Reducing BOBO neck weight

[josswinn]

Since first posting to this forum, I've built four flamenco guitars and made notes on each here:

https://josswinn.org/2018/09/29/my-flamenco-guitar/
https://josswinn.org/2019/05/05/notes-on-guitar-3/
https://josswinn.org/2019/12/28/notes-on-guitar-4/
https://josswinn.org/2020/05/05/notes-o ... -guitar-5/

You'll see that I'm using a traditional Spanish build and although guided by the characteristics of a flamenco guitar that Trevor discusses in his books, I've not yet adopted his build methodology.

I'm still torn between achieving a very light guitar of around 1100-1200g, but benefitting from the approach to construction that Trevor uses, specifically the BOBO neck joint and truss rod. It's very difficult to build a guitar that is 1100g and use anything other than wooden pegs and lightweight samples of Cedrela for the neck, so I'm concerned that the BOBO/truss rod method will make it impossible.

Can anyone who uses those methods advise on how to cut weight from them?

Does the BOBO neck have to have such a fat heel that I see on them in the book or can it be shaped in the traditional lean, sharp flamenco style?

Has anyone used a two-way titanium truss rod like this?

I assume the bolts and T-nuts for the join could be titanium?

Am I right in thinking that the 'wedge' for the BOBO classical neck might not be required for a flamenco, where the desirable saddle height is 7-8mm, rather than 10-11mm for a classical?

If you were set the task of reducing the overall weight of the BOBO neck, how would you do it?

Thanks for any insights and suggestions,
Joss

[johnparchem]

Am I right in thinking that the 'wedge' for the BOBO classical neck might not be required for a flamenco, where the desirable saddle height is 7-8mm, rather than 10-11mm for a classical?

I think you still need the wedge. Just imagine flat, no neck angle, with a 6 mm fretboard 2.5 mm of action will result in the string height of 11 mm. So a 4 mm thick at the nut wedge would be needed to get to 7 mm. A flamenco gets most of its lower string height from lower action not a change from the classical forward angle. I tilt my neck an extra 1 mm.

[kiwigeo]

I wouldn't leave out the wedge....

[kiwigeo]

"Does the BOBO neck have to have such a fat heel that I see on them in the book or can it be shaped in the traditional lean, sharp flamenco style?"

The "chunkiness" of the heel is because of the need to have a bit of "meat" surrounding the 10mm square brass bar that the two bolts engage into. Ive got a half finished classical with BOBO neck and Ive managed to shape the heel to the point where it doesn't look much more chunky than on my previous Spanish method builds. It all boils down to how much meat you want left around that anchoring bar.

[josswinn]

Thanks John and Martin. On reflection, I think the standard bolt-on design with aluminium bolt bar is more appropriate for a lightweight flamenco.

I see also, that an elegant heel shape can be achieved (Fig. 14-23 build book) which isn't as chunky as some of the others.

On the three flamenco plans I refer to when building, the heel at its narrowest point is 19-20mm wide. The mortice for the bolt-on is 19mm, but presumably a narrower one could be used and narrower bolts, too. Trevor's design is to accommodate both steel and nylon using the same set up but I think it can be adapted to a lighter design. It won't be as strong as the 19mm mortice/M6 bolt on, but it doesn't need to be.

It's difficult for me to understand the implications for any changes to this without trying them out. The books contain so much (useful!) information and simultaneously cover both SS and CL, that I get lost in them sometimes.

Cheers
Joss

[johnparchem]

Just a thought, not my idea as it was posted on this site awhile back. Set things up so that you can install the bolt on neck sans fret board before gluing on the back but after gluing on the top. Then while gluing on the back you can set the forward angle by tilting the bolt on neck to the correct angle. For example you can set up the guitar top down on a workboard or solera and shim the body at neck join 1-3mm what every is right for your guitar and make sure the neck is clamped down as well. Better description just like you were doing a spanish heel neck joint.

[Dave M]

Joss I was going to suggest exactly that. The BOBO design needs that large chunk of timber on the head block, and indeed two more bolts.

As Martin says one has to alow for the bolt bar but the other thing is that when doing a cutaway it looks a bit silly to have a Spanish type heel. I always do a cutaway so I end up with heels like this: ( I see the photo didn't go where I intended. Never mind.

A narrow heel is perfectly possible. I have just measured one of mine and even going to 30 mm width can look good. However I am sure a narrower tenon would work. Going down to say M4 could be OK but your build accuracy would have to be good. I would worry a bit about tapping an Al bar as steel screwed into Al is a bit problematic. I note that Titanium bolts are available.

I would think the head block size could be reduced somewhat and looking for a low density sample of the timber used. Heel block similarly doesn't have to be too massive.

Trevor's ideas about lightweight bridges are obviously useful.

I always use rosewood for fingerboards - considerably less dense.

Would you need to incorporate Trevor's mass side loading structures...?

Where Trevor suggests an infill block between the head block and the UTP one might use a veneer of say Maple as some traditional makers did.

Could you save on the thickness of the fretboard? Denser than the Cedrela neck.

I guess with all these ideas you really need to get out the scales and check what weight savings are to be had with each one - if any.

A little example: I had taken to making quite short (rosewood) bridges in an attempt to reduce weight. I realised that the bridge ends would not meet the fan brace under the bridge ends. I thought that adding a thin bridge plate might address the possible structural problem but was surprised to find that that spruce plate weighed more than the extra rosewood wings would. So assume nothing.

Anyway an interesting question and I look forward to other people's ideas and where you end up.

Dave

[Dave M]

So not much response, but I have often wondered about this pegs versus machine heads. What actually is the weight difference? Classical machine heads don't feel that heavy to me...? D

[josswinn]

Hi Dave,

John Ray weighed wooden pegs vs machine heads, including the respective removal of wood and found about 100g difference.

Thanks for taking the time to make all the other suggestions. The neck is the main issue for me and I know I need to be more selective about what samples of wood I choose. I like the idea of a bolt on neck and truss rod, for the adjustability that a flamenco would benefit from to get the action and saddle height spot on.

I’ll think about it a bit more and weigh it all up, if you pardon the pun!

[Dave M]

Just out of interest as I was fitting a two way truss rod today I noted that it weighed 98 g. I haven't weighed the wood removed but it's not going to be much in 'cedrella'.

This was a truss rod bought from Tonetech in the UK.

Dave

[Dave M]

I was wondering around the forum and remembered this thread from some time ago. Joss did you find any answers to the weight issues around the bolt on/off neck joint for a flamenco?

I am still using the simple bolt on joint - really just avoiding the accuracy issues of the full monty

Cheers Dave M

[josswinn]

No, Dave. I’ve been looking at the adjustable (Smallman) neck instead.

For bolt on, I like the look of this:
https://www.cumpiano.com/an-improved-neck-body-joint

It avoids having to have a mortise tool.

Joss

[Steve.Toscano]

Am I right in thinking that the 'wedge' for the BOBO classical neck might not be required for a flamenco, where the desirable saddle height is 7-8mm, rather than 10-11mm for a classical?

To get the desired string height at the bridge the neck must have a forward angle. In my case I put a 2mm drop at the nut.
Another way to do this is to put the neck co-planer with the upper bout of the soundboardand to taper the fingerboard (say 7mm thick at the nut and down to 5mm at the 12th fret). This is an easier way to achieve the same thing as it saves having to scrap the underside area of the fingerboard that's above the body.

If you don't have a solera set up to do the 2mm drop at the nut end, you can just shim under the body at the body/neck join with a 2mm shim when gluing the back on.

Side note: I make my classicals with no neck angle, the neck is co-planer with the upper bout of the body.

If you were set the task of reducing the overall weight of the BOBO neck, how would you do it?

Good question, in my opinion the 2 most important aspects for a flamenco blanca are overall mass and tentelones, in that order, closely followed by side and back stiffness. Something i see even the most seasoned guitar makers getting wrong when making a flamenco guitar.
Personally i don't see how one could get the traditional flamenco blanca sound with BOBO neck AND a truss rod.
As you've mentioned Joss, if needing a removable neck maybe the smallman type neck joint or the cumpiano method might be better here.