The Luthier: Part VII- Skeleton III

Last time I finished the bracing on the back by ripping, planing, and carving thin strips that add much needed strength to the joint and rigidity to its support of the sides. For the top, the process was much the same, although the end result and reason in application is very different. While there is a deal of acoustic design that depends on the shape, number, and position of the braces, I have heard that the instrument gets its tone from the top.

A long while ago I jointed the pieces for the sides and top together, but the back pieces were considerably harder than the tops, so these needed a bit of attention after sitting around for so long. A small bit at the top corner separated outwards, whether from the initial planing or something else, which caused the two edges to misalign. To fix this, I returned the pieces to the bench and paired them together for a light pass with the jointer plane.

To ensure that the two pieces are indeed straight and the edges true to the face of the boards, I turn one of the sides around and clamped them back together. Above you can see that there is about ,25mm difference in height on one end. By having the edges switched around like this, the height disparity will be matched but opposite on the other end, and when the two are perfectly in line again, the edges are straight.

Clamping the long joint together was very difficult, as I do not have any clamps long enough to span the width of the two together. Eventually, I worked it out...

Between the two there was the slightest amount of difference in thickness on one end, which after planing disappeared to leave an invisible seam.

For bringing the surface absolutely smooth, I again used a card scraper, although shortly after this I acquired a scraper plane which I would rather have used. There are a few spots here, although far worse in areas of the neck wood, that make use of an ordinary plane rather difficult. Card scrapers are great, but do not as easily preserve the flatness of the surface as when supported by the sole of a plane.

Having no affiliation with the company, I treat the soles of my planes and surfaces of anything that might rust with Renaissance Wax. I believe I have mentioned it before here somewhere, but it is a micro crystalline wax that was developed for preservation of museum artefacts. On tools, it leaves a very slick finish that reduces friction at an incredible proportion and (as far as I have seen) does not interfere at all with any subsequent finishing. And of course, it helps prevent rust.

With the top assembled, I returned to the final kerfing on the sides. Same as before, I measured bits of the kerfing and matched the angle with the corner blocks.

At this time I also went back for a moment to the inner corner near the heel. I had left the block there a bit oversized and angular, which could use some reduction and shaping. Although not the most significant change, I gradually turned the harsh rectangle into a sort of rounded trapezoid with a block plane. This kept the thickness of the block relatively consistent as it followed the corner.

Serving as a form to hold the shape, the back allowed me to further reinforce the curves and take out a slight bit more of the asymmetric relaxation after steam bending the sides.

A wee bit of the kerfing slipped out of position while the glue dried, which was taken care of with a chisel. Fortunately, the grain direction of the kerfing made this trivial.

Same as before, I scribbled some lines along the entire perimeter of the sides so that I could accurately gage that it was truly flat. This was done in part when I sanded the back, but with the addition of the kerfing, it is important to bring the kerfing perfectly coplanar to the edge of the sides.

Handy spray glue again turned the floor into a sanding block.

A quick test fit shows that the sides are in good shape to align with the top.

Since I used the lot of my cut bracing for the back, I had to make a few new ones. This done shortly after ripping a bow stave out of 4/4 Ipe, and to the saw this spruce felt like cutting air rather than wood. Even still, it was nice to return to the plane for a spell.

Once thicknessed and cut to length, I brought out the same curve template as used on the back. One side, the one used above, leaves a bit of height left to be accepted by a notch cut into the kerfing. The other blends a brace to the surface of the top without having a hard edge.

As before, I measured the centreline and cut notches at the depth of the ends of the braces. This step marks a point of contention in how acoustic instruments are made. Archtop guitars are, as the name implies, curved on the top. That form is also seen in violins, lutes, mandolins, and everything else that has an acoustic body. Even 'flat top' guitars are not actually flat, but employ a shallow dishing to their tops for added strength and rigidity to withstand the tension of strings. I do not, however, have a dished form in which to brace the top, making it extremely difficult to do evenly and accurately. In spite of having thought of a few crude solutions to this problem, I have chosen to abstain from trying to accomplish this. Instead, I will be making the top truly flat and compensating for string tension in other ways that I will explain when relevant.

Once more, I used two small drops of glue on the braces to temporarily attach them to the top, using clamps as guides for the positioning during pre assembly.

Traced, the braces can be pulled free, the positions perfectly transcribed to their respective places on the top. Those marks from the glue are taken down with a chisel to leave the surface ready for the actual permanent assembly.

Similarly, the two braces are jointed with the shooting board and checked for their straightness with the edge of the plane.

The usual collection of hammers makes its way back out in the form of weights for the glueing. Because there are only two braces here, I did them together to ensure that they did not pitch under the weights, supported by those two other boards perpendicular to them.

And there we have the first two braces. A brief note on their positioning. As I mentioned above, the top braces are where the tone is adjusted. These two here are for rigidity and strength across the joint. On the bottom, I brought it to the apex of that curve in the sides traced above. This one provides, as I see it, the most amount of strength with minimal interference to the other tonal braces. On top, I measured where the sound hole will be cut and adjusted to support the rosette.

In order to position the other tone braces, I found the centre of the top. This was, although very close, NOT the centre of the braces.

In order to preserve space for the rosette, I centred the ebony ring and traced it. Around this, there remainder of the braces will be positioned.

First, I needed more braces, so it was back to ripping down the last remnants of that Englemann Spruce plank. Only after the entirety of this assembly was concluded did I realize that I had another plank for bracing, but of Spanish Cedar. That would have, regrettably, produced warmer and richer tones than this.

Before approaching the other braces, I decided to use the space available to reduce the height of the existing lower brace and round over some of the corners.

Long, sweeping curves seemed appropriate. Whether or not this has any distinct effect on the acoustics is yet to be determined.

Next up is the cross brace. Notching the two components at equal but opposite angles to meet at the desired position was an interesting challenge that, for the sake of the limited brace wood I had left, worked out alright. That same scalloping template was used for the ends of these, although it was later adjusted with chisels.

Along with the centre notches, I also had to cut slots in the lower brace for the cross to fit. I could alternatively have cut into the cross brace, but the preservation of the flat edge seemed preferable to attempting to cut an angled, round hole in its length in the hope that it fit perfectly the first time.

Test fit, lying flat.

Now, reducing the excessive amount of material in the cross brace took a bit of drawing and redrawing until I found a curve that looked satisfactory. On the shorter end, there were shorter curves, on the longer end, longer curves.

Based on nothing more than a guess and some visual inspection of classic bracing forms, I devised the pattern I used. It might be great, could quite possibly be awful, but will certainly add strength to the otherwise fragile top.

Looking rather skeletal, the cross brace is fitted, so I turned my attention to the radial braces.

Playing off of a fanlike symmetry, I cut and placed two braces to fit between the bottom brace and upper limb of the cross brace.

And then repeated the pattern on the other side. Another brace will eventually be fitted on the top quadrant of the cross brace just below the sound hole, but I will wait until the hole is actually cut and the rosette inlaid before determining where is appropriate.

The classic upended bag of hammers technique holds the braces firmly down while the glue dries. As bad as it looks, there is a decently even distribution of weight on each of the members and along their entire length.

Finally, a third radial brace is added to the bottom side. This is to compensate for the asymmetry in the sides, there being that swept corner that you can faintly see the tracing of in the bottom right of the above picture.

Once the angle was set, I marked the length by tracing over the angle near the back brace and the meeting with the sides on the other.

With that cut, I was able to notch out a slot in the cross brace for it to slide into. A bit of carving to bring the scalloped end smoothly into the back brace, and it began to look like it belonged.

Concurrently, I also began to shave down the corners on all the braces. This is, as best I understand, something that can also be done before they are attached, but even that slight introduction of stress on the wood can bend it into curves and shapes that I want nothing to do with.

A slight bit of sanding smooths everything out.

All nice and sanded, smooth as can be.

Now that all the pieces are fixed in place, I was able to fit the sides down atop it and mark where the radial braces needed to be trimmed. They could have been notched into the kerfing like the rest, but there was no real need and this was far easier.

And there sits the internal skeleton of the top.

Next up will be figuring how to construct and attach the neck to the body.

The Luthier: Part VI- Skeleton II

Where I last left off, the back panel was jointed and joined, the sides affixed with kerfing, and the lot sanded plane. Now it is time to attach the braces to the back and fit them in the kerfing in a predictable and repeatable way that can also serve as a means to keep the sides in proper square alignment.

To do this, I had to figure a way to mark the sides in a way that would be perfectly perpendicular to the neck. This involved a great deal of trial and error, moving clamps and weights around until I was satisfied with the positioning. A jointed board serves as a line connecting the heel and neck blocks, and with the help of a square and second jointed board, I was able to align points on the asymmetric sides.

The board parallel with the neck was marked in two places where roughly the sound hole would be placed. This is to prevent placing a brace directly under that opening which will look down from underneath the strings. While there is (as far as I know) minimal impact as far as acoustic design is concerned, I did not want to have a brace positioned such that it was somewhere beneath the sound hole and thus awkwardly visible from the top.

With the shooting board, I squared off the ends of the braces I cut in the last post. Although only a small bit of them will be in contact with the kerfing, having it square to the length of the brace makes this process much more predictable.

Evenly spacing out four long braces at intervals on the sides, I marked a line for the bottom edge of the brace, then traced the brace itself for the second line. Between these two I cut a notch that the brace wood fits snugly into. Because the braces were resawn by hand, they are not all the exact same width, and there is also a slight variation from one end to the other of a particular brace. Thus, each individual notch needed to be carefully marked at the precise location along its length. 

Scalloping the ends of the braces allows for some acoustic design that I invented based on seeing other instrument interiors. For these, I made 6cm curves to a depth that extended to a position that makes sense with the bevel cut in the kerfing. The template, pictured above, became the master pattern with which all the scalloping was cut.

Here you can see the braces in place. The top of each brace, as pictured above, is exactly flush with the kerfing. As you might have noticed, I chose to make the braces based off of the configuration of the sides rather than first attaching them to the back. The reason for this is this- Positioning the ends of the braces is much easier and more accurate with access to the sides throughout the process, and the sides themselves are held in their proper position by weights. First attaching them to the back would have introduced a host of complications that would have resulted in a nightmare of calculations and imprecision to the fit, spacing, and pairing of the two parts.

Now to actually attach the braces to the back, I used a trick I picked up somewhere along the way. By applying a very slight amount of glue to the ends of each brace, they will be temporarily joined to the back and lifted out of the sides without moving, then permanently joined later.

"Careful application of weight" at the glue points ensures full contact. One of the benefits of being a blacksmith is that I have an excess of hammers to use as weights...

An hour later the glue has cured enough to hold the braces through extraction. This position is the only position that will now fit with the sides. Fortunately, it is now easy to trace the edges of the braces onto the rosewood. 

Although it is difficult to see, I scribed lines around the entire parameter of the braces, making extra long lines at the corners and ends. Then, all it took was a little shearing and the drops of glue failed, leaving me a perfect outline of where to permanently glue the braces. I should note that I marked each brace as to the number and orientation before removal.

Re jointing the braces and using a chisel to remove the glue from the back, the surfaces are now ready for their final fit.

While it might seem as though this is all needless effort, and that I could have just glued the braces in their final place to begin with instead of only using a small drop of glue, there is actually a reason this is all necessary. Unsupported in the centre, and supported only by a small notch on the ends, I would not be able to apply sufficient weight or clamping pressure while the glue dried to have a secure, reliable joint. Instead, I need full support underneath the brace or back.

Gluing these back into position was tricky and required a bit of forethought. Because the members are so thin, it was entirely likely that under the weight of various hammers and bits of scrap metal, the height could skew to one side or the other, or in the worst case, twist. To counter this, I glued the bottom most brace first, using the unsecured others as support blocks spread out on both sides as pictured above. The one glued (with the hammers over it) then had the lateral support it needed to dry square. 

Second, I did the forward most brace. Now, the bottom one which has already cured, acts as a support, with the other two unglued acting as support on the other side. 

With the first and fourth glued, the middle two would now both have secure lateral support. Through a combination of scraps, hammers, and planes, the middle two were weighted down the lot left to cure overnight.

Success! These braces, paired with the notches in the kerfing, hold the sides squarely without any further use of clamps. What is more, that sideways force of the sides holds it to the back securely enough to handle while dealing with the rest of the interior skeleton.

Although the braces are solidly attached, there is a bit of cleaning that needs to be done. Excess glue on the surface that could not be cleaned off when wet will be removed as well as carving the final adjustments to the shape.

Sharp tools are the greatest.

Four braces would have done the trick when paired with a back joint. However, I am no longer able to see where the actual seam between the two pieces actually lies, and I suspect that it is slightly off of the mandola's true centreline. As an added bit of security, I decided to add four more smaller braces, using the ends of the longer ones I already cut.

With that same scalloping template, I copied the curve, but adjusted it slightly so the ends of the curve converged with the long edge. This is because it will not register with the kerfing and needs to blend into the surface of the rosewood.

Progressively scaling down in both length and height, these braces are proportional to the width of the sides at their respective placement.

A bit of scrap cut down to size is used to centre the braces and keep them parallel to the attached ones. The line on it, paired with the centre of the body, helps ensure that the small braces are also centred from side to side.

Once more, various weights hold down the braces while the glue sets. After curing, they receive the same treatment of chisels to remove excess glue.

Had I been able to find the centre joint of the rosewood back and use it as the positioning line for the braces (which was not possible because of the asymmetric shape and size of the wood), I would have added the back jointing after only fitting four braces. The process remains the same, however.

With a chisel, I split off small sections slightly wider than the distance between the braces. Due to the angle, it was difficult to measure the precise length needed, and so I instead used the shooting board to work them down until the fit was tight but not overly so.

One down, eight to go.

Finding weights that fit between the braces was difficult, so I could only glue them two at a time.

Lastly for this post, I worked down the corners of the braces to make them round and smooth. As I understand it, hard edges are bad for acoustics. If not, they are bad for aesthetics, so they still needed to go. 

In the debate of bevel down v. bevel up, I have found that the only difference is made by the geometry of what you are cutting. For the long, straight edges, I used the bevel up and the back of the chisel to register along the cut, but for the scallops, bevel down so I was able to cut cleanly along the tight radius. To each their own.

And that's it. Next time, I'll be working the skeleton of the top, which is surprisingly very different in design and purpose.