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The Open Harmonic Bars Design of Classical Guitars
http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10101&t=17375		 Page 1 of 1
Author:  CWLiu [ Wed May 28, 2008 11:08 am ]
Post subject:  The Open Harmonic Bars Design of Classical Guitars
Some famous luthiers like Jose Romanillos and Robert Bouchet adopted the open harmonic bars design. I've seen the top of a Montero deformed badly at the open areas of both harmonic bars. It seems to cause enough stress-concentration to expedite some cracks, or not?
Author:  WaddyThomson [ Wed May 28, 2008 11:48 am ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
The Romanillos design, which has all three bars open, staggers the open areas so there is no line of stress to cause a crack. They overlap, but the end of each open area on both ends, ends at a different spot, just for that reason. I can't speak to the Bouchet design.
Author:  Alexandru Marian [ Wed May 28, 2008 2:18 pm ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
Sounds like a problem with that top alone. Too thin and humidity caused stress? I am sure Romanillos would have stopped using it if he had a lot of warranty troubles. It might worth mentioning that he is leaving his tops rather thick in the upper bout at 2.5mm which is rather solid.

Waddy, I heard something similar a while ago from Shawn i think but because of my poor English I did not get it. Do you mean that the walls of the opening are angled, and not perfectly along the grain?

If that is correct, I did that on the open bar top i work on now. Not sure how well visible it is in the picture, but the glue line covers about 3 grain lines.

Not sure if all this is really an issue.
Author:  Alexandru Marian [ Wed May 28, 2008 2:21 pm ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
here is that light bulb going on, I think i got it now. It is the openings which are misaligned each compared to the next one ?
Author:  WaddyThomson [ Wed May 28, 2008 2:32 pm ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
Not, what I meant, Alex. Let's see, how do I explain this? The places where the braces are glued to the top, are not in a straight line. They alternate, purposely so they do not create stress along the grain. i.e., from the center, and these are just examples, not actual measurements, on brace 1 would be 40mm from center, brace 2, 45mm, and brace 3, 35mm, so they do not line up. The outside, ends of the bridged sections fall at different distances from center as well. You can see, a little, in this picture.
Attachment:
P1000381.JPG
Author:  Alexandru Marian [ Wed May 28, 2008 2:35 pm ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
Yes, right, by misaligned I wanted to express something of this sort. WELL, that makes my slanted cut opening a NOVELTY. Going to file for a patent!!! bliss

laughing6-hehe
Author:  WaddyThomson [ Wed May 28, 2008 2:39 pm ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
Why not? Works for me!
Author:  Colin S [ Wed May 28, 2008 4:22 pm ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
Alex has got it right, it's the fact that the foot of the bar is at an angle to the grain line that is important.

Colin
Author:  WaddyThomson [ Wed May 28, 2008 4:36 pm ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
I was not aware of the angled edge point, until now, but the points of contact are also alternated for the same reasons.
Author:  jfrench [ Wed May 28, 2008 7:28 pm ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
I prefer to do both. At the opening the glued bar surface should be diagonal across the grain lines and the openings should stagger a bit.
Author:  CWLiu [ Wed May 28, 2008 8:52 pm ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
Thanks for your explanitions. They are very helpful. Perhaps that Montero has a floppy and thin top, which might suit to his overall design.

I've tried not-so-open bars(a bit like Jeff Elliott's but much simpler to execute) on a experimental top. The overtones of her are quite complex/shimmering to me.

Image
Author:  Alexandru Marian [ Thu May 29, 2008 5:49 am ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
Interesting pic! I don't see any fan braces...
Author:  CWLiu [ Thu May 29, 2008 6:14 am ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
That top is a replacement for a broken all-plywood short-scale alto guitar. Outside the picture there are three fan braces and a bridge bar.

I guess Romanillos won't like Jeff Elliott's idea or mine because the reinforcements are transverse to the soundboard. I thought "what the heck, so is the bridge."
Author:  Alexandru Marian [ Thu May 29, 2008 6:23 am ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
Ah I see. I know the Elliot patch. I actually like the ideea and i might use it on my next, IF I like the sound of this one. I did not use it simply because I made the openings only 3 to 4 mm in height, just enough for the side brace to pass through. I wanted as much meat as possible above to prevent the brace being too floppy. The full height is only 15mm and it gets weak easily.

In any case, being so thin, looks like veneer, it might be too weak to prevent a crack passing through. I mean, if the thing wants to crack, it will probably crack no matter how many stops we put :)
Author:  CWLiu [ Thu May 29, 2008 7:29 am ]
Post subject:  Re: The Open Harmonic Bars Design of Classical Guitars
Hmm... I think my opening height is much taller than it needs to be. Luckily this is only an experimental project so I have nothing to lose.

I glued the full-length strips first, planed them to less than 1mm, then glued the harmonic bars. Next time I'll integrate it with staggering and angled opening.
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