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Thread: Dreadnought build

  1. #1

    Default Dreadnought build

    I recently purchased a set of dreadnought plans from GenOne. It's a pdf download that prints out 5 big pages with every detail I could think of. The only thing that kinda concerns me is the plan calls for a totally flat top with no radius (dome), saying that the string tension will raise the bridge approximately 1/8". Is this common for a dreadnought? And if I put a radius in it wouldn't it change the neck angle slightly? Any help greatly appreciated.
    Danny Gray

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    Default Re: Dreadnought build

    Yes, it will change the necessary neck angle. What kind of neck joint do the plans call for? If it's a mortise and tenon, that's easy to adjust during assembly. I have gone to mortise and tenon necks with 2 bolts through the neck block to make it easy to put the neck on and take it back off during that phase.
    I think you should dome the top. That will change the neck angle. So will the height of the bridge. You can draw it from the side and that should get you close. I don't do final neck angle adjustments until the guitar has been strung up for a week or two.

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    Registered User sunburst's Avatar
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    Default Re: Dreadnought build

    Quote Originally Posted by Dan of SC View Post
    ...Is this common for a dreadnought?
    I don't know how common it is for dreads, but I was surprised to learn many years ago that it is done at all. It is a technique that seems to have it proponents.
    It is true that the top will pull up under string tension, it will do that to some extent regardless of whether you build some induced arch into it or not.
    It seems to me that a true flat top is more susceptible to crack caused by wood movement from dryness, but I don't have any statistics to back up that suspicion. If you add some induced arch it will change things so you are no longer following the plans, but frankly, that is the sort of thing I did quite a few times in my early days of building (back when I more-or-less followed someone else's plans), so it is up to you; you can follow the plans or diverge from them and figure out how to deal with the resulting associated differences in geometry.
    Either way I can guarantee that the resulting instrument will sound exactly like a guitar.

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  5. #4

    Default Re: Dreadnought build

    Dreadnought mandolin, aka "mandolinetto?" If it's a guitar, just go flat top.

    A lot depends on the bridge. Floating bridges are typical for archtops (mando or guitar.)

    Flattops generally have glued-on bridges where the strings end.

    The way the tone is produced is very different, between flattop and archtop. With an archtop, the vibrating strings produce a mostly up-and-down variation in force on the top, causing the top to vibrate mostly up and down (where the top is up and the back is down.)

    On a flattop with glued bridge, the string tension pulls on the bridge and causes the tonewood between bridge and tail to bulge a bit. According to most theorists, string vibration changes the tension on the bridge, causing a complicated vibration pattern in the tonewood mostly behind the bridge (including the wood above and below it, in playing position.)

    The design of the instrument overall should be tailored to the tone production method, so I would hesitate to change it from the original design. If the design has a glued bridge, don't dome it. If it has a floating bridge with tailpiece AND a flat top, well ... I would question the wisdom of that design. (Though I can find some pictures of vintage instruments like that.) My guess is such a design would be geared to lowest cost. I would question whether there would be enough string tension for really good tone.

    Disclaimer: I'm not experienced with mandos, only guitars, and I'm not a luthier. (I'm in the process of finishing my first mando kit, and have completed a StewMac baritone kit. Not impressive credentials!) So, don't take my word for it, just think it through for yourself.

    (And yes, doming it will definitely change the neck angle!)

    If it's flat-top with floating bridge, then it's probably intended to be an easy build, avoiding the work of carving the top and back, but compromising the tone and loudness. Either that or to replicate some vintage instrument. If that's what you're looking for, go for it, but follow the plans. But if you want to carve a top, I suggest you find plans for an archtop. I would advise against using braces to induce curvature in a flattop design. I've never seen a top bowed by braces, in 55 years of playing guitars. Bowing the top would tend to reduce its flexibility and would diminish the tone.

    BTW, the back on a flattop is generally bowed a bit by the braces. It's not intended to vibrate freely, like the top.
    Last edited by JeffLearman; Dec-26-2021 at 12:02pm.

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    Registered User sunburst's Avatar
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    Default Re: Dreadnought build

    Quote Originally Posted by JeffLearman View Post
    The way the tone is produced is very different, between flattop and archtop. With an archtop, the vibrating strings produce a mostly up-and-down variation in force on the top, causing the top to vibrate mostly up and down (where the top is up and the back is down.)
    Now that we have holographic studies of plate motion in acoustic stringed instruments we can see that, in fact, archtop and flat top instruments behave very similarly in the way they transform string energy to air movement (audible sound). Each responds to input from the strings and bridge by moving according to the plate's normal modes of motion, and those turn out to be very similar in everything from classical guitars to jazz archtops to mandolins to etc.. Furthermore, the laws of physics tell us that the normal modes of motion are the only way a plate can move, and any input of energy gives the same result; blast it with sound from a speaker, tap on the instrument, play another instrument next to it, pluck the strings of the instrument, whatever; it will move in it's normal modes of motion.

    Most dreadnoughts, including Martin guitars of which there are quite a few, have some induced arch in the tops and backs. I suspect it is relatively rare to find dreads with truly flat tops as I said earlier.
    Things can get further complicated by the decision to apply the top/back radii to the rim of the guitar or not. Once again, do it as you decide and it will sound like a guitar.

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    Registered User j. condino's Avatar
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    Default Re: Dreadnought build

    A completely radiused ("domed") top will make it difficult to get a clean fit at the neck joint and the fingerboard extension over the body.

    I build them with a radius below the soundhole fading to flat above the upper transverse brace as a compromise of the two.
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    Default Re: Dreadnought build

    I should have mentioned- I do just what James does. For the same reasons. And now I'm too old to think of any other way to approach the issue. LOL.

  10. #8

    Default Re: Dreadnought build

    The cracking of flat vs. domed is plausible, but as John says, might need evidence. If the rim was very rigid, both temperature and humidity induced tension could overcome the smallish strength cross-grain. A dome might just lose height and not crack, which might have been early motivation in violins. All of this might be quantifiable for typical materials and dimensions, but there’s still lots of variability, including bracing and glue to consider. So, sticking to known designs seems justifiable.
    I don’t think it’s fruitful to make too simple a model of bridge motion in order to optimize something. Too complex, too variable, and as said, involves the entire plate dynamics. Fortunately, on a mandolin, with high string loading and low height, bridges are constrained to be fairly similar as transducers.

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    Default Re: Dreadnought build

    I make my guitars with a domed top and back. The top has a 15 inch dome and the back has a 25 inch dome. This is how most guitar makers make guitars, although I know one who does not dome, nearly all the guitar makers I know do dome. Doming does make it more difficult to match the top and back to the sides and as James says, can be tricky with the fingerboard extension. That is with steel string guitars. On classical guitars I dome the lower bout, but the upper bout is flat. Many classical guitar builders don't dome. Classical guitars are different from steel string guitars because the neck angle is different.

    As for how a guitar and mandolin, flattop or arch top vibrates, what John said.
    Peter Coombe - mandolins, mandolas and guitars
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    Registered User bpatrick's Avatar
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    Default Re: Dreadnought build

    I have used the GenOne plan. I purchased it several years ago when the company was named Georgia Luthier Supply. I looked over the plans and didn't see a note about leaving the top flat and 1/8" flex. Perhaps, it has been revised. Page 2 of my plans state, "Guitar Top Plate. Can Be Built "Level" or "Flat", Which Is Also Much Easier To Build. It Is Also A Popular Option To Build An Arch Into The Top By Shaping The Braces With An Arch Shape. This Is A Decision Left Up To Each Individual Luthier."

    I build dreads using a 15 foot dish for the back and a 40 foot dish for the top. I also use the dishes to shape the braces and for gluing them to the top and back in a go bar deck. I flatten the fretboard area using a flat piece of wood with sandpaper double sided to it. The flat wood is elevated at the bridge location and sanded across the fretboard extension area to provide a properly angled area.

    There is a recent series describing Charles Fox's guitar making methods in American Lutherie - issue numbers 139-143. Mr. Fox is a master of jigs for almost every step of guitar making. I don't use nearly as many jigs as described, but nonetheless, it's a good series for reference. The fretboard extension area sanding technique is described in this series.

    A contrasting yet equally valid guitar making approach is presented by John Greven in "How I Make Forty-Eight Guitars a Year". This is available in the GAL 2021 anthology - "Flattop Guitars 2". Mr. Greven is somewhat opposite from Mr. Fox, in that he uses very few jigs and machines. (There are also many other useful articles in the 2021 antology)
    Bryan Patrick

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    Default Re: Dreadnought build

    Bryan beat me to it -- the recent series in the GAL journal on Charles Fox's methods is worth its weight in gold. He has an interesting approach to compensating the domed top to accept the fingerboard. Also a cool idea for lining up and setting the neck. (Among a lot of other good ideas. Mr. Fox helped make me a big proponent of vacuum clamping fixtures some years back.)

    I dome my "flattops" mainly to give them some enhanced resistance to humidity changes. Also sometimes play with the back radius and bracing depending on the musician. For example, I'm currently finishing up a dread for a fingerstyle player (unusual dreadnaught application) that has a very thin and lightly braced back, with the intention of greater warmth and presence. There are many variables to fiddle with.

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    Default Re: Dreadnought build

    Quote Originally Posted by sunburst View Post
    Now that we have holographic studies of plate motion in acoustic stringed instruments we can see that, in fact, archtop and flat top instruments behave very similarly in the way they transform string energy to air movement (audible sound). Each responds to input from the strings and bridge by moving according to the plate's normal modes of motion, and those turn out to be very similar in everything from classical guitars to jazz archtops to mandolins to etc.. Furthermore, the laws of physics tell us that the normal modes of motion are the only way a plate can move, and any input of energy gives the same result; blast it with sound from a speaker, tap on the instrument, play another instrument next to it, pluck the strings of the instrument, whatever; it will move in it's normal modes of motion.
    Thanks for the edification!

    Most dreadnoughts, including Martin guitars of which there are quite a few, have some induced arch in the tops and backs. I suspect it is relatively rare to find dreads with truly flat tops as I said earlier.
    I stand corrected here too. I have learned that flattops often have a radius of from 22 to 65 feet, which wouldn't be visible to the eye. Backs are commonly domed much more, obvious at a glance. Judging from appearance only, my Martin HD28 back is curved in both directions. From neck to butt, the curvature appears induced by the cut of the sides. From top to bottom, the curvature appears to be induced by internal braces.

  17. #13

    Default Re: Dreadnought build

    Quote Originally Posted by peter.coombe View Post
    I make my guitars with a domed top and back. The top has a 15 inch dome and the back has a 25 inch dome.
    Really, 15 inch radius of curvature? This is like a Martin fretboard (16" radius.) That would be really obvious, and I don't think it's how most dreadnoughts are made. Also, I've never seen a dreadnought where the top was more curved than the back.

    For folks who design guitars: would you design it the same way regardless of whether you're using a floating bridge with tailpiece vs glued bridge where the strings terminate? My guess would be that they're rather different beasts, and the requirements would ripple out.
    Last edited by JeffLearman; Dec-27-2021 at 12:22pm.

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    Registered User sunburst's Avatar
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    Default Re: Dreadnought build

    Surely Peter meant 15', not 15".

    Yes, there are different considerations for floating bridge vs fixed bridge, but in general, "flat top" guitars (including those with typical induced arches) are designed for and have fixed bridges while. "Flat top" mandolins often do have floating bridges, and bracing is generally done with that in mind. While dynamic forces are similar with either type of bridge, static forces are different and must be considered.

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    Default Re: Dreadnought build

    Really, 15 inch radius of curvature? This is like a Martin fretboard (16" radius.) That would be really obvious, and I don't think it's how most dreadnoughts are made. Also, I've never seen a dreadnought where the top was more curved than the back.
    Oops, really screwed that one up. We usually work in metric, that is my excuse, ha ha. Should have been 25 ft top radius and 15 ft back radius.
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