Box Joint Table Reinforcement

[Sean P.]

I am building a 72 x 30 x 1.5 inch walnut dining table where the legs will be joined to the top using box joints (or finger joints for some), 6 5 inch boards make up the top and legs. I am also building matching benches that are 65x13.4x1.5 with the same box joint construction. While I know box joints are super strong, I am worried that since this only has three sides unlike a typical box that the box joints alone will not be strong enough to limit any side to side wobble.  I really don't want to put a cross brace under the table nor use any metal brackets, so my thought was to add dowels to the box joints to help strengthen them. However, I am concerned about wood movement across the table top.  I know without the dowels the legs and top should all move the same creating no problems, but will adding the dowels cause too much restriction to wood movement? Creating a floating mortise with a router wont work because of the 5 inch space between the box joints (I tried). My last option I guess would be to hand cut the mortises but that would be A LOT of them. Any help or suggestions are greatly appreciated. Here is a picture of one of the benches and some dowels cut in underside of the bench top with mating holes in the legs. 

[treeslayer]

Well first welcome to the forum Sean, box joints are strong but if it were me i would be adding a 45 degree brace under the benches and the table, while strong and maybe more so with the addition of the dowels i would not trust that joint alone to prevent racking and possible joint failure under load. some 45 degree braces would not show that much and if you morticed them in would give you a lot more strength IMO

[drzaius]

Welcome Sean. The tremendous strength of box joints comes from them having so much glue area. What you have there is technically a box joint, but while very attractive, won't be that strong so your right to be concerned. If the joint spacing was similar to the thickness of the boards then I think it would be fine with no other reinforcement.

At this point in the build, I think you will be best off doing as @treeslayer suggested & put 45* braces underneath. The wouldn't need to be very big and would not be visible unless you got right down there to have a look.

There's some beautiful grain there & I sure hope you share pics of the finished product.

Good luck.

[Coop]

I agree with Dave. If you had more "fingers" to give you more long grain glue surface, it would be better but wouldn't look as good.

Looks like Frank beat me to it!!

[Sean P.]

Thanks Everyone for the responses! Once everything is glued up I'll have to do some testing to see how strong the joint is, but even though I was hoping not at add metal, I figured there was a chance I was going to have to.  I'll add some pictures once complete or you can follow me on Instagram @fallen.timbers

Any thoughts on the movement of the dowels and dowel holes? I would think that each hole and dowel, at only 1/2", would move with the rest of the board and therefore not have the same worry as an entire tenon being glued in place. 

[Mick S]

I would not add a dowel to it. Nothing to be gained. I would recommend heeding the advice above and narrow the box joints to take advantage of more glue surface. If I'm reading your post right, you only have 5 glue surfaces on each end. Narrowing the width of the joints to 2 ½" would give 2X the glue surface and still be divisible by the individual board widths. The wood is plenty strong to hold up at 1.5" thickness as long as the joinery is sound. 1.25" box joints would be ideal.