength of the stub tenons into the case sides — in particular the short side shelves into the center case sides. There isn’t much glue area there, and it’s mostly end grain on one side of the joint. I might need to think about that a little more. I could make it deeper, maybe with twin tenons that went quite deep into the sides. I could thing about a sliding dovetail joint (but that seems like it would really complicate matters). I’m open to suggestions on that joint. Pocket screws? (kidding).
The more I think about it, the more I’m convincing myself that I should change the joinery once more. Through tenons on the top and bottom shelves, and sliding dovetails on the middle two shelves in each unit. That will lock the units together mechanically and there won’t be any reliance on glue strength for the overall structural integrity of the piece.