Francesco Cremonini explains how you can preserve the integrity and customise the appearance of solid wood in one fell swoop.
Anger and delight: these are often the two prominent emotions when we use solid wood to make a top. On the one hand, working with solid wood involves longer processing times and doubts about stability (you have to work out which construction strategies to use so that the surface does not warp or crack over time), but on the other hand there is the solidity and unique appearance of the final result. In fact, a top made of a few boards, even more so if with only one board, always has its own strong personality.
Variations in the grain and colour, inclusions of bark, small knots and cracks are characteristics that can be used to give character to the piece, making it more interesting and less monotonous to the eye. And then there are all the detailed processes, which can only be carried out on solid wood and further increase its interest. Among these is the addition of butterfly bits, an operation that can be performed whenever the wood has a crack.
The Wedge
The wedge is a wooden strip of variable size but with the unmistakable shape of two opposing dovetails. Thanks to this particular geometry, when it is inserted across a crack, it creates a mechanical block that prevents the edges from separating further. It can be more or less robust depending on the proportions given to it, the choice of wood and the direction of its grain.
The ideal is to make the butterfly from a dense and compact wood and from a strip whose grain is arranged along its major axis or at a slight angle. The thickness must be proportionate to that of the top (just under half or more, adding a couple of millimetres to the measurement for finishing), the length is dependent on the width of the crack. The angle of the tails matters less, more than anything else you choose it based on the aesthetics you want to give it: more playful by taking it to the extreme, more refined by keeping it to a minimum (in general, angles between 8° and 40° are used). The neck, i.e. the central point where the two tails converge, must maintain a certain consistency, so it is generally advisable not to go below one- quarter of the size of the short sides of the tails.
The most practical method for creating the dovetail pieces is with a bandsaw. My advice is to start cutting the starting strip with a goniometer (an angle jig). It is of course possible to work without it; the alternative is to cut by eye with reference to marks. Usually, a wood with a strong contrast compared to that of the top is chosen to make the inlay visible once the work is finished. If you’re cutting the pieces by eye, you’ll need to finish the shape with a good dose of patience, using files and chisels. If you want to use a number of identical pieces, it is better to start from a thicker strip, cut the shape and then cut it along the thickness. In this way the uniformity of the inlays is guaranteed.
Once all the pieces have been cut and finished, they are arranged along the split; there is no universal rule on distribution, but to obtain an effect considered pleasant by most it’s necessary to scale both the separation between one inlay and another and their length and width measurements, in accordance with the shape of the crack. If you don’t deviate too much from the 3-2-1 rule (where the numbers establish the proportion between the measurements, of the pieces) and you go up the crack, decreasing the distance between one bit and the other, you remain to be sure, you can always experiment differently.
In the example shown in this article you can see I cheated a little by limiting myself to scaling only the length of the inlays, leaving their width the same. In this way you save yourself from preparing all the rods one by one: you can make them all the same from the start and then shorten them at the ends with a cut by hand.
The Inlay
With the pieces in position on the surface, the inlay work is prepared by transferring their outline with a pencil or marker and a drawing knife. The marker is preferable if you do most of the cutting with a router, the knife groove will provide the reference for the chisels in the final cutting phase.
If you don’t have access to a router that allows a good view of the shape, the initial cutting can be carried out with a drill and Forstner bits; in this case it is necessary to mark the tip in order to have a clear reference to stop the depth and to obtain a surface as level as possible. This process could be difficult if the bits are small or very narrow and require you to work with equally small chisels resting on their bevel.
To produce a precise fit for aesthetic reasons, my advice is to proceed with cutting while leaving a minimum margin of 1mm with respect to the marks and finish off with chisels. When working near the vertices at the meeting point of the two tails, use thin chisels in those places, so as not to dent or scratch the opposite or adjacent profile with the bevel of the chisel.
Gluing and Finishing
The connecting pieces cannot be fully tested in their seats; if this were done it would no longer be possible to remove them to do the subsequent gluing. What you can do is try inserting them a few millimetres or until you are reasonably certain of removing them. Once the fit is correct, any adjustments must be made to the connecting pieces, proceeding delicately with the file or a well-sharpened knife. At some point you will still need to trust in the work done and move on to the final assembly.
After applying the glue in the seats and along the pieces with a brush, tap the inlays into the recesses. If you are working with thin pieces, it is essential to distribute the pressure of the hammer along the entire inlay, placing a block of wood or other material in between so as not to cause the bit to break in the centre.
Once the glue has hardened, you can move on to finishing. The excess part of the connecting pieces can be quickly removed with a gouge or a plane, completing the finishing work with the plane or a scraper. Sandpaper can be used, but only if the wood of the inlay and that of the top have similar characteristics in terms of hardness; otherwise it is better to avoid it.
