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Hey Todd,

Hopefully this helps, reading it back over I wasn't very clear (and maybe my ideas aren't either!):



A ball joint in this case is the rod with balls on the ends which fit into round cavities. I would take a 5/8" wooden ball and drill a 1/4" hole halfway in and cap the inside with ebony then the 1/4" CF rod (LMI has these now) would fit in there (no glue). The other side would be epoxy filled at the end and have a 1/4" radius shaped into it. In the neck block I would rout out a 5/8 diameter opening with a round bottomed bit and in the side support round out a 1/4 opening with a round bottomed bit. Once the sides are in and lined and the neckblock is in you trim the CF rod until it fits in perfectly and whammo. If it ever moves or needs to be adjsuted you remove the strings, pop out the 1/4" end(it would be a shallow accepting hole) and shim the 5/8" end inside the ball. In my head it works but I haven't put it in practice yet.



I mean that the movement is from a distortion of the sides(at the heel) pulling out towards the headstock letting the neck move up without the top caving in. Imagine the radius of the upper bout of the back being compressed(flattened), it means in the flat plane there is more there than there was and something has to move(or crack).



I agree that the upper bout of the top needs to be active to make this worth it, but I was also talking about the upper bout of the back. I build them flat from the heel to the waist (like people do on the top) so there is no radius to flatten out. The pull is shear (kind of, if I am imagining it correctly) against the back plate. When the fingerboard is glued or bolted down it resists that torquing of the neck block very well assuming the upper bout does not cave in. I think you have to compensate for that in the upper bout of the back if you float the extension.



By the spline I mean the maple spline that Kent is adding



The spanish foot is the neck block extending to the first cross brace on the back. By making it stronger I mean doing something similar to what Rick's CF capped brace (and your longitudinal grained and thick center seam brace) does to the strength of the back, especially in the upper bout. I have also really beefed up the waist brace on the back but Kent's cross dowel there is awesome too. I am also trying to make the lower bout active.

I hope that helps, if not let me know!

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

Alright, I was just out putting together a rosette which I use epoxy for (I build them off the sound board ) so I grabbed some brace stock that I had deflected the other day and bonded on a .022 piece of carbon. Tomorrow I'll put it back in my deflection jig and recheck it and report back for all who are interested.

Todd, you are correct we don't make or sell any tube stock.

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Jim Watts
http://jameswattsguitars.com

It's the heel. I was trying to get away with a small heel and a butt joint. That could work with metal in there but I don't want the weight. I wanted to keep it a butt joint because it's a pain to mill the combo of the tenon AND the cantilever. And yes, the tenon goes through the neck. I mill the mortise in the neck with a pin router but it could be done easily on a router table too, or by hand as you suggest. I cut the mortise in the heel with a router table and cove bit. I'm looking for a good ball-nose 3/8" spiral cutter with a 1/2" shank to make that easier. As it is, I have to extend the cove bit further out of the collet than I would like and take lots of passes to get the 1" depth of cut at the wide end.

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http://www.chassonguitars.com

I built 3 "prototype" guitars with floating FBs last winter, all 12 frets to the body. I think Kent is right about the heel bending. That's what happened on my 1st, I can clearly see the end of the FB extension getting about .100" closer to the top when strings are brought to pitch, it almost kisses it. On this one I lap-jointed a tapered piece of mahogany under the floating FB, not unlike an archtop, but no contact at all with the top, 2 x CF rods in the neck and no truss-rod.
On the other ones I used 2 x CF rod dadoed into the FB and neck stock (à la Turner), truss-rods, dadoed Spanish heel butting on the 1st back brace and added a small block of hardwood between the CF rods with a 3/16" insert under the floating extension next to the mortise. It bolts through the neck block "paddle" and the top. The neck block paddle butts onto the UTB, the UTB is dadoed into the top linings with supporting lining pieces under it.
IMHO bolting the very beginning of the FB extension stabilises everything. I think that's why archtops traditionally have the part of the floating FB close to the heel glued onto the top, or rather the part of the top over the neck block.

Regarding laminations on the back centre graft and back braces I've been doing it for my last 6 guitars. I use .040" bloodwood, cocobolo, EIR or ziricote over the spruce. I don't feel the need for CF there.
Just cross laminating the centre graft with this thin piece of hardwood stiffens the back longitudinally, it's incredible and really surprised me.
That and the the capped braces raise the pitch of the plate, make it very stiff and more inclined to keep its exact radius. It also allows me to go even thinner on the back when I dare. I still scoop the ends of the braces and keep about .125", or less, under the linings. I sort of "tune" the back with the length and height of the scoop. I didn't feel it radically changed the tone of the box, it definitely helps with projection, and perhaps sustain.

I never felt there was much going on on the upper part of the top but I am interested in reading what you guys find out, or have found out with the flying buttresses and a "free" upper bout.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

I don't think it's about generating sound straight from the upper bout. I think freeing that area creates some northward expansion of the footprint of the top that's pumping in monopole.

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http://www.chassonguitars.com

I don't have time to read all the new posts right now, but I just wanted to thank James Orr for his info on Rick Turner's approach. Sorry, James, that I forgot to thank you in my post last night.

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Todd Rose
Ithaca, NY

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

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

Ok, in an attempt to provide some objective data to this (ok, really to make up for my poorly worded first post ) I capped a piece of brace stock with a piece of .021 unidirectional carbon fiber at 35% resin content.
Here are the results:

The original piece of brace stock was .714 X .318
It was deflected between 12.0 inch centers with a load of 16.26 pounds.
The brace in it's original form deflected .040 inches, which is equal to a modulus of elasticity of 1.52 msi.

After capping with .021 cf it was .735 X .318, same load and center spacing as in above.
it deflected .025 inches, which is equal to a modulus of 2.23 msi.

Now if we could go back to the original brace (modulus of 1.52) prior to capping and change the dimensions to .835 X .318 we would get a deflection of .025 equal to the capped brace. This is a growth in height of .121. This is based on standard beam deflection calculations.

I hope this is of interest to some and gives an idea of the effect of capping a brace. Carbon of .03 would have a bigger effect while .01 would have a smaller effect. Also, as the braces get shorter the delta in height between the capped and uncapped becomes smaller also.

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Jim Watts
http://jameswattsguitars.com

Hi Jim, interesting to see your results.
I think you would have got a much bigger improvement ( and more relevant) with CF capping the brace if you had tested the brace with a flange equivalent to say a 2" width of top plate material glued on, tested with this flange on the top so that it is in compression and then with the CF capping added to the tension face.
A plain rectangular brace with CF to one flange only is going to be limited in stiffness by the spruce on the other face.

Thanks for describing this, Jeff. Is this a steel string guitar? Does the neck block have a "foot" on either the top or bottom or both (like a Spanish foot, and as Laurent describes)?

I've never seen the inner workings of a Stauffer style adjustable neck joint. What does the upper part of the heel pivot on? And how is the neck set for alignment with the centerline of the guitar?

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Todd Rose
Ithaca, NY

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

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

Thanks for describing this in more detail. I see now that you meant, literally, a ball joint, much like on a car, or our hips and shoulders. Cool idea. I'm still not sure if I follow all the details, but that's okay, I get the idea. My hesitation with that would be that even CF has some flex; going for maximum stiffness, it seems to me that I would want the ends of the rods held captive, i.e. glued firmly into holes rather than allowed to roll on balls, which would then allow more lateral flex in the rods. Follow me?

...But your idea of allowing for some adjustability is interesting. It makes me think of using a turnbuckle or something like that. Of course, this is getting into more complicated and bulky/massive stuff, so it's kind of a crazy idea, but sometimes a little brainstorm like that leads to viable solutions...


Makes sense. Basically, I see the centerline brace on the back (as I described earlier) serving that purpose. A beefy brace, longitudinal grain, radiused on the bottom, flat on the top and capped with CF... so, there's a tension rod of sorts going straight (since the brace is flat on top) from the neck block to the tail block.

Another idea I'm playing with is a separate tension rod, floating above the back, that also serves as the adjuster for an adjustable neck joint. This rod would go through the neck block and thread into the lower part of the heel at the neck end, and on the other end, it would have a head that would bear on the tail block and be accessible from the outside, to be turned with an allen wrench.

Thanks again for your input on this!

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Todd Rose
Ithaca, NY

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

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

Thanks, Kent. Got it.

Yes, milling the combo of the tenon and the cantilever is the bugger, isn't it? I've been puzzling over how best to do this, so it was very helpful for me to see your photos, and I appreciate your further description of how you did it.

When you suggested cutting the through mortise in the neck shaft on the router table, I had a moment. I really don't have any experience with mortise and tenon joinery in general, so thanks for shining the light bulb of the obvious into my dim brain. Anyway, your solution seems very good to me. What I'm now thinking I might do is cut the neck shaft mortise on the router table (I don't have a pin router), then square up the ends by hand with a chisel. That way I could cut the mortise in the heel with a straight bit (or spiral cutter with a square end like a straight bit) and use a square tenon.

Again, lacking experience with mortise and tenon joinery, I've never used a hollow chisel mortiser, so I don't know exactly how they work and what their limitations might be, but I'm wondering if one of those might do the trick. I'm thinking here of having the heel block already glued onto the neck shaft, then cutting the mortise right down through the shaft and the heel block. Any thoughts on that idea?

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Todd Rose
Ithaca, NY

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

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

Thanks, Laurent, for joining this discussion!

When you said:

...there are a couple things I didn't follow. First, the Spanish heel. Do you mean a full-out Spanish heel, i.e. the heel and neck block are integral, with the sides set into slots in the block? And when you say "dadoed Spanish heel", what do you mean? Then, "a small block of hardwood between the CF rods with a 3/16" insert under the floating extension next to the mortise"... I get that this is a block that is joined to the underside of the FB extension, above, and the soundboard/neck block, below, thus solidifying and stabilizing the structure, but I don't get what you mean by "with a 3/16" insert under the floating extension next to the mortise".

Anyway, I appreciate your description of how you've found solutions that basically modify the existing structure of the neck block and upper bout rather than adding an internal CF framework like Kent and Rick T. have done. How have these two guitars been holding up?

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Todd Rose
Ithaca, NY

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

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

And finally (for now), thanks to Jim for the CF cap testing -- and to Jeff for challenging your results! Very interesting and pertinent stuff...

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Todd Rose
Ithaca, NY

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

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

No, just a neck block with a butt-joined "paddle" under the top and a narrower "heel" (plate) dadoed into the neck block (rather than butt-joined), glued on the back and butting on the 1st back brace.


The small block is CA'd between the rods butting on the neck tenon under the FB and has a 3/16" bolt insert, the bolt fastening the very beginning of the floating FB to the paddle on the neck block. This small block's thickness determines the clearance between the top and the FB extension right at the heel. I also dado 2 x 3/16" channels into the top/neck block paddle for the CF rods (I leave them full height, they're inserted maybe .125" into the FB and the rest into the neck stock/top-neck block paddle). Again those are 12fret guitars, so the cantilevered part of the FB is quite long.


They're young, but so far they're fine. I don't want to say, but it seems to me they sound a hair more open than the glued-on FB version. But it may be other factors as well.
My perception is that the neck heel will remain flexible when nothing holds the FB to the top. I am not an engineer at all, but it seems logical and obvious.
Unless, like Rick Turner, you weld a steel rod 90º to the truss-rod buried in the heel.
I forgot to mention that on the 1st guitar I CA'd a hard maple dowel in the neck tenon, didn't seem to help much. Perhaps, there, a short length of CF round stock could stiffen the neck heel…
Unfortunately I don't have a digital camera with me, that would really help…

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Todd, have a look at this for some original Stauffer neck joints.

http://www.euroguitars.co.uk/index.php?topic=229.0

The top of the neck is basically just pivoting on the endgrain of the soundboard.
Seems crude but appears to have worked.

I chose to use a piece of 1mm thick by 3mm wide CF as a pivot but it is still bearing on the endgrain of the spruce top underneath.

I just used a plain headblock but used two diagonal spruce braces to transfer load to the area outside the soundhole and interupt potential shear cracks in line with the edge of the block.

I actually disagree with you here - I think Jim's test is perfectly relevant. It does show that adding the CF cap does have the effect we described, perhaps not to the magnitude we both thought though. A regular spruce brace with no cap will be stiffer attached to the top as will the one with the CF cap.

The next interesting question would be is the capped brace any lighter than the non-capped brace of equivalent stiffness? Yet another would be does a capped brace resist cold creep the same way the sandwiched ones do (i.e. with the hold their shape longer than just wooden braces)?

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

[quote="Andy Birko"][quote="Jeff Highland"]
I actually disagree with you here - I think Jim's test is perfectly relevant. It does show that adding the CF cap does have the effect we described, perhaps not to the magnitude we both thought though. A regular spruce brace with no cap will be stiffer attached to the top as will the one with the CF cap.

Andy we are not in that much disagreement,

I was suggesting testing both CF capped and uncapped with a section of top attached to see what improvement is achieved.
What I would expect is that the magnitude of improvement in stiffness with the CF capping would be greater than Jim found with just testing the brace/CF unit

Once you start glueing components together you really do have to consider the total arrangement rather than taking the components in isolation.

There's another way of looking at this, at least as far as the back graft is concerned.

My understanding of the purpose of capping that brace is as follows:

The red arrows show the direction that the strings are trying to pull the rims. The butresses are there to stop that. If you assume the rims can't pull together at the top, then you've transerred that force to trying to spread the rims at the back, the direction of the green arrow.

A domed back would then want to flatten out over time allowing the rims to spread and the neck angle to change. If the goal of the cf cap is to make the back stiffer to resist flattening, then stiffness is what counts and capping is marginally helpful.

But another way to look at it is that the goal is to have the cf cap in a staight line between heel and butt, like the dotted line. In that case, the cf cap would have to stretch in tension for the back to flatten. And we know that ain't gonna happen.

There are other ways to help keep the rims where they are too. Solid (or capped) linings, larger end blocks, and my personal favorite, rounder ends at the butt and heel that have more inherent stiffness (like an arch).


edit: Of course the another way is to make a back that is domed across the width but not the length. Wood is pretty good in tension too.

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http://www.chassonguitars.com

Jeff - Thanks for the link, the photo of your top bracing, and the further explanation. So, I take it from the X bracing, that this is a steel string guitar. I'm surprised you're not experiencing the crushing/compressing of wood where the heel bears on the block and end grain of the spruce. I've made one neck joint very much like this, and that is one of the problems I've had to overcome. It taught me a lesson in how powerful string tension is.

I'd still like to know how you dealt with setting the neck in terms of alignment with the centerline of the body.

Laurent - Thanks for the additional explanation. I got it all this time.

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Todd Rose
Ithaca, NY

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

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

Kent, thanks for the drawing to help clarify things. Having the CF cap on the back's centerline brace lie in a straight line between the neck block and the end block is exactly what I'm planning, and what I was trying to describe - this brace will be radiused on the bottom, but flat on the top; it will be tall enough to have ends of about 1/8" height where they meet the sides (I haven't yet measured how tall it will therefore be in the middle on a 12' or 15' radius back); the ends will be inlet into the neck block and the tail block.

My guitar shapes are all round between the shoulders (across the top of the upper bout) and at the butt. I've never liked shapes that are flat, or relatively flat, in those areas anyway. I will use reverse kerf linings at the very least; I'll probably go with capped linings like Charles Fox's, maybe capped with CF.

Giving the back a cylindrical curvature, rather than domed, as you describe, is an interesting idea that has some appeal. The problem with that that I see is that it doesn't give the back the same ability to move with changes in moisture content. Then again, a domed back that is stiffly braced with a CF-capped centerline brace, as I'm planning, might not be able to move, either - that brace might not let the back flatten out a little if it gets dry, because that would put the CF cap in tension. Ah, well, humidity schmumidity!

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Todd Rose
Ithaca, NY

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

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

[quote="Todd Rose"]Jeff - Thanks for the link, the photo of your top bracing, and the further explanation. So, I take it from the X bracing, that this is a steel string guitar. I'm surprised you're not experiencing the crushing/compressing of wood where the heel bears on the block and end grain of the spruce. I've made one neck joint very much like this, and that is one of the problems I've had to overcome. It taught me a lesson in how powerful string tension is.

I'd still like to know how you dealt with setting the neck in terms of alignment with the centerline of the body.

I did have the neck alignment slightly off on initial stringing With using the carbon fibre bearing strip, I was able to add on another layer on the low side and file it down till I got perfect alignment.
Yes it is a steel string
Only had it unstrung and unbolted the once so far and did not see any crushing of the spruce endgrain or neck wood, will have another look next time.
Certainly no sign of any distress on this area visible from the outside.
I too was surprised to see those pics of stauffer joints on jazz boxes with just bearing on the spruce, I guess it is strong enough but would surely be easily damaged.
One guy I came across had used a bone bearing strip.
I had a lot of CF strips lying around so used that.

Jim, if you're still checking in on this thread, I'd be interested to know if you deflected the CF capped brace such that the CF was in tension or compression. And, like Jeff, I also think it would be beneficial to see what happens when the other side is capped with a flange of spruce or rosewood, if you're up for doing one more test.

Thanks!

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Todd Rose
Ithaca, NY

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

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

Jeff, thanks for your further explanation. I'm using a strip of bone on the one that I made like this. I think that will fix it.

Another general question for y'all: Does CF sand okay? Does it dull sandpaper quickly? To date, I've only used it as rods to reinforce necks, and I just cut it with a hack saw, lightly rough up the surface for better epoxy adhesion, and install it. I've been told the dust is very bad to breath, so I have avoided any further grinding/sanding, etc. If I use it to cap my linings, there will be a fair amount of sanding in the dish, so I'm hesitant about that.

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Todd Rose
Ithaca, NY

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

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

IME CF dulls edge tools and sandpaper fast. The dust is nasty, and like ebony, gets everywhere. Maybe Jim can give some advice here as he cuts and sells the stuff.
Whereas I see the purpose of CF as rods dadoed in the neck stock and/or FB, I am more skeptical of its use to cap the centre graft or back braces. A dense hardwood seems to do as well IMHO. I think it's the structure (the lamination) that counts more than the material here. What is the difference in stiffness and weight between .020" CF and .040" Honduran RW?
But again, since this is all Turner inspired, I think Rick also uses CF as purfling material to further stiffen the rim.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Thanks for sharing your experience with working with CF, Laurent.

Your point is also well taken about using hardwood lamination rather than CF - another possibility well worth considering. As I understand it, the advantages of CF for applications like capping the back centerline brace (I'm calling it that because, in the design I'm working on, it is decidedly a brace and not just a centerseam graft) are that it has superior tensile strength and that it doesn't "cold creep", or distort, over time.

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Todd Rose
Ithaca, NY

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

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