What is the minimal spacing between edge of leg pommel and mortise and tenon apron (intersected by tusk tenon)

Question

I would like to balance aesthetics and strength for a craftsman-style tusked tenon table frame. Are there rules of thumb to determine how close to the leg corner an apron should be?

(as a preamble, I should mention I'm working with construction grade lumber)

A tenon within a tenon

The particular construction I'm shooting for has one tenon passing through another tenon. And I feel this might require larger margins around mortises to prevent splitting failures in the leg's pommel (i.e. the upper part).

For a visual reference, I am trying to lay out the mortises in the piece (D) from the following picture, and I would like to know how to pick safe lengths for X, Y, Z (in green):

For assembling, piece (C) goes through the leg first, intersected by the other tenon piece (B), and finally locked in with one tusk on each leg. I am planning on chamfering the part of (C) which protrudes on the outer side of the leg.

My personal opinion is that it looks best if the mortises are off-center relative to the leg, closest to the outermost corner of the leg.

Additional details on the objective

The assembly plan above is accompanied with a FRONT+SIDE view diagram where all mortises appear to go through the center (i.e. not offset), as well as a picture where they clearly are offset. Those discrepancies and oversights seem to be par for the course in most of the project-books I've looked at this far. Is this only a matter of personal taste?

I'm trying to to something like the plan, without the lower shelf. In the absence of the shelf and lower rail, the "meat" (Z) between the top of the leg and the top of the mortises becomes more important. I'm doing this in thick pine stock (aprons are long 1.5"x3.5", and legs are roughly 3"x3").

Most other table plans I've seen place the outside face of the aprons very close, maybe 1/8", from the leg's corner (which places the mortises a third of the stock further inwards). But that might be a safe layout only for a hardwood leg. I can't assess from my limited experience.

References

Got the project description from this book: "Tables you can customize. Ernie Conover. published by Popular Woodworking Books. October 1995. ISBN: 1-55870-397-7."

Answer 1

There is no magic ratio to grab and be done with. I'm not aware of a set of 'rules of thumb' that apply universally. It depends on wood selection and maintaining the joints when they start to loosen and on the abuse the table will suffer, at the hands of users less inclined to be gentle (six-year-old boys come to mind).

The strength of the wood in 'area z' comes not only from the height, from the leg top to top of the mortise, but also from the width of the mortise (how much 'roof' there is over the mortise), how long the 'roof' is and how much rocking the joint will be subjected to. The joints can be smaller and lighter if you leave the bottom shelf since the bottom shelf secures the bottom of the leg, thus protecting more than a table built without the bottom shelf.

Having said that, I think for your purposes here, you can take a great deal of wisdom from folks who have travel this path before you - not just from the makers of tables but also from the makers of chairs.

As an oversimplification, chair joints suffer from more abuse than table joints due to having direct contact with the bulk of the weight of a sitter. People tilt the chairs, stand on them, stack them, etc. and the chair often has less material than a table to boot, so take a look a chairs, in particular side chairs (the have less mass then arm chairs). (Yes, stools count but chairs have much heavier use.) I would add the history of the chair is full of designs where elegance trumped strength. The curved sabre-like legs of Sheraton period chairs come to mind - the grain of the leg is short due to the curve and that is where the legs break.

If the table isn't designed to be taken apart and reassembled, then corner blocks will lend strength by reducing the racking and twisting the joint will take. Needless to say, the joint is the place to use straight knot-less lumber, to avoid failure from short runs of grain giving way. If it is designed to be reassembled, you may find after a period of use, the pins holding the joints may need to be tweaked or replaced due to compression.

Given all the above, you would be fine building as the plan suggested. If you feel you want a safety margin, move the mortise away from the edge by 3/4" or 1" from the top and front of the leg, giving values of greater than half an inch for x, y, z. Given the thickness of the leg, the aesthetics of the piece should be unharmed.

Also as Graphus suggested, you can make the top over-long and pare down later. In fact, you can over-build the whole joint and see how the wear and tear works out and pare down over time, if you are so inclined and willing to do the work.