In case you missed it, on November 1st we posted a full-length video from Glen Huey in our Free Videos section (to view the video you either need to be logged into your Free or Premium membership account) on making a true divided light door. In the video Glen talks about how lots of beginning woodworkers jump into making glass doors by making a mortise-and-tenon frame, then routing the rabbet into the frame for the glass (shown in the first photo in this post). And, while that’s one way to make a glass door, it certainly isn’t the best way.
Once the rabbet is cut, you need to go back and square the rounded corners left by the router bit. Although I find it annoying to make those chop cuts, there’s a structural reason to make your glass doors with offset shoulders. If you look at the second picture, you might see the reason.
Wood tends to split along the grain, or with the length of the fibers. By making a door frame and routing the rabbets into place, you’ve shortened the length of the fibers at the shoulder of the tenon. If you run the rabbets first, then offset the shoulders of the tenons to properly mate up (as in the top frame in the second photo), you keep the long fibers intact over the length of the door stile. This makes for a stronger joint and a cleaner look on your glass doors.