I once had a large table that would have been great except for weak joints between the apron and table legs, which made the table wobbly. I wanted to strengthen the table without modifying it, and decided to see if I could retrofit mortise and tenon joints.
After removing the top, I drew in ¼” x 3” rectangles where I planned to add hardwood splines, at all 4 corners of the table.
First, I drilled ¼” x 2” deep starter holes at one end of the rectangles. Then I used my router, with a ¼” upcutting bit, to rout in several passes ¼” wide by 2” deep grooves along the length of the rectangles. Next, I glued and tapped in ¼” x 3” long x 2” deep hardwood splines.
The final result was a rock solid table, finally with strong joints.
This sketch shows the concept; looking down at the 2x2 leg and the 1x4 aprons that support the top. In dark is the added spline.
Step 1: Mark out location for reinforcement
As you see in this photo, I marked with black felt pen where I will cut a groove with the router. The board held with the C clamp serves as a guide for the router. I have a 1/4" straight cut bit in the router.