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JP: not even a scrap from a top offcut? Or was the top pre-cut?

JP, your rosewood strip will look great in there! Serge Poirier38863.3085648148

I thnk that David is arguing that the piece of the other brace, if properly fitted and glued will fill the gap and turn the X back into a single structural member. In that case the cap wouldn't do anything. I think the reality is that this is not so. First, the fit of the other brace is likely to be a little loose and the cap would make it so that that didn't matter. Second, the glue joint is end grain to side grain. I don't know why but end grain joints seem to be very weak. Third, the stress on the joint is a peeling stress. That is the stress you use when you are trying to disassemble something. There is a great deal of leverage available to pull the joint apart and all the force is concentrated on the boundary between where the glue is holding and where it has already released. I consider the joint to be basically unglued for structural purposes.

The static stresses on a guitar may in the long run cause a problem but the real problem is when the guitar is hit with something on the top. Say, for example, when you get up off your stool and bonk the top right into the microphone.

A tiny piece of left over brace stock or top cutoff is more than enough. I use a piece of top cutoff that is oversized, glue it down and trim it to size with a chise. It takes almost no time.

Yea rosewood would look and sound cool for a traditional sound. For a mellow woody tone could use mahogany. For a bright tone and sustain maple koa. Just think, with one little piece of wood can use various woods for just the tone looking for.

"Start with ..."I was just kidding. Hope no one was offended. Anyway, to patch the brace...any scrap of wood with the grain running length wise will work ( about an inch and a half to two inches long). Flush the brace to the other and glue the "patch" on. Your done and believe it or not you have restored a great deal of the integrity to the joint. This was one of the experiments Ervin had us do in his class. As always he was dead-on right! Dave-SKG38867.5795949074

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Dave Bland

remember...

"If it doesn't play in tune...it's just pretty wood"

Where' my beer drinkin' alien when i need him?

OK then ...

So why bother with a lap-jointed X? Make one of the X's solid end to end and glue it to the top. Then on the other X cut a slice right out of the middle. Glue the two sides onto the top butted against the solid X brace. Then hey, no worries, put a little cap across joining the two cut sides as this magic piece of wood gives the cut brace the stiffness it would have had had it been solid on the beam principle Q.E.D

The stifness right in the centre might be the same but I'm not sure about over the entire length of the brace. There is a reason that the braces are usually notched half and half at the X joint.

Stress riser I buy, weak end grain jointing I buy, and insurance against future joint failure/brace splitting I buy - but not the "magic bullet" stiffening impact of this small piece of wood that can be implied by the half brace height to the power of 3.

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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

[QUOTE=stan thomison] Yea rosewood would look and sound cool for a traditional sound. For a mellow woody tone could use mahogany... [/QUOTE]
Heh! Won't you be embarrassed if it turns out to be true? CarltonM38863.6240277778

Alright...now...I guess that I need to weld in a steel plate using a Ferrous-Wood Filler Rod...you can get those next to the Alumi-Steel Filler Rods used to weld Aluminum to Steel (welding joke for those who like burnt metal)....

So I do have just barely enough spruce if I cut the mold ears off of my top (the top was a Martin factory reject...so it has mold ears...mickey mouse ears)

I'll be able to scab just enough for a 1.5 x 0.25 " cap...although I'll have to make a flat landing across the x-bracing to accept the cap (I already shaped the braces...so they end up about 1/16" shorter in height in the area of the cap...so I can get the cap glued in.

Thanks for your help with this...
JP

I'll post pics once I get done...

[QUOTE=Dread1916] although I'll have to make a flat landing across the x-bracing to accept the cap[/QUOTE]
Should be okay--remember, you'll be adding strength with the cap.

If you like the idea of asymmetrical bracing, one option is to make one leg of the X shorter than the other. Then, only notch the taller one to go over the shorter one. I know of at least one excellent builder who does this.

Or, if you didn't want it to be asymmetrical, you could theoretically do what I just described, then carve off all the extra height on the taller brace except where it crosses over the shorter one. This would be, in effect, the same as Dave's thought of leaving one brace solid, butting the other one against it on each side, then capping it.

Maybe Dave's no-lap butting-and-capping idea is actually a very good idea. One brace would be solid and the other would be like a beam with a hole in it. Why not? Significantly less strong than the capped lap joint? I wonder...

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Todd Rose
Ithaca, NY

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

https://www.facebook.com/ToddRoseGuitars/

Mike:
End grain glue joints tend to be weak because of the structure of the wood. It's like a bundle of straws that are much longer than they are wide. The actual area of wood on the end cut surface is much smaller than on the side cut one, and it's nearly impossible to fill the straws up to any depth with most wood glues. Slow setting epoxy seems to do pretty well if you use enough clamping pressure, but how are you going to get clamping pressure on a lap joint?

Dave:
The stiffness added is certainly subtantial. I use 'glitter pattern' testing on my top and back plates to 'tune' the stiffness and mass distribution before I assemble the box. If you leave off that little patch you'll know it, fast! The modes are extremely asymmetric without it. It may not be 'as stiff as' an uncut beam of the same height, but it's darn close.

Dread1916:
Some folks are drilling braces. I question the utility. _All_ of the bracing on a steel string top weighs about 30 grams, compared to the 150 or so of the top itself. Could you drill out as much as 5 grams? Besides, as you know from aerospace standards, the holes would have to be perfectly clean, with no splinters or rough spots to form stress risers. I'm not sure I want the assignment.   

In the ASIA 2005 Symposium voicing workshop that Ervin taught, Judy (pictured with Ervin above) said that she had done tests with patches as thin as 1/64th of an inch and it still made a difference. Another person used the anology of an I-beam.

Todd, good point about the grain orientation of the soundboard.

I'm still thinking about the butted-and-capped joint idea.

First, let me clarify terms, because it took me a couple readings to get which brace is which when you all were talking about notch-up and notch-down (you meant "up" with the guitar assembled and lying on its back, where I was thinking of the top lying on the bench while gluing on braces). I'll continue with your up/down orientation and clarify it thusly: on a normal capped lap joint, the "notch-up brace" is the one whose notch is bridged by the soundboard, while the "notch-down" brace is the one whose notch is bridged by the cap.

Okay, back to the butted-and-capped joint idea. If we leave one brace solid, let's think of that one as being analogous to the notch-down brace - except, of course, that it's not notched at all, so its "virtual notch", if you will, is bridged by itself (rather than by a cap crossing the notch-up brace, as is usual). Good and strong, no problem there.

Now, if we butt the other brace to each side of the solid one and cap it, let's think of that one as the notch-up brace. Once capped, that's essentially what it is; it's just taller in the area of the joint, and notched more deeply - these being the only differences between this brace and a normal notch-up brace. Just the same as a normal notch-up brace, the compressive load is carried by the soundboard as well as through the joint. On the other side, where the tensile load is carried, the cap could be made as thick as we want, though it doesn't seem to me that it would have to be very thick at all to do its job. It seems to me that the distance from the neutral axis gives the cap great mechanical advantage, whereas the normal notch-up brace has its wood nearer the neutral axis. Maybe, for this reason, this butted-and-capped brace is actually stronger than a normal notch-up brace...??? Or am I missing something here?

I haven't yet learned about the concept of a "stress riser", so I'm not sure what relevance, if any, that has to this line of thinking...      Todd Rose38866.3078703704

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Todd Rose
Ithaca, NY

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

https://www.facebook.com/ToddRoseGuitars/

Todd,

I claim the patent

Th major difference that I see is in terms of stress risers. Although the soundboard is bridging the gap, there are the two edges of the brace halves glued to it. These could let loose with a sharp blow to the top. On the notched brace there is a continuous seam along the bottom of the brace. How much of an issue this is in reality I don't know.Dave White38866.347650463

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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

Hi Dave

I've seen a three piece X Brace done on a Seagul Guitar, I think someone posted it here, but from memory I don't think it was capped.

[QUOTE=RussellR] Hi Dave

I've seen a three piece X Brace done on a Seagul Guitar, I think someone posted it here, but from memory I don't think it was capped.[/QUOTE]

Russell,

It must have sounded terrible then

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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

Thanks everyone for this great thread. I have learned a ton and it has me thinking....which is probably not good.

Of course everytime I see the thread title I keep thinking (in my best gangsta rap voice), "I'm gonna bust a cap in your x-brace!"

Jeff

I canceled this post. Please see below.Todd Rose38866.9041203704

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Todd Rose
Ithaca, NY

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

https://www.facebook.com/ToddRoseGuitars/

Seaguls are a touch hard to bend as they peck like crazy when you try to get them between the slats, although it helps if you rap the bill in Kraft Paper

Do you think they sound better when they wear a cap ?

LMAO here Russ!

[QUOTE=Dave White]

Although the soundboard is bridging the gap, there are the two edges of the brace halves glued to it. [/QUOTE]

This is the same situation with the notch-up brace in a normal lap joint, right?

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Todd Rose
Ithaca, NY

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

https://www.facebook.com/ToddRoseGuitars/

[QUOTE=ToddStock] I think you've got it. The one issue with the butted X joint is how the soundboard will carry the compressive load on the notched brace, which is an issue only because of grain orientation.

[/QUOTE]

Isn't this the same issue with the notch-up brace in a normal lap-joint X, whose notch is bridged by the soundboard, i.e. the soundboard is carrying the compressive load? If you're suggesting that this would more of an issue with the butted-and-capped brace in our hypothetical model, I'm not following you.

BTW, thanks for the explanation of "stress risers". That was pretty much what I figured it meant, but your explanation was very helpful in clarifying it for me. I'm still having a bit of a hard time getting my mind around the case that Al was discussing above - how any slight irregularities in holes drilled through a brace would create stress risers. Hmmmm....

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Todd Rose
Ithaca, NY

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

https://www.facebook.com/ToddRoseGuitars/

Thanks again, Todd, for taking the time to share your knowledge. I appreciate the education.

_________________
Todd Rose
Ithaca, NY

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

https://www.facebook.com/ToddRoseGuitars/