Official Luthiers Forum!

Hi! For those of you who have built from the Cumpiano book(or read it and just did the math; you never know around here) , does anyone know what the top and back radius for the guitar built from it would be?
Cumpiano does not give them, he simply shows how to get the radius for each by the use of three screws or nails and a thin piece of metal rod.
The nails/screws are placed in a line so that the one in the center is off set by a certain amount. The the rod is placed lengthwise across the two outside nails/screws and pushed up to touch the center one, forming a radius which is then traced onto whatever the radius template material is.
My math skills end with addition, subtraction, multiplication and division, so I've no clue how to work out what the radius is for each.
Anyone know?
Edited to add: It will work whether I know the radius or not. I'd just like to know what they are. Thanks.

_________________
Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

OK, more info. I went back and re-read that section.
For the top the span between the outer pins(sorry, he uses push pins, not nails or screws) is 18". The center pin is offset by 1/8".
For the back the distance between the two outer pins is 16", and the center pin is offset by
1/4".
Anyone know what the respective radius is for each? In feet(for example, 25', 28', etc)?
Once again, he uses a piece of flexible rod stretched across the span of the two outer pins(in the case of the top, 18" apart, for the back 16" apart), and places a pin in the center of that imaginary line, but offset in the case of the top by 1/8", and the back by 1/4".
By pushing the rod up in the middle until it contacts the center, offset pin, a radius is described for each.

_________________
Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

Here's a handy tool to do just that:
http://liutaiomottola.com/formulae/sag.htm

Looks like ~27ft for the top, ~11 for the back.

Thanks, Dennis! Much appreciated.
By the way, 11' seems like a pretty healthy(huge) back radius, does it not?

_________________
Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

11' is a pretty hefty radius. That'll make for a stiff back. I don't think mine turned out that large. But then again I didn't always follow the instructions exactly. I just made my own template that I felt was a good radius. I'm thinking out was more along the lines of 20' or so

Zeke, what I'm wondering about is the top radius. I'm particularly concerned about the 1/8" thickness of the cork rim on the workbord that supports the edges of the top, and the cork shim located under the upper transverse brace. Maybe I'm over thinking it, but my concern is whether or not a change to top radius would necessitate a change in the thickness of the cork rim and utb shim, or whether there's enough give to compensate without putting undue pressure on the top arch when clamping, etc.
In other words, can I use that workboard as described for different top radius builds?(say, 25')
Since the back is glued on using the roping method, and does not contact the workboard face, I reckon you could use whatever radius you want for it.

_________________
Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

It seems to me that with a larger radius you would need a thicker shim. The cork will give some so it would only need to be adjusted if it was a difference of say 1/8" around the edges. It seems you could make another shim an just stack it on top of the existing one tour necessary and take it off if doing a shallower radius. The point of the cork shim is just to have good even pressure on the edges of the top. So it's not a precise science at all

A better shim material is hardboard. I'll take some pictures of my Cumpiano style workboard and shim with some details.

_________________
Old growth, shmold growth!

Hmmmm. It would seem like the shim material's purpose, at least under the upper transverse brace, is to cushion the soundboard when any type of downward force is applied. It would seem hardboard is a poor material for that application, but maybe I am mistaken about what hardboard is. In any case, I think I read somewhere that you build in this style, so any insight would be welcome.

Would love to see it.
I'm also wondering if a thicker shim along the outline of the workboard, effectively raising the instrument up so that the top makes no contact at all except along the rim, would work better?
After all, the top and back bracing is glued on free of the workboard using clamps. It seems to me the workboard is used mainly to support the ribs/sides. Thoughts?

_________________
Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

I do not know if having a raised shim so that the top does not touch at all will make things easier but I don't see how it could hurt. You are correct that all you need is pressure around the edges. I think it would suit the purpose well.

Here's the current shim.

It's 1/8 inch thick hardboard, 1/4 inch wide.

The outline of the guitar shape is the middle of the outline, 1/8th inch out (or more if you like) and 1.8yh inch inside the outline.

The neck end edge is is trimmed flush with the front edge of the soundboard, which by this stage should be trimmed as well in preparation for gluing on the neck block.

There's two platforms built in, one for the neck block, and one for the heel block, to support them during clamping operations.

The inside edge of the shim is slightly beveled to accomodate the top radius, inclufding the neck block, but the tailblock shim is not radiused.

When the soundboard is clamped down, it does not touch the workboard, or at least it shouldn't.

Here's why:
Even though there's an 1/8th inch offset for the top, it's over the entire 18 inch span of the brace. When glued down, the full 18 inches is not utilised, therefore the arched of the portion of the soundboard attached to the braces never achieves the full 1/8th inch offset, and therefore shouldn't touch the workboard, and indeed my soundboards DON'T touch the workboards.

It's correct that you only need the soundboard to touch the shim on the edges, as that is where the clamping pressure is needed when gluing the sides.

_________________
Old growth, shmold growth!

TGW, thanks for posting that! Can I ask a few questions, just so I'm certain I'm clear?
1- You're using 1/8" thick hardboard?
2- You are NOT using a 1/16" thick shim to support the Upper Transverse Brace and the appex of the X brace?
3-Cumpiano uses a plain workboard with no side shim for some operations, and the cork edge shim glued to a piece of heavy paper is used on top of that when glueing top and back.
Is it safe to assume you perform all operations on the workboard pictured(with side/edge shim attached)?
Which brings me to
4- You say the shim for the headblock is also radiused in the inside.
In that case, how do you get the headblock to sit straight and square when glueing it down? IOW, how do you keep it from tipping inward toward the inside radius on the shim?

Sorry if I've asked too many questions, and not trying to steal any of your ideas. Just looking for the best way to approach this way of building, and I like the look of it.

Zeke, I'll probably go with a 20' radius for the back also. I think that would be much more comfortable.

_________________
Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

Yup.

Nope, just this workboard shim pictured

I start with no shim same as Cumpiano. I use the workboard flat to glue the bridge patch, transverse graft, soundhole braces, and the finger braces, all of which are unradiused. Then I use the slats to glue the X, the tone bars, and the upper face brace. After the braces are carved I install the shim onto the workboard.

Actually, the neck block tips back slightly with the sondboard. Recall that the UFB is arched to a 1/16th offset. I leave the center 2.5 inches of the upper face brace flat where it glues onto the soundboard, for the fretboard to rest on. The offset causes the soundboard to tip back, which is how the neck angle is derived. The neck block remains orthogonal to the overall soundboard arch, so that the neck block and the neck tenon/fretboard junction remains 90 degrees as the neck block is tipped back at the correct angle for the bridge height. The part of the shim that the neck block rests on is beveled to account for this, so that the geometry remains correct. When the sides are glued on, the actual shoulders of the guitar are tipped back as well.

The practical effect of this whole procedure is that the neck angle is correct for the bridge height, the surface that the fretboard is glued to is flat so that the fretboard can be perfectly straight from the nut to the end of the fretboard with no falloff, there's no flattening of that area of the soundboard necessary to achieve the straight neck in the way that is necessary with truly domed tops, and there's no fussing about with measuring and transferring angles to make sure the neck and body match up as there is with truly domed tops, as the neck block and neck remain 90 degrees.

It takes less time to assemble the box and fit the neck.

The workboard/shim assembly is easier and cheaper to make than the go-bar deck with radiused dishes.

_________________
Old growth, shmold growth!

It seemed to me when I was reading Cumpiano that the headblock tilt (which sets neck angle in this build method) is entirely governed by how much you torque down the work board bolt, given that the text doesn't indicate the thickness of the "added shims" and they are compliant, anyway. I'm looking at Cumpiano figs 10-1a and 10.1b. A half turn too much or not enough would be enough to screw up the neck angle.

I noticed on his website that he now recommends a scooped out workboard in recognition of this problem, which some builders I know "invented" themselves before switching to radius dishes. This will help fix the head block tilt problem as it eliminates compliant shims (and is why Cumpiano went this way) but the workboard is now all compound curvatures rather than simply flat, so has to be made by hand or with some pretty smart CNC machining. It's easy to see why so many people moved to radius dishes and go-bars.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Thanks, TGW! Much appreciated!

_________________
Mike

The only thing nescessary for evil to thrive is for good men to do nothing.

@ Trevor

The current workboard shim has a hard support under the neck block, and the current shoe has a mechanism to prevent neckblock back tip. The neck angle is derived strictly from the amount of curvature added to the upper face brace. As the soundboard is wrapped around the brace, the area of the soundboard to which the neck block will be glued is tipped back in relation to the string side of the soundboard, by a precise degree determined by the geometry of the upper face brace curvature and the point at which the upper face brace is glued in relation to the neck block. A 14 fret guitar has a less curved upper face brace than a 12 fret guitar, as the 12 fret guitar has a longer fretboard extension and therefore requires a slightly greater degree of curvature to maintain a straight line from the nut to the end of the fretboard, otherwise a small degree of falloff would be introduced.

The neck angle is predictable, controllable, and easily repeatable, and requires no transferring of the angle from the guitar body to the neck heel, as both remain 90 degrees, saving quite a bit of time, requires no bulky and special mortising and tenoning jigs, and the soundbox assembly takes no more time than when using a mold. The fretboard from the nut to the end of the last fret is perfectly straight, with no falloff, and little to no flattening of the gluing surface under the fretboard neccessary, leaving the soundboard at full thickness under the fretboard.

I've included a picture of the current workboard shoe. The soundboard is clamped to the workboard and shim by the small amount of soundboard overhang (lightly, not seen in this picture as I had only clamped the lower bouts) and the shoe is butted up against the neck block and screwed to the neckblock from the neck mortis through the pre existing bolt holes in the neck block. The shoe also has the neck angle and in my guitars, butts up against the neck block precisely while resting on the x and ufb, with no gap, before being screwed down to the neck block and then the soundboard/workboard, and the correct angle is precisely maintained during all remaining assembly operations.

Interestingly enough, Cumpiano DOES address the precise concern you raise, and indeed does discuss occasionally using a workboard with a scooped out area, and mentions also using a different shim, here: http://www.cumpiano.com/Home/Book/textbook.html under item 11 "clamping shoe caveats", but the system he teaches his students is the one I use, and the one he currently uses in his builds. The web page deals mainly with updates and corrections to the text of his book without going into too much detail, as the corrections are intended to clarify things to the users of the text without confusing them by introducing too much new information. I'm sure some of these updates are out of date in relation to his current build technique, which understandably has evolved in the almost 30 years since his book was first published.

_________________
Old growth, shmold growth!

Charles, that's good to see and looks much better than the original. Maybe you could persuade Bill to post a pic on his site, because I can't see a reference to this improvement there.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

According to his blog DolceCano, which contains his most up to date ruminations, he is planning a full video online version of GT&T, which will hopefully be making it's appearance mid 2013, on a modest subscrption basis. I'm sure all the updates and changes will be present there.

Anyone interested can read his blog here: http://dolcecano.blogspot.com/

Hopefully I'm not doing him too big a disservice by revealing some preview information, but I feel that some of the concerns raised in these forums pages need to be addressed, some regarding misconceptions regarding how his technique is supposed to look, and some regarding actual realistic concerns for the beginner reading his book and attempting to implement the process for the first time, such as the shoe concern you raised.

I still consider the time I spent with him one on one in his workshop to be worth every penny, as I feel I've benefitted greatly, especially in comparing his build method to other modern build methodologies and geometrical models. His mentality is, I feel, very close to mine, in terms of mechanical thinking and visual/spatial mental imaging is concerned.

I know from experience that a guitar made from his techniques will play just as good (or better) as any other build methodology, and be just as consistent and repeatable moving forward, as any other method, depending of course in part on the skill and mental acuity of the builder, but that's true of ANY method.

Different strokes for different folks!

_________________
Old growth, shmold growth!

Interesting. I've built quite a few guitars on Cumpiano/Natelson work boards with cork shims. I've never had a neck angle issue but maybe that's because I was always paranoid about torquing down on the shoe nut too hard. But that new shoe design looks pretty tight and I would say that the hard board shim would probably be better as well. You can get a lot of clamping force while clamping the sides down and if the cork crushes then your sides will be clamped at an angle, i.e. not 90deg angle with the top (negating top radius).

I build dead flat now so I no longer have these otherworldly problems

I'm quite sure a top quality build can be executed using strictly the build methods in the book, one just has to be a little more conscious of what's going on each step. That actually appeals to some people more.

Interestingly enough I've never actually tried to build an acoustic from the book alone. I knew a tutorial would speed up the learning curve, and I'm glad I went. It was fun, and I learned from a celebrity of sorts.

I don't think you could ask for a more patient teacher.

The new shoe design also supports the neck block while sanding the ribs in preparatoin for gluing the back.

_________________
Old growth, shmold growth!

I read about this method some time back on this forum, but I can't find anything using a search. Are there any books available for this type of guitar building?

Alex

_________________
"Indecision is the key to flexibility" .... Bumper sticker

Well, Sloanes original book was built as a true flat top I think but as great a book that was it's not really good technique by today's standards. I don't know of any books but it's really just about the same. The only problem with it again is getting the right neck angle but then that seems to be a problem for people building with radii too so... Who knows? I also build an A-Brace around the sound hole to the head block to strengthen that up a bit and I don't scallop the main braces but rather feather them to the edge.

The issue with building flat and the neck angle is that you can potentially get too much fall away from the body joint to the end of the fretboard. This can be fixed by canting the top from the waist forward. So I guess in that sense one could argue it's not dead flat but for all practical purposes it is since this angle is only so slight. I don't actually measure this accurately because it's just very difficult to do so. So far I have built 4 guitars flat and each one has a very minimal fall away in combination with a canted top. If I run a straight edge from the bridge along the top all the way to the nut the neck falls back about 1/32nds inch. I used to built with 1/4in fretboards and now I bump that up to 5/32nds so that gives me further boost at the bridge.

Actually, the book this is derived from is called "Guitarmaking: Tradition and Technology, by William R. Cumpiano and Jon Natelson".
It's available from Amazon.com here: http://www.amazon.com/Guitarmaking-Trad ... 0811806405


I would recommend also visiting William Cumpiano's web site, as the book is apparently out of date www.cumpiano.com

He has a list of updates and changes made to the book, including details on his new hardware based neck joint that a lot of people are apparently using too.

_________________
Old growth, shmold growth!

The Cumpiano method does not use a flat top. It is radiused

That's what I thought he was asking about.

_________________
Old growth, shmold growth!