Acoustic Guitar – Completing the Sound-Box

Completing or closing the sound-box is essentially a simple affaire. Glue the top and back to the sides and bingo you have the sound-box. In practice there are a lot of things that make the process more demanding than building a simple box. The main thing to remember is that the back is not flat and has curves along it’s width and length. This means that the back meets the sides and kerfing not at 90 deg. but more like 95 deg. and this angle decreases to about 91.5 deg. for the join between the top and the sides. The Stew-Mac instructions call for the construction of a sanding board to accomplish this task.

This is my sanding board, it has coarse sand paper on one end and a wedge on the other. Both sides are used one with the 5 deg. wedge for sanding the rims for the back and the other side has a 1.5 deg. wedge (or as close as i could get to it) for sanding the front rims.

This sanding board pivots around the wedge end and sands the kurfing flush with the sides at the correct angle. The wedge end is moved around oftem to avoid uneven sanding.  

It does not take too long to get the kurfing and sides sanded down and using the sanding board is quite satisfying as it’s inherent weight seems to do most of the work for you.

As you can see from the photo above the for ends of the x-brace and the brace towards the front all over hang the sound-boards surface. The same thing goes for the four back braces also.

Notches must be cut into the sides and kerfing to accept the overhanging braces. This creates a very strong structural bond between the top, back and sides. The process for cutting the notches is quite simple. The back or top is placed on top of the sides and the position of all the braces marked onto the sides in pencil. The notches are cut slightly under-size using a small saw and then enlarged using a file to ensure a snug fit. 

Before the top and back can be glued to the sides I check that the angle of the neck and tail blocks are exactly correct. Here I use a small spruce wedge between the interior cardboard mold to ease the top of the tail-block out to the correct angle of 90 deg. 

This is a big moment! any thing that does not fit correctly now will introduce stresses into the structure and may cause damage later on. Being careful to apply a constant pressure on the whole circumference of the back I use a combination of lightly applied cam-clamps and very long elastic bands wrapped around the body of the guitar to apply clamping pressure while the glue dried.

This is what things looked like after drying overnight. You can see that the back still overhangs the sides, this excess will be trimmed of later using a laminate trimmer. I was pleased with how the first major construction had gone and I must admit took great pleasure from just looking at for an hour or so after getting it out of the clamps. After that i glued small spruce side reinforcement strips every 20cm or so around the interior of the sides. The strips run vertically between the upper and lower kurfing and are supposed to stop the side splitting should they dry out once the guitar is finished.

Next the top is glued onto the sides, a really momentous occasion this. The guitar starts to resemble a guitar for the fist time. It’s slightly sad to have to say goodbye to all the work that has gone into the bracing, as (hopefully) this will never again see the light of day once the box is closed.

These two photos show the closed box after trimming the overhanging braces, sides, and top. I used a laminate trimmer for this task and was quite nervous about it as it’s a fairly powerful tool, somewhat like a router which could very quickly ruin all the hard work done so far. One slip and goodbye top, sides, or back. The top and back must be trimmed in a special pattern using 4 climb-cut’s where the bit of the laminate trimmer is always rotating towards the wood-grain. This minimises the possibility of wood being torn out of the top or back as it moves around. Luckily things went smoothly and there was no tear-out at all. Again I allowed myself some time to enjoy the fruits of my labour before embarking on the next step which i was absolutely dreading.

These illustrations from the Stu-Mac instructions show the relative positions of the binding and trim for the sides and top of the guitar body. The binding is both decorative and functional as it protects the soft edges of the body from bumps and dents. The bindings fit into a channel routed into the sides and top of the body. The kit came with long strips of plastic binding that bend easily to the contours of the body but I didn’t like the idea of having plastic binding so decided to opt for wooden bindings. I thought real wood would look better but because the wooden binding is less flexible than it’s plastic counter-part it needs to be bent into shape before it can be fitted.

To compliment the colours of my side and top woods I ordered lengths of curly maple binding and a beautiful herringbone purfling for the top trim. The purfling is made up of a repeated pattern of very tiny wooden blocks arranged into a sandwich and then sliced into lengths. 

Before the binding can be fitted channels have to be routed along the edge of the guitars body. The channel has to stay parallel to the top or back and it’s depth must remain consistent. For me it’s one of the two jobs I was most afraid of messing up. I think part of my problem was that I was not sure of the best way to use the laminate-trimmer to create constant channels. And I was very aware that when using power-tools things have a tendency to go wrong much faster than when you are using hand tools. I constricted a guide for the laminate-trimmer (shown above) which followed the sides of the guitar. As long as I made sure that the bare wood part of the guide was always in contact with the sides of the guitar I knew that the channels would at leas be parallel to the sides. The depth of the cut was attained by trial and error on a scrap block of pine. 

It took me several days to summon the courage to start routing the binding channels. During this time the weather became colder and colder and when I was finally ready to begin I woke to find snow on the ground. Routing cannot be done inside the house as the bit rotates very quickly and throws of dust and debris. Undeterred by the awful weather and sure that if I didn’t do the binding today I might never do it I set up an umbrella above my workbench, took a deep breath and got going. I applied varnish to the edges I would be cutting to try and minimise chipping and tear-out.

The result from the laminate-trimmer was pretty good, much better than I was expecting. That said the channel was inconstant in a few places so I tidied up with a very sharp narrow chisel. You can clearly see the two channels for the top binding and trim in the photos above – they look like two steps between the top and the side. 

Unfortunately routing the channels revealed a problem with some of the the back braces. As you can see here I left the back brace too high and the cut I made in the side to accommodate it goes above the binding channel when it should be below the level of the binding.

After consulting the fine people at luthiersforum.com I cut small blocks of similarly coloured wood from scrap and glued them into the holes. The photo shows the block glued in position before I had filled it and reduced it to the level of the original binding channel. This fix worked quite well and you have to look really hard to see it on the finished instrument.

Next I make two cuts in a V shape in the back of the guitar. This is were the the decorative tail wedge will go. In this photo Im halfway through chiseling out material from between the cuts. I want to go down to the back block which is under the sides.

 

After getting the surface as smooth as possible I begin work on the tail wedge.

I wanted quite a decorative tail-wedge so I sandwiched two thin strips of maple between three strips of ebony. Binding matching that which I will use on the sides will go on the outside and will be mitered so they join up with the side biding.

This is how it looked after gluing, the overhanging parts will be trimmed to fit the binding.

 

 Before the bindings can be glued into position the must be bent into shape. Bending the wooden bindings without first softening them would break them so I used a bending-iron which I made from a length of steel tube and a butane torch. The Iron pictured here is not mine as I forgot to photograph it, this one belongs to the ‘Big Island Ukulele Guild’ who have instructions for making your own iron here. I soaked the binding in water for half an hour and then used a damp cloth placed over the hot iron to bend the bindings. The bending process involves heating parts of the bindings and rocking them over the hot iron and applying just enough pressure to bend them but not enough to snap them. The steam generated softens the wood enough for it to bend into shape, when the desired shape has been achieved the pice is allowed to cool at which point it retains it’s shape. It’s quite a difficult process to describe because there is so much judgment involved in getting it right. The temperature of the iron, nature of the wood and amount of steam and the pressure applied all must be monitored and continually adjusted. My bending went well apart from a slight crack that occurred because I had applied to much pressure where the wood was not supple enough. I left the crack and glued it when fitting to the body – it’s not visible in the finished instrument.

 

Here Luthier and instructor Robert O’Brien shows how to bend guitar sides by hand on a bending iron. He makes it look very easy in the video. This process is almost the same as for bending bindings though side bending takes longer as the sides are bigger than the bindings and have to absorb more heat before they bend. 

I test fit the bent binding and trim to make sure they fit the binding channels. I noticed during the dry run that the blue tape kept slipping. 

I glued the lower and upper bindings separately. Here the upper binding and trim are shown drying. It’s quite tricky to keep track of the binding and trim as they follow the curves of the body! I initially used tape to secure the bindings but there were some areas where I could see that the bindings were not snug so I used the bands again to apply more pressure. 

After removing the bands the bindings sit nicely in their channels, there are a couple of tiny gaps which can be filled later. The glue residue will be scraped of later and the height of the bindings brought down to the level of the top using a cabinet-scraper.

This detail of the tail-wedge shows how nicely the black/white/black strip goes from the tail-wedge to the side binding on the right of the image. It did not come off as well on the left and was impossible for me to fix as the gap is less than one millimeter wide.

After a considerable amount of carful scraping and sanding to remove glue residue and to smooth the sides I have a completed body – Lovely!

Now I’m ready to construct the >> neck and fingerboard.

 

 

 

1 Comment to Acoustic Guitar – Completing the Sound-Box

kevin
June 12, 2013

Great work !! Love this blog

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