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Thanks, Hesh and Todd!

I'll take something from each of you as I brace up several backs just sanded. (Two have wedges made of bookmatched material, and the wedges are too wide IMO to apply one huge patch, so I'll get the chance to try your ideas over and over and over...)

Dennis

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Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

Great thread here guys! Thanks Hesh for taking the time to do this excellant Tute.
Great Braz. back BTW ! I can't wait to see this one finished!

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Anderson Guitars
Clearwater,Fl. 33755

Hesh, perfect timing. I have a project in process and was ready to do the center seam brace on the back. After reading your post I made a radiused caul and it worked great - much better than my past approach. I also use a year stick to align to bracing materials during the glue process so combining that with the radiused caul worked out extremely well. Thanks for sharing all that you do. I know how time consuming it is to make an effort like you do and I think I speak for the entire group when I say "we appreciate the effort"

[QUOTE=ToddStock] If you think about an I-beam in bending, the top flange is in compression (it has to get shorter as the beam bends), the bottom flange is in tension (it has to get longer), and most of the material near the center of the beam has been removed because it does not carry any load (does not need to stretch or shrink). The point in the center of the beam where there is neither compression or tension is the neutral axis, and any time we can remove material here, we get very little decrease in load carrying ability, but a big decrease in weight.

Laminating carbon fiber into the center of the beam allows only the fibers furthest from the neutral axis to carry much of a load, so the superior performance of the material under load is not fully utilized. We could remove 80% of the fibers near the neutral axis and see no changes in load-carrying capability.

Using a layer of carbon fiber as a flange (like Rod's approach) allows the extra weight of CF to be more than offset by the improved load carrying performance - the entire piece is in tension or compression, so it is more structurally efficient than the earlier example.

Clear as mud, right?[/QUOTE]

I'll just correct one thing here.

It's Rick's approach, not mine both Rick and Todd are smarter than me

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ok, i guess if im going to highjack, i might as well just go for it. thanks for the reply todd. im pretty sure i understand what you are saying as far as compression and tension are concerned. i think what i was/am confused about is, where is the neutral axis located in relation to the load or force that is to be borne (im not sure about that spelling, but i think you get it)? it seems like the load in a guitar back or top is mostly perpendicular to the plane of the plate. am i seeing that correctly? so to me, the neutral axis would be located somewhere between the suface of the plate and the surface of the brace farthest away from the plate. which is why you would want your "I" beam to sit on the plate like this: I and not like this: l-l. like you said, clear as mud. so what im thinking is carbon fiber laminated vertically (or perpendiular to the plate) actually bisects the neutral axis. so i guess i get what you are saying about the CF being out away from said neutral axis, but it would be thin and wide. i thought we wanted braces tall and skinny. what am i missing here? sorry for jacking your thread hesh.

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sweat the small stuff.

And why do you want conventional braces tall and skinny? Because that puts material as far away from the neutral axis as possible.   That part of the brace that's up at the top is doing the work...another reason not to taper it much.   

Also, if you think about the load on the back of a guitar...the neck is applying leverage to the neck block which is attempting to stretch the back and flatten out the doming. That means that the static load on the back puts the back surface in compression and the tops of the braces in tension...a perfect application for graphite topping. Add to that the fact that most players are pressing the guitar back against their bellies, and you add to the compressive load on the surface of the back and the tensile load on the brace tops.

In my back joint reinforcement, I'm using either spruce or cedar in the usual cross grain arrangement, and it is essentially the neutral axis.   It's light weight, decent in compression, and the grain is helping to keep the joint together.   The "flanges" of the I beam effect are the hardwood back and the graphite composite on the top surface.   I suppose I could inlay a back stripe of graphite and make the thing even more radically strong.    Hmmm, maybe I will on the one I'm building now...   

I can say that I really like combining wood and graphite in these ways. I think we can have the best of both worlds.   Ken Parker is doing a lot of this in his own way, and to get the strength and stability of graphite with the sound of wood is a worthy goal.

Hesh,
Again propf that you are a true Mensch sp.
Thank you again for such a great tutorial-your visuals are always great and very helpful.

I just do mine like Todd, straight edge and hide glue in the go-bar. Leave it a few minutes for the HHG to gel then take the straight edge off and clean up. Never had a problem.

Colin

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I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

[QUOTE=ToddStock] In the shot below, the unloaded beam is on top and the loaded beam below it. You could think of this as a back brace, where the guitar is resting top-down on a bench and you are applying pressure on the back (engineers - please ignore the simply supported ends, non-uniform section, etc....it's just easier not to explain degree of fixity, I, etc. right now).

Notice the upper surface has to shrink a bit to carry the load, while the lower surface stretches a little...this is what causes tension and compression forces in the upper and lower portions of the brace. I cut out a small element of the uniformly loaded, rectangular brace, and you can see how both tensile and compressive forces decrease as you move toward the neutral axis, as Rick indicated in his note. The arrows indicate the amount and direction of force being applied to the beam element...I eliminated reaction forces in the upper drawings to simplify things.

[/QUOTE]

Todd,

How strong are these forces given that most people profile the braces to the surface shape of the back rather than fit flat braces and curve the back to tension the braces as your diagram sort of suggests? I would have thought that the main forces on the back will come form string tension on the neck pulling at right angles to ladder braces (subtley different for X braced backs) trying to flatten the long arch, and the pressure of the air pumping in the top which should go in cycles in two directions.

I'd love to see a scientific measurement of "beer belly" force on the back I can see how it would dampen the backs vibration but how much pressure does it exert. Mind you if you did Rick's equivalent of standing on it via a "beer belly slam dunk" the that would be an altogether different matter

Rick,

Nice to see you posting here more, your work and ideas have been a great inspiration in my building development. I find that you can get great projection and volume using bigger arching on the top, and leave the back responsive. Even though my more than ample "beer belly" is doing it's utmost, I still get to feel the back vibrating which I like a lot. Probably as much pschological as scientific I know.

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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

[QUOTE=rich altieri] I also use a year stick to align to bracing materials during the glue process so combining that with the radiused caul worked out extremely well. [/QUOTE]

Rich, is that "year stick" 365 days long?

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http://www.presnallguitars.com

Hesh, it seems to my inexperienced mind that after the radiused caul is removed after gluing the reinforcing strip on the back, that the back will return to its original flat form.  I don't think the strip is thick or strong enough to retain the radius of the caul.

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A higher purpose for wood.
Rich Smith
Issaquah, WA

A conventionally done back seam strip reinforcement will not hold the curve of a domed dish, but if you put graphite on top of that strip, it will hold about 50% of the arc.   It's pretty amazing.

As for the back vibrating...well that may be great for the player, but if you get out about 30 feet away from the guitar and in front, I can only think that what's going into your belly is not going out front. That is energy that's going back, not projecting out forward. I think too few of us get the chance to listen to our guitars out front and at a distance. I make a point of doing that. A big reinforcement to me was hearing one of my guitars played in a jam at Healdsburg about six years ago.   I was about 30 or 40 feet away, and my guitar dominated the mix with a couple of other fine makers' instruments being played. The other guitars seemed louder to the players, but mine had the projection I'm going for with this particular series of instruments.   

I found similar effect when doing some very controlled recording for the Mama Bear project. I was clamping guitars into a rack and we had the mic position and distance consistent to within about 1/8". In the mic position, about 11" off the face of the guitar, Ms. Antarctica (the Henry Kaiser guitar) was 3 dB louder than a 1934 D-28. However if you played the two guitars, the D-28 was louder from the player's position. You simply cannot judge "projection" very well playing a guitar; you have to hear it from a listener's perspective.

Whatever you think of the Smallman sound, his ideas re. projection vis a vis a stiff set of sides and a very rigid back really work. Paul McGill and I have had some interesting chats re. blending Smallman back and side theory with a more traditional top bracing for classicals, and it sure works for Paul. He gets projection and warmth at the same time. That's what I'm trying to get with steel string guitars now, and to give a bit more back to the player, there's always a side port.

Of course, not everyone wants a unidirectional cannon of a guitar. For playing in small rooms or for playing primarily for one's own amusement, a guitar with a floppy back and thin sides may be just the ticket for player satisfaction.   It's probably easier for luthiers to sell guitars that are player-pleasers, too. As for myself, I'm just happy to know that we have a range of options on how to design guitars for different players and different purposes. I may very well design a "player's guitar" that is a more omni-directional instrument at some point using what I've learned about the stability of graphite reinforced wood but going with thin resonant back and sides.   Actually, I'm working on a couple of "extreme" ukes right now on which the backs and sides are down at about 1 mm, and the back seam reinforcement is graphite topped and the three back braces are graphite topped balsa.   I'll have at least one of them at H'burg.   

Hesh, you should write a book.

ok, so 1) hesh, that is looking good my friend and thank you a second time for posting the tutorial. 2) thank you todd and rick for responding to my questions. i think what helped me most was viewing the guitar as a whole. back plate, wood brace, and CF all forming what is essentially an I-beam. i at least get that part now. no more questions... for now. thanks guys!

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sweat the small stuff.

[QUOTE=Rick Turner] A conventionally done back seam strip reinforcement will not hold the curve of a domed dish, but if you put graphite on top of that strip, it will hold about 50% of the arc.   It's pretty amazing.

As for the back vibrating...well that may be great for the player, but if you get out about 30 feet away from the guitar and in front, I can only think that what's going into your belly is not going out front. That is energy that's going back, not projecting out forward. I think too few of us get the chance to listen to our guitars out front and at a distance. I make a point of doing that. A big reinforcement to me was hearing one of my guitars played in a jam at Healdsburg about six years ago.   I was about 30 or 40 feet away, and my guitar dominated the mix with a couple of other fine makers' instruments being played. The other guitars seemed louder to the players, but mine had the projection I'm going for with this particular series of instruments.   

I found similar effect when doing some very controlled recording for the Mama Bear project. I was clamping guitars into a rack and we had the mic position and distance consistent to within about 1/8". In the mic position, about 11" off the face of the guitar, Ms. Antarctica (the Henry Kaiser guitar) was 3 dB louder than a 1934 D-28. However if you played the two guitars, the D-28 was louder from the player's position. You simply cannot judge "projection" very well playing a guitar; you have to hear it from a listener's perspective.

Whatever you think of the Smallman sound, his ideas re. projection vis a vis a stiff set of sides and a very rigid back really work. Paul McGill and I have had some interesting chats re. blending Smallman back and side theory with a more traditional top bracing for classicals, and it sure works for Paul. He gets projection and warmth at the same time. That's what I'm trying to get with steel string guitars now, and to give a bit more back to the player, there's always a side port.

Of course, not everyone wants a unidirectional cannon of a guitar. For playing in small rooms or for playing primarily for one's own amusement, a guitar with a floppy back and thin sides may be just the ticket for player satisfaction.   It's probably easier for luthiers to sell guitars that are player-pleasers, too. As for myself, I'm just happy to know that we have a range of options on how to design guitars for different players and different purposes. I may very well design a "player's guitar" that is a more omni-directional instrument at some point using what I've learned about the stability of graphite reinforced wood but going with thin resonant back and sides.   Actually, I'm working on a couple of "extreme" ukes right now on which the backs and sides are down at about 1 mm, and the back seam reinforcement is graphite topped and the three back braces are graphite topped balsa.   I'll have at least one of them at H'burg.   [/QUOTE]

This is very interesting.... Thanks Rick.

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Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Though I'm late to the party (again!), I wanted to say well done, Hesh. BTW, how about if you bordered each side of your backstrip line with blue tape before you spread the glue? I would act as a stop for the strip, and catch much of the squeeze-out.