Official Luthiers Forum!

For those of you using an adjustable neck with the carbon bars/tubes running to the sides:

Are you using bar stock or tubes?

What size?

Thanks,
Bruce

Rectangular bars, 1/8" x 3/8" or 1/8" x 1/2".

Thanks, Rick. Just to be sure, I'm needing the sizes of the ones running from the head block to the sides, not the ones imbedded in the neck shaft. Re-reading my original post I don't think I was very clear.

Thanks.

What Rick said.
Check out Los Alamos Composites (sponsor here) for all your needs.

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Hank Mauel

Those are what I call "flying buttresses" after the engineering principles used in Gothic cathedrals that kept the walls from blowing out and the structures collapsing. That innovation made possible the incredibly height of the walls and a roof without structural truss chords.

Anyway, I digress...

I use 1/4" solid CF round rods that I get from the Graphite Store on-line.   I'm now using two per side.   One could use 1/4" tubing, I suspect; the buttresses are loaded in compression, and what I'm doing is probably overkill, but it sure works.

I get my normal bar stock from Jim at Los Alamos.

There will be an upcoming photo essay in Fretboard Journal on how we build, and there are great photos of how the buttresses go in.

Bruce:
Another source for CF is:
CST-The Composites Store
I ordered some stuff from them and was happy with the service. They have tube and rod at good prices, If you use tube in a bit larger size you can get the same stiffness with less weight than rod- though none of these braces weigh much anyway.

Howard Klepper uses tubes in this application:
Klepper neck block bracing

And a pic from the archive of one of Rick Turner's:


I put a couple of these braces into an eBay 'project' guitar recently- they sure lock that neck block in place..thanks to Rick, Howard, and the other folks I got this idea from!

BTW, the inside of this thing is no prize, but I got it a bit cleaner-looking than the pic would indicate!
Cheers
John

Bruce,

What Rick said. I use 2 per side as well on the basis that the those at the top are in compression and those at the bottom in tension:

_________________
Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

Thanks for the help and the links, guys.

I started doing these in around 1990 using wood flying buttresses.   I rebuilt half a dozen flattops into baritone guitars for various customers, and as long as I was re-topping them, I figured I'd might as well stiffen up the whole box. It worked like gangbusters.

The little archtop mando-guitar in the picture was from '97, and on that I used two CF rods. Now I use four, all in compression to the top of the neck block and splayed to get some triangulation, and I also top the center seam reinforcement with CF to get the tension strength to the bottom of the neck block.   The CF on the back strip also adds incredible strength and stiffness to the back.

Rick, Dave, et al,

I can see the value, structurally, of the brace from the top of the neck block to the back/side/waist/kerf interface.  The triangulation is transfering the compressive load of the string/neck moment to a very strong area of the guitar body.  What I may not understand is the value of more than one brace per side such as the one pictured above with four rods.  I visuallize the string tension trying to rotate the neck block top toward the bridge. 

I see the strength in a set of rods from the top of the neck block to the top/side/waist kerf interface.  This also resists the movement of the top of the neck block.

On a side note, is this how a floating fingerboard extension body would be reinforced since the fingerboard would not rest on transverse braces under the top?

Thanks,

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Steve Walden
Aspiring Builder,
Bartlesville, OK

It's a double triangle and that just makes the whole thing that much more resistant to twisting, torquing, etc.   In fact, it's a quad set of triangles, and then with the CF topped back seam reinforcement, the tension on the back is taken care of as well.   

I dado CF into the fingerboard and relieve it only as much as needed over the body. It's plenty strong.

Steve,

I'll give you my reasoning. The neck under string pressure pushes in on the top of the neck block and pulls on the bottom of the neck block - potentially over time the lengthwise back arch can be pulled straighter, with the neck block and neck angle changing. The top pair of rods under compression resist the push in of the top of the neck block and take the string load into the sides. My little brain says "fine, but the bottom of the neck block could still move out over time due to the pull and flattening of the back arch". So I use a bottom pair as well that are under tension. It may not be such a big risk but it is pretty easy to put in another pair as insurance.

If you look at the gap in the neckblock in my picture, this is where my flaoting neck extension goes. Rick uses dadoed cf rods under the fingerboard which is way cool but I just have the neck extending under the fingerboard like this:

.

People like Rick, Mike Doolin and Howard Klepper have been inspirations in developing my instruments this way

_________________
Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

Dave, I highly recommend to you that you try topping the back seam reinforcement with a thin strip of CF. I think it's much more effective in maintaining the arch of the back...and it's stupidly easy, and as some of you know, I'm a lazy SOB, and I like easy.

Dave - Thank you for the explanation.  I was thinking that the neck block is trying to pivot around an axis perpendicular to the neck and about 1/2 to 2/3 of the way from the top to the back.  This would put your top rods in compression and the bottom ones in tension.  Depending on how the rods are attached I would think that the bottom rods wouldn't contribute much to structural integrity.

Rick - are you refering to the cross-grained back joint reinforcement?  I can see that keeping the back from stretching.  However, does the CF give much resistance to flexing it flat?

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Steve Walden
Aspiring Builder,
Bartlesville, OK

Yes, I top the cross grain spruce or cedar back joint reinforcement with .021" x .375" CF. We run that all the way through the end block and the neck block up to the binding, so it's well locked in. We also top the back braces with CF.   I have literally stood on the center of a back when it was glued to the sides.

The flattening of the back and the rotation of the neck block is one of the chief reasons why most traditionally built steel string guitars will eventually need a neck reset. Most people think of it as changes in the top, but in fact it's the distortion of the entire guitar body. This was verified to me by Tim Teel, one of the main guys in the C. F. Martin R&D department, who said, "Rick, if we could build with dead flat backs, they'd never need resets."

A back with CF reinforced strip and brace tops will not flatten out in any degree measurable.   Or so I believe... But then, I make guitars with adjustable neck angles, so it hardly matters to me in that sense. But you do get tremendous projection with a guitar made this way.

Rick - Thanks again.  When you lay down the CF on the back strip do you epoxy or HHG?  Is the CF you use just the fiber, i.e. you have to add the matrix?  I have done a fair amount of fiberglass repair and was wondering if the CF is more of a cloth before you adhere it to the wood.  And if so, saturation with epoxy must be needed.

As to the strain of the guitar body over time, I liken it to a piece of glass.  Glass, technically being a liquid, will 'flow' due to gravity over decades.  You have to measure the difference in thickness from top to bottom in ten thousands of an inch.  Wood will stretch and/or compress due to the forces exerted on it.  Geometry tells us that it doesn't take much movement of the neck to get the action too high...But, you already kow that!  I guess I'm talking to the others that may see this.

Thanks again, between you and Carruth I am learning so much of the physics and acoustics of our instruments. 

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Steve Walden
Aspiring Builder,
Bartlesville, OK

I'm using strips I get from "the Graphite Store" on-line. they're already cured unidirectional CF in epoxy. I glue the CF to the spruce braces with CA glue, and then do the wood to wood joints with HHG, though LMI white glue is fine, too.   Frankly one of the things I like about HHG is that it is eminently marketable. Clients like the idea of it, and they really respond to the idea of tradition meeting the future.

This is, after all, a business for me...

I have been compounding the radius of my backs to try to offset the distortion. In the lower bout, I radius about 1/4" across 16" and work to 1/8" at the waist and then 1/16 at the upper bout so when it meets the neck block it is flat. I love the idea of the cf topped backstrip and braces, but I have been following some advice Al gave about matching the pitch of the top and back before gluing on the bridge and in doing that I find that I need the back to have quite a bit of give in the lower bout. Maybe I could run the CF only to the waist. I have been wanting to build with the buttresses since Rick mentioned the harmonic sustain he encountered with the active upper bout. I have been trying to imagine what force is still exerted up there with the buttresses carrying much of the load and daydreaming about bracing structures. It will be fun to experiment and see what can happen. Thank you for posting some photos of those buttresses.

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Burton
http://www.legeytinstruments.com
Brookline, MA.

While I understand the whole bit about "tuning the back", what I'm going for here...and getting...is more of the Smallman style projection that you get with a very stiff and reflective back. I've heard my guitars played in jams with others' guitars from 30 to 60 feet away, and mine carry and project as I want them to as more of the energy is going forward.   Loose backs tend to radiate as di-poles, and a lot of energy goes back to the player. With these particular guitars I'm looking for a more cardioid response.

I do have an order coming up for a 14 fret 15" mini-jumbo...nylon string guitar. The owner wants this as more of a living room instrument, so I'm going to go quite thin with the back and sides. He wants a fair amount of "snap", too, so it'll have a stiff neck structure and probably a spruce top. It'll get either a 664 mm scale or a fanned fret board.

Rick Turner wrote: "I have literally stood on the center of a back when it was glued to the sides."

Well......  You may not want me, at 275 lbs, standing on the back of your guitar!  You may get to do what the one poster did with his broken guit..... Burn it!

As in the other thread on bracing and the 'cube' equation,  I can see how the addition of the CF to the joint and braces would add incredible strength and stiffness.

I am wanting to design a 'floating' fret board extension with minimal bracing in the top upper bout.  I realize that the UB does not add to the sound pressure level or tone much.  I am also thinking of sloping the top from the center of the sound hole toward the neck block about 1/2" to 5/8" and cantilever the FB over it.  I know several builders do something similar.  Using the CF rods to counteract the string tension would allow me to do this.

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Steve Walden
Aspiring Builder,
Bartlesville, OK

Don't give up on the tone production from the upper bout until you've optimized it. Curving it down may just kill it.

[QUOTE=Rick Turner] Dave, I highly recommend to you that you try topping the back seam reinforcement with a thin strip of CF. I think it's much more effective in maintaining the arch of the back...and it's stupidly easy, and as some of you know, I'm a lazy SOB, and I like easy.[/QUOTE]

Rick,

Thanks. I'm all in favour of stupidly lazy Do you use the cf as a continuos strip along the whole back seam and then cut the other back braces to fit over this? I use the bottom pair of braces not as an "immidiate" solution to the impact of big forces but to counter the drip, drip pressure over a long time of the back arch being flattened to a degree - I suppose you would have to stay standing on the back for a year or so to fully test this

I just think that the rods straight in tension and anchored to the sides - may do this best rather than the cf in the plane of the arch of the back. With cf rods in the neck the truss rod will still alter the neck reflief. My engineering theory isn't that sound and a could be way off the mark there though. Time alone will tell and again like you I usually use an adjustable neck system in conjunction.

I want my backs responsive as I'm mostly building for living room players that want to hear as much as possible themselves and probably play mostly sitting down with the back allowed to contribute. Having been influenced by the ideas of the great Stefan Sobell I find that arching the top gives a great amount of forward projection and volume. Yours would still probably be louder though.

_________________
Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010