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I bought a large stash of wood from another builder, and among it all was what I first thought were two pieces of Brazilian rosewood for a guitar back. On closer examination, I realized someone had glued two pieces together for each piece. I think the guy I bought it from must have bought it on eBay. I am building a resonator guitar and the back will be considerably thicker than a flat top, around 150/1000", and will be cross ladder braced as well, but I am wondering if I should also reinforce the glue joint in each piece as well as the center. The joints are very good now, nearly invisible. I am thinking of using a less wide spruce strip than the center 3/4" one, maybe 1/4" to 1/2" wide. It's not the prettiest wood I've ever seen, but it's got a great, very long lasting tap tone, and I don't want to kill too much of that, but I don't want to have the guitar come apart on me either.

Any and all thoughts are appreciated!

thanks,
Damon

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Damon Wack
https://www.facebook.com/LindamonGuitars

Definitely add cross grain strips to reinforce all back joints. I made a few 4-piece backs of perfectly quartered and very old Brazilian. Thought I could get by with only reinforcing the center join. Bad idea. A fair amount of warranty work ensued.

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Stay with the happy people.
--Reynolds Large

Yes, I would definitely put some reinforcements on those joints.

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Pat

Yes, reinforce for sure, but for a join without purfling the cover strip need not be so wide as a joint with purling.

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nigel

http://www.theluthierblog.com

Thanks guys, I kind of figured it would be a good idea. Nigel, mine looks just about the same width as yours, so now I feel better!

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Damon Wack
https://www.facebook.com/LindamonGuitars

I have pondered this many times myself. In fact, I've pondered the conventional cross-grain reinforcement of the center back strip. I do realize that it's the traditional thing to do and I realize it's the "conventional wisdom", handed down through generations, and it seems as though it has always worked. But my thoughts take me to reinforcing with long grain strips, oriented WITH the back grain rather than across it. Here's why: If the instrument is ever subjected to harsh swings in temperature and humidity (and we've all seen vintage instruments subjected to those conditions) it seems as though a long grain reinforcement would be much more able to move with the back, thereby lessening the chance of a crack starting. Further, we already have a lot of cross grain reinforcement across the back via our major back braces. Lastly, we seldom if ever see full length cross grain reinforcement on our top joints. I don't believe I've ever seen a guitar built that way. We rely on our top bracing schemes to keep the center joint closed. We occasionally see some small diamond cleats between the bridge plate and tail block, or, on fan-braced instruments, we sometimes see one of the fan braces riding long-grain right on top of the center joint. But we don't see full length cross grain braces on top joints. So...is this conventional treatment of back joints truly necessary? I can't convince myself that it is TRULY necessary. What works for the top joint ought to work as well for the back joint. Yes, I DO realize I've just poked a hornet's next by posting these thoughts. And my mind is open. Some of you folks can probably point out an essential fact that I am missing. But, right now, I'm visualizing the top and back plates as two units that are much more likely to get thicker and thinner with wood movement (assuming quartered grain) and less likely to get wider and narrower, and I just can't see a reason to add more than a parallel grain reinforcement to the back joint. Help me out here, folks.
Patrick

I have a parlor guitar from the 1800's with a four piece back. It has not failed at the unreinforced seams.

Clay, did all else on that guitar look typical? Plate thickness, back braces etc?



Not sure if Patrick is still on here, but yeah, what about parallel grain back reinforcements? Not so much for expansion tracking, just in general?

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Pat

Laravee does not use BJR (back joint reinforcement) and we have never seen one fail.

I would use them anyway especially if you are not sure that your technique for shooting.... this important joint is perfect or near perfect. It's cheap insurance and again I doubt that the millions and millions and millions (channeling Carl Sagan RIP ) of guitars built with BJRs that their makers did it simply to increase complexity and costs.

I had an old Bay State parlor guitar with long grain reinforcement on the back that cracked right along with the back. I am not seeing much use for long grain. The reinforcement is only useful if the joint is bad. Then the reinforcement needs to be oriented in a way to give it strength. My two cents.

I have made quite a few 4 piece back guitars as I had come across a beautiful stash of Imbuia floor boards in the 1990's that I still have many sets of left. In fact I just pulled a set out the other day to build a guitar on my spare time to hang in the shop. But anyway, I have always used matching cross grain cut offs for the outside two joints and a spruce or Mahogany one for the center seam. You can hardly see the outside ones and the center one makes it look 'normal'.

FWIW I never use a marriage strip unless there’s an external decorative back strip, or unless I’m making a more expensive guitar and don’t feel like explaining to a client that it’s not strictly necessary.

As Hesh mentioned, it does require a perfect joint, but we always do that anyway, amiright?

As we know, glue lines are always stronger than the wood, so if there’s a fail, it won’t be on the seam, unless it was jointed poorly.

As for a long grain reinforcement, there’s no point. If it cracks it just means you’ve got two things to fix.

As Hesh also mentioned, there’s hundreds of thousands of Larrivee’s out there without it, and I’ve also never seen one fail at the joint. I expect they add them now because they’re tired of explaining to people that they’re not necessary.

So IMO it’s not needed unless you have an external back strip, in which case it becomes best practice, particularly if you employ the method of butting in a back strip rather than inlaying it.

A violin maker/repairman I knew once pointed out that he'd seen a lot of fiddles with lines of studs along cracks. In many cases the piece they're on cracks along the end of the line of studs, because they're too stiff to move with the wood they're glued to. Sometimes the studs just peel up, again, because they're too stiff. But he said he'd never seen a broken stud. They're always thicker than they need to be.

With studs, and cross grain reinforcements in general, what matters is the gluing area. Glue lines are weak in tension, in general, and shrinkage across the back over time produces a tension load on the joint. The glue line between a cross grain patch and the back is in shear, which is not a bad way to load a glue line. Making he patch as thin as possible, and feathering it down to nothing at the edges, reduces the stress riser.

One of my students repaired an English guittar (a wire strung cittern, that was built (by Harley!) ca 1820. The back had a narrow (1 cm?) and vanishingly thin cross grain reinforcement and had held up pretty well, with just a hint of some stress along the edges where the back grain ran exactly parallel to the edge of the strip. The fact that the back had been tooth planed to thickness, and left rough, didn't help. Interestingly, the top was X-braced. It sounds really good.

Suitability for use comes to mind. We had a Beardsall in from what the boys termed the 'Stump-wood Era'... 60 running inches of cracks and cross-grain failures on the back (sides were laminated). Adding insult to injury was the use of a back bracing system seemingly optimized to concentrate stress on the weakest, least stable areas of the Brazilian rosewood stump-wood back plate. Nearly $2K later in bench time and 70 running inches of very thin cross-grain reinforcements and a full refinish, I think they called it done. I believe they strongly recommended back replacement as a cheaper, better alternative, but apparently there is something magical about a back-as-wooden-time-bomb. In any case, no warranty offered on the repair for very obvious reasons.

Summary: Consider whether the material is suitable for use... some of the material sold online is anything but. If not, there is always a need for headplate stock, or consider crossgrain lamination (aka plywood or 'double back' construction). I was taught that conventional construction evolved both as a labor-saving method and as a way to allow the use of a wide range of sometimes marginally suitable wood in back construction, so consider whether that clever fanned jumper strut system with all those integral stress risers may actually be a slow fuze lit to a weaponized bit of unstable timber.

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For the times they are a changin'

- Bob Dylan

Ed I have never heard the term "marriage strip" before and it reminded me of my wedding night(s) two of them

Hi Pat,
The plate thickness on that old parlor guitar was typical for those of that era - slightly thinner than what you might see today. The guitar itself was smaller than most guitars built today and built for gut strings. Bracing was also typical for a guitar of that era, and it did have a back graft over the center seam, but no reinforcement over the two additional seams.
When that guitar was built most houses did not have central heating and experience the wide fluctuations of indoor relative humidity we have today. If I were to build a four piece back that had 3 full length seams I would consider using back grafts over all of them. If I just added small "wings" I might not reinforce those seams. I think back grafts should be made cross grain to the seam - it is unusual for wood to crack across the grain or change in length in that direction.
I have built instruments that omitted seam reinforcement, and also back bracing and they have held up fine (10 years so far). They were multipiece backs "cylindrically" curved and the lack of cross bracing allows them to rise and fall with humidity changes. If I build more of them I might consider parchment or thin linen reinforcement as some lutes and mandolins use - but so far so good!

Hey, at least you had two good nights, amiright?

Woodie, would love a fuller description of that dangerous bracing pattern…monkey see, monkey avoid…

Very interesting that Larivee and Halcy have gone hundreds of thousands of guitars without any bjr and no reports of failure!

Ed, any truth to them using ca for that joint? Also, what is the jointing method? Assuming a power jointer.

I am pretty confident with my hand plane for jointing and have always assumed it would be (potentially) more accurate than any power method. Benchmark would be no visible light at all. I've heard others mention that they get great results from a power method. Maybe it's the setup/machine? I have been eyeballing a bench top Jet spiral cutterhead unit. I know, not as sexy as Chrome vanadium honed to 8K scantily peeking from a lignum vitae sole ripping . 0005" fillets. Also not as rich for content creation. (Seems like some are creating more content than guitars tho lol, but that's another topic)

I appreciate all the real world feedback on what people have seen and experienced. John, I like your idea of using matching back wood reinforcements that would not be so noticeable on the two outer joints. The ones that I have seen that have 3 spruce patches look very busy and might create even more consumer questions. I was reading that someone just used round dots in the middle of unsupported spans.

Woodie, good point about the back material to begin with. And Alan, good point about creating stress risers. Not sure what violin studs are but perhaps beveling or rounding whatever kind of reinforcements would be wise.

Pat

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Pat

Itzhak Perlman probably qualifies.

Backs were glued with a specially formulated yellow glue of some PVA variety until California decided it was causing the end of the world as it was known. Don’t know about these days. The jointing was done on a power jointer by one guy who was the only guy allowed to do that job.

While they were never glued with CA, the seam on rosewood backs was flooded from the outside with thin CA once the body was assembled.

I don’t think it matters whether you use a hand plane or an electric plane (jointer) or a sanded joint (as long as the grit it fine enough), what matters is if you get the joint right or not. Technically a planed joint is better for gluing surface energy, but there comes a point where enough is enough and better is just superfluous.

There are some that espouse a slightly sprung joint, I prefer total contact down the entire seam. I use titebond.

Drawn employing the usual convention: plate viewed from outside of the instrument and braces viewed with x-ray vision.



To the best of my recollection: the discontinuous x-brace and single upper transverse brace lines show the areas where the bottom of the braces were relieved from the back plate. The solid brace lines are relatively low, light braces similar to fingers. The orange lines are the approximate crack locations, with the distinctive hook-shaped areas showing the consequence of all that reaction wood in the stump-wood-sourced back plate.

I do not think this modified X-and-fan design is necessarily flawed, but in combination with the usual poorly seasoned BRW stump-sourced stock of the early 2000's, the guitar will always going to be somewhere along the spectrum of undesired self-disassembly. Depending on how diligent the new owner is in humidification and keeping up with what is a perpetual motion machine in terms of new cracks, we forecasted that in another decade or so, the back plate would be a de facto double-back courtesy of the dozens of reinforcing grafts and doublers.

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For the times they are a changin'

- Bob Dylan

Thanks for that! Hopefully the cracks were inevitable with the wood rather than a result of the bracing pattern.

I can't afford BRW, but I can afford cheaper varieties of "interesting" grained wood, and I'm glad I have steered clear of it. I've always worried that it would warp and shift around too much, and Woodie's assessment confirms my fears. Give me boring, straight grained, quartersawn backs and sides, please. Liven it up with purflings and bindings.

I think those discontinuous braces are based on a flawed concept of how a plate vibrates.

There are too many stress risers on that back to count.

Hi Don,
Brazilian rosewood need not be expensive - if you are willing to laminate. As I've mentioned before, I laminate veneers in the same sequence they were sawn so they look and act like solid wood. The cost is a fraction what a decent set of BRW is selling for (typically less than $100 for materials).
Stump wood often has wild figure, which like crotch veneer is unstable reaction wood. If you totally impregnate it with epoxy so it can no longer absorb water there is a chance you can stabilize it.
Pictured is a guitar I built several years ago with laminated back. For those who wish to use this material you may want to consider this option.
Below it is a picture of the Cockatoo's latest effort at brace carving. What she lacks in finesse, she makes up for in speed!