Official Luthiers Forum!

[QUOTE=LanceK] I always buy them from LMI - why - cuz im lazy [/QUOTE]

WHAT!!?? How many do you need? I bet all of the sponsors here will cut some from tops that have pin knots or other defects that make them otherwise unusable and get them to you cheaper than LMI when you place an order with them. Just let us know about these things and we will make them availble!

Eeeek!

Shane

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Canada

Dave,

Another consideration for the gross grain issue is that most (but not all) backs are quartersawn. When wood is such then most expansion and contraction occurs in thickness, not in width, as would be the case in flatsawn wood. I agree about the modern glues and feel as you do. But then again, these are thin bits of wood (the backs) and the centre seam is the longest part so would be the most vunerable. So it is likely just a bit of insurance, cheap at that!

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Canada

Carlton, you don't seem to have responded to what I said about stress
risers or bookmatches. I'm not talking about glue joints separating. I
have never seen a glue expert comment about the stresses in a thin
bookmatched panel, or claim that such a panel, however well glued,
functions as a single piece of wood.Howard Klepper38644.9451851852

_________________
Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

Dave,
there are several things to consider here. As previously mentioned, inlay
strips weaken the joint. If you look at many of the inlays used, you will
see they are made up of 100s of tiny pieces of wood. If you hold one of
them in your hands and bend, you will soon see how easily they break
apart. They are often quite fragile. They are decorative only. No matter
what the glue is you use, they are still fragile.

As PaulB points out, the woods we use are seldom, if ever, very
homogenous. The grain is usually quite irregular. Another consideration
is the use/abuse an instrument receives over the years. A bang or bump
on the back places most of the stress right on the center.

Your question I think, is can you eliminate the back reinforcement? Try it.
See if in 10 years the seam has stayed together. That's the only way you'll
know. I would suggest though, that these practices come about because
there is a recurring problem (like split back seams) that luthiers long ago
learned to prevent by adding a back reinforcement. We're not talking
about a huge amount of wood here. By not using a reinforcement strip I
don't think you'll gain anything acoustically or weight wise. Just my 2c.
Craig

[QUOTE=Howard Klepper] Carlton, you don't seem to have responded to what I said about stress
risers or bookmatches. I'm not talking about glue joints separating. I
have never seen a glue expert comment about the stresses in a thin
bookmatched panel, or claim that such a panel, however well glued,
functions as a single piece of wood.[/QUOTE]

Howard...A multi-piece table top is a good example. Everything I've read about edge-joining boards says to treat the completed slab as one piece when planning for seasonal movement. Now, of course, there are differences between a, 1" thick panel and one that's .085", and that's a major reason why we use (and continue to tinker with) bracing schemes. For seasonal movement, though, that center glue line completely and truely joins the wood into one board, and that's how it will behave as it shrinks and expands. It will always shrink toward, or expand from, its new center--the glue line. All the stress of seasonal movement is now at the outside edges. If extremes are reached, the wood will crack where its grain is the weakest; and that could be anywhere on the back. If a crack appears anywhere near the center join, it's coincidental.

Warping and twisting can occur at any time, regardless of humidity, and, I've read, usually happens as wood loses its cellular moisture content and stresses are relieved in the board. The (should be) dry wood used in lutherie isn't likely to be affected in this way. The twisting/warping stresses have already been relieved, and the board is stable. If you can get bookmatched pieces flat and straight enough for a clean center join, the one-board effect takes over and the whole back absorbes or loses moisture as one piece, and moves as one piece. There are no opposing forces, as the wood was dry and stable when joined. If not, it shouldn't be used. If left as a flat panel, it would expand and contract with humidity changes without any further twisting or warping--just as the unjoined pieces would have. When formed into a box, though, new stresses are added which restrict its free movement, so we brace it and hope for the best; but this should result in no more stress applied to the center join than to any other area of the back.

I'm not necessarily opposed to using a joint reinforcement on the back, but I do question its true usefulness. I'm keepin' an open mind, though.

I hate to appear dense, but could someone define "stress riser" for me?

Thanks!

_________________
53% of all statistics are made up on the spot
http://driftwoodguitars.blogspot.com/

[QUOTE=CarltonM]
I agree! I seems the back strip is a leftover from the days of poor jointing and weak glues. [/QUOTE]

Whenever I'm looking through an old guitarmaking book, it always puzzles me when there are comments like "with modern glues such and such is unnecessary," implying that modern glues are stronger. Yet most seem to favor hide glue, which has been available for quite a while, to say the least.

So this begs the question - is modern hide glue any stronger or better than what was available in the 16th and 17th centuries? You can get it strong enough to rip apart glass all by itself as it shrinks back and dries. That is some strong glue.

I also can't resist the temptation to ask: What were the days of poor jointing? The achievements of instrument makers in the past don't seem to pale in comparison to those of today...

_________________
Instagram: @jfrenchluthier
Web: https://www.jfrenchguitars.com/

"If you can get bookmatched pieces flat and straight enough for a clean
center join, the one-board effect takes over and the whole back absorbes
or loses moisture as one piece, and moves as one piece. There are no
opposing forces, as the wood was dry and stable when joined."

[I wish I could figure out the quote in a box thing]

The thing is, the example of a table doesn't support the claim that there
are no opposing forces in a guitar back. A table top is not
glued to a frame at its edges. It isn't analogous to a guitar back for our
purposes. Any book on furniture construction will tell you not to not to
join a table top rigidly to a perimeter frame--that would make it prone to
cracking when it shrinks in dry conditions. Same thing with a wide panel
in a door; you need to let it float in its framing. Dry, stable wood is still
going to expand and contract with humidity changes

The guitar back's attachment to the sides leaves the center (both
lattitudinally and longitudinally) as the place under the most stress from
shrinkage (longitudinally because the back is glued on its ends as well as
its side edges, so it doesn't shrink across the grain as much at the ends
as it does in the middle).   With a bookmatch, each side is shrinking
equally away from the center line. But the bookmatch isn't the critical
part. You could have a one piece or multipiece back, and the maximum
stress will still be more or less in the center (it may be a bit off-center
because the material on either side varies more than with a bookmatch.

Agree with JFrench, BTW. Howard Klepper38646.0156481481

_________________
Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

I just recieved some very nice tops from Shane, the tops came sandwiched between some pieces of wood with defects in it for protection, my first reaction was "more back grafts". I sometimes buy low grade tops from Allied or LMI just to cut into back strips.

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Jim Watts
http://jameswattsguitars.com

i bought a bunch of martin seconds tops from ebay for the same purpose.

Has anyone used the four-piece Madagascar rosewood backs from LMI? They have two long unsupported seams. Do you use grafts on those seams, too? Probably not. But even if 95% of all back seams would survive for a long time without the graft, it's not worth the risk. Since the ends of the back are glued crossgrain to the sides and kerfed linings, there is a lot of stress on the back during changes in humidity and some woods are more prone to cracks. More good reasons for the insurance. But, personally, I just like the way it looks, kind of like exposed plumbing.

_________________
Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

[QUOTE=WayneC] I hate to appear dense, but could someone define "stress riser" for me?

Thanks![/QUOTE]

A stress riser is an abrupt change in the surface of a material, such as a notch, corner, hole, etc. In the back inlay example, Howard was talking about the point in the back where the channel for the inlay is cut.

Any load on a material causes a certain level of stress in the material. The stress level is distributed throughout the material. The nature of the stress distribution is influenced by the shape of the part. Stress risers have the effect of concentrating stress in localized areas that increase the likelihood of a fracture starting at that point.

In mechanical design, we always try to spot potential stress risers and reduce their effect when possible. A common example is any rotating shaft with diameter changes along it's length. You will normally not see a sharp, square change at the smaller diameter, there will be a round fillet where possible.TomS38646.2636574074

[QUOTE=Howard Klepper]
[I wish I could figure out the quote in a box thing]
[/QUOTE]

Simple: use the 'quote' button in the top-right of every individual quote to do the whole message. Basically, it's:

[ QUOTE=Author's Name ]
Text You Want To Quote Here
[ /QUOTE ]

Only without the unnecessary spaces. Also, the name bit (=Author's name) is optional.

[QUOTE=Ken Franklin] Has anyone used the four-piece Madagascar rosewood backs from LMI? They have two long unsupported seams. Do you use grafts on those seams, too? Probably not. But even if 95% of all back seams would survive for a long time without the graft, it's not worth the risk. Since the ends of the back are glued crossgrain to the sides and kerfed linings, there is a lot of stress on the back during changes in humidity and some woods are more prone to cracks. More good reasons for the insurance. But, personally, I just like the way it looks, kind of like exposed plumbing. [/QUOTE]

Ken, I have not used the ones from LMI, but I have cut madagascar rosewood myself and made several guitars out of it with four piece backs. I only used a reinforcement on the center join, with no problems to date.

_________________
Instagram: @jfrenchluthier
Web: https://www.jfrenchguitars.com/

Tom, thanks for the education!

I agree with those who say using a back graft is cheap insurance. Especially until I have more confidence in my own skills. I would like to think that the guitar I build today will still be structurally sound 10 or 20 years from now.

As for the aesthetics, I kind of like the look when the braces and backgraft are a contrasting color to the back. Someone recently posted a picture of a guitar with a logo inlayed into the back graft so it was visible through the sound hole. It was a really elegant look - something I'm contemplating for my current build.

_________________
53% of all statistics are made up on the spot
http://driftwoodguitars.blogspot.com/

[QUOTE=Ken Franklin] Has anyone used the four-piece Madagascar rosewood backs from LMI? They have two long unsupported seams. Do you use grafts on those seams, too? Probably not. But even if 95% of all back seams would survive for a long time without the graft, it's not worth the risk.[/QUOTE] Ken, I recently saw an Eric Monrad classical guitar made of this wood, and he had used 3 back grafts, one for each seam.

[QUOTE=Howard Klepper] The thing is, the example of a table doesn't support the claim that there
are no opposing forces in a guitar back. A table top is not
glued to a frame at its edges. It isn't analogous to a guitar back for our
purposes. Any book on furniture construction will tell you not to not to
join a table top rigidly to a perimeter frame--that would make it prone to
cracking when it shrinks in dry conditions. Same thing with a wide panel
in a door; you need to let it float in its framing. Dry, stable wood is still
going to expand and contract with humidity changes

The guitar back's attachment to the sides leaves the center (both
lattitudinally and longitudinally) as the place under the most stress from
shrinkage (longitudinally because the back is glued on its ends as well as
its side edges, so it doesn't shrink across the grain as much at the ends
as it does in the middle).   With a bookmatch, each side is shrinking
equally away from the center line. But the bookmatch isn't the critical
part. You could have a one piece or multipiece back, and the maximum
stress will still be more or less in the center (it may be a bit off-center
because the material on either side varies more than with a bookmatch.

Agree with JFrench, BTW. [/QUOTE]

I only used the table top example because, when several boards are edge-glued together they then move as one--toward or away from the center of the new, larger, panel, not toward or away from each glue line (this, of course, assumes that each board used is equally dry). A guitar back will behave in the same way, regardless of the number of pieces used to create it. As I mentioned in an earlier post, yes, stresses ARE introduced when the back is glued to the sides, but the greatest stress will be at the PERIMETER of the back, not the center. Why? Because, like a table top, that's where all the movement needs to happen (most importantely, perpendicular to the grain). The edges of the back want to pull away from, or expand into, the sides, and the sides are stopping them from doing so. The center of the back (i.e. the vertical glue seam) is the eye of the storm--it remains relatively calm. Any movement there is microscopic, because the center is where all the action moves toward or away from. It will be virtually static. This is the most important point I've been trying to get across: What used to be two (or more) pieces of wood now behave as one piece. If the wood is dry and stable when glued, there will be no opposing forces at the glue line, because it's as if the glue line doesn't exist. What used to be two edges are now part of one big, happy board, and that's how it will behave! Even if the glue line is not in the middle of the back, the middle is still where all the action DOESN'T happen! The glue line is irrelevant. The new middle won't move much. Bookmatched or not, it doesn't matter.

Bracing is designed to resist the stresses caused by this movement, and it does a good job when the instrument remains in humidity/temperature conditions similar to those when it was glued into a box. When conditions get too extreme (especially too dry), cracks will happen; however they will happen where the grain is the weakest--ANYWHERE on the back, not predominantely in the middle.

Look, I'm not opposed to using a graft on the glue line, I'm just questioning its efficacy.

It's clear that we have opposing conceptions of how the stress is
distributed in a guitar back (or top). My conception is that shrinkage is
about equal throughout a free panel. If you don't allow motion at the
edges of that panel, the stress accumulates toward the center, where it is
greatest. Most cracks that I have seen from drying out of a guitar are at
or near the center line. Perhaps someone with engineering background
can give a less intuitive and more scientific evaluation.

_________________
Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

[QUOTE=Howard Klepper]   Perhaps someone with engineering background can give a less intuitive and more scientific evaluation.[/QUOTE]

I've been hoping for that, too. I'd definitely like to know if I'm just blowin' smoke! C'mon, you guys with wider experience in wood movement! Give us the scoop!