Cutting Mortises and Dovetails

Now that we have the posts built, it’s time to contemplate how we’re going to connect the cross beams that will connect each pair of posts on the headboard and footboard. In our inspiration plans, this is accomplished with three fairly thin pieces of wood, attached with pocket screws and then covered with 1×4 slats to create the headboard. This method concerns me in a few ways in terms of structural quality. First, we know what pocket screws will constrain the wood movement and could cause warping. But also, there’s going to be a lot of force coming down on those posts from the weight of the bed and occupants. Pocket screws exhibit excellent pullout force, meaning it’s very hard for two pieces of wood to pull apart when screwed together. But, they don’t have as much shear force, which is the amount of force applied perpendicular to the screw. In this case, both are important for us. There’s the possibility of the cross beams separating, but more important (I think) is the shear force being applied to the bed. The solution? Dovetails and mortise and tenon joints.

What is a mortise and tenon joint? Here’s an excellent article, but essentially it consists of a peg of wood from the cross beam going into a hole cut in the posts. A dovetail is similar, in that it uses two pieces of wood cut in a way to join them. However, it’s tapered so as to prevent the joint from separating. We could use a mortise and tenon to connect both beams, but we wouldn’t have anything to prevent the tenon from pulling out over time. And we can’t really do a dovetail for the bottom rail, since it’s in the middle of the post. So, we’ll do a dovetail/tenon hybrid on top, and a mortise and tenon on the bottom.

Here’s a look at the assembled product to help you visualize how everything goes together.

20180219_204925.jpg

Since we’re going to use 6/4 lumber, the broadest the wide part of the dovetail can be is about 1.25″. To give it a nice slope, and ease of measurement, I found up with 3/4″ at the narrowest, and around 1″ at the widest. That gives about 1/8th of an inch on each side of the wides part of the tenon to do the chisel work. But, we’re going to actually cut the mortise first, since it’s easier to fit the tenon to the mortise. Like the picture above, find the center of your post on each side and draw a reference line down the center. Then, off the part that will be the inside edge, measure 3/8″ in each direction (that will create a total 3/4″, centered measurement). Measure 1/2″ from the center on the other side, then connect the points to create a wedged design. Finally, transfer those lines with a square down 2.75″. Why that far? Well, when we create the cross beams, we’re going to have a 3/4″ groove that we cut into them to accept the plywood panels. If we made the mortise/tenons a full 3.5″, then this groove would show up on the other side of the post. Instead, we’ll notch the tenon and make the mortise/dovetail smaller so it will all be hidden (this will make more sense as you see everything come together).

Important note: make sure when you cut these that you are cutting the mortises perpendicular to the glue joint. Otherwise, the tenons might split the joint when we hammer them in.

Now, for sawing. Let me start by saying I fully intend to show my mistakes here. You’ll make mistakes too! It would be easy for me to pick and choose the best pictures, but the point is to humanize this process and make it accessible. So, you can see where I’d originally made my dovetail too wide and started cutting before I realized it. What I want you to notice from the picture below though is how I sawed the lines. Using your pull saw, start the cut on the corner and try to align the blade so both perpendicular lines are visible. If you follow both these lines at the same time, you’ll get a great cut. Flip the piece constantly so you’re always making a nice, straight cut.

As Bob Ross would say, just a happy accident


Second time turned out much better

After you have those lines cut all the way down, use your chisel to cut out the waste. It should look like this when finished:

For the mortise, follow a very similar process. On the footboard posts, we want to measure 4″ up from the bottom and mark there across both posts. That’s because our side boards are going to be that high, and we want consistency. On the headboard, the bottom will be 16.5″ up the post (so when you add the 3.5″ rail, the top will be 20″ from the bottom, exactly matched with the side board). Mark the width to 3/4″, and try to keep the lines identically centered to where your dovetails on top are. 3/4″ is probably a little thin for this mortise, but I wanted to keep consistent with the top. These will be through mortises, but they don’t have to be; you could stop an inch in or so. Chisel out the waste. This will be tedious. If you have a drill press, use a bit to take most of this out and then clean it with a chisel. I tried to use a hand auger, and it worked but the mortises didn’t come out as clean as if I’d just chiseled everything. Oh well! If you have access to a mortising machine, like you would as a member of a woodworking guild, then use that!


Here’s what the footboards will look like once you’re all done!

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