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Actually I did have a serious question on this - is there any possibility that the top itself acts as a cap on the brace with the open slot against the top? Maybe not as strong as a spruce cap with grain running in the same direction, but how strong does it need to be? Then by capping the gap facing away from the top, all bases are covered?

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Damon Wack
https://www.facebook.com/LindamonGuitars

Yes Lindamon that is exactly the correct way to look at it.
Murrmac, I just would not want to be handling a brace with 2/3 of more notched out of it, just gets a little fragile

The cross-grain glue joint at the bottom of the top brace and the top of the bottom brace - if it's a good, tight joint -is second in strength only to an edge to edge glue joint ( where long grain is glued to long grain). And since a tight glue joint is said to be just as strong as the wood itself (excluding end grain, butt joints), I don't see why your idea of 2/3 - 1/3 would provide more strength than 1/2 - 1/2.

BTW: I've really enjoyed this thread.

Why, when the glue joint is as strong as the wood itself?

80/20, 70/30, 90/10. What difference does it make? (Assuming the joint is good, of course.)

Going back to the glulam analogy, the benefit of that process is it allows the use of structurally inferior wood to equal the strength of a solid beam of structurally sound timber of the same overall dimension just as long as the top and bottom cords in the lay up of that lamination are equal in grade to that used in the solid form. Think about this for a minute and it will tell you right away that moving the depth of the lap joint up and down either way should have little, if any affect whatsoever on the integrity of a completed capped brace. This is because it is really only the top and bottom 20 to 25% of a beam that is in tension/compression when under load and the rest is pretty much just holding those surfaces apart. This is the principal which makes box section is so strong.

Cheers

Kim

I think what Todd Stock is referring to is a junction which is uncapped.
In that situation, minimizing the notch depth in the open notch member will make a difference.
Once capped, it makes minimal difference

When we evaluate glue performance at my company, usually ASTM D905, tensile strength is rarely evaluated. The reason? The glue joint is far stronger than the wood and you will always observe 100% wood failure. As a side note: cross grain laps are effectively identical in strength to parallel edge grain joints. However, I would never recommend using an end-grain butt joint for much of anything because they are extremely weak. A well-fitted cross-lap is effectively ideal.

R. Taylor's asymmetrical X gives another perspective on this joint:
http://www.rtaylorguitars.com/Woods-Bracing.aspx
That seems to imply that structurally, the guitar may only need 1 leg of the X to be continuous through the joint. And a flexy joint for the other leg is ok, and could be beneficial.

That isn't to say a regular X can be made poorly w/o structural issues: moving R. Taylor's offset leg back to the X and joining it there will increase its stiffness, and so also increase the torque that leg experiences, and so also increase the demands on that part of the joint. I.e., loads follow the paths of greatest stiffness, so the stiffer parts need better structure. (For the engineers--the regular X is statically indeterminate since there are 2 legs/paths for the bridge torque. In stat. indet. problems, increasing the stiffness of any given path increases its share of the load.)

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David Malicky

I use Chladni patterns to 'tune' my tops, and they give a pretty detailed picture of the distribution of stiffness and mass in the top. The patterns definitely change if you leave out the patch: they becaome more asymmetric showing that the two legs of the X are of unequal stiffness. I have not tried one with a cloth patch; another experiment to do some time!

The last three 00 sized guitars I made had top weights, without the bridge, of 177 gm, 185 gm, and 194 gm. Add in 25-30 gm for a pyramid bridge, and I'm in the ballpark of those 'amazingly light' old 00s at 220 gm. I don't think I'm close to under building: small tops can, and should, be light.

As long as we're talking about alternative bracing patterns I've pondered the result of catenaries attached only to the bridge plate on the top and ribs in the back. All other bracing of the top could then be eliminated allowing for more extensive vibration.

Well just think how having runout in the X brace and not capping it will also effect the strength. I cap it because IMHO the cap does replaces and moves the compression /tension area of the brace to where it should be regaining the strength of the full brace heigth or near to it. I want that joint to be strong . A lot of players like me also thump on the bridge with the palm of the hand while strumming and that can't be all that good on a weak joint either. And how much time and hard is it to cap the joint anyhow? To me it's hard to believe someone wanting to build a better instrument then a factory guitar would want to gamble with that joint to save 5 minutes work if that.
For the beginner and as I see it, if you can't make a good fitting lap joint on a X brace and take the time to cap it as 90% of the Luthiers do then maybe this isn't the hobby for you.
But this is just my opinion on it and each to his own.

I've always used a cap because it just seemed to make sense to me but I think it's making a mountain out of a mole hill to suggest that it is even necessary. When you think of the braces glued to the top along it's whole length you are talking about a weak spot less then a centimeter long. And like what many have said when fitted properly you got nothing to worry about. Having said that I think the but joint as the OP described is not a good idea.

Oh and I'm not an engineer I just play one on OLF

Hugh Evans wrote:
"As long as we're talking about alternative bracing patterns I've pondered the result of catenaries attached only to the bridge plate on the top and ribs in the back."

Do you mean cantelevers?

All of the guitars I've heard where the top bracing was drastically reduced or elliminated for 'more sound' were tubby or bassy, and most folded up pretty quickly too. I think the guitar is an example of a very well worked out design, where very few major changes turn out to be improvements. I'm not saying it's not possible, but I don't think it will be easy.

jfmkenna wrote:
"When you think of the braces glued to the top along it's whole length you are talking about a weak spot less then a centimeter long. "

Such small weak spots are known in the engineering business as 'stress risers', and they go to great lengths to avoid them. All of the stress tends to be concentrated in that one spot, and all the more so when the structure is otherwise beefy.

Aside from any theoretical considerations, anybody who has done any amount of repair has most likely seen an un-capped X brace that has split at the level of the lap. The fact that you get away with it most of the time doesn't make it good practice. As has been said, it's so easy to do the patch, and makes so much sense, that it's hard to understand why anybody would want to skip that step. That said, I've seen supposedly 'high end' production boxes that have split there because they did without it. I guess that, when you make tens of thousands a year, and over build them anyway, the added warrenty load is not enough to make it worth the extra production step. We should do better.

It seems to me that C. F. Martin must have believed that the cloth reinforcement was the best way to finish off the joint. I can't believe that he was trying to save work by not using a wood cap. After all, this is a man who found the time for the "appendix" brace.

What else were cloth reinforcements being used for in the 1840s?

John

There is a luthier doing just that, on the continent somewhere (Switzerland?). I don't have time to find the link now, may post later.

As I remember, tops are two layers, thinned heavily towards the edges, and a cantilevered support under the bridge area.
He says it frees up the top to vibrate.

Colin

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Lukas Brunner http://brunner-guitars.com/flash/ Click on the "Flying Top" button along the top navigation bar.

Honestly in over 15 years doing repairs I've never seen an x-brace fail at that joint. That must be why I am of the opinion that when done right it's not a problem. Having said that I always cap it none the less because it takes about 3 minutes and seems to make sense to do it just in case.

That cantilever design is very interesting. It looks like there is a veneer of sorts around the top along the edge.

Or rather the face is cedar and has a large spruce patch on the interior?

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Build log

Blind faith is better suited to religion than guitars, although faith in Martin's infallibility is a religious matter for many.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

I'd like to have a better idea what you are talking about. I do know what a catenary is.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

I don't know what the top and the interior laminate/patch/second top is but it seems some what like a laminated archtop of sorts. Different strokes for different folks. I think I've found that by removing the UTB and replacing it with an "A" type bracing and having a cantilever FB extension will seem to open up the upper bout some. But I'm sure others have done this and could speak on it better then I.
I'm not sure what the end result will be as it's not finished yet but here's a pic of the Chladni Pattern at 194 hz. This would more then likely be the ring and a half if there was a UTB. You can see where the node follows the upper bout A braces. I can get a full ring pattern also at an other hz. So I'm a little skepical about the pics on the site camparing the free top bracing to the X brace. Too many varibles as to what an individual is after when voicing a top. I don't see where the free top bracing can really claim to extend the vibrating surface. But interesting none the less.

I think Hugh meant to say "CANTILEVER" , Howard, rather than "catenary". Lapsis linguae.

( Although it's not even really a cantilever either, it's more of a flying buttress ...)

But what I really wanted to ask was whether there is any record of a builder having tried doing the X-brace with one brace running all the way through, and the opposing brace pocketed into the continuous brace from either side, possibly even (horror of horrors) with offset mortises ?

What I mean is a mortise cut into the side of the continuous brace ( even 3/32" deep ) with a corresponding tenon on the other two braces. This would surely get rid of the "stress riser" problem.

Stronger than a butt joint, for sure, while retaining the integrity of the continuous brace.

Well that sure seems like a more difficult and timely joint to produce with no advantage over a capped lap joint.

I can't think of a good reason not to include a cap on the X. It's easy, fast & almost a guarantee that the joint won't open up. I too have repaired (only one) X brace that had partially failed in this area, so I'm pretty conscious of the benefits.
As with so many other details in this craft, it's easy to overthink. This easily solves a potential problem with a minimum of fuss.
One of the main objections seems to be the appearance of the cap... Why not make it a "feature"?

Wow, difficult ???

I think I could cut that tapered mortise on both sides in like, 2-3 minutes max...

Glue down the the continuous brace, then offer up both abutting braces ... would that really be all that time consuming ?