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Those of you who use CF sheet stock to top your braces, what thickness do you recommend, and why? On a guitar I'm beginning work on now, I'm planning to use a relatively thick (by comparison to a typical centerseam reinforcement) brace running down the center of the back, with the grain running to parallel to the back grain, and gluing a layer of CF sheet stock on top of it. Any suggestions on thickness of CF to use would be appreciated.

I am also planning to use CF struts, or whatever you want to call them, between the neck block and the waist. There will be 4 of these, all anchored near the top of the neck block, with two of them going parallel to the soundboard and anchoring near the top of the waist blocks, and the other two angling down to the bottom of the waist blocks. I am thinking of using tubes for this, and would appreciate any suggestions on dimensions of CF tube for this purpose, as well as recommended suppliers.

(This guitar will have a floating fretboard extension.)

Thanks!

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Todd,

Give Jim Watts a call. Los Alamos Composites in the link above. Jim is an engineer and guitar builder and his prices, for the volumes we use, I find are as good as it gets. He has been quite helpful with me in the past.

Shane

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Canada

Hey, Shane -

How are you doing?

Thanks for your reply. I've already PM'd Jim to call his attention to this post. I thought I'd do it this way so that his response, as well as anyone else's, would be here on the forum for all to see. I may still call him at some point to talk it over.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Todd, sounds like Rick Turner is the guy you want to talk to. Didn't he come up with that scheme for keeping the curve in his backs in combo with the butresses?

Below is what I've been doing. The cf is 1/4" tubes. I want an active back so I chose this arrangement. The tube that spans across the waist is pinned and meant to keep the waist from spreading and flattening the back.

I remember Rick mentioning that he used to use two bars from the neck block to waist and then went to four but I don't think he said why. I can't imagine needing four. With two, this frame is unbelievably stiff. But my oldest one of this design is only a little more than a year old. Time will tell.

As for the cantilevered fingerboard, you really need to beef up the heel so it doesn't flex, particularly if you leave a lot of meat under the fb.

I'm still debating the tradeoffs of that whole cantilevered design. It is a lot of extra work. There are some real benefits but some headaches too.

I just glued one up with more heel reinforcement. I'll post some pics later if you are interested.

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http://www.chassonguitars.com

I bought arrows from the local sporting goods store for my last guitars' reinforcment shafts, cheap, light, readily available. Whats not to like? Mikey

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The Biggest Little City, Nevada
www.hammondguitars.com
I love building guitars!

Thanks for the replies, Kent and Mikey.

Kent, thanks for the photo of your innards! That looks awesome! I may get in touch with Rick, but I think I've got a pretty good handle on how he does his. What I'm planning will be similar to his, but also similar to yours. He doesn't use CF struts from the waist blocks to the tail block, like you do and like I am planning to do (I didn't mention that in my first post). My back center brace will be different from his as well. I'm still working out the details, but the basic idea is to make that a substantial brace, grain running lengthwise, radiused on the bottom (to create and maintain the back's radius, longitudinally) and flat on the top, topped with CF. Like his, the rest of the back braces will be ladder-style and notched to bridge the center brace.

My thinking on the four-strut structure in the upper bout is this: it seems to me that, without the upper two struts running parallel to the soundboard, the force of string tension could tend to fold the guitar up at the waist. Imagine the box as you've built it, with the top removed. I see the structure as having a sort of hinge at the bottom of the waist blocks, where the two upper bout struts and the two lower bout struts come together. The sides, then, under string tension (if we can visualize imaginary strings applying that force without the soundboard on), would want to buckle, and there's nothing keeping the top of the waist blocks and the neck joint area from collapsing towards each other (except the strength of the sides themselves). With the soundboard on, it (the soundboard) is carrying that load. I'd like to relieve the soundboard of that duty and give that load to a pair of upper struts (and possibly a second upper pair from the waist blocks to the tail block, i.e. four struts in the lower bout as well). Am I making sense? That's how I visualize it, anyway. I could be wrong, and you seem to be having success with the structure as you've designed and built it.

Your strut that spans across the waist makes sense to me. I think I can achieve the same thing by putting a brace there, but it might be good to think about anchoring it solidly in the waist blocks - something analogous to your pins... in fact, maybe I'll use pins! I'll have to think more about this same issue (spreading the waist and flattening the plate) with regard to the upper struts and the soundboard. I'm not going to have a transverse brace running across the top at the waist. Hmmm.....

With your cantilevered FB design, you've discussed a lack of rigidity that is causing your neck angle to move under string tension, and attributed that to flexing in the heel. How do you know the heel itself is flexing, rather than the neck joint/upper bout structure? Are you measuring actual flex in the heel itself?

I really appreciate all your input on this. I am largely following your lead (and Rick's), and just tweaking the design in ways that make sense to me. Thanks again!

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Hi Todd

Rick tops his braces with .021" and runs the same lench under his linings. I'm running it outside the linings because it's what works with my skill set right now.

The back strip is topped with CF of the same thickness and runs the length of the back, inlet into the neck and tail blocks. It's glued in with the back in the dish. The braces are rabbited to go over the back strip.

The pillars that the 1/4" round rods go into rest against the back brace, opposite side of the neck block.

The back strip takes care of the neck pull, which keeps the back domed.

The gap under the fingerboard extension changes. I start out with an even gap and after several months, one of them started to close up at the soundhole. Here's my original design.



There is a through tenon in the heel.

Here's the latest version. I went to a tapered tenon so it could be much wider. I also went with maple instead of mahogany. It's a pain to make but it is very stiff!

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http://www.chassonguitars.com

Hey guys,

I am working on something like this too. I am trying to do ball joints for the buttresses so I could either remove them or shim them at some point just to retain some level of correction. I haven't gotten too far with it though. As for the floating fingerboard warp, I was not gluing or bolting down the fingerboard extension for a few and on all of those guitars I got a very defined bend right at the body join. The top had not caved in at all, measured across it was the same. It had to be the back and side joint coming up and out. I have changed how I brace and radius the upper bout on the back and it helps a lot, but just even 1 bolt under the fingerboard is much more effective (the stiffly braced top upper bout resisting that pull). I have been trying to come up with a way to do a floating fingerboard and not still have that problem but am stymied so far, I will be very interested to see what your results are Kent. Other than beefing up the spline, I was thinking about a real spanish foot and what Todd is thinking, making that intersection of upper cross brace and longitudinal brace much stronger. This is a good topic!

Todd, if Jim can't help you you can also look at graphitestore.com. I ordered some stuff recently and they stock some strange sizes, which minimizes the need for cutting it, which is good!

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

Todd, As far as capping braces goes with cf people seem to like the .02 - .03 stuff. I'm not sure how effective this really is from just a stiffness standpoint and here's why;
While the stiffness of unidirectional cf in a matrix of 35% resin is about 12-14 times stiffer than spruce, on the other hand you pick up geometric stiffness at the cubed rate as the brace grows in height. For example a piece of cf that is .03 thick has the equivalent stiffness of a piece of spruce that's .07 thick or so depending on your spruce.
There are other effects also however. Carbon fiber has a much better stiffness to weight ratio than spruce, one of the very few materials that does to my knowledge, and as such the sound waves will travel through it differently. Carbon fiber is also extremely stable which may help you here. Some time in the future I'd like to set up some models and experiments to try to flesh this out for folks who would be interested.
I think the tubing for the neck block is the way to go.
To try to end on a more encouraging note for capping braces, earlier this year I made a Smallman type top for a steel string and it sounded great , I don't believe I would of got that sound with just spruce bracing.
Kent, your box there looks great. I've also been kicking around using tubes to help support the neck block, but haven't done it yet.
BTW Todd it was a pleasure meeting you at the GAL conference in June.

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Jim Watts
http://jameswattsguitars.com

When you are using CF as a capping it is not the bending stiffness of that piece of CF that is important, it is the stiffness under tension that matters.
By capping a brace (a beam of spruce with a flange of top or back wood) with carbon fibre on the other face you have provided an extremely strong and stiff material at the exteme fibres location on the beam, and that is why it makes a difference.

Jeff,
I'm probably missing something here, as I'm pretty tired and may not be thinking clearly. Are you saying your concerned about the tensile strength of the brace, for example with carbon fiber you can flex the brace a lot more prior to it breaking? Which is obviously true.

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Jim Watts
http://jameswattsguitars.com

You've got to think I beam type of construction here.

If you take a spruce brace say .5" high and cap it with CF .03" thick, you get roughly the equivalent of a solid CF brace .530" thick (if I'm remembering my statics correctly).

This is why an i beam is the shape it is. Bending is dictated by something called the second moment of inertia. it turns out that when you do the math only the extremes of the beam are really "doing" anything to keep it from bending so it's o.k. to remove all of the material that's not at the extremes save enough to pass the shear force to the extremes.

At the "top" of the brace, the CF is completely in tension where it does best - hence the stiffness should go up a lot.

Not the best explanation I know, but that's basically how it works.

That's why I always thought it funny that people were lining the sides of their truss rod channels with CF when if stiffness is what you're after, lining the bottom would have the biggest effect (if the neck is thick enough to handle it and total stiffness is what you want)

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

No Jim, I am saying that when you are attempting to bend a capped brace (by pushing from the outside) the extreme fibres of the inside of the beam end up in tension and if you have the carbon fibre glued there, then it is acting in tension.

When you produce a glued up beam CF/ spruce/ plate you have to look at it as a combined system rather than considering bending stiffness of each component.

That is why placing such a small amount of CF as a capping in the right place can make a big difference. Place it at the neutal axis and it would do very little.

Hey Andy, thats why my last neck has a carbon fibre rod epoxied as far to the back of the neck as I could. Combined with a Cooktown Ironwood fretboard it is very stiff.

Andy and Jeff, You are correct and I left an important part out of what I was saying, my apologies to all. As I said I'm tired these recent days.
I'm pretty certain it won't look like a solid piece of carbon however due to the extreme differences in materials. I think I'll cap a piece of brace stock tonight and deflect it prior to and after capping. One of the points I was trying make is that you can make up stiffness with height. The .03 thickness example was a bad choice on my part as you can't carry it over to capping.

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Jim Watts
http://jameswattsguitars.com

No, it won't be quite a stiff as a piece of solid CF, but it will be very close. A spruce beam capped on both sides would be even closer.

The calculations to figure this are rather straight forward actually - the problem is it's been about 15 years since I've done them! I'll bet that they're in "Left Brain Luthiery" If I can find my copy I'll check.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Andy, I actually have it also, it's like doing a cored panel with the spruce as a core.

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Jim Watts
http://jameswattsguitars.com

Couldn't that be happening because of distortion of the box, rather than flexing of the heel? I understand you have found the frame, as you've constructed it, to be extremely stiff, but I still wonder if, over time, due to the "creep" of the wood, you may be seeing the effects of some distortion of the box (perhaps because the structure really needs the additional upper struts) as much or more than flexing of the heel. Do you think I'm out to lunch and just being a pesky thorn in your side, or could I be onto something here?

In any case, it makes a lot of sense to me to make the heel as rigid as possible. That would certainly seem to be another important piece of the puzzle, and I appreciate you calling attention to it. Rick, as you probably know, welds a steel rod to the truss rod; that rod goes down through the heel (and is what his bolts thread into). I've been thinking about doing something very much like what you've done with the tapered tenon in your latest version. I'm guessing the tenon goes all the way up through the neck shaft, i.e. you can see the end of it flush with the surface the fretboard will be glued to. Am I right? That's what I'm thinking of doing. Did you cut the through mortise in the neck shaft (if I'm guessing right) by drilling holes and then paring away with a chisel, or do you have some kind of mortiser that does that? At this point, I'm not planning on tapering the tenon, but I am planning on making it thicker than yours, and making it project a bit deeper into the neck block. It seems to me that that added "meat" should do the trick with stiffening the heel. Then I'll put barrel nuts in it to bolt on the neck. For this next guitar, I'm actually thinking of bagging the adjustable joint plan, so this would be essentially a traditional mortise and tenon neck joint, but with the elevated/floating fretboard extension. Even without the adjustability hardware in there, it will still make a neck reset very quick and easy by not having to deal with the FB being joined to the top; plus it will have the effect on sound that I believe an elevated fretboard has, due to the different angle of the string pull on the soundboard.

Thanks a lot for posting these photos of your heels! Very helpful.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Quick question:

Does it make a difference in stiffness whether or not you cap the top or bottom of a brace, or a truss rod slot. I understand that on any bend you have tensile resistance on the outside of the bend and compressive forces acting on the inside. Would the contribution to overall stiffness be the same either way?

-Clint

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-Clint Beacock

Revealing my ignorance, here... what's a ball joint?


Not quite sure what you mean by that.


My upper bout will not be stiffly braced (the soundboard, I mean). That's the other part of why I want to pursue this design, to have the whole top be fully acoustically active.


By "the spline", are you referring to the heel tenon, or - ?


I'm not quite following you here, either...

I appreciate your participation in this, Burton, that's why I'm trying to understand you better. Thanks!


Thanks for the lead. Jim supplies CF sheet, but not tubes - right, Jim?

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

A top or back brace already has a big cap (the top of back plate) glued to one side of it so you want to put the CF on the other side.

On a neck all the tensile force is applied on the front by the strings so ideally you want something strong in compression at the front of the neck (such as a stiff fingerboard)
and something strong in tension towards the back of the neck

Yes, it makes a difference. The bottom of a brace is already effectively capped by the plate (the soundboard or the back). So, capping the opposite surface is what makes it like an I-beam. In the case of the truss rod, it's a similar thing. The fretboard is analogous to the top of the "I", while CF placed near the bottom of the shaft would be analogous to the bottom of the "I".

The only problem, as I see it, is that, with a relatively slim neck profile and a truss rod, it's not practical to try to squeeze some CF in there at the bottom of the shaft. It seems to me that putting a couple 3/8" tall rods on either side of the truss rod is better than none at all. It will still stiffen the neck somewhat simply because the CF is stiffer than the neck wood. Rick Turner uses 1/2" tall CF rods and cuts grooves in the underside of the FB. I might go that way myself.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

I posted simultaneously with Jeff, who put it more succinctly and precisely.

Thanks to Jeff, Jim, and Andy for their informative posts in this thread! Much appreciated.

My understanding of one of the other benefits of using CF is that it doesn't "creep" over time, so it provides a certain amount of limitation to how much things can distort. I see it as kind of putting a lock on things, rather than just submitting to the inevitable effect of string tension on wood over the years. If any of you more knowledgeable folks would care to elaborate on that (or correct me if I'm wrong) for the benefit of all here, please do.

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Todd Rose
Ithaca, NY

https://www.dreamingrosesecobnb.com/todds-art-music

https://www.facebook.com/ToddRoseGuitars/

Todd,
I have little practical experience on this , but just strung up a Guitar with elevated fingerboard a week ago.
I have a background as an engineer but have not practiced for 15 years and did not do any calculations, just went in the directions I felt appropriate.

I went for a Stauffer style adjustable neck with the neck heel set about 5mm into the body secured by a single 6mm bolt. I beefed up the neck heel thickness a little since I did not have the extra heft of a dovetail block.

I was going to do CF rod braces below the soundboard, bought all the rods, but then chose just to add some diagonal spruce braces glued to the top from the top corners of the neck block to the UTB a little outside the soundhole bracing. Not quite Martin A style.
No popsicle brace obviously.

No long term experience, but I am confident