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RE: traditional back curvature/shape (in reply to Andrew)
A simple and to me the nicest way is to use a constant curve for the whole back. This can be achieved by using a radiused dish. I use a 15ft dish. and when lined with sand paper can be used to shape the sides, slipper and bottom block. For the actual taper I think I have an 8mm taper from end block to heel. The main bulk of the wood will come off from the waist to the heel.
Heres a picture od roughly what I am on about the same dish can also be used to help glue the back on with the aid of a few clamps.
Images are resized automatically to a maximum width of 800px
Posts: 149
Joined: Jul. 14 2011
From: Newark, DE, USA
RE: traditional back curvature/shape (in reply to Andrew)
I've been using a 15 ft radius for classical guitar backs lately (Romanillos plan), and I have a sense that the guitars look more like steel strings than classicals when there is a significant taper from tail to heel. There is a dramatic fall-away from waist to heel. Do you get that sense, relative to what a more cylindrical back would look like?
RE: traditional back curvature/shape (in reply to Jim Kirby)
With a radiused dish you end up with the longitudinal curve being much more pronounced than the traditional curve. Traditionally each bar has a radius, the way I was taught the upper and lower bouts have the same radius and the middle one is deeper. The longitudinal curve then depends on how "high" the waist is left when planing down. If you use a taper to the heel then the waist doesn't end up any higher than the endblock. In terms of the actual planing you can make an inclined plane from along the central part of the sides (stretching from upper brace almost down to lower brace ) which copies the slope of the central bar where it meets the sides. Then plane the heelblock and endblock to make a slope that ends up around 2cm above the other end when extended with a ruler. Then it is a matter of smoothing out those transitions with the plane. It is not difficult once you do it a few times but extremely difficult to do at first and as you can see from the above not very easy to explain. This ends up being more cylindrical than spherical.
RE: traditional back curvature/shape (in reply to Andrew)
good post John. I work the same way and I agree with Jim that the radiussed backs look pretty steel string. On the other hand, I have no idea if it influences on sound
RE: traditional back curvature/shape (in reply to Andrew)
I agree with John Ray and Anders, but a tip of the hat to Eden because he's fine too.
Radiusdish changes the way the plane of the back sits in relation to the top. In America if you go against The Dishers they will attack you and call you old fashioned. I've been saying it for years; the dish changes the look of the guitar by taking the back or top out of plane.
If you look at baroque guitars with vaulted backs you can see the extreme effect of how a high arched back sits on the ribs. The radiusdish produces a similar effect only more subtle. It is still noticeable.
The dish built guitar probably functions the same as the guitar with the back braced to fit to a plane, but it is not traditional in the modern Spanish school of building since Torres. A radiusdish produces a back which is a section of a sphere, some builders like the geometric regularity, but on the other hand it produces the exact same type of arch that every other builder who uses the dish makes. You can't modulate your arch by making each individual brace a different curvature.
From a buyers standpoint there is nothing to fear from either method; and frankly most buyers don't understand the difference until it is pointed out to them in person by showing an example of both styles of arching side by side. This is one of those points that guitar makers mull over that piss them off when other builders clamp down on them for doing it. The real difference between using a radiusdish and traditional brace profiles is largely philosophical and personal according to each builder. There is structurally nothing detrimental to either method.
Using a radiusdish puts the builder more into the camp of using "workmanship of certainty" because they use a jig which creates a uniform result.
Traditional brace profiles where the maker modulates the shape of the arch through the curve of each brace is more hands on artistic and could be called "workmanship of risk".
Two distinct ways of seeing the same problem. The traditional way would be through workmanship of risk.
I hope that helps.
******** In post script the only caveat I would seriously put on dish vs. building to a plane is that those who use dishes and do not know how to fit the back to a plane should not do back off restoration work on older instruments until they understand this method or know the difference. It's super important to know the working method of the original builder before working on touchy pieces.
Taking guitars apart and putting them back together in the same manner they were originally constructed is fairly important. On inexpensive instruments it does not matter as much, but on high end guitars it makes a big difference in style and structure. If you had to take the back off of a Barbero or a Santos or Romanillos for example, it would be malicious damage to rebuild the back with a radiusdish method.
Posts: 597
Joined: Jan. 14 2007
From: York, England
RE: traditional back curvature/shape (in reply to estebanana)
quote:
Using a radiusdish puts the builder more into the camp of using "workmanship of certainty" because they use a jig which creates a uniform result.
Thats a nice way of putting it. I've been very tempted to go down the dish route for just this reason; uniformity and certainty. But so far I've resisted and I've used the traditional method.
Actually, this thread has convinced me to stick to the tradional method.
RE: traditional back curvature/shape (in reply to Andrew)
If you were to use a 25' radiusdish you would be more likely to have the look of a traditional guitar (length ways) with all the ease of the dish method.
I Have to say I didn't particually like the more 'hit and miss' method of the more traditional way. I've seen examples of this method produce dimples in the back where the point between two back bars is lower.
It doesnt look too much like a steel string as they radius both the front and the back. It doesn't look too traditional either. It is easier, faster and the most accurate way of doing it.
From these measurements can I deduce the general shape of these backs?
So what is a common traditional longitudinal curvature in mm's? Why do most makers have a longitudinal curve, why not straight will all the back braces arched the same?
RE: traditional back curvature/shape (in reply to Stephen Eden)
SEden,
You misunderstand the process of fitting a back (in the traditional manner) when you say hit and miss. When I was taught to fit the back I made eight measurements of height, six measurements of angles, a flat board placed at an angle on top of a section of each side to register high spots, and a strip of newsprint to confirm that the back was in contact with different parts of the rim especially the slipper foot. Surely you meant to say that for the BEGINNER "the dished method is easier, faster, more accurate" or that for yourself this is the case. For someone who has learned to do it correctly the traditional method is just as easy, probably faster and: 1)leaves a planed surface which is much better for gluing, and 2)it makes a perfect curve. If you have never learned to do this or have seen bad examples of this I think your are correct in doing it another way (which might be just as good). I also take exception to Estebanana's "workmanship of uncertainty"; done correctly the process is highly repeatable. There is a huge amount of misinformation on building available on the internet and in books. I think that those among us who make guitars professionally have an obligation to not add to that misinformation.
RE: traditional back curvature/shape (in reply to johnguitar)
I have only done the traditional fit a handful of times at the beggining of my career they all worked out ok though. I got into the dish method mainly because of the ease, but also because I loved the look. Its all about the angle and the curvature you add to the linings, the slipper and the end block. How do you know you have them absolutly spot on at everypoint. This is why described it as hit and miss.
you make 8 hieght measurements and 6 angle measurements. I take 6 Height measurements and 0 angle measurements. So already the traditional method is seemingly more complicated. I use the same radius for all of my bars you use different. I am sure that once you have done it a few (20-30 times) the process does become easier to do. However the dish method is easy from the word go and for a professional its a brainless task (time to think what to have for dinner) if you will.
Whether it's faster or not is another thing. Surely that depends on your overall working pace, the skill level of the individual and the fitting process. I like to think 15-25 minutes for an intial fit (Prior to fitting the linings) is pretty quick.
Which brings me to accuracy. Using a dish, as long the sandpaper hits every part of the surface, it is perfectly fit. 100% Accurate. So Perhaps I should rephrase it to at least as accurate if not more.
Posts: 149
Joined: Jul. 14 2011
From: Newark, DE, USA
RE: traditional back curvature/shape (in reply to Stephen Eden)
SEden,
I don't disagree with your reasoning at all, as I do the same thing myself, I was mainly commenting that I think the 15' radius produces a back that looks too arched in the lengthwise direction. I am curious whether you've tried anything flatter. I used an 18' radius on the back of two Reyes-style flamencos as well as my earlier classicals, which were all Rodriguez-style and really large!, and I'm pretty sure I will go back to that as the default radius. Even that gives a back with that characteristic sort of hump or fall-away over the upper bout, but nowhere near as obvious as with the 15' radius.
I'm not sure why I decided to try the 15' radius on the Romanillos-style guitars in the first place - probably since they are so small, but I'm not sure why that would be a deciding factor.
RE: traditional back curvature/shape (in reply to Andrew)
Hey Jim.
I worked for a couple of years with Pablo Requena who uses a 25' radial dish.
I was taught that the traditional back curvature follows a 25' curve length ways and a 15' width ways. I also know a couple of steel string builders and loved the look of that highly arched back they used over the 25' that Pablo uses. I think that is just preference really
RE: traditional back curvature/shape (in reply to Andrew)
SEden John said that the eight measurements of height and six measurements of angles was somthing he did when he was taught how to fit the back.
I´m not going to argue how John does things because Im sure he´ll do that better than me. BUT I dont understand your argument. This hit and miss idea of yours. Where do you get that? Could you please explain. Trad. Spanish building is often done with a scooped out dish. And yes, its easyer to buy a radiussed dish but when you´ve made your homemade traditional dish, it works forever...... There´s not much difference. I prefer the look of the trad. way, but I´m not going to argue what sounds the best because I have no idea.
RE: traditional back curvature/shape (in reply to Andrew)
So you sand yours too?
The Hit and miss comes when shaping the lining the heel block and the slipper. Because of the varying curves that are being produced by the differing curves the angle of the lining will alter differently as you go around. If you dont get it exactly right I'm sure nothing too bad will happen because you can increase glueing pressure to close any very minor gaps. But If you dont realise you have gone considerably wrong which isnt always that obvious you can end up with some obvious asthetical issues.
All I'm trying to get across here is that with a dish you negate those issues, It is idiot proof! (probably also why I get on so well with it )
RE: traditional back curvature/shape (in reply to johnguitar)
quote:
. I also take exception to Estebanana's "workmanship of uncertainty"; done correctly the process is highly repeatable. There is a huge amount of misinformation on building available on the internet and in books.
In saying "workmanship of risk" ( I never wrote workmanship of uncertainty) I was using David Pye's idea that there are two ways of looking at or classifying methods of craftsmanship; one is "workmanship of certainty" the other is "workmanship of risk".
Workmanship of certainty means the maker uses a method or fixture which results creating the piece over and over again in the same shape without risk of the piece ever being injured during its making.
Workmanship of risk means ( loosely) the same object can be created and variation into its shape texture or composition can be introduced during it's making by the skill of the maker.
One uses a jig with the purpose of creating a sure method of certainty to keep variability from entering the process of making. So I call that workmanship of certainty. The dish is a fixture which ensures certainty.
The other uses a learned set of skills which correctly done in the right order render an arched back in which the maker can introduce his or her own variability in how the arch is modulated over the span of the back. This is workmanship of risk because it does not use a fixture which creates the same arch over and over. Workmanship of risk means in part that it allows the maker to change the piece while it is being worked on.
As I said before "workmanship of risk" and "workmanship of certainly" are both good ways to work and neither of the them should suffer being considered a bad method. Because one is called "workmanship of risk" does not mean that skill or experience is not important. On the contrary workmanship of risk implies that the maker is going on skill and using their knowledge of their chosen working method.
At least if you think that David Pye's ideas of how to talk about craftsmanship are interesting. Here is the book I'm speaking of. It's not a guitar making book, but more of a book that talks about the importance of making things by hand in a mechanized age. He divides skilled tasks into to categories of workmanship of risk an workmanship or certainty. I think the ideas are worth reading about, but this book does foster a lot of disagreement from some artisans.
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