USE THREE SPACER BLOCKS to position the second board for routing and clamp it in place. Make the spacers 1/32-in. narrower than the cutting diameter of the pattern bit, so the second routing pass will remove only 1/32 in. from the board.
ROUT THE 1/2-IN.-DEEP GROOVE for the spline in several passes, using a 1/4-in.-wide slot-cutting bit (above).Tip: To avoid tear-out, make a couple light scoring cuts, about 1/8-in. deep, moving from right to left. Then move left to right, full depth. Stop the cut 2-in. from each end of the board, so it won’t show.
TEST FIT the spline. It should slip into the groove freely, but without being loose. It can be one long piece or several short ones. Rounded ends match the ends of the slot. Dry fit each joint to make sure the spline fits inside without binding.
ROUT THE MATING EDGE of each joint, while keeping the router firmly held against the straightedge. Because the cut is made opposite the bearing point, the edge will be ruined if the bit wanders away.
JOINTING WITH A ROUTER
Step 2: Cutting the Mating Edge
WHILE MAKING the second cut, the pattern bit bears against the same straightedge, but it cuts the second board, which has been moved into position on the opposite side (Photo 5). Keep in mind that jointing the top boards reduces the width of the top about 1/16 in. per joint, so lay it out oversized.
STACK THE BOARDS as you glue them. Put a bead of glue at the bottom of the groove of the first board and add the spline. Roll a layer of glue onto the edge of the adjacent board, add a bead in its groove and assemble the joint. Don’t put any glue on the splines. The glue causes them to swell and make the joint impossible to fit. Glue the top together in stages, two to four joints at a time.