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The Woodwright's Workbook: Further Exploration in Traditional Woodcraft By Roy Underhill
Like so many operations In joinery, success in dovetailing depends on accurately bringing the material true and square. Without this to build upon, the rest of your work will be as useless as barking at a knot. See that your stock is right before you go on.
i. Find your gauge (the old scantillion) and set it to a bit more (Vie inch) than the thickness of the boards. This extra measure will be planed off the completed joint. Now run the fence of the gauge against die end grain of each of the boards to scratch the extent of the tails and pins all the way around the two ends of each of the four boards. The length of common dovetails is determined by the thickness of the wood. The width, however, is a matter of taste and tradition. I will say only that you should not make the pins and tails the same size—that is what machines do.
2. Dividers or a diagonally placed ruler will give an even spacing to the dovetails. When you have spaced them as you wish, take a try square and, at each point, extend two lines spaced about 1A inch apart across the end grain. These lines are the distance between the ends of the tails. The lines that border the space between them diverges from these points down to die base. This angle is easily set on an adjustable bevel by laying it against a square so that in 6 inches of length it deviates i inch from a right angle. You need to draw these lines only on the face side of the boards. Careful sawing will ensure that they are true on the back side as well. (If you saw the tails in pairs then it is wise to lay out the stock more fully.) Mark the waste to be removed with xs so there can be no mistake. It has happened before to better folk than you and I.
3. Saw the diagonals of these lines first, then bring the saw around to cut square. The short length of the cuts can be done quickly with even your finest saw. (The eighteenth-century Seaton tool chest held a dovetail saw with 20 teeth to the inch, measuring 18 thousandths of an inch thick.) The saw cuts for the tails determine the size of the pins.
4. Set the sawn but unchiseled tail board on the end grain of the pin board. Take the saw and set it in the kerfs, drawing it back hard enough to leave a good mark on the underboard. Don't cut, just mark into the wood. Be sure that the two pieces don't move until all the marks are made.
5. Now take the try square again and extend straight lines back down the face side. Think and mark the waste areas to be removed with xs. Saw these from the face side, cutting on the waste side of the line. Too much on the waste side and you will have to pare away and trim before the joints will it; too close, and the joint will be too loose.
6. Check the edge on your chisels—they need to be sharp. Set the chisel a bit inside the waste area from the line scratched with the gauge. Drive it straight down, pull it out, and then dig back to it with the bevel still facing the end grain of the board. Cut halfway through from one side and then turn the board over and finish from the other side. The narrowing spaces between the pins may have to be done entirely from the wide side. See that the wood is well supported underneath so that it does not splinter out. Check the joints to see that they will fit properly before you apply the glue. Put the four sides together and check them with a square before they set up.
"The Woodwright's Workbook: Further Exploration in Traditional Woodcraft" By Roy Underhill
© 2012 The University of North Carolina Press