‘Lapped’ & ‘Double-lapped’ Dovetail Joints

The following articles on lapped and double-lapped dovetails (aka “half-blind” and “blind” dovetails) are excerpted from Volume 3 of “The Woodworker: The Charles H. Hayward Years.”

The third book in our “The Woodworker: The Charles H. Hayward Years” series covers all types of woodwork joints, including how to design them, cut them and fix them when things go awry.

It’s difficult to overstate the importance of the book “Woodwork Joints” by Charles H. Hayward (1898-1998), which was first published in 1950 then reprinted many times and in several different editions of varying quality.

The compact 168-page book is beautifully illustrated by Hayward and contains the kind of spare prose that made him the best woodworking author of the 20th century. Like a good woodworking joint, Hayward’s text contains nothing superfluous and lacks nothing important to the task at hand.

In addition to Hayward’s take on joinery, this volume also contains the perspective of other British writers of the day that Hayward published in The Woodworker, including J. Maynard, Robert Wearing, K.J.S. Walker and C.A. Hewett.

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A WOODWORKER SUPPLEMENT – LAPPED DOVETAIL JOINT

This joint is used chiefly for carcase construction and has the advantage of being entirely concealed at one side. To enable the joint to be started easily the inner corners of the tails can be chiselled off. The ports should not be joined together too often before gluing as this tends to loosen the joint.

1. Square the ends, and with a cutting-gauge, gauge dovetails and pins. The lap should be between one quarter and one third thickness of wood. For the pins set gauge to work from inside face. Mark both pieces. Re-set to thickness of tails and mark depth of pins.
2. Mark out dovetail with template or sliding bevel.
3. Using a fine saw cut dovetails. Do not let saw run past gauge line. Some find it easier to fix wood so that cut is vertical.
4. Clamp work to bench and make a sloping cut at gauge line. Chop down 1/16 in. from line, and remove chips by chopping in from the end. When about half way pare down to gauge line Reverse wood and repeat process.
5. To mark pins fix wood in vice and, with dovetails in correct position, scribe round as shown.
6. When sawing the pins hold saw to the waste side of marks. Inset shows part of waste cut away with saw.
7. With chisel slightly in front of gauge line chop down, and remove waste by chopping with the grain from the end. A 1/8 in. paring chisel is used for the corners and a bevel edge chisel for final paring.

Editor’s note: The image at the top of this blog post corresponds with the numbers above.

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HOW TO CUT THE LAPPED DOVETAIL

The great advantage of this joint is that it is entirely concealed on one surface and at the same time is extremely strong. This gives it a special value for carcase work since the joint does not show at the sides. The top and bottom surfaces where the dovetails are exposed are out of sight Drawer fronts are also lap-dovetailed (though the spacing is rather different), and here there is the added advantage that the wedge formation of the joint resists the pull which is the chief strain the drawer has to withstand.

It makes no difference whether the dovetails or the pins are cut first; it is mostly a matter of personal preference, though choice may be determined by other considerations. For instance, the top and bottom may have to be glued up to make the width, and it would then likely be convenient to cut the pins in the ends whilst the joints are setting.

Marking out. Trimming the wood to size is the first procedure. The ends in which the pins are cut are obvious; they are the finished size of the carcase as shown in Fig. 1. It is clear that the top and bottom must be short of the over-all width by the combined thickness of the two laps in the ends. This lap size has therefore to be decided straightway. In Fig. 1 the required over-all width is 18 ins. Assuming that the lap is to be 1/8 in. it is clear that the top and bottom will have to finish 17-3/4 ins. long.

Use the cutting gauge to mark the extent of the joint as shown in Fig. 2. Set the gauge to work from the inside of the ends, the required lap projecting beyond, and mark both sides of top and bottom as well as the edges of the ends (see A). In this way the pins are bound to be the same size as the dovetails. Since the top and bottom sink their full thickness into the ends, the gauge is now re-set the thickness of these and the inner surface of the ends marked as at B, Fig. 2.

Dovetail positions. The tradesman usually roughly pencils in the dovetail positions and then saws straightway. Practice enables him to cut the true slope without exact marking out. The inexperienced man should either make the simple marker given in Fig. 3, or set an adjustable bevel to the slope (5/8 in. in 3 ins.). The spacing for normal small work is given at A; B is suitable for a wide carcase in which the small end dovetails resist any tendency for the ends to twist away at the corners. Drawer front dovetails are given at C.

Sawing the joints. Fig. 4 shows the dovetails being sawn. If the wood is 3/4 in. or more thick it is advisable to cut each joint individually, but in thinner wood two or more pieces can be fixed together in the vice and sawn together as shown. Most men saw straight across dead square and it is necessary to mark out first with the trysquare. Some prefer to make a slight taper fit and then the saw is taken at a very slight angle as at A—not more than the thickness of the pencil line. If this is done it is obviously important that all face or outer sides are to the front. Otherwise, instead of showing a close joint there will be a gap. A, Fig. 4, is given in exaggeration for clearness.

Marking the pins. Before the waste is chopped away the pins are marked out. Fix the end in the vice, and place the top upon it in the relative position it is to occupy. The inner end can be supported upon a block of wood of suitable thickness. Hold the top firmly down with the left hand, and, placing the saw in each kerf in turn, draw it backwards as in Fig. 5, so leaving a mark which corresponds exactly with the dovetail.

Sawing. When sawing the pins be careful to place the saw on the waste side of the mark as at A, Fig. 5. Don’t overdo it, but just leave in the marks. This will ensure tight joints without forcing. Judgment on this point is probably the most important point in the whole procedure, and it is something which comes with experience. The thickness of the saw has to be taken into account, far less allowance being necessary for a fine saw than a coarse one. Beginners are strongly advised to cut an experimental joint using the same saw that will eventually be used for the actual joint, and try it together. They will learn more in this way than reading a dozen articles.

Of course, the saw can only be taken down diagonally as shown in Fig. 6, and here again the inexperienced man should mark down with the trysquare first. Part of the waste can be sawn away as also shown in Fig. 6. It all helps to lessen the chopping-out with the chisel.

Another way of reducing the waste is to bore it partly away as in Fig. 7. Except for the thickest wood it is seldom practicable to use the centre or twist bit because the centre point is liable to emerge through the lap, but the Forstner bit is ideal as shown in Fig. 7. It has scarcely any centre point.

Chopping. The procedure is given in Fig. 8. Clamp down the work on a flat, spare piece of wood to avoid bruising, placing it over a solid part of the bench such as a leg. At each socket make a small sloping groove against the gauge line, and then chop downwards about 1/16 in. short of the line. Ease away the waste at the end with the grain, and repeat the process. Finally put the chisel right on the gauge line and cut down.

For clearing out the corners a bevelled-edge chisel will prove invaluable because it will work close in. Many men keep an old stubby bevelled edge chisel specially for the purpose. A, in Fig. 8, shows how the waste is gradually removed. Note that if the chisel were placed on the gauge line at the outset it would be forced in beyond the latter because of its wedge formation.

The dovetails are chopped in the same way, but of course, the chisel is used from both sides. The waste at the corners can be sawn away. When the pins are cut first the marking of the dovetails is done with an awl as shown in Fig. 9. Once again the saw must be used on the waste side.

To enable the joint to be started easily the inner eDges of the dovetails can be pared off. This will permit the joint to be partly put together to see that it fits. It should not be driven right home until it is glued as this is liable to loosen it. Place a piece of waste wood over the joint when knocking it home and strike this. Otherwise the wood may split owing to the local pressure, and in any case it may be bruised.

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THE DOUBLE-LAP DOVETAIL

THIS IS SLIGHTLY more complicated than the single-lap joint, but is simpler than the mitre dovetail in that there is no mitre to bother about. In fact, the experienced man can glue up the two parts straightway without first assembling them dry. This is risky in the case of the mitre dovetail because almost inevitably a certain amount of trimming at the mitre is unavoidable.

The joint can be in either of the forms given in Figs. 1 and 2, and to an extent it depends upon the stresses to which the joint is most liable to be exposed, because the dovetail shape resists the The joint can be in either of the forms given in Figs. 1 and 2, and to an extent it depends upon the stresses to which the joint is most liable to be exposed, because the dovetail shape resists the pull more in one direction than the other. The more usual joint is that in Fig. 1, in which form it is often used for cabinet carcases. At the top the joint is entirely hidden, and at the side shows only as a thin line of end grain.

Preparation
It is immaterial whether the dovetails or pins are cut first in the joint in Fig. 1, but in Fig. 2 the pins should be cut first as otherwise it is difficult to mark the one piece from the other. Whichever method is followed, the first essential is to square up the wood to the finished size, remembering to allow for the lap when calculating the length of the parts. This scarcely arises when just an isolated joint is cut, but in the case of, say, a carcase the sizes are obviously important. The rule to remember is that the piece with the projecting lap is always trimmed to the finished size, whereas that with the flush lap is less in length by the thickness of the lap on the other piece.

Marking out
Assume that the joint in Fig. 1 is to be cut. The first stage in marking out is that of gauging as shown in Fig. 3, because until the thickness of the projecting lap is decided it is impossible to trim the wood to the finished length. Set the cutting gauge to the dovetail thickness (Z), which is the thickness of the wood less the thickness of the lap. Mark the end of the piece with the dovetail, the gauge fence working against the inner surface of the wood, and also the inner surface of that with the pins. Decide on the thickness of the flush lap (it is usually the same as the other), and mark the inner face of the dovetailed piece, and the end of that with the pins (see Y). Lastly set the gauge to the thickness of the wood with the pins, and mark the inner face of the dovetailed piece (X).

Cutting the dovetails
The outer mark on the inner face and the end mark give the extent of the rebate to be worked on the dovetailed piece. Saw down across the grain and remove the waste by chopping with the chisel at the end. It may be necessary to trim the rebate afterwards with the shoulder plane. Note the sloping channel chiselled on the waste side of the line as shown inset in Fig. 4, to provide a convenient path for the saw. Mark out the dovetails and saw down as far as it is practicable to cut as in Fig. 5, taking care not to let the saw touch the projecting lap as any such marking would show badly later.

The rest of the work must be done by chopping with the chisel as in Fig. 6, and for this a bevelled-edge chisel is desirable to enable it to reach into the corners. Follow the usual practice of chopping down first short of the gauge line (see top arrow), and do not bring the chisel right on to the line until the majority of the waste has been removed. The latter is done by chopping in from the end (lower arrow). Since the saw cannot reach right into the corners owing to the projecting lap, it is necessary to cut down on the line of the dovetail, but only light taps with the mallet should be given as otherwise the wood may split. Clean out the corners and make sure that the bottom is level.

Marking the pins
Transferring the marks to the pins is the next job. The one piece is held in the vice and the other placed upon it as in Fig. 7. A block of wood to the rear will make sure that the wood is level and is steady. Press firmly down to make sure that there is no movement, and pass a marking awl along each side of every dovetail.

The process of cutting the pins is given in Fig. 9. The saw is held immediately to the waste side of the mark, and is taken down as far as the diagonal. Part of the waste can be sawn as at (2), but finally it is chopped away as described last month. The stages are shown in Fig. 9, but it will be realised that each socket is not completed individually before the next is sawn. Rather, all sawing is done, then all chopping.

Finally the inner corners of the dovetail are lightly chiselled away to ease the assembling of the joint as in Fig. 8. Be sure to do this after the marks have been transferred. If the parts are lightly started together it will be obvious whether they fit or not. It is better to avoid knocking completely together dry before gluing.

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