Why do the plans for this bench require a side panel oriented on an angle from a larger piece of wood?

Question

Wondering if you might be willing to weigh in with any guidance you are willing to offer on the situation below.

The bench I am working on calls for a side rail panel made from two 4 ½” wide pieces that otherwise meet specs in the photo attached.

There is no explanation as to why they are executing the creation of the component parts by cutting down a 1 1/2 “ thick piece of stock, though they are clearly recommending bandsawing the piece to get the ¾” part. My thinking is they took this approach in order to make it a simpler process of cutting the tenons and not having to deal with the cutting of a compound tenon from a piece of ¾” stock, but I am not certain. I am very limited on 6/4 stock though I guess I could laminate some ¾ stock then execute the procedure as they show, but it seems to me that I should be able to calculate the angles needed and cut this from ¾ stock, saving me from wasting so much wood and having to worry about how the glue line of a laminated part would come into play if I had to bandsaw across it.

How would you attack this?

Hope I explained it well enough. Appreciate any wisdom you can offer.

EDIT: added illustration of the bench

Answer 1

Without seeing the rest of the design it would hard to explain why the angled rails need to be present. The mortises that would partner with this board are not directly opposite of each other for whatever reason so the board has to have the angle. I can however weigh in on why they would suggest that you cut that out of one and half inch stock.

The strength of the tenon

They are asking you to do that so that the grain of the tenon is parallel to the tenon cut. Aside from the size and fit the strength of the tenon comes from the grain direction.

Looking at the picture I can see that the tenon is cut parallel to that of the original board.

If you were just to use a smaller board and cut the angled tenons you would be creating weaker ones than what is pictured in your instructions. I don't know what kind of wood you are using but for something like that quarter-sawn lumber would be valued. Cutting the tenon on an angle would go against the inherent strength of that lumber.

While your bench could suffice without following those instructions it would surely last longer if you did.