Thursday 21 August 2008
Robert Ingham investigates this strong and aesthetically pleasing technique
The lap dovetail joint is a fascinating combination of man's ingenuity in developing a practical and reliable means of forming a junction between two pieces of wood, and the aesthetic evolution woodworking has undergone since its beginnings.
The first large-scale use of dovetails in carcass furniture was applied to drawer-making. These through joints were employed to resist the stress imparted to a fully loaded drawer, when the front pulled the sides out of the carcass. Many saw the pins as a distraction from the overall appearance of the piece, and some form of applied decoration was often used to disguise the joint. As the actual quality of craftsmanship in the construction of these early pieces was not very good, the application of an applied moulding or inlay was seen as an acceptable way of overcoming the problem.
An emphasis on decorationMany of the finer pieces were very ornate, with the emphasis placed on decoration rather than construction. In fact, it was often the case that if some aspect of the piece did not show, then the quality of craftsmanship didn't really matter. With the increasing use of imported timbers and a realisation that the colour and grain of the wood was an important contribution to the appearance of a piece, the problem of the through dovetail in drawer-making came under scrutiny. By this time in the evolution of design and construction, veneers were in use, and sawcut examples could be laid over the drawer front, thereby covering the front corner joints. It was not long before the economic advantages and the ingenuity of the observant craftsman, together with improvements in tool technology, resulted in the lap dovetail joint being cut from the solid. This meant the entire drawer front was made from one piece of wood. Very soon the joint was being used not only for drawers, but also for larger scale carcass construction. In principle, lap dovetails are very strong joints with good location and large gluing areas. They are best suited to situations where the appearance of the joint on show surfaces would compromise the aesthetic appearance of the piece.
Marking outWhen making a lap dovetail, the preparation of components is essentially the same as that for any carcass construction employing dovetail joints. Once again, as most of the marking out is done from the ends of the components, it is essential to carry this out with care. A reliable dimension saw is the best tool to use.
The main difference between lap dovetails and through dovetails is in the cutting of the pins. Again the question of tails or pins first is a matter of personal choice. Whichever choice is made, it is advantageous to work the marking out of both aspects of the joint in a harmonious sequence.
The shoulder linesAs with all the dovetail joints I have described, the shoulder lines are best marked out with a cutting gauge. In fact, it was as a result of the marking out of this aspect on a very deep drawer I made many years ago, that I first decided to use a cutting gauge instead of a marking gauge, as the profile of the blade ensured a clean, square corner by getting the orientation of its faces right. Make sure that the flat face of the blade is away from the face of the cutting gauge stock and start by marking out the shoulder lines for the tails. There is a proportional consideration to be made here, which affects the thickness of the lap as it appears on the end-grain of the drawer front.
I used to employ a ratio of 1:4 but I now rely more on intuition and decide on the thickness of the lap very much by eye. This thickness contributes mainly to the appearance of the joint, so long as the lap does not get too thin, running the risk of being cut through when the waste is being removed later. Having marked out the shoulder lines for the tails, repeat the process from the inside face of the pins. If you have made one of my paring jigs remember that it's only necessary to mark shoulder lines on the inside faces of components bearing tails, as all the paring is done from the inside! Next, mark out the shoulder line for the depth of the pins. The cutting gauge needs to be reset to slightly less than the thickness of the tail-bearing components. This reduction in the dimension is absolutely necessary for small objects and for drawer-making. After the joint has been glued-up, without the need for slotted cramping blocks, the extra thickness can be easily removed with a smoothing plane. This is not the case for a large carcass, so for these allow the pins to project slightly and clean them off later, without having to remove a lot of material from the larger surface areas of the carcass.
Pin profile sawingThis is probably the most demanding aspect of cutting lap dovetail joints. The first requirement is to emphasise the end-grain profile of the pins. This is best done with the same back saw used to cut the tails. As the pins are constrained by the lap, the only way to saw the profile is to start on the nearest corner and saw down at an angle of 45 degrees until you reach the two cutting gauge lines defining their limits. Don't be tempted to saw away from the line and pare back to it later. A well-sharpened and set saw, directed so that it halves the scalpel line, will produce a clean, accurate cut. If you put off this challenge and rely on a chisel to cut back to the line, you run the risk of the chisel following the grain.
Perhaps through a lack of confidence in your ability to saw accurately, the likelihood is that you will never attempt to use it directly. In general, if it's necessary to cut a profile with a chisel, avoid cutting into the end-grain as the wedge action can cause the chisel to follow the direction of least resistance, so drifting off line. This is less likely to happen if the cut is made across the grain.
Chopping outBefore the router was introduced, the waste between the pins had to be removed with a chisel and a mallet, a crude but effective technique. If this is your only option then the sequence will be dictated by the nature of the wood. Hold the component down with a cramp, on a part of the bench top that will withstand the impact of a mallet blow. The end-grain of the component needs to be level with the edge of the bench top.
Choose a chisel slightly narrower than the slot being produced and make the first cut across the grain a little way in from the end-grain. The depth of this first cut should be equal to the distance in from the end. It's likely that because this first cut is close to the end that the wedge shape of the chisel will cause the waste material to simply split off. If not, pare in freehand from the end-grain to remove the waste. Repeat this process until the slot being produced reaches the limits of the angled sawcuts that produced the profile of the pins. To proceed beyond this point, you will need to cut down across the grain in the corners with a chisel narrow enough to be held at the angle of the pins.
WasteRemove the rest of the waste in a series of manageable cuts until the cutting gauge lines are reached. I made myself a pair of left- and right-handed skew chisels by regrinding the cutting edges to angles slightly less than the internal angle in order to produce clean inside corners. The last end-grain cut per slot is the most critical.
The chisel can be located positively in the cutting gauge line so there is little chance of it slipping. Don't try and remove too much material for this cut. I aim to leave about 1mm (1/32in) for this last cut, in the knowledge that I can push the chisel forward and counteract any tendency for the cutting edge to follow the grain, should there
be any tendency for it to do so. Finally, I hold the component upright in the vice and pare back to the shoulder line. Nowadays I carry out this stage with my paring jig.
The process of chopping out the waste between the pins is noisy and tedious, but can be speeded up with a router. The bulk of the waste can be removed by plunging a cutter in a series of cuts into each slot, controlling the distance from the end with a router fence. Set the fence to leave 1mm (1/32in), which can be pared back later with a chisel. The depth of plunge also needs to be set to leave enough to be cleaned back with a chisel. The internal corners can be cleaned up either with a narrow chisel or with skew chisels.
AssemblyIf all the cutting and removal of waste has been carried out by working carefully to the marked lines, the joint should fit together without any tweaking or fiddling. However, bearing in mind that at least four components will have to be assembled when gluing up - possibly more if the construction is a cabinet that may have partitions and shelves - anything that can be done to make this stage easier is worth considering. With mortise-and-tenon joints, the ends of the tenons can be chamfered. Although it is not possible to do this to through dovetails, it is possible to chamfer the inside edges of the tails of lap dovetail joints as these will not show after assembly. Bear in mind that the inner surfaces need to be cleaned up and treated with a couple of coats of finish before the final assembly. As with through dovetails, I only apply glue to all the mating surfaces of the tails when gluing up to reduce the open assembly time of what is often a stressful stage in the construction of carcass pieces.
Aesthetics and context
A final word on the appropriate application of the lap dovetail to a piece of furniture. When designing a piece, give some thought to the visual contribution that joints make to its aesthetics.
In my opinion, the lap dovetail joint is often used out of context. The very fact that it is available in the extensive arsenal of joints we have inherited and developed over the years encourages some craftsmen to use it to create an impact, often for the wrong reasons! The fact that the dovetails can only be seen from one face suggests that it is the other side that should be emphasised. This is perfectly demonstrated by its application, for example, to drawer fronts.