Mark (TheCraftsmansPath.com) on April 6th, 2008

Just when you thought that the bracing tasks were complete there was more bracing to do. Whereas the back braces are a bit more structural in nature, the top actually has a more complex bracing pattern that keeps it strong but also allows it to vibrate giving the guitar its unique tone. Bracing the top was my most recent task in the workshop.

Laying Out the Brace Pattern

There are quite a number of braces that must be attached to the underside of the top. As I mentioned, because these braces are both structural and also directly affect the sound of the guitar, the braces must be carefully laid out and applied.

Front_Brace_Layout.JPGTo do the layout, I overlaid the guitar top onto the full-size pattern. I was careful to align the center line of the top and the sound-hole with those on the pattern because all of the brace positions utilize these elements as a reference. There are several sets of braces – the main structural braces are the X-braces and they form and X across the top. In the lower bout of the guitar, stemming from the X-braces are the tone bars. Around the sound-hole are some reinforcement strips and above the sound-hole in the upper bout are the main transverse brace and another reinforcing brace. Last, there is a flat bridge plate just under where the X-braces cross.

To layout all of the braces onto the back of the guitar top, I started by transferring the locations of the ends of the X-braces to the top. With a long straightedge I connected these points giving me the X-Brace locations. Next, I marked for the ends of the tone braces and the upper transverse braces. By extending the lines of the shorter tone bars across the pattern and then replacing the top onto the pattern I was able to extend the end marks for the tone bars to the points where they intersected the X-Braces. The upper transverse braces were straightforward to mark across the top and the sound hole reinforcing strips and bridge plate were done in a similar manner to the tone bars. The picture (click for larger view) shows the completed layout on the back of the guitar top.

Notching the Braces

First_X_Brace.JPGX_Brace_Notches.JPGCompleted_X_Brace.JPGBefore the braces could be applied to the top, some work was required on the braces themselves. A half-lap had to be cut into the X-braces and notches were required at the intersection points of the tone bars, sound-hole reinforcing strips and bridge plate. I started with the X-braces. With one brace clamped to the top, I marked the intersecting points of the other X-brace. Then, by overlaying the other X-brace I transfered the marked location to denote where the joint needed to be cut. From there it was a matter of some quick cross-cuts to half the depth of the braces and cleaning out the material with a sharp chisel.

Marking_Brace_for_Chamfers_1.JPGMarking_Brace_for_Chamfers_2.JPGBrace_Chamfer_Closeup.JPGIn addition to the half-lap on the X-braces, the intersection points of the tone bars, sound-hole reinforcement strips and bridge plate needed to be notched into the X-braces. With the X-braces clamped to the top, I used the pattern to mark out the locations of all of the intersection points onto the X-braces. I then scored the lines with a knife and filed small chamfers at the marked locations. With a small chamfer applied to the ends of the tone bars, reinforcement strips and bridge plate I was ready to apply the braces to the top.

Applying the Braces

Completed_Fitting_and_Chamfers.JPGClamping_X_Braces.JPGIn the first picture you can see the top with all of the braces laid out, fitted, notched and ready to be glued to the top. I applied the braces to the top in a couple of stages. First, the X-braces were applied. In the second picture you can see that I again used the Go-bar Deck to clamp the braces. Because of the breakage of a couple of the wooden dowel rods I used the last time, this time I decided to use fiberglass rods as the Go-bars. These were originally sold as stakes to mark out driveways during the winter here so that the snow plow driver can see where the boundaries of the driveway are. The stakes came in 4 four foot lengths so, I just cut them to size and applied protective screw covers to the ends. This was a much cheaper alternative than buying the fiberglass Go-bars sold by the guitar specialty shops and it worked very well. Again, while clamping the X-braces you can see that I used small strips of stock under the corners of the top to maintain the radius of the top while gluing the X-braces to the top.

Clamping_Tone_Bars.JPGAfter the glue on the X-braces was dry, I moved on to applying the rest of the bracing to the top. I did this in several stages just to keep things manageable. First came the tone bars, next the sound-hole reinforcement strips and the upper transverse braces and last the bridge plate. In the picture you can see the clamping of the tone bars and how many Go-bars were used. Notice that I used some small blocks of scrap to hold down pairs of tone bars. When I did this I just left the X-braces clamped to keep the top in the same position and then carefully clamped the tone bars.

Trimming the Braces

Trimming_Braces.JPGCompleted_Front_Braces.JPGAs I mentioned, these braces are attached to the top of the guitar for structural reasons. However, the idea is to reinforce the top and make it stiff without adding weight or hampering its ability to vibrate. As you can see in the pictures, the most of the braces were already scalloped along their length. This scalloping minimizes their weight while retaining their strength. To further allow the top to vibrate, the ends of the braces were trimmed where they will eventually intersect the kerfed lining when the top is attached to the guitar sides. The ends of the X-braces and main transverse brace were trimmed to a thickness of .100″. The ends of the tone bars were trimmed to nothing where they meet the kerfed linings. In the picture you can see the top with all of the braces attached and their ends trimmed. Next up will be attaching the top to the sides.

21 Responses to “Dreadnought Guitar: Bracing the top”

  1. Mark,

    Another great installment! Not only am I learning the clamping technique using go-bars, but also learning the the WHYs of the bracing.

    1) Can you please elaborate on the reason for having a radius on the top of the guitar?

    2) Do you incur a sound quality “penalty” if the guitar top does not have a radius?

    I appreciate all the detail and the care it takes make this instrument!

  2. Hey Al,

    The reason for the use of arching or a radius in the top and back of the guitar is the same as it is in architecture and engineering, to strengthen spans. An arched-braced structure is stronger than a similar flat braced structure. So, the arches help to stiffen the guitar top (and/or back). An increase in stiffness will improve the high-frequency response of the top, making the guitar brighter. It will also allow the top (and/or braces) to be thinned further, reducing its mass and increasing the volume (loudness) of the guitar. The increased stiffness will also help to reduce the effects of humidity on the top (humidity will tend to bow the top out during periods of high humidity and dish the top in when the humidity is low ).

    As far as flat-top guitars, there are some that are made that way. I’m not sure what kind of “sound quality penalty” there may be. However, from the reasoning above (all other things being equal), it is probably reasonable to believe that they are less stiff and have thicker tops and as a result are not as crisp sounding or as loud as an arched-top guitar. Of course, other things may be designed into a flat-top guitar to achieve similar sound attributes to an arched-top.

    There is a real art to shaping and trimming the braces on a guitar to alter its eventual sound qualities. The process is something that looks to take many years of practice and experimentation to acquire – certainly not something I have on my first guitar!

    –Mark

  3. Hi Mark,

    A quick question… Generally speaking, is the bracing on the inside of an acoustic guitar the same on left-handed models – or is the bracing reversed?

    Many thanks,
    Kevin

  4. Kevin,

    Thanks for visiting the blog.

    That’s an interesting question that I had not really considered before (I’ve never built a left-handed guitar).

    Thinking about it, because there is a bit of asymmetry in the bracing design of this guitar, I would believe that the correct thing to do is to reverse the bracing. Most likely it would affect the bass and treble response of the top based on the locations of the strings on the bridge vs. the bracing pattern if it were built as is for a left handed version with the current bracing.

    –Mark

  5. Typically on a left handed guitar, the bracing is a mirror image of the pattern you are using. It is not necessarily a requirement, however, as there are plenty of right handed guitars that are modified for lefties after leaving the factory, and enjoy the same sturdiness and sound it did as a righty intrument.

    Some of the pros have theoried that it is more important that the guitar bracing in the lower bout be assymetrical than oriented for a right or left hander.

    Great job on the guitar!

  6. Ken,

    Thanks for you comments an clarification on the left-handed bracing. Given its asymmetrical nature, it makes sense that it would be reversed on a lefty.

    –Mark

  7. Hey Mark, Jimmy here . I have been looking all over the internet for some information on bracing materials . My question is …Why not use oak ,cedar , or maple to brace top and back ? Why mostly spruce ? I live in the country with a lot of trees of different variety that need to be trimmed periodically , and I just hate to waste anything .

    Guitar builder wannabe
    Jimmy

  8. Jimmy,

    Thanks for visiting the blog.

    While I am no expert on the subject, I can give you some thougths with regard to bracing materials.

    The critical things in bracing the guitar are the stiffness-to-mass ratio and the elasticity of the wood. This is a fancy way of saying how strong is the material without being too heavy and how well will allow the strings vibrate the top and back to radiate sound from the guitar. This will affect both how well the guitar is braced for strength as well as how it will sound.

    Having said that, Spruce is common on spruce topped guitars. It is relatively light while being strong. Cedar is also often used as a bracing material – especially on classical guitars that may or may not have a Cedar top. As far as Oak and Maple go, they would certainly be strong enough however, they would also be heavy. I have only seen Maple bracing on a Maple topped guitar. That’s not to say you couldn’t try it though. Many of the techniques that we study today derived from experimentation of luthiers long ago. They certainly did not know what would happen when they tried various bracing materials and patterns. What we see today is the results of them trying many things. If you’re willing to experiment a little, your efforts to try other things might yield something great!

    –Mark

  9. Hi Mark

    What is the best wood to use for the bridge support as this is a key component in preventing the top from lifting?

    All bridge supports I have seen have the grain running purpendicular to the strings. What would be the effect of having the grain running parallel with the stings for strength?

    Gavin

  10. Gavin,

    Typically the bridge plate is made from a hard wood (usually Maple or Rosewood) and is about .100″ +/- thick.

    The plate is glued on with its grain running across the guitar or perpendicular to the strings. A plate that thin cut with grain running the other way would have the tendency to split along the grain (possibly even before it’s installed). The bridge plat often also has its corners tucked under the X-braces for strength.

    –Mark

  11. Hi Mark

    I dont know what wood is used on a Monterey for the support, probably pine! LOL!. As I wont be able to remove the original damaged bridge support, would there be any issues with gluing a rosewood support over the original support?

    What is your opinion of fitting a JLD Bridge Doctor?

    Gavin

  12. Gavin,

    I’m not familiar with a Monterey or with repairing something like this first-hand.

    I know that an old bridge plate can be removed but it takes some heat and humidity (depending on the original glue used) and you have to do it blind by prying it off with from through the sound hole (if that’s even possible on a Monterey).

    Using another piece over the existing one presents at least two possible issues. One is that the existing bridge pins will probably be too short to work. Second, the added mass and stiffness of the additional piece would certainly change the tone of the guitar.

    One method I’ve seen done is to remove the bridge and then drill out and plug the holes from above with new wood. Then the bridge is replaced and the new holes are drilled and reamed from above through the bridge. This takes a lot of care and accuracy to remove the bridge and re-install it at precisely the correct location for proper intonation and string action.

    If you are not real familiar with this operation then it is probably a job for a competent guitar repair technician.

    –Mark

  13. HI MARK I AM A LEFTY DOES BRACING HAVE TO BE DIFFERENT OR IS IT THE SAME BOTH SIDES

  14. Jeff,

    Thanks for your question. Take a look at the previous comments and responses. Look at http://thecraftsmanspath.com/2.....g-the-top/ for a prevuious response on bracing for a lefty.

    –Mark

  15. Jeff,

    Sorry for the previous link – just look at my response to Kevin above for comments on a lefty setup.

    –Mark

  16. Hi,

    Did you end up radiusing one or both ends of the large x-brace? The handbook I’m following suggested radiusing the two lower bout fan braces and as well as both ends of the x-brace peices. If so what did you radius them to and what method did you use to get a good consistent arc? Thanks!

    Tim

  17. Tim,

    I’m not exactly sure of what you mean when you say radiusing the x-braces.

    Since this was a kit, my brace material already had a radius or crown along to top edges. SO, I did nothing there except to sand them until smooth enough for my liking. If I was starting with square stock and wanting to do this I would probably just use a block plane to do multiple small chamfers until I approached a radius and then sand it until smooth.

    If you are referring to tapering the ends of the x-braces down to the sound board plate, then yes I did do that. This was done with a bench chisel with the bevel down so that I could work this into a smooth curve. Just take a little at a time and when very close to the desired taper then just dress with a little sandpaper.

    –Mark

  18. Dear Mark,
    Being an electronic engineer, I can not imagine how a definite shape/ size and pattern for guitar braces are not yet defined. Nor a tangible/measurable definition for sound parameters ( brightness, warmth, etc. etc.) seems to exist.Using an audio spectrum analyzer all such parametes should be defined. What seems worse is that having to actually build a guitar before you can decide on the outcome of sound quality seems tremendously annoying to me.( Obviously uou can not test the soundboard before assembling the whole guitar )!!!!
    Am I right in my above conclusions, or am I missing a lot ???
    Appreciate your comment.
    Farhad, June 2011

  19. Farhad,

    Thanks for your comment.

    I think that the tools are definitely available to analyze how a guitar sounds after it’s built. However, there are many variables: what kind of wood is it made of?; how well is it constructed?; what is the shape of the braces?;etc… well, you get the idea. So, even thuogh there are rules of thumb on brace shapes and placement, so many variables affect the actual sound of a finished guitar it really is a process of refinement of technique to get something that sounds good and is reproducible.

    With a trained ear, you can actually test the sound board before the guitar is completely assembled. It’s a process called tap-testing. Again if you have trained your ear, you can tap on the soundboard and if held correctly you wil hear a distinct ring from it. Varying the braces, etc. will change that ring and ultimately how the guitar will sound.

    –Mark

  20. Farhad,
    Unlike a 14 gauge piece of copper wire, every piece of wood is different,even the the same species and even from the same tree according to the the grain and density. I’m in my first build and had to order an extra piece of spruce from a different supplier than where my kit came from and am amazed at the difference in stiffness. Even in the same size piece of wood that feels to be even a similar weight. Moisture content of the wood and seasoning also adds another variable.
    Don

  21. Mark… I understand that an acoustics back braces are essentially structutal in nature. However, the back does in fact move in relation to what the top is doing during play… So… My question is, even if it is only in a minor way, why are all back braces (all four of them) basically the same height and thickness when none of them span an equal distance across the back of the instrument? Does this not in fact create quite a difference in stiffness (or flexibility, whichever you prefer) that, even though having very liffle effect on tone, if any, would seem to me to have an noticeable effect on volume and projection? Wouldnt stiffer sides and a more evenly looser back be a more desirable attribute (providing the instrument isnt held too tightly against the body) while being played as most people tend to do?

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