My double top soundboard experience.

Construction and repair of Classical Guitar and related instruments
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mike.janel
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Re: My double top soundboard experience.

Post by mike.janel » Sun Feb 28, 2016 7:17 am

Very interesting.
When should this project be ready?
Michael
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2013 Amalio Burguet 3M (Cedar)
1989 Yamaha CG 110 (Spruce)

Echi
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Re: My double top soundboard experience.

Post by Echi » Sun Feb 28, 2016 10:02 am

Marcus Dominelli wrote:It had a spruce top, with red cedar bracing.
The guitar was a monster. Really responsive and explosive. I think the key to using cedar, is you have to treat it like cedar, not spruce. His braces were bigger than typical spruce bracing you'd see on a "Granada School" flamenco. There was very little top distortion at all, mainly because the string height was only about 8mm off the deck, which is about right
Francisco Manuel Diaz is a great maker.
I saw quite a lot flamenco guitars of the Granada school of the 70ies, as I'm fond of that kind of guitars, and must say that in some cases (of course not all) the tops supported with cedar bracings just gave in.
I have actually a couple of guitars of Manuel Bellido and Josè Bellido and both the tops show som signs of bowing. Some years after they passed to use spruce instead of cedar.

AndreiO
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Re: My double top soundboard experience.

Post by AndreiO » Sun Feb 28, 2016 11:00 am

I prefer always a spruce bracing thinner and stronger than cedar, also from my humble point of view spruce is far superior than cedar from acoustic point of view.
also the replace of spruce with balsa/carbon fibre/ cedar (western red) instead of spruce change a lot also the sound of the guitar.I respect spruce more than other top choices...but is my choice!
Spruce is a wood that you can trust in it, Wred cedar is not so sure as resistence to some forces...
www.classicalguitars.ro

VuTran
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Re: My double top soundboard experience.

Post by VuTran » Sun Feb 28, 2016 12:09 pm

@ AndreiO:
Sorry but I’m not sure I understand you correctly. So on the last post, you prefer the Spruce over Red Cedar in sound quality.
How can I get a good light density European Spruce for less than $60 w/o shipping and tax ?

The density of this rosewood is about 1.05 ton/m3. It also depends on where it's grown.

I decided to brace it like a traditional top, using James Lister bracing pattern. All bracing should weight about <28 grams. (German spruce 0.4). The top will end up at 137-138, I guess. I would like to use Red Cedar if I find a good one and runout of spruce :).

@Marcus Dominelli: I used Titebond original glue. I can’t figure out how to use hot hide glue to laminate the two skins together. Do you have any suggestions ?
The gluing surface is larger than Nomex. It'll be very hard for the top to be delaminated. When It happened, I know it’s my mistake :).

The CNC tolerance is +-0.05 or so. It saves time and makes less noise on the tasks that need a router, good tool to make tools.

To avoid all the troubles I'll have with this top in the near future (the sound, cracking, delamination). I decided to use bolt on neck joint with elevated fretboard for easy repair. Which is also another experience.

@ mike.janel: The project will be started in March. I took a picture to show the separate neck and heel (the red part).
Image

Thanks everyone for your comments. I'll keep you updated.

Vu Tran

AndreiO
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Re: My double top soundboard experience.

Post by AndreiO » Sun Feb 28, 2016 12:19 pm

@Vu tran, Sorry I know is your topic, is about ideea to made a good guitar.It is really difficult to find good spruce, but you can find not so hard good engelmann lower in density than your double top.I have the tops thinned at 2.3-2.7 mm arround 122-135g (western red cedar sometime 110g) to 140-145 grams on heavy spruce that is 0.42-0.44 density.So you can find for less than 50$ a good engelmann top AA grade or A+ that can be also 0.38-0.39... and you will have around 130g so will be lighter than your actualy double top...
Ok you should be very carefull with bracing...I will brace thicker and if is not what I wish I can reduce more after....but this is my aproach.
Take care with western red cedar for braces...At least try to find somenthing with more grains/inch to be a bit stronger...
:merci:
www.classicalguitars.ro

AndreiO
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Re: My double top soundboard experience.

Post by AndreiO » Sun Feb 28, 2016 3:42 pm

:desole: Just to know...the ideea of double top aim is to reduce mass of the top, replacing a part of the wood with a material that is light and strong...I refer to nomex....usual if you have a top 2,3mm uniform thickness using double top technique you can reduce the mass with somenthing 20-30%.
If you have a heavy top that at 2,3mm is arround 150 grams you can have a top of 120g, but also the ideea is to have tops under 100g including the nomex and also the second top.with wood you can have a reduce in weight of 10-15% , no more, but is enough for also extra work?
I prefer to try to find lighter wood instead of gluing all this layers, but I am curious how will sound a guitar with a top of 100g?
I have a cedar that is near 110 is the lightest I found untill now ....but I heard about 75-85g with DT....so consider the efforts you are far away with 138 grams you have at least 7-10-15 grams more than you should be consider a double top with wood layers.
Succes I wish you! I am curious about sound also from this strange back :mrgreen:
Please don't be upset because I told this,please continue your work, but also light wood is a bit more expensive than ordinary wood that is on the heavy side.Some sellers know what they sold so the price is more! :(
Best regards
Andrei
:bravo: :bye: :merci: :merci: :merci: :merci:
www.classicalguitars.ro

Marcus Dominelli
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Re: My double top soundboard experience.

Post by Marcus Dominelli » Sun Feb 28, 2016 4:29 pm

VuTran wrote:@Marcus Dominelli: I used Titebond original glue. I can’t figure out how to use hot hide glue to laminate the two skins together. Do you have any suggestions ?
The gluing surface is larger than Nomex. It'll be very hard for the top to be delaminated. When It happened, I know it’s my mistake .
I thought you could get away with using a water based glue, based on the surface areas, and more importantly, the increased thickness of your inner skin, which is why I asked.
You probably used a roller to apply the glue evenly to the perforated top, then vacuum clamped it. It will probably not delaminate.
I don't know how it could could be done with hide glue. I am sure it's possible, but a bunch of methods would have to be tried.

I use nomex, and .5mm thick skins, so water based glue is out of the question...

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bacsidoan
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Re: My double top soundboard experience.

Post by bacsidoan » Sun Feb 28, 2016 5:49 pm

Antonius Mueller uses hide glue for his Lamellendecke top using spruce slats instead of Nomex or CNC cut wood plate. His guitars are well sold with a 6 year waiting list.
lammelendecke.jpg
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Frederich
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Re: My double top soundboard experience.

Post by Frederich » Mon Feb 29, 2016 5:08 am

Nice work, Vu Tran. May I suggest you that to make the top 2mm thick, go down 1,4mm with the holes and the second skin about 0,5 - 0,6mm thick.
A good WRC top before lamination should be about 72g - 80g. Try to use polyurethane glue (Gorilla) instead of Titebond. Use a rubber roler and put glue only on the top, don’t put on the thin skin. This will eliminate any tendency of distortion when cured. If you have a CNC machine why don’y you try to build a vacuum table? I’ll be afraid to use tape. The photo is from a guitar that I work on right now.
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VuTran
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Re: My double top soundboard experience.

Post by VuTran » Mon Feb 29, 2016 2:49 pm

Frederich wrote:Nice work, Vu Tran. May I suggest you that to make the top 2mm thick, go down 1,4mm with the holes and the second skin about 0,5 - 0,6mm thick.
A good WRC top before lamination should be about 72g - 80g. Try to use polyurethane glue (Gorilla) instead of Titebond. Use a rubber roler and put glue only on the top, don’t put on the thin skin. This will eliminate any tendency of distortion when cured. If you have a CNC machine why don’y you try to build a vacuum table? I’ll be afraid to use tape. The photo is from a guitar that I work on right now.
Thanks so much, Frederich! Your advises is worth more than a thousand dollars.
Gorilla is not available in Vietnam. Can you tell me why it's better than water-based glue? Do you use silicone film on the vacuum to glue the skins? I think I should leave the inner skin thicker than 1.5mm, then plane it down to 0.5-0.6.
Because the top was thick, so the tape works fine. For 2mm plate with 1.4mm holes, I have to use the vacuum table.

How about the dimension of your bridge? 180mm? How much does it weight?

Your input has made me change the direction in this topic. I have to wait for 2 weeks on the Gorilla glue from Amazon.

Vu Tran

VuTran
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Re: My double top soundboard experience.

Post by VuTran » Fri Mar 25, 2016 1:31 pm

I did lots of trails, almost everything I can think about with the soundboard on the CNC. Here are my updates on this topic. I followed Frederich's input. Except I used Titebond to glue the two skins.

Image
It's my final version of "honeycomb shape" The gaps are 1.3 mm. 1.4 mm depth. 5 mm in diameter. The plate is about 2mm thick, weights 78 grams.
Image
It's a honeycomb shape, 7mm, lots of tears out by the end mill.
Here is my DIY vacuum press, using silicon film. The pressure is about 9 kPa. Actually it's the vacuum cleaner.

Image
I leave the inner skin at about 3.5mm thick. Made one side flat and smooth and applied the Titebond glue directly into this surface. Then placed the top on the cloth, put it into the vacuum press. After everything held up. I turned the vacuum press up side down and hope for the fillet.
Image
After a day, I put it back to the cnc. Planed it down to 2.55mm. The final inner skin is 0.5mm after sanding.

The final plate before sanding:
Image
The outer skin:
Image

The final weight is 97 grams. Still stronger than my traditional top.
Last edited by VuTran on Sat Mar 26, 2016 12:24 am, edited 2 times in total.

Alan Carruth
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Re: My double top soundboard experience.

Post by Alan Carruth » Fri Mar 25, 2016 3:57 pm

Serge DeJong used a spruce spacer in double tops instead of Nomex for a while. He cut slots in his by hand (whew!). As I understand it, he stopped using that system because there was too much tendency to crack along the grain. I note that you put the inner skin on at something of an angle, which should help.

I did some tests of Nomex years ago, specifically checking on the issues of poor glue bonds and delamination. I know that the FAA held off in approving of Nomex sandwich construction in aircraft for several years until the makers could devise ways of testing for poor bonds and delamination. I made two test panels that same size of standard rotary cut mahogany veneer and Nomex, with epoxy. On one panel I waxed areas of the veneer to produce poor bonds, and marked those on the outside so I'd know where they were.

After making the panels up I tested the resonant modes, to see if there was any difference in stiffness, and found none. Nor could I find any other sign of the poor joints. I then glued a piece of wood across the end of the test panel with the poor joints, clamped the other end down, and used a signal generator and 'stinger' driver to vibrate it as hard as I could fer several days. Removing the glued on piece I tested the vibration modes of the panel again: no change. However, I did note that when I pressed on the poor bond spots I could hear creaking sounds, as the loose Nomex moved across the veneer. Further vibration caused these loose areas to expand beyond the limits of the waxed parts, but there was still no sign of reduced stiffness in the vibration patterns.

Driving that plate was noisy, so I stopped the test before it reached the point of destruction, but it seems clear to me what is likely to happen over time. So long as the loose bond areas are 'small' and tight around the edges there would be little buckling of the skins, and the overall stiffness of the plate would be largely unaffected. However, over time it's likely that one of those loose areas would reach an edge, in which case it would be much easier for the skin to buckle. At that point I'd expect the plate to fail. Since the only warning you might get would be some creaking or possibly slight buzzing this would be a catastrophic failure: good one day and gone the next.

I'll note that in a normal bond, where there was no wax or other material to impede things, it's easy enough to repair small delaminations. All you need to do is find the spot, drill a small hole in the skin, and inject a little thin CA glue. The trick is, as the FAA realized, finding the problem in the first place. Since aircraft use aluminum skins for the most part they can use ultrasonic sensors to detect these, but I'm not at all sure this would work on wood skins, even if we could afford them.

I've mentioned this several times on various lists over the past few years. Generally the response has been unwelcoming. It's usually said that this system has been in used for a decade or two, and there are few, if any, reports of such problems. Recently I got an e-mail from a well known dealer and repair person who said that this is actually not an uncommon issue, its simply not reported very often.

I built two Nomex core double tops myself. Both failed due to issues with the epoxy: the first after about a year of playing, and the other in the process of building. I'd had enough experience with epoxy before that that it came as no surprise, although it was certainly an unpleasant one when it happened. The one that worked sounded pretty good while it lasted. I doubt I'll do any more with Nomex.

FWIW

rounie
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Re: My double top soundboard experience.

Post by rounie » Sun Mar 27, 2016 5:32 am

Chris,

I play a Howell too and they are amazing!!
Let Music Unite All............
Richard Howell No.408 Cedar/Indian-2007

Tim W
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Re: My double top soundboard experience.

Post by Tim W » Mon Mar 28, 2016 11:33 am

If you have access to a thermal imager, (there are some relatively inexpensive ones available these days) you can probably find areas of de-lamination as the thermal capacitance will be different from the well-bonded areas. This method is regularly used to find bonding issues in industrial and scientific processes. Set a very narrow span and very briefly apply a heat source to the top, (hair dryer, incandescent light, etc.). The goal is to momentarily create a temperature differential and that differential will be clearly visible with IR.

VuTran
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Re: My double top soundboard experience.

Post by VuTran » Tue Mar 29, 2016 1:54 pm

Hi Alan,

I think if the is the delaminations then it's because of the thin joints between the top and the Nomex. On this type of construction, the joints are 1.4mm and compared to some light bracings with 2.5mm wide. I don't think the delaminations will happen unless there is not enough glue on the skin. I added more glue than It need to be after weighting the Titebond when it dried.

My vacuum cleaner went to the heaven after two hours of hard working.
Image
Image
It's still working but It's better for my ears to let him go.

Vu

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