Official Luthiers Forum!

Approaching my first acoustic build, I am planning everything in advance, and just got to wondering about the traditional cross lap joint in the X-brace on the steel strung acoustic.

The way I look at it is that it would be just as effective, and probably even more so, to have one of the X-braces running all the way through, un-notched, and the other brace butted up tight on both sides.

That way, the integrity of one brace would be totally uncompromised, whereas with the conventional joint, the brace with the downward facing notch (ie notch facing the back of the guitar) is seriously weakened, and the integrity of the other brace depends totally on the accuracy of fit, so that the sides of the notch have to compress onto the sides of the opposing brace.

Why not just do away with the notch, butt joint the two halves of the "opposing" brace up to the "main" brace and cap the joint with a strip of glued on spruce to maintain joint integrity ?

Gibson tried that approach and I don't believe it worked very well. The notch in the x-brace, when done correctly, is apparently stronger than butting two pieces up against one.

Glen is correct. The old Gibson guitars and others had butted X-joints and the tops sunk in front of the bridge as a result.

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"Building guitars looks hard, but it's actually much harder than it looks." Tom Buck

Once you cap the lap joint, integrity is reinstated. If you butt joint, even mild impact with the top would probably sheer the components. Take a look at what happens at the other end of the "X". If you don't let them in to the linings, you need to tapper them to nothing to prevent de-lamination down the track and that de-lamination does not take much even if the ends are left mildly proud of the top plate.

Cheers

Kim

In '78 I built a guitar using Brosnac's book on steel guitar construction. He advocates one leg going through and the other coved and butted. Maybe 2 or 3 months after I strung up my effort, there was a valley in front of the bridge deep enough to rappel down. Apparently, Brosnac also advocates stringing his guitars with ultralight monofilament.
In '79 I built a guitar with a lapped x joint (and 24' radius). The top is still essentially where it was when I first strung it up. To address your concern about one of the 1/2 laps spreading, either do that lap 1/3 deep and the other 2/3, or cap the joint--or do both. Either way by all means, use a lap joint.

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michael propsom
www.propsomguitars.com

Was that joint capped? Of course without a cap, you'd want a lap joint. But I think the question here is whether the "bottom" part of the lap helps the strength of the brace that has the cap running along it. I almost think not. Without the cap, then the downward force of the bridge is just peeling the butt joints apart directly and will clearly fail. But with the cap tensioned under the force, I think the base would actually be compressing against the sides of the continuous X arm, so no real benefit to having continuous fibers there. Purely theoretical visualization though, I have yet to string up my first guitar.

I'm going to be doing a re-topping exercise soon, where I rip off the top of a cheapo guitar and make new ones to try on it. Maybe I'll do the last one with the capped butt joint and let it sit for a year with mediums to see if it sinks.

Hate to be critical but Bosnac's book has to be one of the poorest ever attempted on guitar construction.
Tom

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

I know many like the wood patch but I prefer the cloth reinforcement patch. This is plenty strong to he honest and the wood patch does add more mass. I have never seen a cloth patch fail but I have seen wooden one fail. I think this is more a matter of personal preference . This isn't a floor beam so we don't have to think it in that fashion .

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

We build all our student guitars as described above (capped butt-joint for 1 brace, continuous for the other). About 50 guitars so far... no issues. I discussed it with Bob Taylor and he thought it was fine (assuming the cap).

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David Malicky

I think there's no question that the cap strengthens the joint, but I don't like the way the cap looks
through the sound hole.

So I've started inlaying it about 1/4" deep and then flush on top. I believe that
inlaying it, as long as it's a tight joint all around, makes it stronger still.

So you wanna know why we notch X braces...

Well its really quite simple... technology hasent figured out how to get two different pieces of wood to sharing the same x:y:z ordinance in time and space.
Yes we can overlap the molecular and atomic structure of liquids and gases into the same space, but unfortunately we can't seem to do it yet with most solids...perhaps tomorrow.

So we have to notch...well maybe you do...me don't do no X bracing.

Now aren't you glad you asked?


blessings

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Audiences and dispensations on Thursdays ~ by appointment only.


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I built a butt joint xbrace several years ago. It has held up well, but I have always thought it sounded rough and unfocused. Maybe for some other reason.

Actually, it will be weaker because you've effectively shortened the height of the other brace by 1/4" (If I'm understanding you correctly).

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

If the work were to have some historical significance then I could see your point. If that were the case then Tod's suggestion to cover a .020 cap with cloth would solve the issue. But to consider sacrificing the structural integrity of the instrument for the sake of how a component looks which you cannot even see unless you make a concerted effort to do so seems a little extreme to me, but then again to each his own and if that is what turns you on then more power to ya.

Cheers

Kim

My opinion only, but it seems a butted joint (especially without a cap) is weaker than a lapped joint with cap. If the top vibrates up and down, when it deflects upward it puts tension along the bottom of X-brace arm. The butted arm has end grain glued to the other X-brace arm which is surely weaker than a half-span of continuous grain. I realize that the leg is glued to the top along it's length and the top plate carries a lot of the tension when the top deflects upward.......but it doesn't give me a warm fuzzy. Just judging by the failures/successes listed in this thread, seems you are on the edge. Maybe a heavier built student guitar is enough to keep one on the safe side.

What is so difficult about cutting a lap joint? Seems the only ones that would consider it is a production or classroom situation where you need to cut assembly time. For hand built, custom instruments, it seems odd to consider a small time saving shortcut that may or may not compromise integrity over the years/decades. If the cap came loose for some reason, you definitely have a weak joint that I doubt will hold up.

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Formerly known as Adaboy.......

Yeah, I agree with you: to sacrifice structure for aesthetics wouldn't be smart.

But I'm not doing that. I'm inlaying the cap making the joint even stronger than simply gluing the cap on top of the X.

What's the formula for figuring how thick the cap needs to be?

I've been inlaying the cap about 1/4" on my guitars and perhaps I don't need to be so deep.
Although I've never done it, it would make sense to me that inlaying from both the top and bottom
would make it all that stronger.

A glued up tight fitting x joint should be stronger than a single leg. And will deflect less when a cap is installed. I glue a 1/16" cap tapered to nothing at the brace on mine and they work good for me.

Unless somehow structurally unsound to begin with, removing wood from a beam/brace and replacing it again will do nothing whatsoever to 'increase' strength regardless of how tight you make the joint.

If you research laminated beam technology you will find that a glulam beam can only be made 'as strong' as a 'clear and better' solid beam of an identical species of wood in equal dimensions. The advantages of laminating these beams is that wood of a 'structurally inferior grade' can be used to achieve this same level of strenght just as long as the top and bottom chords of the layup are of the same structural grade as the solid beam and form around 25% of the total.

On the other hand gluing a cap 'on top' of the lap joint increases the height of the brace and so to its strength. In accordance with the cube rule, a little goes a very long way.

Cheers

Kim

Martin has used cloth since 1833 . I don't think this is really all that critical .

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Martin also used "T" bar truss rods for far too long as well. I don't see a company 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.

I 'have' seen cloth X brace bridging fail. I have also read comment of the improvements gained in tone that have been obtained by people who have moved away from cloth altogether and adopted wood to couple the X brace lap joint of their guitars....Not really all that critical? To each his own, but don't knock it until you try it I say. Just because a method or process has stood the test of time, it should not enter the realm of 'never to be improved upon'. To think that way is to steal all potential from yourself and your instruments, and that is just too boring to contemplate.

If Martin are the be all and end all of acoustic guitars, then why is it that so many hand built guitars, even those made by the very new to the craft, can sound so much better?

Sometimes tradition is no more than an anchor.

Cheers

Kim

Inlaying a cap is a really really bad idea.
It will adversely affect BOTH of the braces

I don't think the cloth patch achieves anything structurally, isn't it just to cover gaps at the joint from Martin shaping the profile of the braces before joining them?

I do think some people are making the timber caps a little short to properly transfer the load, and if you see failures with timber caps, this may be why.

No one has suggested lap joint with wood gussets -- little angled blocks glued into the 4 corners of the X, then profiled with round file. I've built a few that way -- haven't convinced myself that it's better, but the joint is solid as hell. Maybe "too" solid.

Creating that lap joint with a good tight fit is extremely satisfying

J

I have tried top caps and prefer the cloth . In 10 yrs of repair I have yet to see one fail but I have had to reglue many wood caps . Simon Patrick guitars seem to be the worse for this . Just my 2cents. I think the bracing shape is more important the the capping .

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it