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I once used a prefab brace set from Grizzly, and there was no way to put a cap on top. The X braces were radiused on the top, even at the joint. Not sure if they still do them that way.

Anyhow, I think I should put a gauze patch on. Good idea.

[QUOTE=fmorelli] .....I can see no reason for fillets (strength is in the wrong direction with a fillet.....[/QUOTE]

Please don't think I'm picking on you, I'm not. Others have said pretty much the same thing. I don't understand the idea of strength in the wrong direction. A fillet doesn't add strength in just one direction. The fillets would enhance strength in the same directions that the glue inside the joint adds strength, yet no one would ever not glue the joint.

I'm not trying to be argumentative.
I'm not even bought into the need for fillets. I'm just trying to discuss and explore the idea.
The objections to the idea, at least to me, don't seem as sound as the idea.
Perhaps I'm a little slow and just don't get it. Perhaps I'm not. Who knows?   

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

[QUOTE=SteveS]   I don't understand the idea of strength in the wrong direction. A fillet doesn't add strength in just one direction. The fillets would enhance strength in the same directions that the glue inside the joint adds strength, yet no one would ever not glue the joint.
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Steve:
I'm with you on this. If you think of two fillets and the intervening brace wood, they are reinforcing in just the same way that a cap does.
I've used fillets (epoxy) in boatbuilding (a lot) and they stiffen the joints in all directions.
I don't see any problem reinforcing the 'X'- this is surely one of the most rigid areas of the entire top, so any reinforcement wouldn't be likely to degrade the sound much, I'd think.
I may do a few tests with some scrap. This is interesting.
Thanks for the thread.
John

Steve – I’m certainly no expert… but IMHO I think your idea has great merit and your reasoning for doing it is sound.

Of course everyone who points out that the greatest strength from adding the fillets to the X brace would be where it flexes horizontally is 100% correct. But don’t let that notion overshadow the fact that strength is also being added on the vertical plane as well. It might not be the normal reason for using this method of joinery, but we shouldn’t deny its other benefits.

To me, you are adding strength vertically by virtue of the additional glued surface used to mate the pieces - locking the two pieces more solidly together at the intersection.

In my eyes it is kin to the principle that Alan uses when he mates his back plates with the faces to be mated having been planed on matching angles – more glued surface, added strength at the joint. The argument could be made that - everyone else in the world (figure of speech) planes the matching faces at 90 degrees – and there are guitars over 100 years old that have not failed. Is that a good reason for not adapting more sound construction methods?

I think Evin would suggest that we should make our evaluation based on the strength gained against the additional weight we are adding. I don’t see any significant weight added by using fillets. And if the added strength makes it too stiff at the X brace, that only means other opportunities are presenting themselves – to shave more here or there,etc.

Unless you are on a tight production schedule and budget, I say go for it. I see little chance for it causing a problem. And the fact that you have identified this little “thing” means your build will be unique – a little more “you” in it. And that would be a good thing – IMO.

Filippo wrote:
"Since my X brace was (classic Martin) scalloped and equal height, I mixed wood glue and wood dust, and simply fill the gap until it was solid from top to bottom. Looks good, easily as strong now in both directions. "

It may seem so, but it's not.

We're talking about a 'box' or 'half-lap' joint here.
If the brace is 6mm wide and 14mm tall, you cut notches in each brace 6x7mm at the crossing point, one notch in the 'top' of the brace, that faces toward the top plate, and the other in the 'bottom', that faces into the box. Being a good craftsman, you cut these tight, and glue the joint on all surfaces, of course.

The notch that faces 'up' is bridged over by the wood of the plate. The notch that faces downward is open. Of course, it is filled in by the wood of the crossing brace, but that's only effective in resisting compression loads. On the completed guitar this part of the joint is in tension, since it's in front of the bridge, and the front edge of the bridge is pushing downward, trying to open the notch.

In your filled joint, the only thing resisting that tension load is the glue and sawdust mix you put in it. Wood glue is not really very strong in tension, nor does it stick well to end grain, due, in part, to the shrinkage as it dries. So there's not much holding that part of the joint together.

Once that lets go (and it will) you've got a situation where there is a lot of stress concentrated in a small area. This is because the stiffness of the brace goes as the cube of the height. At the notch the brace is only 1/2 as high as it is on either side, so it's only 1/8 as stiff. It 'wants' to bend a lot more right there. This puts a very high load on the wood just at the corner of the notch, where the stiffness changes suddenly, and it is not unusual for the brace to crack right at that point. Once it does that whole area of the top is much less stiff in that direction than it is supposed to be, and it dips down. I can't tell you how often I've seen this on old guitars, or even ones, like my student's Taylor, that are not all that old.

You can also see the effect of that stiffness difference if you look at the way the 'free' plate vibrates. Ones without the patch over the joint have quite asymmetric modes, and they clear right up when you add that little patch. It's not as obvious on completed guitars, of course, but it probably makes some difference.

As I say, for braces like those in kits that are pre-rounded, or tapered in width near the top, the cloth patch makes a surprisingly large difference. It's not as good as a wood patch, but it's sure better than nothing.      

Alan -you speak in volumes!
Mike Collins

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Mike Collins

I was thinking to myself, my next guitar is getting some fillets. Just then the thought popped into my head that I don't have to wait, it wouldn't be that hard to add fillets to a couple of guitars I have. I can get immediate feedback.
Hmm.....



Anyone else want to try?

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

FWIW - I like the idea. Think I'll join you. Simple enough addition.

I see fillets with a small footprint and with the grain running perpendicular to the grain direction of the braces. That should also help to prevent the brace from splitting under stress at the "x" intersection where they have been notched.

Hmmm,
Next time before I disclose a good idea, I'll patent it first.    

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Steve, Just a suggestion here, but if you try this on your next one, I'd leave one of your x-braces.....well, as an example only, let's say your braces are .250" wide. If you left one about .75" wide within say .25" of either side of the intersection, then carve the fillets once the X is glued, I would think the joint would be even stronger with that much gluing surface!

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DES - Farwell, MI

Here's what I mean (no, I didn't take this long to draw it! ).

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DES - Farwell, MI

One problem with that setup, Darin, is that you're still gluing end grain to side grain, and the joint will have little or no strength in tension unless it is fitted very well and glued with epoxy. Even then.

In fact, the whole idea of fillets seems less and less good to me the more I think about it. The only way to avoid having some end grain joints would be to make the fillet pieces run across the grain of the braces, and then they will not be nearly as strong in tension as the wood they're glued to. By comparison, the 'patch' over the open side of the box joint can be as strong as you want to make it, with little, if any, added weight, and it is simple to do. Why make things more complicated _and_ less strong?

Ya, I see what you're saying. K.I.S.S.! Plus, how tedius would it be to carve the fillets from the brace itself?    However, I think Steve's original idea of fillet pieces, with the end grain facing the top to provide a better gluing surface toward the brace could help the joint. The question remains, "Is it worth the time making those little piece for possibly a minimal gain?" I like the idea nonetheless, Steve!

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DES - Farwell, MI