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The Woodwright's Guide: Working Wood with Wedge & Edge By Roy Underhill
Through Dovetail - Tails First
Dovetails are cut by superimposition. I wrote earlier about laying out mortise and tenon joints by superimposing one element on the other and transferring dimensions. This makes some people uncomfortable. Good cogs in the mass production machine are supposed to work from a measured drawing, manufacturing each precise part and then fitting them all together. But try to find a measured drawing from the days of the great cabinetmakers — there are none. Their forms emerged from the constraints of classical proportion, customer demand, and mechanical necessity. They built furniture largely with superimposition shaping each piece of the emerging whole.
The immediate question for dovetails is, which do you cut first? Which part gets to be the super of the superimposition, the tails or the pins? In secret miter dovetails, we have to cut the pins first, so that’s settled. But we’re making through dovetails now, so we’ll do them tails-first.
The wide boards of this chest will have lifting handles on the end. Thus, the tails will go on the sides and the ends will get the pins. Dovetailing can be no more precise than the boards are square and true. Square all the end grain with a block plane, leaving it smooth to allow clear marking.
Assuming you are joining equally thick boards, set the gauge 1/32 inch greater than the thickness of the boards and run the fence of the gauge against the end grain of each of the boards to mark the extent of the tails and pins. This extra 1/32 inch gives you something to plane off the completed joint. Lightly trail the point of the gauge across the grain rather than scratching and tearing. A lightly held cutting gauge serves even better.
Now decide the spacing of the tails and pins. If the chest has any skirting around the base or lips to meet the lid, consider their locations so the dovetails won’t begin or end awkwardly. If the boards will have a groove plowed down their inside to hold a bottom, that groove should not overlap the joint between a tail and pin. Make these grooves first so they won’t be a surprise later on.
To fight confusion, I’ll call the piece with the tails the tail board, and the one with the pins the pin board.
We will cut the tails first, meaning we are working on the broad sides of the chest and defining the sockets for the pins between the tails, the pin spaces. Custom demands that dovetails end on a half-pin, although you may find otherwise in old pieces.
The size of the pins relative to the tails is an aesthetic/mechanical tradeoff. For a heavy chest, you could make them equal in size, but even when equal spacing would be stronger, a chest looks far better when the pins are about half as wide as the boards are thick, and the tails are about three times as wide as the pins.
By this guideline, the thickness of the wood determines how many tails and pins will fit within a given width. For 3/4-inch-thick, 11-inch-wide boards, this comes to 8 dovetails separated by 7 pins, bounded by a half-pin on each end. We thus need to divide the 11-inch board into 8 equal spaces — not measuring from edge to edge, but from the centerline of one outer half-pin to the centerline of the other.
At their widest points, these half-pins should be the same width as the full pins, at least half the thickness of the wood, in this case, 3/8 inch. This puts their centerlines at 3/16 inch back from the edges. (If you think the corners are going to take a beating, you can make the bounding half-pins a bit wider.)
Back to dividing an 11-inch board into 8 equal spaces. Hold a ruler diagonally on the board with the zero end crossing the centerline of one half-pin and the 12 crossing the centerline of the other. Put a pencil mark every 1 1/2 inches down the diagonal ruler. Carry these marks up to the end using a try square and a pencil, dividing the width into 8 equal parts.
This works for any width or number of divisions. You would place a mark every 2 inches if you wanted to divide the board into 6 equal parts. For 7 equal parts, position the ruler at zero and 14 and mark every 2 inches. For 5, go to 15 and mark every 3.
Anyway, where each pencil line crosses the line marked by the gauge, measure out the width of the pin space, 3/16 inch on either side for a total of 3/8 inch. This is the widest part of the pin space, the narrowest part of the dovetail, and the slope starts here.
For most work, find the slope angle for your dovetails by setting the sliding bevel to cross the one and the six inches on the square. A one-in-five slope looks more robust; a one-in-seven looks more delicate.
Whatever angle you decide upon, set the beam of the sliding bevel on the end grain and draw the converging sides of the pin spaces up from the gauged line with a sharp pencil. Mark the little spaces with Xs so you will know what you will be cutting away. Carry all these lines square across the end grain and use the bevel again on the back side. The lines across the end grain are the most important. You can have variation in the slopes between different dovetails — they’ll just look funky. But if the lines aren’t sawn square across the end grain, the assembled joint will have gaps that weaken the whole works.
Sawing at last. A dovetail saw is simply a fine-toothed backsaw, but use what you have. Saw precisely parallel to the lines, touching them but leaving them intact. Saw diagonally at first and watch that you leave the line on both faces.
"The Woodwright's Guide: Working Wood with Wedge & Edge" By Roy Underhill
© 2012 The University of North Carolina Press