Winter Build-Off 2022: Bamboo EUB

[Max Bogosity]

I gave myself a profile gauge for Christmas and put it to work tracing the neck profile from my old King Mortone bass. I chose two sections, one near the nut and one near the heel, and gauged the neck profile from all four directions at each section. You can see where I did the back profile twice just to make sure I wasn't fooling myself by presenting the gauge off-square, but no, it really is that asymmetrical. Or at least I made the same mistake consistently. You can also see I was too lazy to take the strings off. If I were to do this step again, I'd pull out or mark one pin on each side to indicate where the fingerboard-to-neck joint lies.

Then, on parchment paper, I marked out a rectangle with the overall width and thickness of the neck at that location and traced the four profiles inside it, aligning them with the rectangle and each other. I may or may not go over it again with my grandfather's french curve and smooth things out a bit more.

I couldn't believe how thick the fingerboard is compared to the neck, but it really is like that. The fingerboard I bought for this bass is even thicker at slightly over 20 mm at that section. It's tempting to thin it down to match, but on the other hand there's a pretty strong argument for leaving it thick so there's room for adjustment later. Speaking of adjustment:

From what I've heard, upright fingerboards are tricky. They need a certain "scoop" to stop string rattling, and this can take double bass luthiers years to master.

I've heard that too. Putting the board up against a straightedge suggests that unless the right shape is a drunken sine wave, I'll need to figure out the scoop for myself. On the other hand, the straightedge says that my King isn't scooped correctly either and that hasn't bothered me, probably because my action is relatively high. I'm going to leave it alone for now, then we'll see how it looks after the neck is together.

 

[Flamingo21]

Yes, it's probably best to figure out the scoop (or non-scoop) much later, when the bass has come together and all the variables are known.

 

[Bruce Johnson]

I've heard that too. Putting the board up against a straightedge suggests that unless the right shape is a drunken sine wave, I'll need to figure out the scoop for myself. On the other hand, the straightedge says that my King isn't scooped correctly either and that hasn't bothered me, probably because my action is relatively high. I'm going to leave it alone for now, then we'll see how it looks after the neck is together.

If you want to know more about that subject, you can read through this classic thread:

Even fretless fingerboard levelling tips

We get into the meat of it on Page 2, Post #40. I call it "hourglassing" the fingerboard. Yes, you'll need to do it on this fingerboard. It's part of the process of radiusing and shaping the fingerboard surface. Nothing magical about it, once you understand the geometry.

 

[Max Bogosity]

Thanks Bruce. Beautifully clear explanation.

I guess the additional complication in an upright bass, and what I maybe incorrectly understood ‘scoop’ to refer to, is that without a truss rod the relief along the length of the string has to be created manually.

I’m imagining the order of operations might be something like
1. complete the rest of the instrument first
2. flatten the individual string paths
3. string up, play test, see if and how much additional relief is needed on each string
4. remove strings, remove material to create relief along each facet
5. Iterate steps 3 and 4 as needed
6. Smooth out facets to recreate the radius across the fingerboard.

Or basically what I understood you to say in the linked thread, with steps 3-5 added. I guess if you don’t do step 6 between the two lowest strings then you end up with a Romberg bevel.

 

[Bruce Johnson]

Thanks Bruce. Beautifully clear explanation.

I guess the additional complication in an upright bass, and what I maybe incorrectly understood ‘scoop’ to refer to, is that without a truss rod the relief along the length of the string has to be created manually.

I’m imagining the order of operations might be something like
1. complete the rest of the instrument first
2. flatten the individual string paths
3. string up, play test, see if and how much additional relief is needed on each string
4. remove strings, remove material to create relief along each facet
5. Iterate steps 3 and 4 as needed
6. Smooth out facets to recreate the radius across the fingerboard.

Or basically what I understood you to say in the linked thread, with steps 3-5 added. I guess if you don’t do step 6 between the two lowest strings then you end up with a Romberg bevel.

Yes, that's it exactly. Nicely written! On an upright bass, because there's no truss rod, you have to cut the relief curve into each string path. That's the Magic of shaping an upright bass fingerboard. Each string path needs to have a gentle relief curve, while being smooth and true; no lumps or dips. That takes some skill and experience to do it right.

On electric fretless basses, we can cheat because of the truss rod. Moderately tighten the truss rod, then cut the string paths flat (which is much easier to do). Then, when it's strung up, back off on the truss rod to adjust the amount of relief curve.

Either way, all that matters is the shape of the narrow strip of wood directly under the string. The rest of the fingerboard between the strings is just there for decoration. Smooth it out and make it look pretty. Without touching the string paths.

 

[Max Bogosity]

A smarter man might have done some testing before announcing to the world that he was going to build a bass out of bamboo. But that man isn't here.

There are plenty of sources stating that bamboo can be up to twice as strong as hardwoods. But when you starting looking at data it becomes clear that the word bamboo includes dozens of different species, some of them not very strong at all. Here's a good summary. Which one do I have? Who knows!

Another question is, are the stiffness and strength the same in the quarter sawn and flat sawn directions? Who knows!

A third concern is that what I have is not big solid planks of bamboo (which don't exist), but a composite of little strips laminated together. So the bulk properties depend on how good the existing glue joints are. How good are they? Who knows!

The simplest useful test I could think of was to compare the bending stiffness of bamboo beams to identical beams made of other woods. So I made some 13 mm (~1/2") square test pieces from bamboo, maple, and cedar, and devised this very sophisticated rig to apply a carefully calibrated load to the end of each beam.

When cutting up the bamboo planks it became apparent that the existing joints between laminations are not great. There are frequent separations on the order of a couple of inches long and wide enough to get the tip of an x-acto knife into.

The good news is that it doesn't make any difference where the bamboo beams are loaded in the quarter sawn or flat sawn direction. They deflect about the same amount regardless of orientation. The bad news is that it was quickly obvious that while some bamboo may be very stiff compared to hardwood, this is not. Under the same load, the bamboo beams bent slightly more than the cedar and about twice as much as the maple. I hoped I was doing something wrong, so as a verification of the method I decided to try to calculate the stiffness and see if I was getting reasonable results for maple and cedar. Measuring the deflections and working out the stiffness (with some help from ye olde first year mechanics of structures textbook) yields modulus of elasticity figures for maple and cedar which are in the right ballpark compared to the data in the USDA Wood Handbook. It looks like my crude test rig is doing its job, which is great, but my bamboo is roughly half as stiff as maple, which is not great at all.

I'll have to consider where to go from here.

 

[2groggy]

Well, seeing as your bamboo is already a composite material, what about adding some glass or carbon fibre for stiffness? I don't know how deep into your mechanics text you need to read to find the bit about how reinforcing is only useful on the parts of a beam that are in tension, like the top surface of the test piece in your picture.

Edit - I decided to make that last bit into something resembling a sentence.

 

[Max Bogosity]

Yeah, I did start thinking about running carbon fibre along the back of the neck to compensate, but it seems a bit like putting lipstick on a pig. The point of the bamboo was supposed to be that it was stiffer, which might make up for the fact that it's a bit miserable to work with. If it's actually less stiff, then I'm using a worse material just for the novelty.

Not sure I'm contrary enough for that. But I'm not sure I'm not, either.

 

[Will_White]

Yeah, I did start thinking about running carbon fibre along the back of the neck to compensate, but it seems a bit like putting lipstick on a pig. The point of the bamboo was supposed to be that it was stiffer, which might make up for the fact that it's a bit miserable to work with. If it's actually less stiff, then I'm using a worse material just for the novelty.

Not sure I'm contrary enough for that. But I'm not sure I'm not, either.

A fiberglass layup on the back once you shape it would make it stiffer and still let you see the grain of the neck.

 

[RichterScale]

Do you think it will be stiffer once a bunch is laminated together?
You can always route slots for a couple carbon fiber bars.

 

[dwizum]

I guess it depends on criteria for "better" - sooner or later we may all need to be using materials that are as renewable as bamboo, so it's good in that manner!

I'd also guess that a lot of the stiffness in a typical upright bass neck comes from the stupid-thick ebony fretboard. You may not actually be all that far off once your bamboo is laminated to a chunk of ebony.

 

[RichterScale]

Do uprights have truss rods?

 

[Reedt2000]

Do uprights have truss rods?

Nope

 

[Reedt2000]

You could really bury any stiffening material (wood, CF, steel) in the center of the neck, no? Without a truss rod stiffening is all you care about, so there's no worry about making it too stiff, right?

Visually it would be all bamboo

 

[2groggy]

I guess it depends on criteria for "better" - sooner or later we may all need to be using materials that are as renewable as bamboo, so it's good in that manner!...

You can't recycle composites because you can't effectively separate the components. So make it last forever.

 

[Flamingo21]

When I was a kid, we went to France every summer. On one of these trips we visited "The Bamboo Forest". It was not a natural forest, it was all planted, and it had this souvenir shop with all kinds of stuff made of bamboo.

There were also some large information signs, and though my French wasn't up to scratch it became clear to me that there were over a hundred kinds of bamboo in the world, with impressed me quite a bit. And the forest reflected this fact, it looked extremely varied.

If I'd thought of this sooner I might have warned you, but the memory only just surfaced.

 

[Phil Mastro]

Measuring the deflections and working out the stiffness (with some help from ye olde first year mechanics of structures textbook) yields modulus of elasticity figures for maple and cedar which are in the right ballpark compared to the data in the USDA Wood Handbook. It looks like my crude test rig is doing its job, which is great, but my bamboo is roughly half as stiff as maple, which is not great at all.

I'll have to consider where to go from here.

I vaguely remember talking with some small skateboard or ski manufacturers, and what I was told about bamboo is that the word "bamboo" is akin to the word "wood", in the sense there are hundreds of species of it with different properties. Not many types were structurally useful, with one of the usable options being moso bamboo.

And as far as I can tell, it's basically impossible to tell them apart, and manufacturers of products (flooring, chop boards, whatever) rarely have the species info available.

Maybe your fingerboard is thick enough to make your neck stiff enough anyway.

 

[1n3]

I sub'd to this thread at "bamboo". Published mechanical specs for Bamboo are intriguing, but there are obvious obstacles of processing, and transforming a tubular material to usable forms.

If this project goes no further, it was worthwhile for your stiffness test. Thanks for that! It would also be interesting to know how density compares in your samples; strength/weight.

 

[Phil Mastro]

So it turns out my skier manufacturer contact said that moso isn't particularly dense and may not be appropriate for instruments, and that more appropriate species may be Dendrocalamus Asper and Guadua.
...
I copy pasted those last few words, I know nothing about them

 

[Flamingo21]

Dendrocalamus sounds like some part of the brain.