Official Luthiers Forum!

I am a new guitar builder. As you might guess I have many questions about building. I have been a working furniture/cabinet maker for 35 years,so many of my concerns about traditional guitar construction are contradictions with my training and experience. Let me start with the back seam reinforcing strip. Why is the strip crossgrain to the back? Wood expands and contracts primarily across the grain, so cross grain construction is a no no in furniture construction. Why not support the back seam( probably not necessary if the back joint is properly executed) with the same species as the back and with the back grain?? The back seam strip has always looked like a mistake being covered up! I have other concerns I will address later.
Thanks for the input

It is cross grain because that is the direction you need to reinforce, and the cross grain direction means you can make it very light and small and still have tremendous strength. No rule says that you cannot use the same species as the back for this, but I think it is sort of natural to use the same wood for both reinforcing strip and bracing. It is so narrow that it does not represent a problem as far as different wood movement goes. However, the braces, which are glued across the whole span of the plates can represent a problem if the instrument is subjected to extreme humidity conditions and cracking can occur. Building in a humidity controlled environment is a good insurance.

_________________
Rian Gitar og Mandolin

Yes. I agree. I think the main disadvantage with all the grain running the same way is that you may be more prone to crack both the back and the center seam.

The head and tail blocks usually have the same cross grain orientation.

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com

Like stiches on a deep cut, the cross grain help hold the the seam closed. also spruce in commonly use for it's strength to weight ratio. weight saved is resonance gained, kinda simplistic satement but you get the drift

Dave, when you think about it, in furniture & cabinetmaking, how often do you build something with wood that is edge-glued and has a final thickness of often as thin as .075"?
Obviously, this is material that cannot support itself over much of a span. It is inherently weak, and especially so at the center, where in the case of a guitar back, is where it also happens to have a seam. If you reinforce it with wood going the same direction, as others have said, you risk fracturing right down through both layers.
The cross-direction piece is so narrow, that it will have almost no effect on the back as it expands and contracts ever so slightly. It's the best way to reinforce this type of seam in this application.
I regularly use the same wood species for my reinforcement strip.

_________________
"I want to know what kind of pickups Vince Gill uses in his Tele, because if I had those, as good of a player as I am, I'm sure I could make it sound like that.
Only badly."

This is a timely topic for me. I have just started guitar #2 and I'm still purchasing materials for it (as well as #3). Where is a good source for backstrip material? Or do people make their own?

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

I usually get mine from the end (cutoff) of either the top or the back.

_________________
Tickle your guitar daily, and it'll tickle you back.

Wayne,

I have found that most top sets are long enough where the top template can be positioned so that you should be able to get at least one strip out of it wide enough for a back strip. The strips are long enough for a classical with a Spanish heel, but might be a bit short for a dreadnaught with a standard heel block. Still, there should be enough of the top wood remaining after the top has been cut out to add the necessary small section to the strip.

Best,

Michael

_________________
Live to Play, Play to Live

Yeah, just make your own. About 3/4" wide give or take.

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com

I too get mine from top drops

Mine are from top leftovers too.

_________________
Dave Rector
Rector Guitars

Mine are left overs also, but can get them at LMI sort of pre shaped

Thanks for the help! I have not measured the top I bought, but it seems like I should probably have enough left over.

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

I always buy them from LMI - why - cuz im lazy

_________________
Support the OLF! Bookmark our STEWMAC link Today!
Lance@LuthiersForum.com

In furniture construction you would never glue beams across the grain of
a panel. For example, a table top needs to be able to expand and
contract. We violate that rule as a matter of standard procedure in guitar
making. We do it out of structural necessity, and get away with it (but not
always) because the panels are so thin, they can distort without cracking.

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

When all else fails, clean the shop.

[QUOTE=Dave]   Why not support the back seam( probably not necessary if the back joint is properly executed)... The back seam strip has always looked like a mistake being covered up! [/QUOTE]

I agree! I seems the back strip is a leftover from the days of poor jointing and weak glues. As the experts frequently repeat, if the joint is properly executed, the glue line is stronger than the wood it joins. That joint just ain't gonna come apart! Back cracks happen frequently in improperly maintained guitars; so if we follow the logic of the seam reinforcement, we'd have to slap down a cross-grain veneer over the whole back--oh, wait--some people already do that--it's called plywood! Now, some might say, "Yeah, but it's been done for a hundred years, and few guitar backs have split at the seam!" But isn't that like saying, "I don't have elephants in my house because my dog scares them away."? It would seem that the dog, or the reinforcement seem, has nothing to do with it, really.

Please, everyone, show me how I'm wrong (it wouldn't be the first time)!

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

I used to buy them like that too.... but then... I got to thinking that you really can't apply even pressure on these when you glue them after the tops are radiused. (or at least I haven't quite figured out how.)

So now I apply them flat and radius them after they are glued on.

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com

If you have a PERFECT join, it won't matter. It is insurance for those 2 out of 10 that aren't perfect. The added mass on a top might hinder sound, but another 3/4" strip of spruce on the back, I think, isn't going to interfere at all.

Ron

_________________
OLD MAN formerly (and formally) known as:

Ron Wisdom

Somewhere in the middle of Arkansas......

Brock: I think you'll find a lot of people gluing blocks (headblocks, anyway) with grain running parallel to the sides. Tailblocks ditto, unless they're laminated/plywood.

As for reinforcement strips/cleats, I get enough out of top offcuts, plus an large additional 'buffer' of pieces from a very nice, high-quality looking Italian spruce slice that, sadly, had two giant knots running through it. The wood between the knots was fine. Thing came as a protective backing board with some of my other tops :-)

"Please, everyone, show me how I'm wrong (it wouldn't be the first time)!"

A few things: if you inlay a back center strip, which is commonplace, you
have .050" or less of glue line, which isn't much. You also have a stress
riser at the edges of the inlay strip. Next, with bookmatched woods, the
two sides of the joint will each be pulling away from the center when the
wood shrinks. Even if the joint is stronger than the wood, the likeliest
place for a crack (where stress is highest) is right alongside the joint.Howard Klepper38644.6364351852

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

When all else fails, clean the shop.

[QUOTE=Howard Klepper] "Please, everyone, show me how I'm wrong (it wouldn't be the first time)!"

A few things: if you inlay a back center strip, which is commonplace, you
have .050" or less of glue line, which isn't much. You also have a stress
riser at the edges of the inlay strip. Next, with bookmatched woods, the
two sides of the joint will each be pulling away from the center when the
wood shrinks. Even if the joint is stronger than the wood, the likeliest
place for a crack (where stress is highest) is right alongside the joint.[/QUOTE]

But if the glue experts are correct, the back would then behave as if it were one wide piece of wood, not two pieces. As it shrinks, it moves as one piece toward the center and away from the sides, not away from the center. The stress is at the edges, not the center line. Now, if the perimeter joints are strong and the back wood wants to shrink too much, cracks will appear in the back, but they won't be at the center glue line unless it's poorly executed, and they're just as likely to appear five inches from the center line as one-eigth of an inch. The instrument's caretaker (i.e. "owner") has the responsibility to assure that doesn't happen. The builder can not make a good guitar if it's armor plated. I'm not quite sure how a center inlay strip would be a stress riser. Again, if the inlay is properly cut and glued, it will behave as if it's one piece with the back wood.

A decorative center strip may actually add to the strength of the back by increasing the glue surface but I still use a reinforcment strip.

By the way I make mine by cutting them from either the top or the back wood and I have a flat board with a 45 degree saw kerf in the length of it. I lay the pieces in the saw kerf in a row and run the whole thing thru my thickness sander just takin off the edge then turn them over and run it thru again. I end up with a nice 45 degree angle sanded into both sides and a uniform width. I glue them on in my concaved dish in the gobar deck using a thin wooden cawl to even the pressure.

_________________
Tickle your guitar daily, and it'll tickle you back.

[QUOTE=CarltonM]

But if the glue experts are correct, the back would then behave as if it were one wide piece of wood, not two pieces. As it shrinks, it moves as one piece toward the center and away from the sides, not away from the center. [/QUOTE]

If both halves of the back were from a piece of wood with perfectly straight grain, and no runout, that was cut perfectly, with uniform rates of expansion/contraction at every point across the back, then yes it might behave as one piece. But most backs aren't like this, especially the zooty ones.

Any internal stresses in one side of the plate is the oposite on the other side of the joint. It'd depend on the individual piece of wood used as to where on the center seam the stress would be, but it's what you get with book matching. These stresses are opposing on each side of a thin glue line so that they have a cumulative effect at the joint, where the stress will be double what it is on either side of the joint.

With some backs that are really uniform, you might get away without the strip. But the guitar buying public are used to seeing one there.

Yes, cross grain gluing is a no,no in woodworking, but as others have said we don't have much choice, we need to re-enforce the thin plates. Using well dried quarter
sawn wood and keeping an eye on humidity as you build will help a lot with stability problems.

   Hi Dave,
            All wood will expand and shrink with humidity changes,as you well know.Nothing can stop it!
   The arch or dish imparted on the top and back are there for that very reason It also gives the plates a lot of strength as in the egg shell affect.
In high humidity , the arch or crown increases. It decreases in low humidity.
   So, you can expect an instruments pitch to go sharp on wet days and flat on low humidity.
   If the plates had no crown (as in furniture) you could certainly expect a crack to develop
The crowns imparted give the wood "room to move"
    The reinforcing strip is only around an inch in width and insignificant in humidity changes, although even it , would have a slight profile change from time to time,albeit minute.
I wish you well on your journey into guitar making.It becomes an addiction, a love affair, sometimes a love/hate relationship, other times it's the most forfilling and exciting venture you may ever have experienced. Above all though,, It's an obsession!!

Kind regards KiwiCraig

_________________
CRAIG LAWRENCE of AUSTRALIA
_____________________________________________