Bolt-on bolt-off neck joint

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Dominic
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Bolt-on bolt-off neck joint

Post by Dominic » Fri Feb 17, 2012 1:33 am

Hi Trevor, a question about your bolt-on bolt-off neck joint. Is there a structural reason why the fretboard extension needs to be bolted down? Most of the stability of the neck would (in my mind) come from the neck block and its supporting structures, the sides, the top and back. I've thought through this a lot and I can't see how bolting the extension to this structure helps but I am not an egineer and am happy to be corrected. I would think that if the neck block does tip in at the top a bolted down extension would just follow it. So whats the point of the extra work?

I've been using carbon fibre cloth and wood laminates under the extension to stiffen it up and add mass for the sound and just letting it it sit on the top, not gluing it down. Recently I moved to a raised extention (~12mm) with a wedge and I can target any bridge height I want without worrying about the angle of the upper bout. The next step is to make it adjustable. So I suppose it goes for adjustable necks as well.

If my thinking above is wrong, do you think you think guitar designs with floating extensions like an adjustable neck need some other type of reinforcing inside the box to aid in neck stability?

Thanks
Dom
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Trevor Gore
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Re: Bolt-on bolt-off neck joint

Post by Trevor Gore » Fri Feb 17, 2012 10:51 am

There's certainly enough strength and stability in the joint not to have to bolt down the fingerboard extension. You could probably take the top bolt out of the heel, too. The main reason I do bolt it down is the same as the reason there is to glue down a conventional fingerboard extension - to ensure plenty of stiffness and mass beneath the upper frets to avoid "dead" notes, i.e. an acoustic reason. You'll know what I mean if you've played a note on a fret overhanging the sound hole. If the "tongue" is wide enough and deep enough it's probably not necessary to bolt it down; it can float like on an archtop guitar. Once "tooled up" the extra work for the bolts is trivial and the bolts make sure the extension seats firmly on the neck block and the edges of the fretboard sit down onto the top. And as you say, this design is well on the way to a fully pivoting neck, which I think is more of a builder convenience (avoiding some precise alignment) than a player convenience. If you have the rest of the guitar designed/built right, the action doesn't change with the weather, etc..

Would you like to post some pics of your raised/wedge extension?

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Re: Bolt-on bolt-off neck joint

Post by kiwigeo » Fri Feb 17, 2012 11:14 am

Could one argue that that the floating fretboard on an archtop and also on some steel strings are not well designed from an acoustic point of view?
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Trevor Gore
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Re: Bolt-on bolt-off neck joint

Post by Trevor Gore » Fri Feb 17, 2012 2:13 pm

One could certainly argue that. However, with their typical ebony fretboards and a big lump of dense maple underneath, they probably mostly get away with it, though I've played some that don't.

You're usually asking different things of an archtop, though, and they are mostly acoustic/electric which brings in a heap of other variables.

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Re: Bolt-on bolt-off neck joint

Post by Dominic » Fri Feb 17, 2012 4:15 pm

Thanks Trevor, so it is really about acoustics and consistency above the neck joint.

This the best pic I have at the moment and both guitars are away from home at the moment. Its a bit hard to see it here. I let it touch the top above the neck block then just tapper it a bit so it rides above it for most of its length. this pic is during setup so I'm still tweaking things. I made 2 like this and I liked how they came out.

The neck on most of my guitars has a steel truss rod under most of it but not under the neck extension. (These classicals have light weight aluminium truss rods). I wondered about adding some steel or brass mass under the fretboard around the extension to try to even it up the sound a bit. Obviously more important on cutaways. This would be simple to set a decent thickness plate into the extension, then I can concentrate on the neck adjuster mechanism and not compromise sound. Any thoughts?
Classical 015 (Custom).JPG
Classical 015 (Custom).JPG (81.83 KiB) Viewed 11157 times
BTW, My archtop based on Bob B design has a pretty thick extension, and its square so its bigger than the neck down at the nut so I'd have to say there is nothing wrong with that design. But any neck joint where the fretboard is glued to an unsupported top with a few braces across it would suffer which is most Martin style designs.

Cheers
Dom
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Trevor Gore
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Re: Bolt-on bolt-off neck joint

Post by Trevor Gore » Fri Feb 17, 2012 6:25 pm

That looks pretty neat, Dom. To my eye, it looks like it might be a bit floppy by the time you get to the 19th. Have you got it strung yet? You'll know straight away if you have a problem.

If you do, it'll be a case of going "millennium" on the next build, or cutting a pocket into the upper bout so you can get more thickness of neck wood beneath the high frets.

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Dominic
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Re: Bolt-on bolt-off neck joint

Post by Dominic » Sat Feb 18, 2012 5:10 pm

Thanks Trevor, I have strung it and its brother up. I made a narra one for myself and this one for Julie. They both came out really nice but Julie's is better sounding.


Anyway, do you think it is stiffness that we want in the tongue, so a light but very stiff tongue - thinking carbon fibre, or is it having mass that matters most so a steel plate? Or both. I am thinking about violins which have a curved fretboard tongue so it is stiff but it still hangs out there without support and doesn't seem to bother the purists.

I just don't like the idea of cutting a hole in the top of the guitar with an adjustable neck and I do like the raised tongue, I just want to make it work on a deep cutaway.

Cheers
Dom
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Trevor Gore
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Re: Bolt-on bolt-off neck joint

Post by Trevor Gore » Sat Feb 18, 2012 6:01 pm

What you want is low admittance in the tongue (so the wave in the string is reflected back). However, if it plays well and you can't hear a difference above 12, why change it?

How old is Julie?

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Dominic
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Re: Bolt-on bolt-off neck joint

Post by Dominic » Tue Feb 21, 2012 1:52 pm

She used to be 25, didn't we all?
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Re: Bolt-on bolt-off neck joint

Post by Trevor Gore » Tue Feb 21, 2012 7:00 pm

Apparently! If only I could remember...

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