Why would you double the workload, just to hide it inside a table leg? "Because it's awesome"
A double mortise and tenon joint also offers twice as much surface area for glue, making the joint stronger. It is not something you'll need for every project, but it will be handy to know for some.
In this video, the Wood Whisperer uses a router to cut the mortises and a tablesaw to cut the tenons.
Cutting the mortises:
- Mark start and stop points of the workpiece (the edges of the tenon stock).
- Draw the tenon shoulders, beginning 3/8 inch in from each edge and 3/8 inch from each outside edge that was drawn in the previous step. The wood inside these lines is what gets cut out with a router. But not all of the wood.
- Use a 3/8 inch router bit and a straightedge to cut the 3/8 inch mortises.
- Tip: Plunge a hole at the beginning and end of each mortise. This allows you to cut the middle out without cutting past the line.
- Plunge cut the meat between the holes, in multiple passes, cutting deeper each time.
- Flip the workpiece around and repeat the process. The edge guide ensures that both mortises are equally distant from the outer edge of the board.
- If you are cutting a lot of mortises for a table, or another woodworking project, cut them all now, to keep the router set up identical.
Cutting the wide tenons
- Find a piece of scrap stock that has the same dimensions as the workpiece. Make a jig with this piece.
- Now that all of the mortises are cut, you can safely change the router fence set up to cut out the wood between the mortises, on the setup piece of stock. This piece is used to mark and adjust the tenon.
- Place the tenon workpiece on the setup piece and mark the outer walls of the mortises.
- Establish a cut line around the perimeter of the workpiece using a marking gauge. This severs the grain to avoid tear out after cutting on a tablesaw.
- Set tablesaw blade at (or just below) the marks made two steps ago.
- Set the fence to place the blade just inside the shoulder cut made two steps ago.
- Use a miter gauge to slide the workpiece through the saw, cutting all four edges.
- Cut all other tenons now before changing the blade/fence setup.
- Raise the blade to cut up to the shoulder cuts made previously.
- Use a tenoning jig to cut the edges of the workpiece up to the shoulder cuts.
- Test fit on the setup workpiece — the one with the wood cut out from between the mortises.
- If it is snug, adjust the tenoning jig and test fit again. If it is loose, go back to the woodpile.
- When the width is dialed in, reset the tenoning jig and cut the shoulders off.
- Do this to all tenon pieces before changing the tablesaw setup.
Cutting the double-tenons
- Line up the extra-wide tenon with the actual double mortises and mark the inner walls of the mortises.
- Set the tablesaw blade to be just slightly wide of the marks and cut the wood between the tenons using the tenoning jig.
- Test fit.
- Clean the edges and corners of the tenon with a sharp chisel.
- Use a narrow chisel to clean the space between the tenons.
- Use a shoulder plane to flatten and floor of the joint and do the final cleanup.
- Test fit.
- Bask in glory.
Now that you know how to make this fine piece of woodworking joinery, you can assemble it, so that it will be hidden from the world forever.
—The WoodWhisperer's YouTube channel provides education and entertainment for the modern woodworker.