Inlay adds lots of time to the oud's construction process. For some of the instruments I've made, I would guess that the inlay aspect alone takes as much time as the rest of the entire instrument. I also won't use an inlay technique that either yields an unsatisfactory result, or that I'm not proficient at. So I'm always looking for ways to speed up the process while maintaining or increasing the quality level.
A few years ago I read an article from an old marquetry text describing the technique known as "double bevel marquetry". This is a well-known technique among marqueterians where the two pieces of inlay material are sandwiched together and the pieces are cut simultaneously at a slight angle. The result are two wedge-shaped pieces of material. The piece from the bottom layer (the wider wedge) raises up into the cavity formed by removing the top piece (the smaller wedge). The kerf (saw cut) made by the blade, combined with the tilt angle create the perfect amount of "slop" between the two pieces and they fit together flawlessly. Technically this is not inlay, the results however are the same. This is a very quick method since it completely eliminates the need to scribe and mortise the background material.
The "inlay" material, in this case, bone, is tack glued to the background material (here, rosewood) only at the ends, so once the cuts are made, the pieces are easily removed.
In this sample the background piece is pushed up to the level of the inlay, and because it is tapered, it wedges in position, flush with the inlay material (the bone). This is where tweaking the angle comes into play. Once the perfect angle is found for the material thickness and blade kerf, the background piece will seat perfectly flush with inlay material, leaving very little levelling necessary once the design in finished.
I attach the pattern with spray adhesive directly to the inlay material. Here I've already cut out the inside portion of this design and pushed the background up to the level of the inlay. The fit is perfect.
This is a view of the backside of the design with both background and inlay pieces removed. The taper can clearly be seen in the small curly part of the cavity.
The background piece is inserted into the cavity, and bottoms out flush with the face of the inlay piece.
I use a high-end scroll saw on its slowest setting to do the cutting. I use jeweler's saw blades. I also use a magnifying lamp which greatly improves my ability to cut exactly to the line.
The inlay is cut using the same bevel angle, but the direction of cut is the opposite as the background (interior) pieces, so the instead of the background rising up to fit within the inlay (for the interior pieces) the inlay now drops down to fit within the background.
I always cut out the interior pieces first (in this example, it's just the one fleur shape at bottom right), then fit them into position without glue (the wedging action keeps them in place firmly) while cutting the inlay portion. This keeps the inlay portion strong while cutting.
The paper pattern is removed and the waste pieces are discarded or saved if they are large enough for future use
The background piece (or pieces) is glued into the inlay piece.
Then the inlay piece is glued into the background.
A light press and it will seat flush.
If the angle was right, the inlay will seat almost perfectly flush. Since all the cutting of both layers was done simultaneously, the grain matches perfectly. And if the cutting was done with smooth continuous motion, the fit will be perfect, with no gaps between the inlay and the background. I can even deviate from the pattern if I like during the cutting with no adverse effects to the fit. There is however one gap that must be filled, and that is the hole drilled to insert the saw. With the fine jeweler's blades I use, this is not an issue, since the holes are so tiny. I try to find a small curved corner of the design to drill this hole to further disguise it. The fine curved lines in the leaf portion of this design are simply saw kerfs only, will be filled with glue, and take on a dark appearance once the final piece is installed in the instrument.
The piece is then cut to final shape and inlaid into the instrument in the typical manner. Scribing around and wasting away the mortise with a router or router plane.
The finished piece.