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help me understand a concave top, please
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=47698		 Page 1 of 2
Author:  mqbernardo [ Thu Apr 28, 2016 5:48 pm ]
Post subject:  help me understand a concave top, please
thanks for looking

don´t have pics at the moment, but the top on the guitar i´m building has sunken in. strange thing is it happened after i glued the sides to the top (spanish solera and dentellones) and RH was kept constant at 47-50 %.

of course there was water introduced into the top from the HHG, but this stuff never happened before. i´m puzzled.

anyway, thanks in advance for any musings.


best,
Miguel.
Author:  mqbernardo [ Thu Apr 28, 2016 5:54 pm ]
Post subject:  Re: help me understand a concave top, please
alright, found a pic on my phone. this is a small guitar, for my 4 yo daughter.

to make things worse, it has a sort of Tornavoz, which will make clamping and bridge cauls not a solution.

oh well
Author:  DennisK [ Thu Apr 28, 2016 6:15 pm ]
Post subject:  Re: help me understand a concave top, please
Maybe could be caused by movement of the sides if they didn't have time to acclimate after bending before being glued to the soundboard? I wouldn't expect that to be so symmetrical though. It definitely looks like a dry soundboard. That shouldn't be the case if it's never left humidity control, but if you originally took it from high humidity into the control room, then braced it, and then exposed it to low humidity and brought it back to the control level, that could explain it. Maybe try exposing it to high humidity for a few hours and then bring it back to the control level and see if it remains convex.

Also, you could clamp the bridge with a thick caul before closing the box.
Author:  johnparchem [ Thu Apr 28, 2016 6:18 pm ]
Post subject:  Re: help me understand a concave top, please
I will bet that with a bridge with a radius and strings the top will have just the dome you hope for. The bridge will act as a radiused brace to hold the dome and the string tension tends to pull the top out.
Author:  truckjohn [ Thu Apr 28, 2016 6:37 pm ]
Post subject:  Re: help me understand a concave top, please
Do you use a spruce bridge patch? If so it could be that it was more dry than the top. Then - humidity hits it and it expands and the top sinks like that.
Author:  Tom West [ Fri Apr 29, 2016 9:56 am ]
Post subject:  Re: help me understand a concave top, please
mqbernardo wrote:
thanks for looking

don´t have pics at the moment, but the top on the guitar i´m building has sunken in. strange thing is it happened after i glued the sides to the top (spanish solera and dentellones) and RH was kept constant at 47-50 %.

of course there was water introduced into the top from the HHG, but this stuff never happened before. i´m puzzled.


Miguel : This just screams RH problem to me. The area of the glued surfaces is too small for the moisture in the HHG to have any effect. If you are absolutely sure your RH was measured correctly during bracing ,how about where the guitar has been since being assembled. Here on the east coast of Canada we have a very large swing in RH throughout the year. Very low during the coldest of winter to very high(high 90s)in the summer. Because of that I brace at about 35% RH. This gives a bit of defense in the winter and has not been detrimental during the wetter weather. Guitars exposed to low RH are in much more danger then those exposed to high RH. Again I would suggest a check of your RH control. That's about all could add. Take care.
Tom
Author:  mqbernardo [ Thu May 05, 2016 5:50 am ]
Post subject:  Re: help me understand a concave top, please
thanks for the replies guys. I still have my head scratching over this one. All wood used was in my RH controlled room. both my abeon and DIY-all-wooden-cheapo hygrometers showed no deviations in RH... but RH outside did drop quite a bit
i also wonder if the sides couldn´t have introduced some distortion to the top. i only use a bending iron and this bloodwood (at 1.9 mm) wa a real PITA to bend and had some spring back. against better judgement i decided to force the last few inches of it against the end block (which was already glued to the top, in the solera). but that can´´t explain all...


anyway, i´ll glue the bridge before gluing the back, which makes me a bit nervous, but anyway

cheers!
miguel.
Author:  bluescreek [ Thu May 05, 2016 6:00 am ]
Post subject:  Re: help me understand a concave top, please
you have an RH problem. When you glue braces on the top run the RH down to about 40%. You also may have a point if you didn't check the sides before you glued the top but my money is on the RH. If you were 55% and it is 45% your top moved with the RH.
drier air pulls the top down moisture up. I have seen worse but you have to consider this when gluing braces.
Author:  truckjohn [ Thu May 05, 2016 6:47 am ]
Post subject:  Re: help me understand a concave top, please
Flip the guitar over and give us a shot of your top bracing.

You have some cross grain bracing reinforcement piece in the lower bout that is driving this.

Thanks
Author:  mqbernardo [ Thu May 05, 2016 10:33 am ]
Post subject:  Re: help me understand a concave top, please
well, now i have, i glued some bits of spruce under the bridge area to see if i could make up for the thing (silly idea, but in dispair
i´ll try anything), when the thing turned concave i only had 5 fans.
Author:  mqbernardo [ Thu May 05, 2016 10:37 am ]
Post subject:  Re: help me understand a concave top, please
Ok, here it is, with the afterthought (and I'll thought) multi patches
Author:  mqbernardo [ Thu May 05, 2016 10:39 am ]
Post subject:  Re: help me understand a concave top, please
that was supposed to have written ill-thought, oh well. also a pic of the (calibrated) abeon. it proves nthing, i know, but makes me feel better.


cheers,
Miguel.
Author:  kencierp [ Thu May 05, 2016 11:46 am ]
Post subject:  Re: help me understand a concave top, please
Quote:
I will bet that with a bridge with a radius and strings the top will have just the dome you hope for.


I agree, very good advise - but I'd use a flat bottom bridge.
Author:  WendyW [ Thu May 05, 2016 12:46 pm ]
Post subject:  Re: help me understand a concave top, please
I have had the same thing happen with 2 guitars, both Torres bracing patterns, both had cross grain bridge patches. Another 2 guitars with same bracing pattern but no bridge patches did not do that. My RH was steady in both cases. Interesting that you added the cross grain bridge patches after the fact, so my theory of the cause is proved wrong. 1 guitar I went ahead and finished and glued the bridge sanded to a 25ft radius. You can still feel a little dip in the top right at the center. The 2nd guitar I I thought about dampening the top and clamping with an inner and outer caul for a while. Then gluing the bridge in the white. I put it aside and am also debating a top replacement. It's been a few years and haven't done either yet.
Author:  kencierp [ Thu May 05, 2016 3:38 pm ]
Post subject:  Re: help me understand a concave top, please
Likely a whole different thread but I don't understand the need for a bridge patch on a standard bridge classical guitar. The bridge itself is a brace, its glued to the sound board, the fans control roll and there's nothing inside that's going to get damaged since there are no pins. Seems like added weight?
Author:  truckjohn [ Thu May 05, 2016 4:51 pm ]
Post subject:  Re: help me understand a concave top, please
My experience is that crossgrain patches generally do not help concave top problems. They don't really add much stiffness. One single 1/4" wide x 1/2" tall brace is about 2x the stiffness of a crossgrain patch like that.

I think I would rather shave off the bracing starting with the center fan and see when the top comes back up. Then replace with newly carved braces.

Barring that I would remove the crossgrain patches and install a diagonal transverse lower bout brace like Lacote or Fleta...
Author:  DennisK [ Thu May 05, 2016 5:36 pm ]
Post subject:  Re: help me understand a concave top, please
kencierp wrote:
Likely a whole different thread but I don't understand the need for a bridge patch on a standard bridge classical guitar. The bridge itself is a brace, its glued to the sound board, the fans control roll and there's nothing inside that's going to get damaged since there are no pins. Seems like added weight?

My theory is that it lessens the seasonal change in action height (counteracts the bending force generated when the soundboard expands underneath the bridge), and possibly reduces the chance of splits, or prevents them spreading. It can be super thin, so very little added weight. But I don't think it's really necessary.

I'd say remove the patches, glue the bridge with clamps, and finish it up. It will probably have a sort of recurve shape (domed in the middle, dipped between the bridge wings and perimeter), but you could carve down the fan braces and call it an experiment in geometric stiffening :) A while back Nigel Forster was building guitars with a shape sort of like that intentionally (unfortunately the photo links in the old threads are dead).

My guess at the cause of the problem is that the wood was acclimated to high humidity before going into your control room, and never got dried below the control level so its moisture content remained on the high side. Or if you use water to clean up glue residue between gluing each brace, that can also make your effective bracing RH higher than intended.
Author:  mqbernardo [ Fri May 06, 2016 12:24 pm ]
Post subject:  Re: help me understand a concave top, please
i´m not the one to be too sure about it, but i generally agree with Denis re:bridge patch, it helps to balance out seasonal changes and top curvature. But although there are probably better ways to add stiffness in that area, all little bits add up - it does create a ply effect right in the center of the soundboard.
I´ve add patches that amounted to a rough 5% increase in the main resonant peak of the bare plate, but i´ve also had patches that actually decreased the frequency of the main top resonance (added mass won over added stiffness). the chladni shapes also chage quite a bit. if all this makes a difference in a finished guitar is obviously debatable.
another advantage of a bridge patch is that it seems to mitigate the telegraphing of the fan struts, specially if you use the thin and tall kind of braces and a pronounced dome.

regarding the reasons for the concavity, all i can say is that both the top and the bracing and patch stock had been in the RH controlled room for at least 6 months, so they should´ve been secured. but obviously something went wrong there. I did clean the glue residue with a damp cloth, but i´m not sure it added more water than the glue itself.

anyway, thanks for the interest. I´m gonna leave the hasty bridge patches (too much trouble getting them out at this point) and will glue the bridge before the back goes on, using a flat caul. should be fun!

all the best,
Miguel.
Author:  Jeff Highland [ Fri May 06, 2016 2:13 pm ]
Post subject:  Re: help me understand a concave top, please
mqbernardo wrote:

I did clean the glue residue with a damp cloth, but i´m not sure it added more water than the glue itself.

all the best,
Miguel.


I'd say that is the reason.
Your carefully acclimatised soundboard has gone to a high humidity.
Best practice is to scrape any squeezout of glue with a sharpened piece of spruce when it is gelled enough not to smear.
Author:  bluescreek [ Sat May 07, 2016 8:39 am ]
Post subject:  Re: help me understand a concave top, please
Let me say this about your brace set up
You have nothing that is contiguous across the grain. I do not remember ever seeing a classical without a bridge patch and the one you put in, is in pieces so there is no true cross grain support.
It is not uncommon to see top go up and down with RH changes. Yes the strings will pull up on the top and the fan braces will help with some of that you do need something to go across the top. Without that you have no support . I would address that part of the structure.
Author:  kencierp [ Sat May 07, 2016 9:01 am ]
Post subject:  Re: help me understand a concave top, please
Image
Author:  bluescreek [ Sat May 07, 2016 9:24 am ]
Post subject:  Re: help me understand a concave top, please
you may be correct but many do have plates. Here in these examples have contiguous support across the top .
The fan braces here should have been started closer together so there was more support across the top.
Author:  mqbernardo [ Sat May 07, 2016 11:20 am ]
Post subject:  Re: help me understand a concave top, please
I beg to differ, respectfuly . If anything I think this instrument is over braced. It's 350 mm long and 270 mm wide at the lower bout, only a little bigger than the traditional Portuguese cavaquinho which has _no_ bracing under the lower harmonic bar whatsoever. String length is 487 mm. I've made this sized guitar before and thought it could be loosen up, and needed a bit more bottom end, hence the open harmonic bar and the kinda-tornavoz thingy. Of course, with such a small box you can't expect miracles!
The patches under the bridge area were an afterthought and in fact I wanted them to add the least cross grain stiffness possible (in which I failed, cross grain modes rose by roughly 5 %, which was a surprise).

Regarding bridge patches , I often use them for the already stated reasons, but no harm in not using them. Torres surely didn't , neither did Santos (at least on the most famous models). Hauser I popularized it but Romanillos , for example, dispensed it after using patches for a while.
Author:  bluescreek [ Sat May 07, 2016 6:20 pm ]
Post subject:  Re: help me understand a concave top, please
It isn't the over bracing it is the layout. If you think of the top , the stiffness is with the grain not across it , so you need to plot braces so you can reinforce the top east to west.
Also if you brace up at 55% and afterwards it hits 40% you will see a dip. if you don't have a radius on the braces , the flatter the top the quicker they dip. In the examples ken shows you can see that the braces run north and south but also the start at a center and fan out at a stronger angle. This supports the top across the grain.
When you glue up braces pull the rh down to 40%. I have been doing that for the last 15 yrs and found that the tops are less prone to sink and seam separation.
Classical design also works a different neck angle then steel strings.
Author:  Clay S. [ Sat May 07, 2016 6:41 pm ]
Post subject:  Re: help me understand a concave top, please
"i also wonder if the sides couldn´t have introduced some distortion to the top. i only use a bending iron and this bloodwood (at 1.9 mm) wa a real PITA to bend and had some spring back. against better judgement i decided to force the last few inches of it against the end block (which was already glued to the top, in the solera). but that can´´t explain all..."

Hi Miguel,
If you squeeze the sides in toward each other (until parallel) does the soundboard flatten out?
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