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Radiused tops?
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Author:  mgcain [ Mon Dec 17, 2007 1:15 am ]
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Ok, so I'm confused...My first build was a kit and i didn't have to know about or deal with radiusing the back or the top - so I'm giving myself a pass!


i see that folks are radiusing the sides for the tops - using dishes with sandpaper to sand a radius into the top of the sides...the top on the guitar I made was dead flat, and where it attached to the sides was also dead flat...where does the top radius come into play? 


I sure hope John Mayes DVD's get here soon and hopefully clear up some of my confusion...


 
Author:  Hesh [ Mon Dec 17, 2007 1:36 am ]
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Mitch buddy we radius our tops and backs so that when the humidity/temps change the guitar can expand and contract without it resulting in a cracked top or back.  Another way to look at this is that the "domes" that we create on the tops and back allow room for expansion and contraction of the guitar without it resulting in a crack.

Many builders, myself included, also believe that a slight dome on the top enhances the mid range.

A dome is inherently stronger then a flat surface - ever look at the roofs of buildings?  Many have some dome or peak to them.

If your first kit was a Stew-Mac kit you did indeed dome your top but probably did not know it.  Stew-Mac kits come with pre-radiused braces that will pull the top into a slight dome even though the sides were not prepared to match the dome.  But the sides were prepared to have the kerfed linings angle up toward the dome - remember the 2 X 4 sanding board that they have you make?  If you didn't build a Stew-Mac kit forgive me for digressing - I do it often.....

So the dishes help us sand the dome into our braces, hold the dome when gluing the braces, sand the sides to match the dome, and provide a stable platform to do many things like carve the braces.

Not all builders build domes into their tops and backs either - just one of the many ways to build a guitar.
Author:  jonhfry [ Mon Dec 17, 2007 1:36 am ]
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If your top was flat, then you built a 'flat top'.


Nothing wrong with that, I think Jim Olsen's are mostly flat tops.


Some do choose to build a radius top though.
Commen top radii are around 30 ft' while backs are somewhat less in the 15-20 ft' range. Just depends on preference, and your ultimate goals.
Author:  Bill Greene [ Mon Dec 17, 2007 3:44 am ]
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...and now for another opinion:

Many builders build "flat" tops, and Jim Olson has been mentioned above as example of just one. The thought process goes something like this: if you artificially stress the top, via a radius, you may be artifically limiting the tops ability to vibrate freely, which may (or may not) affect the sound of the guitar in a manner to your liking. As Hesh said, domed tops are reported to accentuate the midrange - although my tin ears have never been able to pick that out. And personally, I have never heard that a flat top is any more or less likely to crack based solely on the radius. Cracks are usually related to abnormal humidity fluctuations, stresses in the wood itself, poor structural composition, accidents, etc. In fact, for those that do radius their tops, the radiuses are frequently different: 25, 28, 30, etc., so it is hard to define exactly what the "perfect" radius is, or might be. It's an imperfect science.

I build strictly flat tops myself, since that was how I was originally taught by a terrific builder named Brad Nickerson. I did, however, send Kevin Ryan an email asking him to explain why he builds with flat tops vs. those with a radius. I'm paraphrasing, but his response read similar to this: "it has never made sense to me why someone would artificially inhibit the ability of the top to vibrate as freely as possible, and I've never heard an audible reason to change, so I haven't. If you want your guitars to sound like everyone else's, build them like everyone else."

Whatever you decide will likely work for you. Best of luck.

Bill

PS: I built my first from a StewMac kit...yep, flat top. (and it sounds fantastic )
Author:  WaddyThomson [ Mon Dec 17, 2007 4:10 am ]
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Radiused or flat, when you put strings on it, it becomes radiused.  If it doesn't you braced it too heavily.
Author:  mgcain [ Mon Dec 17, 2007 4:23 am ]
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My kit was from US Guitar Kits (www.usguitarkits.com) and it came with the back and sides pre-assembled, kerfings in place, and top braced.  I still had to carve the bracings.  So, for the most part, I missed out on all the fun!


Its flat, and it sounds great - I have no opinion on how strong and or acoustically different a flat top will be from a radiused top, but since I will probably never build two guitars exactly alike, I'll probably never know.  


Thanks for all the input - this is a great forum


Mitch
Author:  FishtownMike [ Mon Dec 17, 2007 7:54 am ]
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[QUOTE=Bill Greene] ...and now for another opinion:

Many builders build "flat" tops, and Jim Olson has been mentioned above as example of just one. The thought process goes something like this: if you artificially stress the top, via a radius, you may be artifically limiting the tops ability to vibrate freely, which may (or may not) affect the sound of the guitar in a manner to your liking. As Hesh said, domed tops are reported to accentuate the midrange - although my tin ears have never been able to pick that out. And personally, I have never heard that a flat top is any more or less likely to crack based solely on the radius. Cracks are usually related to abnormal humidity fluctuations, stresses in the wood itself, poor structural composition, accidents, etc. In fact, for those that do radius their tops, the radiuses are frequently different: 25, 28, 30, etc., so it is hard to define exactly what the "perfect" radius is, or might be. It's an imperfect science.

I build strictly flat tops myself, since that was how I was originally taught by a terrific builder named Brad Nickerson. I did, however, send Kevin Ryan an email asking him to explain why he builds with flat tops vs. those with a radius. I'm paraphrasing, but his response read similar to this: "it has never made sense to me why someone would artificially inhibit the ability of the top to vibrate as freely as possible, and I've never heard an audible reason to change, so I haven't. If you want your guitars to sound like everyone else's, build them like everyone else."

Whatever you decide will likely work for you. Best of luck.

Bill

PS: I built my first from a StewMac kit...yep, flat top. (and it sounds fantastic )[/QUOTE]
Bill when did the use of radiused tops and backs first come about? Was it private luthiers or the likes of martin or gibson start using this technique first. Is this a more recent technique that has been used. I myself have been thinking of using a trully flat top but I need to do a little more research on it before I decide. Mike
Author:  Michael Dale Payne [ Mon Dec 17, 2007 8:18 am ]
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Someone may have said this and I may have missed it, but if not thought I would mention.

As Hesh correctly stated, a dome is stronger in weight to strength ratio than the same top and bracing in a flat plane. One of the areas in which this is a reall plus is in helping eliminate bridge belling with age. as the guitar gets older the sound hole side of the top at the bridge want to deform inward and the tail side of the top at the back of the bridge wants to deform outward due to the rotational forces of the strings at the saddle/bridge junction. All things being equal, domes resist this loading better.
Author:  MikeP [ Mon Dec 17, 2007 8:22 am ]
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from what I've read, and it makes complete logical sense, a domed top helps prevent cracking from low humidity (i.e. the wood dries out)...as wood dries it shrinks...if the top of already completely flat then the result of shrinkage is stress on the top, and I guess sides, which most likely will end up splitting the soundboard...if there is a dome then that will give some room for shrinkage, flattening out, before the critical point of splitting occurs....I don't think the dome in anyway helps or hurts in the case of wetness, which of course results in swelling of the wood....


Author:  Hesh [ Mon Dec 17, 2007 9:00 am ]
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Mike P I agree, the existence of a dome is like having an expansion joint to cover changes in RH and temp.

Good points Michael IP (I have to sort you guys out since you are both Michael P's) and again the dome benefits the guitar as it ages.

Bill buddy if you built a Stew-Mac kit and used their braces you don't have a true flat top.  Stew-Mac braces are pre-radiused to 25' for the top.  But if you made you own braces then you indeed have a flat top.

Again I am not advocating one over the other - I was just posting what I know about domed tops.

I have built with a domed top but without sides prepared for the dome (Huss and Dalton offers this too) and the guitar is a great guitar.
Author:  tippie53 [ Mon Dec 17, 2007 9:01 am ]
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Martin pioneered the domed top. I have a 1880's that has a domed top. From an engineering stand point the radius does indeed allow top movement without cracking during humidity changes and makes perfect sense.
   To be honest the argument of not stressing the top doesn't hold up. Once the humidity changes you "stress" the top. Humidity changes from the time the braces are glued onto the top will stress the top. As humidity goes lower the top will sink as the wood contacts and rise as the wood expands. The only way you can keep this from happening would be keeping the guitar is a static environment.
john hall
blues creek guitar
Author:  FishtownMike [ Mon Dec 17, 2007 9:08 am ]
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I understand the logic of what your saying, but if the top does shrink because of low humidity and the braces being quartered have less tendancy to shrink end to end then the side to side the top will shrink isn't there still an issue with cracking or braces becoming loose? Even though the braces are radiused to match the top i still see an issue with shrinkage and splitting still being very possible. I guess the only way to not have this problem is to keep your guitar humidified but i have met many people who own acoustics that don't even know what this is and have never have used a humidity device.
Author:  Bruce Dickey [ Mon Dec 17, 2007 9:24 am ]
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[QUOTE=WaddyT] Radiused or flat, when you put strings on it, it becomes radiused.  If it doesn't you braced it too heavily.
[/QUOTE]

Thanks Waddy, what a truism. You made my day. A little levity is a good thing!   
Author:  tippie53 [ Mon Dec 17, 2007 9:43 am ]
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   Not really , as the wood moves across the grain , the braces are able to flex to follow the movement. Bracing will loosen if the ends are not supported in the kerfing , or thinned to zero so then can flex with the top.
   
Author:  BruceH [ Mon Dec 17, 2007 9:49 am ]
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...and some builders build with a radius in the lower bout, but glue the upper bout dead flat.
Author:  MikeP [ Mon Dec 17, 2007 9:58 am ]
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"It seems to me undeniable that we have the Larson brothers Carl and
August -- already mentioned above -- and not the Martin Company or any
other manufacturer to thank for adapting the gut-string guitar's "X"
bracing successfully to the needs and design of the modern steel string
guitar. To repeat: starting in the 1890s, they made the first steel
string guitars sturdy enough to not collapse under the pull of steel
strings, and yet not so overbuilt that they lacked sound. The Larsons
achieved this in part by enlarging and beefing up (with increased size
and laminated construction) the previously too delicate "X" bracing, by
doming their guitar tops, by reinforcing the guitar necks, and by
increasing the size, shape and gluing surface of the bridge. These
design advances notwithstanding, it wasn't until the 1920s that such
guitars were produced in sufficient numbers by factories for them to
become -- as it were -- principal players in the popular market."

The above is a quote from here: http://www.esomogyi.com/ssg2.html

So it would appear as if domed tops are the standard for steel string guitars with X-bracing.  The term flat top is relative as they are certainly not carved arch tops like jazz guitars and such


Author:  Kent Chasson [ Mon Dec 17, 2007 12:35 pm ]
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Tippie said: From an engineering stand point the radius does indeed allow top movement without cracking during humidity changes and makes perfect sense.


I understand the argument and there's some logic to it but I'd like to see some empirical proof. I doubt it is as straight forward as conventional wisdom has it. I think if Hoffman, Ryan, and Olsen were getting guitars back with cracked tops every day, they would have stopped using flat tops long ago.


Mike is correct that gluing one piece of wood across the grain of another is always going to create a potential for splits because of differential expansion/contraction. And dryness isn't the only problem. When wood can't freely expand as the humidity rises, it gets compressed. Over time, it will want to stay in that compressed state (called "compression set") and will then be more likely to split in low humidity.


Maybe a repair person will chime in but most splits I've seen (none of them in any of my guitars yet, knock on wood) start at the rims or the bridge. They are areas where split potential will be essentially unaffected by doming or not.


I do agree that the concept of an "unstressed top" doesn't make sense. An unstressed top would make NO sound. All steel strings get about 180 lbs of stress as soon as they are strung up.


A domed top can attain a better strength/weight ratio than a flat top of similar construction but strength/weight is not the only issue.
Author:  Hesh [ Mon Dec 17, 2007 1:30 pm ]
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Kent empirical proof/data and guitar building are at times like relationships between men and women........  What is true beyond any doubt to one is not true to the other and they have their own supporting evidence to the contrary......

Al Caruth will tell you that what he sees in his test data is not always what he hears with his ears and what others hear.  Guitar building seems to cross the line, when ever it wishes, between art and science.....

I am not advocating doming or not here.  But I think that it is important to note that those who do not dome will do other things to either make the rims more flexible or more stiff.

I often see people thinning tops to what to some will seem extreme.  The problem is that this concept of very thin tops is taken out of the context of an entire structure and system of building that will support a thin top i.e. double sides, stiffeners, Fox style linings etc.

The trick is how ever a builder decides to make their tops to also be knowledgeable of the "system" of how the entire box is built to compliment a thin top, a domed top, a flat top, etc.  To me the top is just part of a particular system and must not be taken out of context.

BTW I do not dome my upper bouts, all braces above the sound hole are not radiused.
Author:  tippie53 [ Mon Dec 17, 2007 1:47 pm ]
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   I do a lot of repairs. It seems that people may be a bit confused as to just what the top is and does. This is not to insult anyone but many early builders don't understand all the loading and engineering of a top . The top is the most important part of the guitar. It is taking the energy of the strings and converting that into work all the while it is structurally withstanding the forces applied to it.
   The top is under a rotational force , a compressive force and a tensional load from the strings . Along with this it is also under stress from the environment.
    I see and repair cracks all the time. Most are humidity issues. When the wood is under stress from humidity , it will shrink across the grain , not along it. When you add the bracing ,it is now being confined and as a result is under stress. Some cracks , often near the sound hole are from neck loading.
    most cracks from humidity will happen at the glue joint behind the bridge or along the grain as this is the weakest point when wood shrinks and as stated out at the edges.
    Yes on occasion a brace will loosen but the braces and glue joints are often stronger than the top and it is the top where the failure will occur. Wood is in flux and doming both natural or built in allow a certain amount of movement.
     The proof is in the construction technique and time. We are dealing with a natural material and it has flaws that we as builders must contend with and apply bracing in a manner that can address the load and inherent ability of the wood to withstand them. If you build too light you will have load failure and too heavy the tone will suffer , so you must find a balance.
     I personally will build with a flat in the upper area of the body and dome the lower. The upper bracing is stronger and is more of a heavy load bearing or structural. The X brace while structural is also tonal . The gluing techniques and construction methods are most influential to the tops ability to carry the load and generate sound.
      To carry the load there have been many bracing systems developed . X Ladder H Fan Lattice. All have their strengths and weaknesses. Wood choice and bracing shape all come into play.
    I don't think anyone person will ever truly master this and while we may often be chasing a shadow , it is fun and a never ending learning experience.
    In my early days I did build flat , and though not all tops failed , some did. Once I started doming ,and building in a controlled environment , the failures stopped. I have learned much and share what I have learned but I am far from knowing all .
      Find your technique . I am hoping to hear more on this . Al Carruth I hope chimes in as he is one that can explain the technical terms better than most I know.
Author:  Zach Ehley [ Mon Dec 17, 2007 2:00 pm ]
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In a FWW article with Grit Laskin and a GAL atricle about Jean Larrivee, they both radius the upper bout and keep the LOWER bout flat.  I thought it was a misprint in the Larrivee article, but the drawing in the FWW article shows a radius on the top of the X brace and none on the lower.  The tonebars and fingers are flat and the transverse brace is radiused.  THis seems to be backwards from everything I've seen.

How do you glue up a brace thats not either all flat or all radiused?
Author:  Kent Chasson [ Mon Dec 17, 2007 3:09 pm ]
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Kent empirical proof/data and guitar building are at times like relationships between men and women........  What is true beyond any doubt to one is not true to the other and they have their own supporting evidence to the contrary......


Ain't that the truth! I just don't like it when what I consider to be misinformation becomes "common knowledge" and I have to fight an uphill battle with a client when I decide that a flat top sound would best suit their playing. If a flat top caves, it was under built. If a domed top caves, it was under built. Either way, the cause wasn't whether it was flat or domed.


If a crack starts at the bridge or at the rims, I don't see how doming the top would have prevented it. If I'm missing something there, I'd certainly love to learn differently. But again, there's a lot of top notch top flat top guitars out there that are not having problems which is enough empirical proof for me that flatness is a fine option if done well.


Tippie, thanks for the repair perspective. I do some but I primarily build new. When you say "most cracks from humidity will happen at the glue joint behind the bridge", do you mean right behind the bridge or somewhere between the bridge and the tail? If the latter, I could see how doming might help that. If right behind the bridge, I can't see how doming will make any difference.


As for doming the upper bout and leaving the lower flat, I had never heard that but it makes sense to me. The dome at the upper bout would be a great way to help support the neck load while getting a flat top sound. And in my experience, after building a lot of side by side test tops, flat tops do have a different sound.


I certainly don't claim to "know it all" either. But I do build domed and flat tops and variations of each depending on the sound I want. And it's a drag to have to defend flat tops from an undeserved bed reputation.
Author:  tippie53 [ Tue Dec 18, 2007 12:09 am ]
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    I have seen humidity crack issues on areas where the wood is "locked". Usually at the bridge and along the glue joint. That is the most common.
     To help you understand why ,you must look at the woods cell. The cell will not change length but will widen and narrow as it looses and gains moisture. So the more it looses the smaller it will get. This is why you see the top fall in.Wood under tension across the grain is one of the weakest points. Yet wood under tension along the grain is one of its strongest.
    The wood being glued to a brace is now fighting with the brace and the movement will cause the unit to arch down. The brace stock is smaller in cross section and is more stable , the top will cause the top to pull on the sides of the guitar and down it will go.
Doming will permit some movement a true flat top won't have the allowance for movement . Once the top goes past a straight line the stress will continue to grow until yield strength is exceeded and the weakest point of the top will fail. Usually the top itself or even a brace. Now if the top is allowed to gain moisture the top usually will arch up. This will cause the bridge area to rise and thus the action will go high. You know you are in a danger area when you can feel the fret ends protruding from the fretboard.
   keeping a humidifier in the body during dry times is a big help. Over time the resins harden and the movement is lessened but for the first few years when the wood is newest can be the most dangerous times for humidity cracks.
     When you build the RH at the time you build can be a help or a hindrance. That is why most builds are done in the winter when the RH is naturally lower. If yo brace a top during the summer months of 60% humidity of higher you can see the inherent dangers
john hall
    
Author:  mgcain [ Tue Dec 18, 2007 12:37 am ]
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Sheesh...and I was confused when I STARTED this post!


All cleared up now though!


Glad to see everyone in total agreement!


I actually thought it would be a really dumb question before I asked it - thank God I did! Sounds like there was some pent up angst out there!


thanks so much for all the good information - As usual, I'll probably study this to death before i put any wood in harm's way, but at least I know I've come to the right place to learn...


wow.


Mitch
Author:  Arnt Rian [ Tue Dec 18, 2007 1:10 am ]
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[QUOTE=tippie53]    
In my early days I did build flat , and though not all tops failed , some did. Once I started doming ,and building in a controlled environment , the failures stopped. [/QUOTE]

John, I suspect starting to build in a “controlled environment” had a lot more to do with the failures stopping than the domes of your tops. There are enough flat topped guitars out there that are doing well to prove that, as has been mentioned.

I am with Kent in that the degree of dome (or not) should be determined by the sound you want your guitars to produce. One type of doming that has not been mentioned in this thread yet (I think) is the “flat sides / domed braces” concept that some builders use. This is the type of doming you get if you build like Cumpiano shows in his famous book, or the Huss and Dalton “Traditional” series guitars.

So now we have: Domed (25’, 28’ and 30’ radius); flat upper bout / domed lower bout; domed upper bout / flat lower bout; flat sides / domed braces… How many variants do you’all think we can manage to list in a single thread??!!
Author:  mgcain [ Tue Dec 18, 2007 1:51 am ]
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My heads gonna pop!
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