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As Jeff Highland has suggested the only reason a wood cap will fail is if it had been made too short in the first place.

Thin is fine, but a spruce cap does need a reasonable footprint. On the other hand a lap joint can fail under cloth and most people will never even notice. This is because cloth can become like a 'bean bag'. The glue will hold the outer 'edges' firmly in place while the lap joint of the X is free to move beneath and sap away precious string energy. To be completely honest, with what we now understand about structure, I can not think of a single good reason to use cloth. It offers next to zero structural integrity and only serves to mask any problems that may occur beneath.

Sorry John it really is not my intention to be disrespectful to you but in my honest opinion rag and glue just does not cut it at such a critical intersection and there is plenty to suggest it is inferior in every way to a 40 to 50mm slither of spruce held in place with PVA or HHG.

JM2C

Kim

That sounds like a great idea to me.

Can the joint really be "too solid " ???

After all, the primary function of the X brace is to counteract the torque exerted on the bridge, is it not ? I would have thought that added rigidity, achieved without undue addition of mass, would be a good thing.

Seems any part of the top could be "overbuilt" to me.......including the stiffness of the X brace........and the X-brace intersection.

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Formerly known as Adaboy.......

I don't see how the parallel you are trying to make with the glue lam analogy follows but maybe I'm missing something.

In a lap joint, the weakest part is where end grain runs into side grain. End grain to end grains joints are the absolute weakest joint; end grain to side grain is only marginally stronger.

Gluing a cap on top, across the joint, strengthens it as does an inlay. And since a tightly fitted, glued joint is just as strong as the wood itself, how could an inlay be weaker? With a cap there's one gluing surface; with an inlay there's three.

Of course the strength of my argument is only as strong as the joint is accurate.

Or, perhaps my argument is about as strong as a butt joint but I don't see it.

Alex Willis uses the gussets with a lap joint in his book .

IMO the cloth caps are there to hide the joint and add minimal strength. Martin has for a long time shaped their braces before notching for the lap joint, so there is no contact between the arms of the X in the upper half of the joint (the half away from the top). They hide this joinery flaw with the cloth. Gibson did it to hide the butt joint, and because they tend to think that imitating Martin is a good thing to do.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

I may not understand what is meant by an "inlay". If I do, seems it would strengthen one X-brace arm but weaken the other one.......so are you better off? Seems a cap would be better as it strengthens the weak arm of the x-brace but doesn't weaken the other X-brace arm.

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Formerly known as Adaboy.......

Yeah, I can see how the word, 'inlay' can be confusing but I don't know a better word for it.

What I mean here is that I cut a rectangular mortise (1/8" deep X 1/8" wide X 1 1/4" long) across the upper piece of the X brace (the brace that runs through on top), and inlay a piece the same size that is flush on top with the surrounding X.

Does that make sense?

I don't follow you when you refer to a "weak" arm of the X brace. In my mind, it's not either of the arms that are weak
but the joint (which consists of 2 butt joints) that needs further support.

Since the height of the brace is the critical factor, by inlaying the cap, you have reduced the height of the other brace, thus weakening it.
At the ends of the inlayed piece, you also have a discontiniuty(end grain to end grain), effectively giving you the equivalent of two 1/16 deep saw cuts.
Just put the cap on top, it is much better.

The lower halves of both braces are in tension, are they not, (and the top halves in compression) so the brace with the downward facing notch is by definition the weaker brace, surely?

I know luthiers hate carpentry analogies, but it is directly analogous to cutting a notch in a floor joist to allow the passage of a pipe. The ideal is to drill a hole in the center of the joist, but where this is impossible, then you notch the top of the joist, never the underside.

Here's the cap I made on Friday. It's roughly 1/16" (0.062") thick and tapers into the shape of the brace. I think it looks all right.

You had earlier stated that by inlaying the cap into the lap joint it would somehow give 'greater' strength than simply gluing the cap onto the top of the brace. My glulam analogy was used to point out that a lamination of any kind within a beam can never 'increase' its strength above that of a solid wood component of the same dimensions, it can only ever be made 'equal' to a solid beam of the same dimensions. The analogy was also used to explain that in order for a laminated beam to 'equal' the strength of a solid beam, it requires a continuance be maintained of defect free wood of equal grading in the top and bottom cords of the layup.

Your idea of inlaying the cap fails to do this because regardless of how well the joint is made it does not reestablished a continuance at the most critical point of the brace. Your reasoning that 3 glue joints are somehow superior to one fails to acknowledge that two of those joints are unnecessary 'end-grain' glue joints which you have acknowledged are the "absolute weakest joints".

The fact that a cap being glued on top of a lap joint has only a single 'side to side' glue joint is its very strength. Such an arrangement 'reliably' reestablishes a continuance to the brace and further more, in accordance with the cube rule, that tiny little bit of extra height provides a disproportionate amount of load-bearing capacity to the most critical point of the brace and that is something inlay or cloth cannot do.

As for 'gussets', I fail to see the point of adding so much extra work over a simple lap joint which has any short comings effectively dealt with by gluing on an even simpler cap, the tail is now wagging the dog me thinks.

Cheers

Kim

Yes, the grain of the gussets runs at right angles to that of the braces, but I'm not sure why you would conclude they would have "little value". Like gussets in any other engineering application, they act as couplers for the parts intersecting at the joint, and the intersection reinforced by the gussets is VERY strong -- as I said, maybe too much so.

I saw Alex Willis' book after I'd made a few like this, and decided that maybe it wasn't such a dumb idea. Despite Kim's remark, it really doesn't take a lot of effort -- only marginally more than a cap. I simply glue triangular cross-section bits into each of the 4 corners, then use a round rasp to shape them at the intersection to almost nothing.

They "look" nice (if that's a concern), but I feel they may create a local "dead" area due to excessive stiffness, and that's my reservation about continuing to use them.

I've gusseted the X with good results. The gussets are very small. I start with a piece of spruce about 1/4-5/16" square, cut it to a triangle, and then use a round file to remove most of that, like Tim.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

I have used the gussets a couple of times and am not concerned about them adding any extra stiffness. That area is the stiffest area on the top and the gussets will not add more that is measurable. But it will add strength to the joint and help to prevent any splitting from the stress riser corners of the lap joint.
Tom

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A person who has never made a mistake has never made anything!!!

I'd like to see a picture of those Gussets, but like Kim, find it hard to see why you would chose it over a simple cap for structural efficiency.

Jeff: I always use a thin cap and have only used the gussets a couple of times. The gussets were similar to those shown in the Willis book. I just think they are irrelevant as far as stiffness goes and might be a bit of overkill.
Tom

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A person who has never made a mistake has never made anything!!!

Thanks everybody for your critiques of my inlayed cap.

I've made my last.

I usually try to stay out of topics like these, but the last post by runamuck pushed me over.
Threads like this have a feeling of "this way is the best and only way, this is why, and you are wrong if you don't conform".
Hundreds and thousands of guitars, both factory and hand-built have been made with every iteration of lap-joint/butt-joint/cloth/cap/nothing, and I dare guess that a very small percentage have failed in any way associated with the joint.
Gibson used a butt and wedge with a cloth cover, at the X from the 30's to the 60's and many of the old L-00's are still going strong today.
You all know what joint Martin used since forever, and don't ask me to cover your back if you wander into a gaggle of bluegrassers and tell them their 'bones are built wrong! It is in no way wrong if it has worked for 60 years.
Maybe the weakest joint that will suffice will be the key to the best tone of all. The only difference between the strongest joint and the weakest joint that will pass muster, is that they are different.

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If you work with hide or fish glue, you can go the extra mile by first sizing the endgrain in the cuts before assembly. Paint the endgrain with a generous amount of glue, let it cure, then redo the fit. Fishglue especially will soak in the endgrain quite a lot.

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Build log

Indeed, but it can also lead to unintended consequences. Don't ask me how I know.

Kim wrote:
"I don't see a company (Martin) being slow on the uptake of obvious improvements to be a positive thing for their customers, rather, when you consider the size of their operation, such resistance to change of process is more likely to be driven by a desire to maintain their low production costs rather than some noble pursuit of a superior tradition. "

Martin is in a rather peculiar position in the market. As has been said:"Anybody can claim to be the best, but only one can be the first".
Martin was the first maker of high-quality guitars in the US in any numbers, and that has formed the entire basis of their advertising in recent years. It works well for them, but it also locks them into some design features that might be better done another way. After all, if they replaced the cloth patch with a wood one, it would no longer be 'just like Grandpa's Martin', and an admission that maybe Grandpa's was not as good as it could have been.

Cloth patches do work. In cases where X-braces have split out from the lap joint, and the tops of the braces are rounded off, I've re-glued the splits and added a cloth patch with good results. It's not as strong as a wood patch, but much stronger than nothing.

The key to the wood patch is not how thick it is, but how long. If you think about the static load on the top, the torque of the bridge is pushing downward on the lap joint. The open part facing into the guitar is trying to open up, so the force there is tension across the gap. Spruce is quite strong in direct tension, and I don't think you need a cross section of more than about 1/16" high by the 1/4" wide of the brace there to take the load in that sense. The limit is more likely to be set by the shear strength of the glue line holding the patch on. The maximum load in the glue line occurs right at the end of the patch, and the longer the patch is the more of the load is dissipated in the glue line before it gets to the end, so making the patch longer makes it stronger. It's sort of the same thing as the 'belly' bridge: making the bridge deeper along the line of the pull reduces the maximum stress at the back edge, and it stays glued down longer.

Very wise words as usual Al, being first really is a peculiar position to be within a market, especially one so steeped in tradition and legend. As you say with each change CF Martin can only ever be seen as either an innovator, or a traitor..There simply is no middle of the road for them.....Bob Taylor must chuckle to himself whenever he thinks about that.

Cheers

Kim

I was thinking that an inlaid cap was stronger than one simply glued to the top of the X. It seems I was incorrect.

So as I said: I stand corrected. I stand corrected on my belief that that inlaying that piece would make the joint stronger.
And I appreciate those who understand engineering principles better that I do in pointing that out.

The issue you raise seems to me to be entirely different than the question of what goes into making the strongest X brace. As you intimated, the strongest may not necessarily be the best.

Perhaps you're right. But I'd rather eliminate the variable of the strength of the X brace in order to concentrate on other issues.