Bookcase Shelves That Do Not Sag

Step 3 A Tight Slot

Using the 3/4" bit on the router, set the guide to cut at least 3/4" from the edge (see the figure in step 5). Now take the 2X4 and cut a 3/4" wide by 3/4" deep slot lengthwise as shown in the figure. You'll probably want to do that in 3 or more passes, going a little deeper each time. If you don't have a router with a fence or a router table, you can use a table saw to nibble out this slot.

The precision of the width of this slot is critical, so take your time and keep things well aligned from pass to pass. The width of this slot should precisely match the thickness of the raw pressed board. A tight fit is good, a loose fit is very bad.

Sand the slotted 2X4 and stain it if you wish. It is much easier to do this before everything is together. You can seal it with a nice polyurethane now too, or you can add a clear polyurethane top coat to the whole thing after it is assembled.