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Locking the A-frame
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Author:  Wes McMillian [ Fri Dec 21, 2012 8:12 am ]
Post subject:  Locking the A-frame
Just caught a recent thread in which this was mentioned and it got me thinking about my own process.

I have used the A-frame upper bout bracing on almost every guitar I've built and intend to keep doing it. I have always inlet the braces through the UTB and into the neck block, gluing and locking it into one unit. It just seemed "right".

But here's what I'm questioning....why? While it sounds right and solid at first glance, I'm thinking now that it would make more sense to butt them up to the neck block instead. Actually, it seems to me that you would be more likely to have a solid, locked-in connection from the neck block to the upper X than trying to get a perfect pocket fit at the end of the A-frame braces. It's much easier to get a solid joint butted up to the neck block.

So, what am I missing? Is there a good, real world reason for inletting the upper A-frame braces into the neck block?
Author:  bluescreek [ Fri Dec 21, 2012 9:00 am ]
Post subject:  Re: Locking the A-frame
do it and see what happens
Author:  Darryl Young [ Fri Dec 21, 2012 10:21 am ]
Post subject:  Re: Locking the A-frame
My thoughts: As long as the glue joint doesn't fail, there would be no difference in strength. You have a good glue joint to the top right up to the neck block. You are gluing end grain to the neck block so keep that in mind.
Author:  arie [ Fri Dec 21, 2012 11:11 am ]
Post subject:  Re: Locking the A-frame
pocketing adds more surface contact for gluing. not counting contact with the top, you theoretically have four surfaces (3 sides + the end) instead of one (just the end). end grain to flat grain imo is not the greatest. were i to use A frame i'd pocket. also you might need not to be too concerned with exact brace length with a pocketing scheme.
Author:  tjp [ Fri Dec 21, 2012 11:27 am ]
Post subject:  Re: Locking the A-frame
Not an expert here by any means, but my take is that popsicle braces have worked for a long time, and pretty much anything you do there is going to be stronger than that. I suspect that if you can make your butt joints tight enought to effectively keep the headblock a fixed distance from the UTB, you're good to go. IMO, anything more, while kind of cool, is overdesigned.
Author:  Colin S [ Fri Dec 21, 2012 1:01 pm ]
Post subject:  Re: Locking the A-frame
I have a slightly different take on A braces and the head block and UTB, one that would probably be understood by those that also make classical guitars. I inlet all of the braces on my tops to the X brace, making a unified bracing system rather than discrete separate braces. The A frame braces are also inlet to the headblock, (as is a neck extension) but they pass through apertures in the UTB uninterupted. The UTB being treated more as an open harmonic bar.

Image

Image

The the neck extension is inlet into the block as well, the FB being glued to this extension, not to the top.

Image

Colin
Author:  Wes McMillian [ Sat Dec 22, 2012 8:28 am ]
Post subject:  Re: Locking the A-frame
So, what's the concern with having end grain butted up to the neck block? And I'm still not seeing where pocketing the braces will keep the neck block from moving any more than butting them up to it. I certainly want that area to be as locked in and solid as it can be, but overkill is just overkill unless it really is a benefit. Not to mention making top removal more difficult. Again, I've been doing them this way, just appealing to the pros and engineer types on this board for input as I rethink my method. Anybody?
Author:  John Arnold [ Sat Dec 22, 2012 6:02 pm ]
Post subject:  Re: Locking the A-frame
Quote:
So, what's the concern with having end grain butted up to the neck block?

It doesn't glue very well.
Author:  Wes McMillian [ Sat Dec 22, 2012 7:39 pm ]
Post subject:  Re: Locking the A-frame
Understood. My thinking was just that this glue joint wasn't actually doing anything in this application. The neck block and the braces are glued to the top solidly in that area, so I don't see down forces that would trying to shear the joint. Particularly with the UTB, whose primary job is to counteract these forces.

In regard to pocketing the UTB and upper X, there are down forces at work against these braces, which is all the more reason to pocket them. Seems to me what we're trying to counteract with any neck block bracing system is the tendency of the neck block to rotate toward the soundhole. That's the idea behind using struts and flying buttresses to lock the neck block back to the waist, right? Neither of these methods are concerned with down forces at all directly at the neck block. They are locking the rotational forces against the waist whereas the A-frame locks them against the UTB and upper X. The UTB, again, is your primary defense against the down forces. I mean, if the purpose of the A-frame is to create resistance to bending, we would be using taller braces, right? The little square braces most of us use would be of little use in that application, whereas if the purpose is to lock the neck block in, this can be accomplished with a fairly small brace. You are simply loading a column rather than being concerned with bending.

It's certainly not a big deal to inlet them and I can keep doing it. It just seemed that butting them to the neck block, instead, would ensure a tighter joint at the end of the brace where the forces are. Not to mention saving a step.
Author:  John Arnold [ Sat Dec 22, 2012 11:31 pm ]
Post subject:  Re: Locking the A-frame
Quote:
You are simply loading a column rather than being concerned with bending.

For that function, I use a 1/8" thick flat plate with angled edges, wedged tightly against the neck block and the UTB. The advantage of that over A-bracing is that it reinforces the top against cracking at the edge of the fingerboard.
Author:  Alan Carruth [ Sun Dec 23, 2012 2:50 pm ]
Post subject:  Re: Locking the A-frame
I've been using the A-frame design for some time now. I have a few guitars with that feature that live on the road, and the owners tell me they are way more stable than anything without the A-frame. One of these guys uses about ten different tunings, and switches several times in a set, with no problems.

Except for the first few that I did I've inletted the upper ends of the A-braces. I've had two of those come back for work. One was a 12-string that got knocked off a table, and hit head first. The A-brace was not fully butted up against the block. The top crackd along the sides of the fingerboard, and the neck shifted in until it hit the braces. I elected to replace the top and do it right. The second, and OM, has had a particularly hard life, and the owner brought it back in to have me fix a hole he'd punched in the side when he walked into a door frame. Judging by the amount of white paint on the sides, it was far from the first time. The neck has held up on that one, and it looks as though it got properly butted. It does not take much of a gap to allow the neck to shift, and it doesn't have to shift much to be a problem. Inletting it greatly increases the chances that you'll have proper contact.

Even with a UTB, there is still some down force on the top above the sound hole, and this will tend to peel the upper ends of the A-braces loose unless they're inletted. Any time you put a stress on an object, you get a 'strain': a displacement of some sort. When you tighten a string to bring it up to pitch it gets longer, and when you stand on a floor, it sags a little. You mght not see the sag, but the down force of the neck does push the middle of the UTB down a bit, and that shifts some of the load onto the A-frame and the top. Being stiffer, the A-frame doesn't bend as much as the top does, so there's a peeling stress set up in the glue line. That's why you inlet things; to keep them from peeling loose.
Author:  Wes McMillian [ Thu Jan 03, 2013 6:45 am ]
Post subject:  Re: Locking the A-frame
After continuing to go back and re-read this and studying my design, I think it's time to redesign my upper bout bracing scheme. But where I was looking to simplify it, I now think I'll be adding an element. Thanks a lot guys!! [headinwall] ;)

So, looking at the two forces at work - bending and rotational - I will continue to inlet the A-frame into the neck block as protection against bending load. But I love the simplicity of the doubler plate that John is using. I've seen that before but had forgotten about it. The inletted A-frame, then, would add a bit of stiffness to the UTB against the bending load in that area (although the typically low height of the braces may not add much resistance), whereas the doubler plate locked tightly between the neck block and UTB (and also inletted to the A-frame) would lock the neck block against rotation against both the UTB and upper X (by way of the inletted A frame).

Seems like a really solid solution. I suspect the doubler plate, alone, would work plenty well since the UTB is still there to resist top deformation and bending. Not to mention serve as added protection against top cracks in the fretboard extension area. And, as a benefit to using the doubler, I really don't have to worry about a perfect fit at the blind end of the A frame where it is inletted to the neck block. That rotational load will be completely carried by the doubler. And, by doubler, I'm looking at something more substantial along the lines of 1/2" mahogany that the A can be solidly inlet to. Still working it out, what do you guys think?
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