Join Date: Mar 2018
Location: Huntington Beach, California
First, accept the fact that wood moves and there is nothing that we can do about it, except.
Second, use as a general rule of thumb, wood at equilibrium in the shop will move about 1/8 inch per foot of width.
Third, an old sage of an instructor told me, "When you have a problem call attention to it." Meaning make the edge as nice as possible. A simple round over is enough many times.
I built a similar coffee table. My design was to put a mortise in the stretchers and a tenon on the shelf. The mortise was about 1 inch shorter than the width of the shelf and the tenon was about an inch shorter than the mortise. Only the center inch of the tenon was glued into the mortise. During the past year we have been through some vicious humidity swings but no problem with the coffee table. A similar coffee table went up to the Portland, Oregon area and again, no problems.
Oh yes, the "except". Something 2 feet wide needs 1/4 to 5/16 inch for movement. That is wide enough to drive a truck through. If you fix the center inch or two of your two foot wide shelf in the coffee table, then the movement is restricted to 1/8 on each side of the shelf or half of the total. If you leave 5/32 on each edge wood movement ceases to be a problem.
In furniture 1/32" is a Grand Canyon