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Bracing comments invited
http://luthiersforum.com/forum/viewtopic.php?f=10101&t=47042		 Page 1 of 1
Author:  Colin North [ Tue Jan 12, 2016 7:42 pm ]
Post subject:  Bracing comments invited
Curious to find out what others feel about this OM soundboard's bracing.
Adi top and bracing, 2.65mm (0.105") thick in the middle, 2.5mm (0.10") at the edges.
X-braces 15mm (0.59")x 7.2mm (0.285")
They cross at 97 degrees (slightly opened from 90 degrees), 1/8" cap
10mm (0.39") upper tone bar, 8mm (0.315") lower.
Bottom of X-braces will not be tucked.
Bridge plate is 2.65 (.105")mm rift sawn hard maple.
Opinion?
Author:  bluescreek [ Tue Jan 12, 2016 8:02 pm ]
Post subject:  Re: Bracing comments invited
you do need to fade the ends of the braces , if they don't go into the kerfing , that edge is a stress riser and I have seen the brace come off the top. One other thing is that the fretboard extension if glued to the top may crack the top , so instead of a Popsicle brace you should consider cleats to help support the area as the hard wood of the fretboard has a different shrinkage rate than spruce.
Author:  Rod True [ Tue Jan 12, 2016 8:54 pm ]
Post subject:  Re: Bracing comments invited
Colin North wrote:
Curious to find out what others feel about this OM soundboard's bracing.
Adi top and bracing, 2.65mm (0.105") thick in the middle, 2.5mm (0.10") at the edges.
X-braces 15mm (0.59")x 7.2mm (0.285")
They cross at 97 degrees (slightly opened from 90 degrees), 1/8" cap cap only needs to be a sliver, can reduce to 1/16"
10mm (0.59") upper tone barNot sure about your dimensions here..., 8mm (0.315") lower Not sure about your dimensions here....
Bottom of X-braces will not be tucked. Good choice IMO
Bridge plate is 2.65 (.105")mm rift sawn hard maple. That seems very thick to me. I've been going thinner on my bridge plates at 0.065-0.075" over the last 5 guitars
Opinion?


See my comments in red

And as John mentioned if you're not tucking braces you need to shave them to nothing at the end to eliminate the stress riser.
Author:  Grant Goltz [ Tue Jan 12, 2016 9:16 pm ]
Post subject:  Re: Bracing comments invited
Why is the not totally tapered end of a brace a stress riser any more than the entire edges of the braces?? And what, really, bad is it going to do? And, hey, there is a stress riser all around the bridge plate. Engineering types make a big deal out of stress riser, but they are all over in wood working and I can't really recall where there is a problem.

Grant
Author:  DannyV [ Tue Jan 12, 2016 9:16 pm ]
Post subject:  Re: Bracing comments invited
Spec guitar or for a specific player/style? Light gauge strings? Stiff Adi?

If it were me I would take the lower X down to 1/4" in the middle the finger braces tapered from the peak out to nothing. And what John said about feathering all the other ends. If it's not for a heavy handed player you could probably lighten up all the lower bracing a bit but what do I know. (disclaimer)

I generally use 1/4" for the x on most guitars.

Nice clean work Colin!
Author:  bluescreek [ Tue Jan 12, 2016 9:29 pm ]
Post subject:  Re: Bracing comments invited
The ends of the braces is the stress riser that is the point where the stress will focus and it will separate and loosen the brace. There is a reason that over time this design is the proven one. Think of it this way. The brace is supporting the top. The end of the brace is not supported so as the top moves that end of the brace will not. Over time the glue joint will fail. If you notice on the bridge plate the ends are tapered down but also has a large glue surface area . It will also have the bridge on top so that becomes a structure of sorts , then the braces are along the side so the free end of the plate has a large enough glue area to control a stress riser.

As are repair shop I have seen many loose ends on such a design.
Author:  Tom West [ Tue Jan 12, 2016 9:39 pm ]
Post subject:  Re: Bracing comments invited
Grant Goltz wrote:
Why is the not totally tapered end of a brace a stress riser any more than the entire edges of the braces??



Grant: There is a stress riser at the edge of the brace but the difference is the size of the area under stress...compare the length of the end of brace to the length along the brace it's self. The stress is concentrated to a much smaller area. Also not much one can do about the riser on the edge that would be over all efficient.
Tom
Author:  Terence Kennedy [ Tue Jan 12, 2016 11:39 pm ]
Post subject:  Re: Bracing comments invited
I think if you taper the ends of the X, finger braces, and lower face braces to nothing you will have a good sounding guitar. Record all the numbers, see how it sounds and make some tweaks on the next one. Do that for the rest of your building career :).

Remember you can still voice the closed box by thinning the edges of the lower bout further before binding.
Author:  Colin North [ Wed Jan 13, 2016 5:37 am ]
Post subject:  Re: Bracing comments invited
Thank guys. Sorry about the original typo in conversion to inches for tone bar. (Fingers braces are 1/4" - missed in original post)
Yes top is pretty stiff, S.G. about 0.47 after baking, but sightly off quarter, maybe 2-5 degrees.
Intended as a finger-style guitar, my first build with Adi.
At this stage of voicing, , the whole soundboard has has just started to loosen up, and wasn't sure if I should go more. As Terence mentions, I intend thinning the edges of the soundboard later. (I've got copious notes BTW!)
Point taken on the braces ends feathering out.

Question about the "popsicle" brace issue and the spruce cracking at the FB because of the differing expansion rates.
The "popsicle brace" (invisible in pic) is about 1 1/4" wide on this, trapezoidal in shape beveled all round.
It has only about 1/8" space above and below, between the UTB and the neck block.
And the neck block is little wider than the FB, so differential rates of expansion between the fretboard (IRW) and the spruce shouldn't come into the equation?
The grain directions of the wide popsicle brace and the neck block make this whole area like a plywood, cross grained. FB edges are inside this area.
Author:  David Malicky [ Wed Jan 13, 2016 2:23 pm ]
Post subject:  Re: Bracing comments invited
Grant Goltz wrote:
Why is the not totally tapered end of a brace a stress riser any more than the entire edges of the braces?? And what, really, bad is it going to do? And, hey, there is a stress riser all around the bridge plate. Engineering types make a big deal out of stress riser, but they are all over in wood working and I can't really recall where there is a problem.
Grant

To add to the good explanations above, think of each brace as a beam. When the top tries to move up/down or the bridge rotates, the beam resists this movement, and the beam is put into bending. The wood fiber directions give this beam high stiffness along its length. If the end of the brace is abrupt, there is a high-stiffness beam meeting a low stiffness top (it is thin, and its grain is only stiff along its length). Wherever high-stiffness meets low-stiffness, there is high stress at the edges.

Good question on the the other (long) edges of the braces. Yes, these are also stress risers, but as mentioned above, there is much more length to spread the stress. Equally importantly, the 'beam' in this case is only ~1/4" 'long', and the fibers are oriented so this 'beam' is not very stiff in that direction (and the top fibers are stiffer in this case). Such a short beam cannot resist top movement or rotation, so this 'beam' experiences little bending, and only small stresses occur at the 'ends' of this 'beam'.

The bridge plate is thin, so it has low bending stiffness. It also has a long glue line to spread the stress.

Stress risers will cause wood or glue to fail only if they are high enough. Most wood products are under relatively low stress. Guitar tops are an exception.
Author:  JSDenvir [ Wed Jan 13, 2016 2:25 pm ]
Post subject:  Re: Bracing comments invited
Hey Colin, it may be a minor thing, but I like to butt the top tone bar right against the bridge plate. I think it helps in conducting vibrations from the strings.
Author:  Colin North [ Wed Jan 13, 2016 3:16 pm ]
Post subject:  Re: Bracing comments invited
Thanks Steve, that's where it is. And it butts up against both x-braces too.
I think what you're seeing is the top of the bevel.
I bevel the edges of my bridge plates all round to about to alleviate stress risers (and to smooth the passage of the vibrations) ;) ;)
Author:  Tom West [ Wed Jan 13, 2016 3:24 pm ]
Post subject:  Re: Bracing comments invited
Colin: I'll jump in the pool and stir the water a bit.
1. Taper the untucked brace ends to nothing. Kills the stress riser and frees up the outer edge of plate a bit.
2. X brace patch can be taken down to .040" to .050". Reduces the stiffness at the center of the X a bit.
3. Taper the edge another .020" or so. Again, this frees the edge of the plate a bit.
4. Too late now........unless your adventuresome...bridge plate could be a lot smaller and thinner. This drops weight in the most crucial area of the top and helps in terms of responsiveness. Hint..........light bridge.
5. You have to judge this of course, but if the top is quite stiff I would have it a bit thinner.
On the other hand, nice clean work and I like the size of the UTB.
I don't know for sure if or how much any of this will help but it is what I would do. Hopefully all the little bits add up to give some tonal improvement. I'll shut up now.........good luck and give the crew some feed back when completed.
Tom
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