I built my 1st mando & banjo kits in 1982 sparking a fire for luthier work that has continued to grow these 20+ yrs now. Not being schooled in fine wood working or luthier, I've had to walk through many a nightmare, I created for myself, like sanding through to the stain during finish sanding. You know, patchin', fillin', filein', scrapin'. My 1st attempt at a handmade mandolin was a flat top & back, round body instrument that folded at the neck & body upon trying to tune up for the 1st time. The neck block & neck just folded like the leftovers of many a domestic dispute. I then realized the importance of the neck block size. I also had the top of an A5 model explode & shove the bridge to the back while being played a little too close to a campfire on a cold Alaskan night.
So if you're a 1st time builder don't be discouraged. Use firewood for your 1st few instruments so it can always be used for its original purpose if you make a mistake. It's not rocket science. It's simple common sense physics. You're creating a wooden amplifier complete with speakers, (top & back) sound chamber (body). When the strings are excited (played) the bridge & neck transmit that energy into the amplifier & depending on its carved dimensions, tone bar placement, etc. It will move air & create a sound wave. As luthiers we try to shape that sound wave to a desired characteristic, usually good volume & a full warm quality tone with a degree of cut as well.
Here's what little I've found out through experience & some common sense deduction. Carved instrument tops are under 2 major wood warping forces. First the tailpiece/end block trying to shove his way to the neck block from the pull of the strings. In doing so, you'll see the instruments top either sag or bulge between neck block & bridge and/or end block & bridge; totally depending on the type of arch the top originally had carved into it.
So by that example you can see that depending on the top thickness dimensions before carving there is a perfect arch line (between end block and neck block). This will push toward the bridge from each end & won't bulge or sag from the string tension.
I'm sure a mathematical mind could tell us what the strongest arch would be in say 5/8" or 3/4" high top (thickness), that spans 12". Those are the basic dimensions of a mandolin top from outside of end block to the 15th fret cross piece & the rise or top thickness. I use my "eyecrometer" , & depending on top thickness before carving. I can see the arch line that will provide that (shove into itself) strength necessary for longevity. The arch line can change depending on where the bridge sits along that line. It seems my bridges all sit a little toward neck block from the center. Slightly changing the arch lines in front of and behind bridge from being the same as each other.
I owe this bit of arch info to the only man I've ever talked to about luthier work. He is a master builder from Australia. I was lucky enough to spend 15 minutes with him at the 1999 IBMA Festival. So I asked him the one question I had been struggling with - How to make an arch top thin but strong as well? He simply said " the strength is in the arch". Wow! I saw a flash of lightening, heard peals of thunder. When my head was clear & could see again, the dude was gone. I never even got a chance to thank him for that revelation. Simple, but powerful words for the arched instrument builder.
Secondly, the force of the bridge flattening out the top right under the bridge from the down pressure of the strings. Old oval hole mandolins prove that very thick tops (some of them have 7/16" thick tops), still fatigue under the bridge with time & pressure. Even with a large brace horizontal to strings near bridge. F hole tops with 2 tone bars parallel to the strings, seem to flatten out a little less even though the tops are a bit thinner. By flattening out the top under the bridge, this can change the perfect arch lines creating a weak spot in the arch. Hence, more potential bulging problems in the long run.
Knowing that wood is always subject to change & with enough pressure, time, heat, & moisture, can be bent, twisted or otherwise manipulated, no matter how thick or dense it is. Since wood is prone to fatigue. I truly believe in the graphite/spruce laminate tone bar, bracing or reinforcement of some type under the bridge of carved string instruments. The stuff is absolute & complete in providing the perfect ratio between stiffness & weight. Allowing string movement to come into play but not the long-term movement caused by overall string tension.
I ordered my 1st batch of carbon graphite from Luthiers Mercantile International in 1991. With hopes of bracing a flat top guitar with it. I got side tracked & used some in an A5 type mandolin using X-bracing (popular at the time) & a thin top. It has a real loud bark but not so thick treble notes. The next two F5 models were set up with the same graphite spruce X-bracing. They basically had the same overall qualities as the A model with X-bracing. All had good full bark & thin treble notes.
I abandoned X-bracing & moved onto parallel tone bars & thru a series of size, position, & material changes I learned how to create full, fat, treble notes while maintaining a full warm overall sound.
An experiment I did with rosewood tone bars & a thin top in an A5 model I built, had real tight cutting sound but lacked the warmth in tonal quality. Even with the koa sides, back & neck it had. Now that I've used some redwood for top material, the combo of rose wood tone bars, redwood top, koa body & neck could produce a warm tone with good cut ability as well.
Thin tops produce more volume than thick tops. Thin tops produce thin treble notes but increases woody or bass note response. Changes in bracing/tone bars position, size, &/or wood density do change & shape the note or "sound wave" that leave the instrument. Soft redwood creates "warm tone" instruments, compared to hard spruce "brighter tone".
By using graphite in the tone bar some might fear that the instruments sound/tone might be "plastic like. I've heard a few mandolins like that with out graphite. That sound/tone is due to the wood being saturated with finish during the 1st few coats. The dry wood will absorb almost as much finish as you want to apply for the 1st coat or two, creating a "plastic" tone. Since I don't use a sealer, my 1st few coats of finish are lightly misted on the top of the wood with out much penetration. Since using this application technique the tonal quality of my instruments are quite a bit more "woody". This seems to apply to the top & back only. The sides can be loaded up early with no adverse effect.
As far as tuning the top or back to a specific note. I'm sure there is a lot to be said about that but it won't be from me (I'm clueless). I tune my instruments to a degree of movement. Once I stiffen the area around the bridge, trying to predict a certain amount of cut ability. I go to the outside "the channel" in violin terminology. Then I start scrapin', sandin', carvin' until I create a "speaker diaphragm" effect and visually see some movement. Trying to bring out the full warm tones.
Dense wood will call for more carving while light wood will need less carving to bring it into play. This applies to carved arched instruments mostly. I also use its basic theory in bracing a flat top.
The back & its effect on tone: A thick back that doesn't move will produce an instrument that will be thin in bark & bass note response, even with a properly designed top. The back being powered only by air from the top & the vibration from the side, needs to be thin enough to be effected by these 2 forces. A real good mando/arch guitar will get quiet when the back is pressed to the player's body & louder/fuller when pulled away & allowed to move freely.
I'm sure this information (or a variation of it) is known by most master builders & is in a book or two somewhere. Since I haven't been to luthier school most of this what I discovered by sheer experience and trial & error. So be encouraged to experiment and enjoy the art of the luthier. In Alaska it's said : There's no disgrace or loss of face for those that try & lose, but say a prayer for the unaware who could conquer but refuse." Stay tuned for my discoveries on neck angle, bridge height and how they effect the tone/playability & why I really like my 3-footed bridges.
Tips #2 Graphite
Wooden stringed instruments are built light enough to allow string vibration to move the wood and create a sound wave/tone and braced or carved in a fashion to prevent overall string tension from quickly deforming and destroying the sound plate. Examples are seen in flat top guitars, when the bridge twists and no longer sits flat/horizontal to the top and strings, but shoves the front of the bridge (toward sound hole) down and pulls the back (toward the end block) skyward. Taking top and X braces underneath for its ride to near vertical final tweak with no glue joint failure. Carved mandolin tops (under extreme pressure) show signs of stress around the F holes as the top sags under the bridge pressure or as the top bulges between end block and bridge and/or neck block and bridge.
Wood is prone to warpage when enough time and pressure are applied. No matter how dense it may be. Even ebony mando bridges concave under string pressure with a little time. There is no one thing that can make a fine wooden string instrument sound good but, there is one thing that can make them last a long time. Carbon graphite 4 times stiffer than steel by weight. It will not rust in neck reinforcement application. It will stabilize sound plates in flat top instruments. For a carved arched top instrument a combination of proper arching added to the graphite spruce laminate tone bar, spring loaded to push up under the bridge and pull down on the ends against the bulge. It will boost string response levels and in the long run continually provide the strength that will never fatigue with time and pressure. It will combat overall long term effects string tension has on wooden stringed instruments.
There's a reason they pre stress the rebar when building an aircraft carrier. It's so upon pouring the cement for the floor it will settle flat under the weight. So in designing anything that spans a distance under pressure and weight , be it bridge, aircraft carrier or mandolin. Pre stressing against weight and pressure is a logical, beneficial and proven construction technique.
Instrument builders who have experimented with minimum tolerances for a given instrument design usually find a point where the instruments sound really good brand new, but self destruct within a few years or so because of the ultra light construction.
There is a certain amount of stiffness an instrument needs to last for years and produce a stable, full/fat single note (tone/sound). Real light tops produce woof/open bass response and thin treble notes.
Real heavy tops produce thin light bass response and cutting treble notes.
I am generalizing here because wood density levels, bracing patterns, the back and neck all contribute to towards tone production.
The use of graphite in the tone bars allows the builder to confidently remove wood to bring out the sound and still keep the stiffness needed for tone and longevity.
Some call it a joke some call it genius, but its just another material at the luthiers command to help take his instruments further into the future. To bless the following generations with that magical thing called music.