A number of bit vendors have specialized bit sets for constructing divided light doors and for making window sashes. A finicky aspect of any divided light construction is the sash-bar joinery. Do you cut tiny mortises and tenons for each joint or depend on the strength of a simple cope-and-stick joint? A crosslap joint is traditional and strong, but it’s tedious to cut accurately.
Not anymore! Unbeknownst to us poor woodworkers, bit makers have come up with a pair of cutters that make this cross-lap doable. And easily doable, too! One bit is a 1/4" straight bit, the other a V-groover with the point clipped off. It cuts a V-groove with a 1/4"-wide flat bottom.
Here’s how the system works. Cut a 1/4”-wide dado across the top edge of one sash bar and across the bottom edge of its mate. The depth of the dadoes is half the stock thickness. Next, use the modified V-groover to cut across both sides of both sash bars. Center this cut on the two previous cuts, and set the cut depth to leave a 1/4" of wood between them.
When the cuts are done, the two bars should slide together. You have a strong and invisible joint.
When you are making a grid of more than four lights, you do have to lay out the crosslaps precisely. Your router table setup must enable you to locate the cuts precisely from piece to piece. Cut the cross-lap joints before routing the profile and the rabbet for the glass. The profiling cuts parallel to the grain, so it’ll clean up cross-grain blowout from the joinery cuts.