Join Date: Apr 2016
Location: Troy Michigan USA
I did quite a bit of searching yesterday about this subject yesterday and came to this conclusion. If you made a groove in your (long grain) traditional breadboard, and made this groove from end to end 2" deep...it would naturally curl and spread open and fail, due to being drier on the outside of the breadboard. To help prevent it the groove (at 2" depth) should stop before the ends, and in fact should be at a 2" depth intermittently to lessen the curling effect. So if you had a top that is a 30" wide glue up you would need an odd number of 2" deep mortices (say 3 or 5) evenly spaced into the bread board's length. The outer ended mortices should not exit the ends of the breadboard...to prevent the ends of the breadboard from curling. The thickness of the tenons should fit snug into the mortice groove...but provide ample room to expand/contract widthwise within the mortice because the table top WILL move within the mortice as moisture changes. Pining the breadboard to the tenons will keep the breadboard tight to the tabletop and requires elongated holes in the tenons for the pins (because of this movement). Glue is ONLY put into the center mortice, thus allowing the table top to freely expand/contract outward from the center of the table top. The pins are glued ONLY into the breadboard to keep them from falling out.
Other method is to first make a shallow groove...say 3/8" deep first into the breadboard from end to end, then create the mortices as described above. This groove would be the same width as your intended mortice. You would chop your mortice within this groove to a depth of 2" into the breadboard. Then when you make the tenons into the table top, you would use a haunched tenon in order to plug the groove in the breadboard.
Woodworking is like wetting myself....Only I know that warm feeling!