A sliding dovetail is a deceptively simple joint. and on top of that, it is a fun one to do by hand! This is often used in case work to connect the shelves to the side walls or even between shelves in drawer dividers. There are many ways to make these, and none of them are right or wrong. This just happens to be my favorite method.
Step 1: Layout the Tail
I use a marking gauge to mark a line on both sides at how deep the tail needs to be. In this case, it is about 1/4" I like to make the tail 1/3 of the thickness of the board it will go into. Then, set that board on the bench and set a fence along that line. I use a holdfast to keep the fence and board in place. Now we are ready for some cutting.
Step 2: Cur the Tail
I use an old dovetail plane I picked a while ago. They are not easy to find, but they are out there. You can easily make one much like you would make a shoulder plane. but you can also buy a new one here.
with the point of the dovetail plane riding along the fence, you can then cut in one side of the dovetail. You should go down till you just touch the outside corner of the board turning that into the tip of the tail. Then flip the piece over and cut in the other side.
this should give you a nice tail running the length of the board.
Step 3: Mark Out the Dado
Next, we need to mark out where to cut the dado that receives the tail. you need to measure the thin point on the tail and transfer that resurgent to the board. I use a caliper for that but you might just hold it up alongside the board and make the marks for it. Then, with a square draw those lines all the way across the board. you want to cut to the inside of these lines with the saw pointing out.,b ut we will cover that in the next step.
The next mark you need to make is the depth stop mark. Your marking gauge should be still set with the mark for the depth of cut from when you marked the tail. just use that to put a mark at either end of the dado.
Step 4: Cut the Dado
You can free hand cut the angle for the dado side wall but I like to use the Dovetail plane as a fence. this will guarantee the angle is the same for the dado and the tail. just like before I place it just back from the lin and hold it in place with the Holdfast. I want it back from the line by the thickness of the saw kerf. this way you will cut on the inside of the line. just keep your fingers on the saw plate holding it to the fence and lightly make your cut down to your depth line.
After this, flip the fence around and make the second cut going the second direction.
Step 5: Remove the Waiste
I like to make a third cut right down the middle to make removing the chips easier. I just stop it short of the depth mark and freehand cut right between the two other depth cuts.
Next, I will come in with a chisel bevel down and remove the majority of the waste. With most of that out of the way a router plane will give you a perfectly flat bottom and the tip of the blade will work into the two corners. of the dado. sometimes a marking knife or chisel will help you clean out any stray fibers.
Step 6: Fit the Joint
At this point, I like to test the joint and remove anything that stops the tail from sliding in. this will let you know if you need to go deeper, clean out stray fibers, or make the dado a bit wider.
If you need to widen the dado the easiest way is with a sharp chisel.
With a little work, it should slide in and fit snugly.