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A recent thread revealed an almost unanimous recommendation to a novice builder to remove and replace X-braces from a soundboard because the half-lap was not 'tight'.

Just for interest's sake-
Is there any logical (engineering-type) explanation for this widely-held idea that the X-brace intersection must be very tightly fitted?
The idea of capping the top of the 'lower' brace to restore the longitudinal stiffness makes complete sense.

However, the prevailing idea seems to be that a very tight half-lap joint (restraining the braces from changing their angle of intersection) is very necessary as well.

This doesn't make sense to me- both braces are glued to the soundboard along their length and there is absolutely no possibility of movement relative to each other.

It is a nice cosmetic touch to have a tight joint here; I'm just wondering if it is a structural necessity.

Fire away!

John

Mine are generated by a router bit, after sanding them to the size of the bit in a drum sander. They squeak when they go together, so they are tight.

I bet if you use a cap on the lap, it would work fairly loose as well. Interesting question. I suppose you could slide in a splint to take up the gap if you have one?

Don't swallow any camels while straining at a gnat.

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Thanks for the input, Bruce.

My brace lap joints are 'acceptably tight', but my standards may not be as high as some.

My question is really theoretical for me; not so theoretical for a novice who is told to chisel off the braces and start again.

BTW, your aphorism went over my head completely. (Not an unusual happening for me.)

Cheers

John

I have been told by my teacher and master luthier that the intersection of the x brace was crucial for a great sounding guitar. Whether he is right or wrong about his teaching I wont argue, but I will tell you that I intend to make this a crucial point in all of my guitar building. The guitars I build have a richness of sound that exceeds any plant manufactured guitar that there is barely any comparison. the only real difference between what I can see in theirs and what I know mine have is a very tight x-brace. I may be right or wrong on that assumption, but I believe it to be a fact.

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Reguards,

Ken H

John -

I think you are right (although it wouldn't have occured to me before reading your post)


Looking at each brace separately: The "lower" brace, if it's capped, does not derive much stiffness - or strength - from the upper. The upper does fill the "hole", but, the brace's stiffness and strength in the bending direction (the important direction, as you say) is not reduced much by the notch, because of the cap.

And, the "upper" brace, although notched at the bottom, is securely glued to a relatively stiff plate (the guitar top) which stabilizes the notch, so the lower notch does not weaken it much.


One could argue that, if the notches in each brace are tightly filled by the other brace, it has to help. And, it probably does, but not much. Remember that the notch-filling part of the brace is much less stiff, because it is being used cross-grain, rather than long grain.

Phil

Suppose we exaggerate and say the joint is so sloppy that the half lap of one doesn't touch the other. Obviously the 2 individual braces in themselves would be much weaker because of the 1/2 lap cut out of it. However a well fitted 1/2 lap joint properly glued would restore the strength of the 2 pieces. I suppose a test with weight to the breaking point would tell what the difference in strength would actually be.

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[QUOTE=Bobc] Suppose we exaggerate and say the joint is so sloppy that the half lap of one doesn't touch the other. Obviously the 2 individual braces in themselves would be much weaker because of the 1/2 lap cut out of it. However a well fitted 1/2 lap joint properly glued would restore the strength of the 2 pieces. I suppose a test with weight to the breaking point would tell what the difference in strength would actually be.[/QUOTE]

Bob-
That's a good way of analyzing the situation.
Think of the two braces separately, one with the 1/16 inch cap on top spanning the gap, and the other with the soundboard spanning the gap.
Isn't this similar to beams with holes drilled in them?
There's actually not much strength lost, I think. I'm not an engineer, so I can't offer any proof of this, but you commonly see steel beams with cutouts in the web. As you say, a test/experiment would be in order.


I usually have a pile of bracing stock on hand, so it's a matter of a few minutes to cut a new set if the lap joint isn't satisfactory (tight; cosmetically acceptable). I'm not sure I could think of a good structural reason to advise somebody to chisel off kit braces and re-do them, however.

John

Phil Marino had already stated in much clearer terms most of what I just said.
Thanks, Phil!

I think there are two things going on in keeping the joint tight.

1 - coupling between the two braces for vibrational transmission.
2 - the brace glued notch down (ie towards the soundboard, say gluing on the go-bar deck) gets the advantage of having the other fill its void during both rocking motions of the bridge during string excitation. i fell the other brace only gets this advantage half the time - that is when the top is rocking in an upward motion, squeezing the opening against the other x brace. When the top is in a downward motion, the tightness of the X has no effect, as the notch is being stretched open (pretend you are pushing down on the X from the top of the guitar) - this is where the X brace cap comes in and holds the braces tight together, plus it restores the full brace height strength.

This joint is not like drilling a hole in the web of an I beam - nowhere close from a structural point of view. You can drill holes into the X brace and it wont affect its strenght as a beam (hole size within reason), similar to drilling holes in floor beams to run wiring and plumbing.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

I'm going to defer to the explanation given by the master Somogyi on his videos that were posted here awhile back...

Remember the cube rule of thickness – the load bearing stiffness is a cube function of it height. Say we have a brace 1 unit high, it has an index of stiffness or load bearing capacity of 1 unit cubed. If we increase the brace to 2 units, the new brace now has 8 times the capacity for supporting the load (2x2x2=8). If something is 3 units high, then it has 27 times the capacity to support weight. 4 = 64. When you cut the notches for a brace, it reduces stiffness in the same way, so, a brace reduced by 1/2 it's height for the notch actually weakens it by 8 times. Somogyi states that a tight fitting X-brace with a cap will restore the stiffness back to equal the original height. If it's a sloppy fitting joint, it will not bring stiffness back up to the original, so what you have is a joint with the stiffness equal to that of a much smaller brace. So in essence, you have way more mass than is required for the same stiffness if it's a sloppy joint.

Does that make sense? It did when I listened to Ervin explain it.

Cheers!

John

Goody, goody - a theoretical debate. These things are fun.

We sort of had this debate some time back in terms of the X brace capping, where - until Todd Stock convinced me otherwise with his logic - I thought that with a tight fighting X brace joint this restored the stifness without a cap. But no - it is the cap that is the magic ingredient that restores the upward facing notched joint's stifness back to the original based on it's height - not the tight fitting joint.

Coupled systems are interesting and I'd be interested to know just what a coupled X brace does - does it help or hinder vibration transmission? Coupling involves constraints amongst other things and means the two X braces are made to operate more as a system - so things can get cancelled where they join as well as added. Is this good or bad? If you could join the X braces say with a few thousands clearance all round with a cap for equallised stiffness, wouldn't the individual braces be more free to do their own thing in terms of vibration transmission?

Just another thought - and this may be totally wrong too - but the X brace intersection may be a node of sorts on the top and a tight fitting capped X brace joint may make a better one.

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

One word.........



McPherson

but then we all know that they sound awful.......

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Thanks, everybody!
It's getting my little grey cells working, for sure.

John

[QUOTE=martinedwards] One word.........



McPherson

but then we all know that they sound awful.......[/QUOTE]

Well there you go - think about something and it's out there already. Someone ought to tell them that the X brace needs capping though

H ey John Mayes . . .

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

After looking at the McPherson X brace I feel a bit better. I was getting worried about my glued up brace because the upper have of joint is definitely not tight. The bottom have of the intersection is tight but the top opens up. I was thinking about putting shims in the open area but no maybe I won’t do that. However, I will put a cap on it.

Philip

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aka konacat

If you think my playing is bad you should hear me sing!
Practice breeds confidence and confidence breeds competence. Unfortunately, I'm stuck in practice.

I like snug, but not too tight. Like everything else, some of a good thing is good but lots is not always better. Take tightness of the x-joint, for example. It is possible to make it so tight that the braces are no longer straight. DAMHIKT.

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now known around here as Pat Foster
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[QUOTE=Hesh1956] When I look at McPherson's bracing I see some very inovative design, materials, and engineering.

However........    It looks contrived to me. My impression is that although a lot of good thinking went into the design and I give them a lot of credit for looking for a new way to make a guitar sound good - I seriously doubt that a McPherson will ever be able to sound as fine as a pre-war Martin.......
[/QUOTE]

I agree, but just to muddy the waters, don't pre-war Martins have open x-brace joints with just a piece of cloth slapped over them?

     I have learned much in the last 10 years of building. I used to be so anal about the notch but when you look at the actual joint and the way the loads are distibuted to the bracing you can see that you can't compare the loads the same all along the bracing.
     The top is under a compression load from the bridge up and a tensional load from the bridge back. The braces support the top in these conditions and they are infact glued to the top. The load applied isn't as much a stress load as we would have as compared to a floor joist.
    I admit that as I got busier and better I don't recommend a sloppy fit but too tight and you will actually cause a reverse bow on the bracing and stress on the joint. I am not concerned of perfection as I reinforce the joint in the manner of martin with a cloth patch that melds over both braces .
     This does ad a great deal of structural integrity. I have never had to repair a brace with a cloth patch but have repaired many braces with the wood overlay.
   As discussed earlier in these post that stiffness is cubed to the height , rememebr that stiff isn't allways stronger.
john hall
blues creek guitars

I think more important than the tightness of the joint is capping it. If it is
uncapped and not tight..then there is some big room for cracking of the
braces. A squared off joint is much more susceptible to cracking than is
a rounded one..the words don't seem to be coming out right but I hope
you can envision what I mean (which is why the McPherson does not crack
in that spot, but I've seen many lapped X-braces cracked..again totally
different system, but some things transfer.) If you cap it then that helps
limit the vertical torque and therefore helps prevent cracking. I learned
this the hard way....

Be gentle on the McPherson comments as it is hard to judge them by a
traditional ruler as they are going for a much different goal. I'm not
saying I'm in love with all the designs, but I think they have a good
foundation to work with.

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John Mayes
http://www.mayesluthier.com

To add to what John said, and in response to what Hesh said, which was, "I seriously doubt that a McPherson will ever be able to sound as fine as a pre-war Martin," I would make this comment: To a person who prefers the sound of a McPherson, or prefers that sound for certain musical applications, the pre-war Martin will never sound as good. Speaking for myself, I love the sound of old Martins, but I consider that sound (not that all old Martins sound the same, but you know what I mean) to be one of only many in a broad palette of great guitar sounds, some of which have not yet been heard, because the guitars haven't been designed and built yet.

More on topic, though... I would second the cautions about making the joint too tight, especially considering that the glue is going to swell the wood. A joint that fits nicely but slides together very easily may work better for you.

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Todd Rose
Ithaca, NY

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

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Yeah, I miss that edit button...

Of course, I meant, "...only one of many in a broad palette..."

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Todd Rose
Ithaca, NY

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[QUOTE=Hesh1956] Todd are you sure that what John said was in response to what I said?   [/QUOTE]

Sorry, Hesh, I didn't mean that what John said was in response to what you said. I meant that I was adding to what John said, and I was also responding to what you said. I have a habit of writing convoluted, run-on sentences that can be confusing to decipher.

I also didn't mean to question your right to express your opinion, but simply to differ with you and express my own point of view.    

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