Tag Archives: guitar side bending

Guitar #30/3

Assembling the box

After days spent reorganizing the shop, which had become next to impossible to move in, I am back at it. Yesterday I bent the sides ( also called ribs) and assembled them to the end blocks. The end blocks had been made some time ago. These determine the rib depths and the correct angles at the four intersections where the ribs join the top and back (neck and tail). The first step was to trim the blanks to just a bit wider than the maximum finished width (depth of the body). I trimmed these to 104mm – the maximum depth being 100mm. That left me room to trim after assembling the ribs and blocks before attaching the plates. The next step was to mark each rib with yellow pencil at 25mm intervals. This is a great aid in checking progress against the pattern, which is also marked at 25mm along the outline. While I did this I had the bending iron heating up.

one rib trimmed and marked, and soaked down for bending

one rib trimmed and marked, and soaked down for bending

body half pattern with 25mm markings

body half pattern with 25mm markings

bending iron hot and ready to go

bending iron hot and ready to go

Bending walnut is not very difficult. You keep it wet and hold in one place for a few seconds, pressing gently, then move a centimeter and press some more. The hot metal pipe dries the wood in seconds and it sizzles when it contacts wet wood. No sizzle means the wood is dry and has to be wet again. I start at the waist, then do the upper bout, then the lower. It takes constant small adjustments but with patience it can be made to follow the pattern shape quite closely. Once it is close enough it gets clamped into the mold, where it then dries and takes the shape in which it is clamped.

rib clamped to mold

rib clamped to mold

Once both ribs were bent and dried in their final shape I trimmed the ends and assembled the pair of ribs into the mold, checking that the joints met tightly and along the centre line of the guitar. Then I glued in the end blocks. The easiest way to do this is without the mold, just one end at a time, clamping the ribs onto the block. Once both blocks are done then the assembly goes back into the mold and is clamped with spreaders.

ribs in mold with spreaders

ribs in mold with spreaders

The spreaders ensure that the ribs are in the right position for attaching the plates. Just a bit of pressure is enough to keep the ribs pressed to the sides and ends. I assembled the back plate first this time. I wanted to try a different method of gluing in the linings. First I glued the back to the blocks, neck and tail. Then I put the whole thing on its back and clamped the back plate here and there to the ribs, so it was tight. I then glued in the kerfed lining strips, rubbing them to increase the initial tack, and holding them pressed into place for a short time. In a few places I resorted to putting pressure with a re-purposed chopstick and a clamp. This saved me doing two steps, where the linings are first glued to the rib, after which the plate is then glued down onto the linings. This way I was also able to avoid having to cut pockets in the linings for the ends of the braces. Instead I simply glued several teeth from the kerfed strip at each brace. This will be sufficient to hold the brace end tight. Note here that I first trimmed and fitted the back plate to the ribs before this procedure.

gluing the linings to attach the back plate

gluing the linings to attach the back plate

After the back plate was done and the glue set, I then fit the top plate to the ribs. Then I glued the linings for the top in place. Since my main top x-braces are tapered to zero they require no pockets in the linings, except for two which hold the upper face brace. These will be cut before the top is glued on.

box ready for the top plate final fitting

box ready for the top plate final fitting

 

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Guitar#29/6

top with bracing complete

top with bracing complete

My top bracing for this guitar is arched. There are only 3 major braces. The transverse one at the top is for strength mainly, so it is 19mm high. It has a hole in it for accessing the truss rod adjustment. The main X-brace is smaller, and tapered down to zero at the ends. This is where the sound is produced, so it has to be a balance between strength and flexibility. The three pieces around the sound hole are off cuts from the top, thin pieces to strengthen the area of the hole against possible breaks.

I also did some spectrum analysis¬† of the plates, using the computer. I hold up a plate to a microphone and give it a dozen taps with a felt covered hammer, similar to a piano hammer. I use a free program called Audacity for this. It has a function to analyze the spectrum of a recording. This shows me a graph (logarithmic) which gives an idea of the peaks and relative strengths of various frequencies. There’s a lot of unsubstantiated hokum around tap tuning, but I do this more out of curiousity than anything. Then I export the results to a text file, import this to Excel, and then generate a graph. This will show me the dynamic range. I then plot a trendline which gives a better overview of the response. But after all is said and done I shave the braces until it sounds good, and is still stiff enough. Better to stop before it’s too late. The top has to support the strings or it’s all over, structurally speaking.

spectrum analysis of the top

spectrum analysis of the top

back bracing complete

back bracing complete

I’ve also done the back. I shaved the braces to drop the fundamental frequency as low as I dared. The back however, unlike the top, doesn’t carry any downward pressure, so the braces can be lighter. Hardwoods are always lower in tone than softwoods, simply due to their density. This back plate is several whole tones lower than the top plate. The bracing design is identical to the top, same arching, same locations. The back braces do not taper to zero however. They are let into the linings at the perimeter. The centre strip is merely a precaution to reinforce the centre joint. Many guitars do not have this, but it doesn’t affect the sound so I put it in.

side bending setup - aluminum pipe with bbq lighter inside

side bending setup – aluminum pipe with bbq lighter inside

Once the plates were done I bent the sides (ribs). I made a template from thin MDF, and marked off 25mm hash marks on it. Then I marked the sides with a yellow pencil at 25mm spacings, and labelled them. This helps immensely with bending. I have the bending pipe clamped in the vise and an old BBQ lighter which has been squeezed when hot, shoved inside the pipe. This is controlled by a light dimmer. Full power would be much too hot. I run it around 50-60%. The bending is not hard once you get a feel for the wood. It takes patience, however. This guitar is made of Black Walnut, which bends very well. Some wood is horrible to bend, like Paduak. Once each side is bent to the profile of the template within 2-3mm tolerance, it gets clamped into one half of the mold. The mold splits in two for this purpose. The sides are bent wet, and the steam generated by contact with the pipe softens the wood so it bends. I use a spray bottle to keep the wood wet. It dries out very fast once in contact with the pipe.

side piece clamped in mold until it dries

side piece clamped in mold until it dries

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