Tag Archives: bamboo

The bamboo guitar Part 5 – laminating the top linings

OLYMPUS DIGITAL CAMERAThe top linings form the joint between the soundboard and the sides of the guitar. Generally guitar makers use kerfed linings like the one above. They’re easy to bend around the curves, and they act like a series of small reinforcing blocks to tie the top and sides together. They work well, and I use them for the back-to-side joint.

But I have a theory that solid top linings are better at transferring vibrational energy from the soundboard to the soundbox body. They also add greatly to the overall strength and structural rigidity of the guitar. So I laminate them out of thin strips of Australian oak, a wood that takes to heatbending well. In future I think I’ll use bamboo instead, because the trees the “oak” comes from, Eucalyptus regnans or Mountain Ash, are precious. They’re one of the tallest tree species in the world:

Eucalyptus_regnans_Sherbrooke

I cut the strips on my table saw from a blank that I have thicknessed to 18mm in my drum sander. I then thickness the strips to 2mm in the other dimension (actually, I stay in this dimension to do it) by feeding the cut faces through the drum sander. Using good old Titebond  glue, I laminate three 2mm thicknesses in a mould to end up with a pre-shaped lining of 18x6mm.

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To make it easier I prebend the strips using the side bender, otherwise they put up a hell of a fight.

Here’s what it looks like out of the mould:

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What I’m doing here is cutting notches to house the top of the side-splints that will reinforce the sides. The splints also add rigidity to the sides, and a bit of useful mass as well.

I then glue the linings onto the sides, after which I can saw the ends flush with the construction mould. After that I bolt the two sides of the mould together ready to take the top and tail blocks that unite the sides. Notice the very clever placement of baking paper to avoid gluing the guitar to the mould. Yes, we guitar makers are a cunning lot.

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The bamboo guitar Part 4 – bending the sides

I use a mould and heat blanket to bend my sides rather than try to do it freehand with a heated pipe. Lately I’ve been thinking about trying a temperature-controlled electric bending pipe instead, in the hope (or is it delusion?) that it might give me more control over the final curvature.

When people ask how I bend the sides, I tell them I use a bender. Here’s what I think flashes through their minds before they hit me for being a smart-arse:

bender-for-website

No, but seriously folks, people are often overly impressed by bent sides on a guitar. It was the thing that intimidated me the most when I started building, I know, but I’ve found that while there’s a very bountiful source of possible stupid mistakes available to choose from, it’s actually not that hard.

My side bender is a hollow mould on a base, with a waist clamp to draw the wood down into the concave bend and two clamps each at the head and tail to draw the side around the convex curves:

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The sides of the mould define the shape, and it’s useful to slightly exaggerate the curves because the bent wood always springs back a bit when it’s removed. Around the edge of the mould you can see the ends of the rods that go from from one side to the other to form the actual bending surface.

The clamps are made from threaded rod and wingnuts from the hardware shop. You can’t see the end clamps that pull the front and back downwards to put the whole piece under tension, but they’re just eyebolts with a wingnut on each that act on the undersurface of the mould base.

There is a thin metal sheet above and below the wood that make up a metal/wood/metal sandwich to stop the grain breaking around the top surface, which is under tension. I use aluminium, though springy stainless steel is better in some ways but very hard to work.

Here are the first two metal/bamboo layers of the sandwich, ready for the blanket and then the second metal sheet on top. I put baking paper in between and give it a squirt of water to help the bend:

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The heat is provided by a silicone rubber thermal blanket connected to a temperature controller. In the picture below you can see the thermocouple temperature sensor and the controller that allows heater to work safely and accurately.

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Here is what the bent side looked like after the first attempt at 180 degrees C, which is what the manufacturers recommend. You can see that the waist doesn’t conform to the mould shape at all well, so I had to re-bend it several times at higher temperatures before it came right, ending up at 230 degrees:

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Here is one bent side clamped into the building half-mould while still warm to allow it to keep the bend:

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The bamboo behaved really well during the bending, and showed no sign of delaminating. So far, so good.

In my scheme of things, the next job is to make up the laminated top linings and fit them.

A bamboo guitar Part 3 – bamboo compared to blackwood

In my last entry, I had just finished measuring the fundamental frequencies of my bamboo panels in three different vibrational modes:

  1. along-grain free beam (marimba) mode: 109.0 Hz
  2. across-grain marimba mode: 226.1 Hz
  3. twist (torsional) mode: 80.1 Hz

What I am aiming at is to compare the stiffness of the bamboo material with my usual blackwood, so I can feel smug that what I get from the bamboo guitar will be worthwhile acoustically as well as using a readily renewable resource. With a little help from Gore and Gilet, I can convert these frequencies into measurements of:

  1. stiffness (Young’s modulus) along the grain
  2. stiffness across the grain
  3. shear modulus, or stiffness in the twisting mode

And if I’ve done the same tests on a blackwood panel (and you can bet I have), I can begin to compare the properties of the two materials. So here are the figures for blackwood:

  1. along-grain marimba: 130.1Hz
  2. across-grain marimba: 182.4Hz
  3. twist mode: 80.1Hz

Before I can work out the stiffness moduli for the materials, I need to allow for the fact that the bamboo and blackwood panels were slightly different sizes as well as different in density.

The next step was to calculate the density of each panel (density is mass divided by volume, measured in kg/cubic metres). I weighed the panels and carefully measured their length, breadth and thickness, and came up with these results:

  1. density of bamboo: 490.3 kg/cubic m
  2. density of blackwood: 697.3 kg/cubic m

So blackwood is about one and a half times denser than bamboo.

Using three more equations from Gore and Gilet (Equations 4.5-2, 4.5-3, and 4.5-4, page 4-59) I get:

  1. stiffness along grain: bamboo 10.6 GPa; blackwood 19.8 GPa
  2. stiffness across grain: bamboo 1.4 GPa; blackwood 1.6 GPa
  3. shear (twist) modulus: bamboo 1.3 GPa; blackwood 2.5 GPa

To sum up (pay attention up the back!):

  1. blackwood is 1.9 times stiffer along the grain than blackwood
  2. blackwood is 1.1 times stiffer than bamboo across the grain
  3. blackwood is 1.9 times stiffer in twist than bamboo

I’m now very close to answering my question about what thickness to make my bamboo back panels to match the stiffness of my blackwood ones. Back to another Trevor Gore equation (4.5-7 on page 4-61), which will tell me how many times thicker the bamboo back must be to have the same stiffness as blackwood. After some nail-biting, hair-pulling writing of the expression into a spreadsheet (Jeez, Trevor!), the answer is:

1.3!!!!

Oh, alright. Settle down. The excitement is too much, I know. Talk about suffering for the sake of art.

A Bamboo Guitar Part 1 – why bamboo?

I’m certainly not the first person to think about using bamboo to make a guitar – Yamaha produced an acoustic 6 string called an FGB1 a while back, and people have used thick bamboo laminates for electric guitar solid bodies.

The first reason I have for wanting to try bamboo is to do with its merits as an easily renewable material, unlike the hardwoods usually used for acoustic guitars. Many of these are very hard to get hold of as humanity tramples across the planet: Brazilian Rosewod and Koa are just two examples.

Most people know how good a flooring material bamboo makes because of its hardness and pleasant appearance. It’s now available in thick laminated panels for benchtops and furniture making as well.

One thing I don’t yet know about bamboo is how stable it will be with changes in humidity and when it comes under the prolonged stress of string tension. Users of the Yamaha FGB1 talk about having to adjust the neck every week to keep it playable.

Another thing I need to investigate is its acoustic properties – particularly the along grain, cross grain, and torsional moduli of elasticity for the panels I’m using.

Other question from a builder’s point of view are:

  • how well I can make the bamboo bend in the side mould?
  • will the formaldehyde laminating glue hold up while bending?
  • how well will bamboo take machining to fit edge bindings?
  • will the formaldehyde glue outgas under heating?
  • what thicknesses should I use for the soundbox?

As a starting point, I’m going to make the sides and back of the soundbox out of bamboo while keeping the rest of the structure and materials the same as I usually use – spruce for the top, and Niugini Rosewood for the neck.

I have now got hold of some 4mm thick by 200mm wide vertically laminated panels:

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I got it from Bamboo Australia Pty Ltd in South East Queensland. You can order from them on line. Durnford is very interested to see how the bamboo guitar turns out.

So using a renewable resource is my first reason to try bamboo. But I have another reason as well.

What I want to find out is how much effect the timber species the soundbox is made from has on the overall tone of the guitar. The reason I want to try and test this is because I have a suspicion that, when stacked up against all the other factors that determine the tone of a guitar (top bracing, soundboard timber quality, soundhole size, strings, neck bracing system, etc) the effect of the wood making up the sides and back of the soundbox is minimally important.

This is not what most people think, so perhaps I’m quite wrong. Some will spend a fortune on a Brazilian Rosewood guitar, for example, because they believe the tone is unique. But then, concert violinists apparently can’t reliably pick out the sound of a Strad in a blind test. Everything we sense is filtered through our expectations – if you don’t believe me, search out some Derren Brown videos on YouTube (don’t be put off by the Svengali stuff – he’s brilliant at illustrating how much at the mercy of our own expectations and senses we are).

I have read of people who claim they can tell an instrument made with hide glue from one made with plastic resin glue by listening to it. Maybe I’ve just got a dud ear.

Making one bamboo guitar won’t give me a definitive answer, of course, but it’s a step on the way.

I intend to document the building of my bamboo guitar on this blog, and I’ll include my thinking on how to evaluate the results – not a trivial question!