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Spin-off from 2 other threads.....really like the idea of a composite CRW/RW top....0.125"x2...plane to 30%CRW/70% RW +/-= 0.125" +/- finish. ... Tap tone the soundboard to taste (ear), brace accordingly, etc. Zach, thanks for your input in the previous thread!

Don't see a need for perfect 2 piece bookmatch on the inside plate, rather a 3 piece plate, same or better acoustics, less $....

For acoustically inert adhesive, I'm thinkin' (but don't like) CA for the bond....titebond is Plan B.....other ideas??? I'm hoping adhesive is my main objective on this 'composite'. Everything else 'should' fall in place...see Hesh's ...worst things in building!!!!??????? Okay! I love 'lessons learned!'

Who's good the zoot (Curly redwood)????? I'm 'in' for a coupla sets or so.....

BobC has one left. Not as nice of curl as the stuff he had before, but he may have more in the zoot cave. The other place I got a top from was OWW. Nothing online right now. Check with both.

Glue, I dont think CA is a good option. I'd guess LMI white or maybe fish.

I would go with epoxy. CA is a bad choice because of cure problems. Fish glue and Titebond would likely curl up your thin tops like a potato chip due to the moisture content. I think the added stiffness of the composite would make .125" too thick. You're probably looking more at .100", but you'll have to make that decision. I build my tops to a load/deflection, not to a thickness, so I'm just guessing at the .100" but it could be too thick also.
BTW - I've been thinking about this because I'll be starting on one this month. I'm going with CRW and Lutz.

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Use Epoxy.
Titebond or LMI white would be my second choice but would wait a few months for the wood to loose the moisture absorbed by the glue.

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

+1 for epoxy - Randy Reynolds uses a thixotropic epoxy like System 3 T-88 for his double tops, this would be less likely to seep into the abundant runout that you'll have in the curly top.

I would also strongly suggest vacuum bagging.

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The member formerly known as erikbojerik....

Do people use epoxy on double tops mainly due to the nomex? How does it compare to LMI, titebond, hide or fish acoustically? I didn't think it was as good in that respect, but don't really know.

Is there a decent way to clamp this without a vacuum bag? I admit its the best way, but i don't have a bag as of yet. It also applies to a question i have about clamping veneers for making perfling. Anyone have a trick?

Epoxy can be used in the thinnest layer without kicking on you before you're ready. Hide would be great, but try spreading a super-thin layer of hide, putting things together, and clamping it up before it cures on you! You can get water-thin epoxies made for laminating that have absurdly long open times, and you'll end up with less glue in the joint than any other method (which means less dead weight, which is better). The glue itself is so insignificant compared to the technique with which it's applied that I'll go out on a limb and say that any non-creeping glue can be used to make a fine guitar if used right. Epoxy is easier to not screw up, and easier to use less of, than the other glues in this application.

Anything that'll give the same results as a vacuum bag on something that size is more expensive. The only thing I can think of offhand is some form of screw or hydraulic press with a convex form.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

Doug,

Go to http://www.dunwellguitar.com/ and click on "double top guitars" or "luthiers pages" to get a good idea how to construct double tops.

I have used T-88 epoxy on several occasions and it mixs easily and works very well. For double tops you only use a very thin application that is rolled on with a brayer.

Ray

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Guitar Building = Continuous Improvement Process

IMO you are planning to sacrifice tone for appearance by laminating a top.

As an aside, I don't see why vacuum bagging, which isn't going to get you more than about 11 psi, should be seen as the best method.

As another aside, the stiffness of a laminate is not going to be any greater than the sum of the stiffnesses of its components. However, saturating wood with some adhesives appears to add stiffness. It also adds weight, and changes the way the material resonates. That's probably why the tonal potential of wood laminates is so limited. Double tops around a nomex core are another story, although I haven't yet heard one I liked.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

AFAIK all glues/adhesives discussed above will penetrate into the wood pores at the very least - and in many cases into the fiber itself. The resulting laminate should thus be more stiff than the same wood of the same thickness. Short of sealing the joint surfaces (why on earth...?), I don't see how to avoid it.

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The member formerly known as erikbojerik....

IMHO a urea formaldehyde veneer glue would be best.
If you dont have vacuum avalable you could glue it up on your gobar deck with a 1/4" mdf sheet on top (with wax paper between)
But I do tend to agree with Howard that this is not a desirable approach for an acoustic.

Totally agree with Jeff, urea formaldehyde veneer glue such as Uni-bond would be best. Dries very hard and rigid.
My vacuum pump and bag can produce up to 1,700 lbs. per sq. ft. That's like putting the weight of 3 cabinet saws on a 12"x12" square. Plenty of pressure to laminate a couple of pieces of wood together.

I also agree with Howard, why sacrafice sound for looks ? Wouldn't you be better off just building with a solid figured Redwood top and bracing to offset the loss of stiffness. You would still be compromising but it seems at least a few folks have gone that route and made decent guitars.

Link

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Cut to size.....Beat to fit.....Paint to match.

Link, you are getting 11.8 psi, which is good for a vacuum system. But the glue manufacturers recommend pressures in the neighborhood of 12-15 times that. Most of us can't actually get that kind of pressure, but we can come a lot closer with mechanical clamping.

Now I think that the manufacturer recommendations are for commercial applications and intended to overcome some poor surface prep. So I'm not saying you won't get decent lamination with a vacuum bag. I was questioning those posters who have said that a vacuum bag system is the ideal way to laminate.

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

Gap-filling is the concept to consider. Epoxy and unibond are both gapfilling glues, and in fact epoxy straight from the package can be squeezed out of a joint , with too much clamp pressure. I always add a little filler to epoxy. The West manuals used to, and probably still do, have a lot to say about thickeners and priming.

Unibond is a gapfiller, and it's thin enough that vacuum pressure can force it through straight-grained maple, never mind the bleeding that can happen with curlys and burls. Unibond offers an anti-bleed-through additive (looks like a superfine fiber filler) that is very effective.

Vacuum does a good job of squeezing two surfaces to full-enough (gap-filling) contact to make these glues work - no voids, no humps, surfaces as smooth as the mold or platen the veneers are in contact with. The finished joint can be so close, less than 0.001" by my estimate, that the glue line nearly disappears.

I'm WAY out of date on boatbuilding techniques, but twenty years ago we were using staples to hold parts together until (gap-filling) epoxy kicked off. We used vacuum to compress and saturate glass-fiber and balsawood layups for structural components in 80 foot offshore sportfisherman, that could do 60 mph.

So, no problem with vacuum working with the right kind of glue, and no problem, apparently, with the right kind of joint. I've yet to use vacuum bridge clamping, but apparently 30 - 40 lbs of pressure does the job.

Why has no-one mentioned polyurethane for double-top construction? Randy Reynolds, in AL #88, covered the subject. No water, works fine with vac, and supplies a lighter top for a given thickness.

Dan

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Dan
http://www.acme-archtops.com

Aww junk...just wrote a nice long, well worded, reply...and a browser error (router blipped) deleted it when I hit send!

Anyhow...vacuum is less pressure, more uniformly. It would be a -very- impressive caul that could provide a uniform 12PSI over an area the size of a guitar top with hand-actuated clamps. Slightly easier, but still hard, if using hydraulics or pneumatics. And expensive. And, if it covers the whole area, a little pressure is enough given that you did the rest of the process right. Don't get me wrong, I clamp the hell out of anything I can, whenever I can, just as a safety measure...but that's to make up for my fallibility rather than the necessity for more pressure. My 'ideal' clamping would be vacuum in a pressure vessel...50 psi uniformly should work quite well!

My theory is that clamping is there to push out excess glue (to an ideal thickness left in joint) and then hold the two pieces together until the glue cures. Better glue spread reduces energy required to push the glue out, better joint prep reduces energy required to push the pieces together. Sum them up and you don't need much pressure. The stuff we do is strictly bush league compared to the work the composites and lamination people do in aerospace and high-performance structural and marine applications, and they use vacuum most of the time. They're also -very- anal about the amount of glue used and the fit of components, so much so that the actual glue content of a laminate of a particular size is as important a QA check as the actual size of the component.

Polyurethane works alright as a laminating glue, and I've used it on some projects, but it's harder to pre-spread as thin as epoxies and some other glues as it only comes in a rather thick viscosity. Bad pre-spread means more clamping pressure needed, and that means that the vacuum might not be able to clear as much glue out of the joint.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services