Through Dovetails - Myth and Mastery

There's great satisfaction in the completion of a well fitting dovetail joint, and perhaps the plain through dovetail offers one of the best visual rewards

Although technically less complex than other dovetail joints, little is hidden in the through dovetail, so we see most of the craftsmanship that has gone into it. By comparison, in the secret mitre dovetail we see no evidence of the 'dovetail' joinery unless the fit is poor (watch my video on the secret mitre dovetail, and you might understand why)
"All dovetails are difficult to make"
Plain dovetail joints, by which I certainly include through and half-blind and probably lapped as well, if joining components at ninety degrees, are fairly simple to visualise in your mind. That's one hurdle out of the way. Now, as long as you can mark out and cut to lines accurately, something which is a prerequisite of any joinery, preparing these dovetails cannot be described as difficult.
Students I've taught have never made a complete hash of their first through dovetail joint. Yes, they may be too tight to start with, and end up too sloppy after over-enthusiastic paring, or be too sloppy off the saw. On a couple of occasions, students have even removed tails or pins, rather than waste. However, these first attempts clearly reveal the student's errors, reinforce the method I've taught, and precede a greatly improved second attempt, mostly! And that's the key--you can see what's wrong, and you can practice on correcting it. It's another mater when you can't see most of the joint or clearly visualise how it should be--the secret mitre or bevel dovetails for example.

For me, mastery of dovetail joints is achieved once you can design them around the situation (for example when making a box that will be split to produce a top and bottom half), lay them out as designed, cut one half of the joint cleanly, transfer the layout to the second half accurately, and cut the second half to fit. Notice that I didn't include speed--once mastered, repetition will increase speed to a certain point, after which quality of fit often suffers. Initial component preparation and final clean-up of the joint, usually by hand plane, I consider separate skills although necessary.

So, lets talk about producing a through dovetail.

The thickness of the pin board should be knifed or gauged (preferably with a cutting gauge) around the end of the tail board, and the thickness of the tail board, likewise, marked on the two faces of the pin board.
If there is any doubt about the squareness of the ends, then a try-square and knife should be used, and the measurement taken from the shortest point. This ensures full length tails and pins to start with.

Setting out the tails--and I prefer to start with tails--is initially all about accommodating the situation the joint will be in, avoiding potential weaknesses in tails or pins due to edges, grooves, holes, etc. that may exist already, or may be planned for later. Once these are dealt with, the aesthetic can be considered. I could write a whole article on that, so let's take the simple case of an even layout of tails and pins.
Select a chisel that is a little under the width you would like the thick end of the pins to be. Place it's tip in the depth line, previously knifed, on the face and at one edge of the tail board. Slide the dovetail marking gauge along the end of the board, up to the chisel, and mark in the first slope and square lines.
Using a pair of dividers, set them so that you can step-off exactly the number of tails you want between the first square line and the far edge--giving a setting of exactly one tail plus one pin (as measured on the end of the board). It's now easy to step-off this distance from each end, as many times as there will be tails, and mark in the positions of all the square lines. Use the dovetail gauge to square these lines across and mark in the slope lines of the tails. Mark in what is waste.
Saw the slopes of the tails, concentrating on keeping the saw square to the width of the board--the slope lines are just a guide for angling the saw to make a tail, and you shouldn't change the angle you saw these during the cut, even if you're off-line. Saw to just shy of the depth line, and never beyond.
Now remove the waste. The two edge pieces of waste can be cut away with the back saw, but those between tails will need a fret saw, or can be wholly chopped (see the video for my method of doing this). Once the majority of the waste has been cut away, use a chisel to pare from the knife lines into the centre, fractionally undercutting inside the joint. At no point cut below the knife lines on the faces or edges of the board, or all you previous good work will be wasted.
Hold the pin board in a vise and rest the tail board on top such that the base of the tails align with the inside end of the pin board, and the two edges line up. A small sliver of light coming through evenly across the joint is a good indication that the pieces are square. Hold firmly while you transfer the outline of the tails to the end of the pin board. I haven't decided whether a sharp pencil or a knife is better for this, with results being equally good, but visibility of either line will depend on the wood being used. Square lines down from the pin slopes just marked, to the depth line. Mark in the waste.
Saw the pins, up to the lines but not over, down to just shy of the depth line. Remove the waste with either the fret saw or by chopping, before carefully paring from the knifed depth line into the centre. Again, a slight undercut in the middle is useful, and ensures no hang-ups when assembling the joint.
When chopped, the centre may break up a little in some wood, as shown in the photo'. Just make sure none of this is higher than the depth line. Narrower chisels make cleaning these situations up easier.
If you get all the basics right, then you should be able to assemble a well fitted through dovetail joint. Once glued, clean-up from this point is just a case of removing any visible pencil marks, and knife lines (although some like to leave these) with a sharp hand plane.
Oh yes, if you're long sighted like me, wear your glasses!

I have a long and short version of my latest video on preparing a through dovetail available on YouTube now.

For another challenge, take a look at the rising dovetail (half shumioshi)

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