My double top soundboard experience.

Construction and repair of Classical Guitar and related instruments
VuTran
Amateur luthier
Posts: 44
Joined: Tue Jan 21, 2014 10:18 am
Location: Dalat, Vietnam

My double top soundboard experience.

Post by VuTran » Fri Feb 26, 2016 7:35 am

Hi Delcampers,

I'd just like to say hello to everyone. This is my first entry on luthiers box and I want to share my new experience with you all.

After a few years of making guitars, the idea about double top guitar has been in my mind (since 2010). On the Lunar new year holiday, I spent lots of time on google reading delcampers’s posts. I decided to stop thinking about double top and start experimenting with it. I followed the idea of Frederich Holtier luthier on making solid wood double top guitars.

I don’t want to pay for the “expensive” top on this experiment. So, I started with the dark color Red Cedar top that costs around $1.5, well quarter sawn.

The first cuts are 2000 holes on the CNC with 4.5 mm diameter and 1mm depth. It took 3 hours on 4mm end mill. The top thickness is 2.2mm in the lower bout and 2.5mm in the upper bout. The “Honeycomb” gaps are 2mm. The weight reduction is 100 grams compared to 111 grams. It didn’t achieve my weight loss goal and I lost the top stiffness. It bends like a rubber top.
Image
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$1.5 was left on the wall for decoration.

I was scraping the binding while the CNC machine was making noise.
Image.


Then I tried it with a new top, changed the honeycombs composition a little bit. It’s now 4.5 mm in diameter, 1.3mm depth. The gaps are reduced to 1mm with 2600 holes. I left the top plate at 2.5mm thick. The final results are 17 grams weight loss. The top is now 103 grams. It’s 0.32 Western Red Cedar.
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This photo is showing the original (bottom) and the new (top) design:
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Then I glued some Red Cedar fire wood (4 pieces) at a 5 degree angle and planed it down to 1.8 mm for the inner skin. This plate weights 51 grams (0.385). I want to use 0.5mm skin so It should be 13 grams after planing and sanding. The gluing surface would add a few grams. The final top is 120 grams at 3mm thickness in the lower bout.

Image

After gluing the skins, the top was a real surprise. Now I understand why Frederich added one transverse bar on the top and how it works. At this weight and thickness, the top is so much stronger than the another 2.2-2.3mm, 0.36 Red Cedar and the weight is almost equal. I have the 2.6mm Englemann plate (0.365) and compared their stiffness (by hand) while I planed down the outer skin to 0.8-0.9 mm. The plate is 2.7mm in the lower bout, 2.5mm in the upper bout. The weight is 110 grams.

This image is showing the Red Cedar (0.36). By adding "2.mm" in front of the numbers, you will get the correct dimension.
Image

I left it as a successful test. But the top is so cheap and I want to make another one using the costly top. But wait, on decamlp, there is a story:

The thread “Making a < -$100 classical guitar” by Arnt Rian made me think about making under $100 guitar with this double top. Which is $1.5. It’s stronger and much lighter than my $135 top from LMII (AAA Italian spruce, 0.4, weight 142g at 2-2.5 mm).
Image

One of my clients rejected the back and sides in the shop. This set cost me about $20 in 2009 from a local luthier. He told me it's Red Trắc wood (Dalbergia Cochinchinensis)

The reason my previous client rejected the original set was because, according to him, it was too thin.
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This back is 400 grams at this dimension with a high density rosewood.

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The picky client rejecting my original set turn out to be a happy accident for my new <$100 double top guitar project.

To be continued....

Vu Tran
Last edited by VuTran on Fri Feb 26, 2016 12:05 pm, edited 2 times in total.

simonm
Amateur luthier
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Location: Germany, Kronberg (near Frankfurt).

Re: My double top soundboard experience.

Post by simonm » Fri Feb 26, 2016 10:05 am

Very interesting. Looking forward to reading more about the project. I like the rosette. What kind of cnc set-up have you got?
For some reason I cannot see the last photo - not loading for me.

DennisK

Re: My double top soundboard experience.

Post by DennisK » Fri Feb 26, 2016 10:10 am

How can you tell anything about the stiffness of that first top if you never glued the second layer over the honeycomb? It will always be floppy at that stage.

Glad to hear the second one is going well though. Sometimes the cheapest wood sounds the best :) And yeah, 400 grams of rosewood is way too much. Clients shouldn't really be involved in decisions like wood thickness anyway...

Good luck on the rest of the build. Let us know how it sounds once you get strings on :)

VuTran
Amateur luthier
Posts: 44
Joined: Tue Jan 21, 2014 10:18 am
Location: Dalat, Vietnam

Re: My double top soundboard experience.

Post by VuTran » Fri Feb 26, 2016 10:38 am

@Simonm: I've changed the image address. It works now.
My CNC is a CNC router runs by Mach3. I bought from Ho Chi Minh city in 2010. To machining the thin top, it's best to have a vacuum table. Because the cutting force on Red Cedar is light so I used double sided tape to hold the top.
@DennisK: Yes, I can't tell about the stiffness but the weight loss is quite small so I decided to make another one. I have plenty of time.

Vu Tran

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petermc61
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Location: Sydney, Australia

Re: My double top soundboard experience.

Post by petermc61 » Fri Feb 26, 2016 10:43 am

A very interesting post, thanks.

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tom0311
Amateur luthier
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Location: Witney, Oxfordshire

Re: My double top soundboard experience.

Post by tom0311 » Fri Feb 26, 2016 10:46 am

Cool project - keep us updated.
“There are two means of refuge from the misery of life - music and cats.”

wLance
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Joined: Sun Jan 31, 2016 1:32 am

Re: My double top soundboard experience.

Post by wLance » Fri Feb 26, 2016 3:55 pm

This should be interesting to follow.

AndreiO
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Joined: Thu Dec 31, 2015 2:06 pm
Location: Romania (fosta Dacia)

Re: My double top soundboard experience.

Post by AndreiO » Sat Feb 27, 2016 12:15 pm

You know that the honey comb of Holtier...is to variable...I ask him some long time ago...and he told me that he don't made more because wood is wood, and is quite to variable to be sure...will resist in time.Also Damman I understood that in some guitars the layers become unglued.
So for me (personal) double top is even worth than a good natural top of low density, but is my personal choice. What glue you use there?
You have there a good back, what density you have as average at that rosewood?
:merci:
www.classicalguitars.ro

Echi
Posts: 233
Joined: Fri Jan 07, 2011 10:14 pm

Re: My double top soundboard experience.

Post by Echi » Sat Feb 27, 2016 3:53 pm

I guess is not that easy to find a low density top with the same strength ratio you would get from a double top (while I agree that it's well possible to make fantastic guitars also with conventional top).
I also don't think delamination is really a common issue of the double top guitars.
I know the legend of Scot Tenant saying that Damman double top can suffer delamination after some years of stress but I'm not sure he diagnosed correctly the problem (I suspect instead that the thin cedar struts can play the bigger part here).

Nice job. I'm very interested too.

MessyTendon
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Re: My double top soundboard experience.

Post by MessyTendon » Sat Feb 27, 2016 7:36 pm

bracing?

AndreiO
Posts: 169
Joined: Thu Dec 31, 2015 2:06 pm
Location: Romania (fosta Dacia)

Re: My double top soundboard experience.

Post by AndreiO » Sat Feb 27, 2016 9:01 pm

Western red cedar is to weak for bracing, but this is based on some article...from internet, and is somenthing personal.I will never use, in fact I prefer spruce for all bracing in my instruments..
Also is difficult to made a good guitar with one top, for double you should co relate all this factors and also nomex, wood as interrnal layers, and glue also...
I suppose with a bit of effort you casn find light wood that will made a wonderfull guitar, but this also is my personal choice
www.classicalguitars.ro

Echi
Posts: 233
Joined: Fri Jan 07, 2011 10:14 pm

Re: My double top soundboard experience.

Post by Echi » Sat Feb 27, 2016 11:45 pm

Dammann used to make the bracing (already very flat and light) in cedar wood.
Given that the top is completely flat and bent under the tension of the strings the light struts grow loosing strength with the time and eventually give up.
I noticed the same phenomenon in some light flamenco guitars with cedar made bracing.

Marcus Dominelli
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Joined: Thu Mar 08, 2007 4:52 pm
Location: Victoria, B.C. Canada

Re: My double top soundboard experience.

Post by Marcus Dominelli » Sun Feb 28, 2016 3:26 am

AndreiO wrote:Western red cedar is to weak for bracing, but this is based on some article...from internet, and is somenthing personal.I will never use, in fact I prefer spruce for all bracing in my instruments..
I would disagree. I use spruce too (just for the record). But I recently played a great flamenca blanca made by Manuel F. Diaz, one of the better Granada makers. I don't know if he's still building anymore, as he's pretty old. But the flamenca blanca was made in 1971. 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.

The Dammans are in a category all their own. They are probably not intended to last more than a couple of decades.
When you build that light, with that kind of top distortion, you are not going for longevity. It's a trade off. Cedar or spruce bracing used in this manner, would make little difference...

Marcus Dominelli
Luthier
Posts: 2767
Joined: Thu Mar 08, 2007 4:52 pm
Location: Victoria, B.C. Canada

Re: My double top soundboard experience.

Post by Marcus Dominelli » Sun Feb 28, 2016 3:31 am

Vu Tran,
Nice work. I am impressed with the accuracy of your CNC set-up.
Were you able to use a water based glue? or did you use epoxy or polyurethane?

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Chris Sobel
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Location: Vancouver, WA

Re: My double top soundboard experience.

Post by Chris Sobel » Sun Feb 28, 2016 4:13 am

Marcus Dominelli wrote:
AndreiO wrote:Western red cedar is to weak for bracing, but this is based on some article...from internet, and is somenthing personal.I will never use, in fact I prefer spruce for all bracing in my instruments..
I would disagree. I use spruce too (just for the record). But I recently played a great flamenca blanca made by Manuel F. Diaz, one of the better Granada makers. I don't know if he's still building anymore, as he's pretty old. But the flamenca blanca was made in 1971. 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.

The Dammans are in a category all their own. They are probably not intended to last more than a couple of decades.
When you build that light, with that kind of top distortion, you are not going for longevity. It's a trade off. Cedar or spruce bracing used in this manner, would make little difference...
I play a Howell that has WRC braces and sounds great. I think the strength of a brace is proportional to the cube of its height, I think you can get low density cedar to approach the s/w ratio of spruce.

Chris
CE Sobel Guitars

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