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I have started on my first double top guitar yesterday and would like some pointers from those with experience (or anyone really). I got some 2nd grade cedar at LMI to experiment with. I have routed out two 'D' shaped cavities in the lower bout to accept the Nomex core leaving the upper bout solid and a solid line down the center to the tail block. (have pics but not uploaded yet). The top thickness at this point is .140 and the Nomex is .08. I routed the cavities down to .100 so once I sand it flush I'll have .120 thick top. I'm thinking that the laminate that seals off the Nomex should be about .040 to match the thin top over the Nomex which is also .040. That's a total top thickness of .16 which is pretty thick but I don't dare go any lower then .040 over the top and bottom of the Nomex, that's like paper thin at that point. Maybe I can go thinner?

So that' that.

My question is on lamination, do you use epoxy over the whole area or only on the Nomex? IOW should I use Titebond for the wood to wood surfaces and epoxy over the wood to Nomex surfaces? Or just epoxy the whole thing (this is what I am leaning toward doing)

Secondly, this is for a Classical guitar so I wonder if anyone could suggest a bracing. I've only built Torres style fan bracing classical guitars out side of an experimental guitar I did several years back. I like the simple fan bracing design. For this guitar I understand Nomex is quite strong once epoxied in place so instead of 7 fans I was thinking of doing just 5 very light fans and bracing the upper bout the same way. I was even considering 3 fan braces.

Any advice is welcomed.

Regards.

I'm still in the 'planning and research' stage of my double-top project, so you are 'way ahead' of me.

One thing I've noted in my reading is that most makers seem to thin down the laminate with the nomex before adding the final skin. This is a way to control the overall thickness, and also to 'shape' the top thickness if desired. So this may be a way around your .160 problem.

I wouldn't mix glue types in a lamination lay-up.
I read somewhere about laying a very thin bead of epoxy along the nomex/wood joint to avoid problems with cracking at the transition, but this is all theory for me.

Fortunately, a lot of people who build with double tops seem to be quite open about their methods, so plenty of stuff out there to read. (we probably have a lot of the same web pages in our browser bookmarks.....).

Keep posting info as you go! I'll be watching!

Thanks
John

I don't know much about this, but glue control is one of the most critical issues, as I understand it, the glue will create more problems than it solves, and it's heavy. Anyway, go HERE, and watch the video's if you have not seen them. He talks about controlling glue. I thought they were very informative, though it didn't make me want to build one.

_________________
Waddy

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Sound Clips of most of my guitars

> I have started on my first double top guitar yesterday and would like some pointers from those with experience (or anyone really).

If you haven't done so already, read Randy Reynolds' excellent treatise on double top construciton in American Lutherie no. 88.

>I got some 2nd grade cedar at LMI to experiment with. I have routed out two 'D' shaped cavities in the lower bout to accept the Nomex core leaving the upper bout solid and a solid line down the center to the tail block. (have pics but not uploaded yet). The top thickness at this point is .140 and the Nomex is .08. I routed the cavities down to .100 so once I sand it flush I'll have .120 thick top. I'm thinking that the laminate that seals off the Nomex should be about .040 to match the thin top over the Nomex which is also .040. That's a total top thickness of .16 which is pretty thick but I don't dare go any lower then .040 over the top and bottom of the Nomex, that's like paper thin at that point. Maybe I can go thinner?

IMO, to bring out the responsive qualities of a double top, you need to thin your skins to at least 0.8 mm (0.040" = 1.0 mm). That sounds scary, but you'll find that with skins down to 0.7 mm, the final sandwich is still very robust and not fragile at all. As John mentions, the outer skin/nomex assembly can be thinned on a drum sander, and further thinned by hand sanding at the periphery. I shoot for 1.3 mm thick Nomex in the center.

>My question is on lamination, do you use epoxy over the whole area or only on the Nomex? IOW should I use Titebond for the wood to wood surfaces and epoxy over the wood to Nomex surfaces? Or just epoxy the whole thing (this is what I am leaning toward doing)

Yes, I use T88 epoxy for the whole sandwich, rolled with a brayer onto an acrylic plate, then transferred to the wood and nomex. I have calculated that the weight of epoxy transferred to the edges of the honeycomb is 0.6 g on each side. My upper bout is also solid wood, so after the transfer I use the brayer on the upper bout again to make sure that the spread is even.

>Secondly, this is for a Classical guitar so I wonder if anyone could suggest a bracing. I've only built Torres style fan bracing classical guitars out side of an experimental guitar I did several years back. I like the simple fan bracing design. For this guitar I understand Nomex is quite strong once epoxied in place so instead of 7 fans I was thinking of doing just 5 very light fans and bracing the upper bout the same way. I was even considering 3 fan braces.

This is where I believe it is key to have done deflection testing and Chladni analysis on your conventional tops to know where your sandwhich lies in terms of relative stiffness in both axes. Based on this, you can adjust your sandwich thickness, amount of bracing, top doming, etc. My sandwiches for classicals are stiffer than my conventional plates, so the bracing is lighter and the doming is less. Be careful: if you have a solid wood island under the bridge, this area can get extremly stiff, so you have to thin and brace accordingly.

Good luck and have fun. The effort is well worth it.

For more detail and information please look at
http://dunwellguitar.com/

The Double Top button will take to to some general information. The Luthier Pages button has much more info. Both the guitar and bouzouki building series are double tops.

FWIW,
Alan D.

Thanks all for your input. The top is complete now. I'll post another thread with pics. Alan I came across your web site via a Google search a month or so ago and essentially used those methods to rout out the pockets for the Nomex. IT worked quite well.

Here's how I did mine:

Step one join the plate like any other top



Step 2 cut out the Nomex using the mold.





Step 3 rout out the cavities



Step 4 test fit the nomex.



Check out how thin the pockets are in the light.



Step 5 epoxy the nomex



The nomex in place:



Step 6 epoxy on the bottom skin. To do this I slightly arch the MDF clamping caul for two reasons, one, it guarantees a good amount of pressure in the middle of the top when clamping the skins together around the sides of the cauls and two, it will actually slightly arch the top.



Step 7
Then I thicknessed it up on the drum sander. There is a nice arch to the plate and quite frankly it feels like a braced top. I need to figure out what I want to do for the bracing. I'm tempted to do no bracing at all because of the way it feels.

Here is how i ended up bracing this top:



It's super thin bracing, I just used the leftovers from the top plate so they are like 3/32's thick or so. BUt the tops feels super stiff in fact I'm a bit concerned about it.

I am going to bind the guitar in lacewood so I made a simple lacewood rosette: