Official Luthiers Forum!

Hi again everyone,

This might sound like a stupid question, in fact it almost certainly will sound like a stupid question, it may even be a stupid question, but I’m not yet qualified to say…

As I’m preparing my set up for my first build and thinking through the processes I’m going to be learning, something odd struck me about radiusing braces. Unfortunately as I don’t have you all in the same room as me, a pencil and a bit of paper I’m going to have to attempt to use words to describe this, so I apologise in advance if what I say makes literally no sense.

Ok here we go: So, the braces are radiused on the part glued to the top/back with the idea being to introduce a subtle dome to the shape of the wood. I can’t remember the exact figures offhand, but IIRC it’s something like 25’ and 30’.

So here’s where I close my eyes and picture the surface of a sphere with a radius of 25’ with the top of the guitar as a section of the surface of this sphere.

Now, if the braces were not scalloped and/or paraboloided (not a real word, but you get the idea) then the flat side of those braces should, unless I’m very much mistaken, fall on the surface of a plane which intersects the edge of the sphere. The maximum distance in from the surface of the sphere should be the depth of the most central brace used.

With me so far? (not sure I would be if I had to read this without illustrations!)

Ok so in order to actually have a radiused top, the braces should, unless I’m very much mistaken, be sanded in the radius dish at the relative position they will occupy on the final sound board, if you were to hold a spirit level on a top across the braces. That would mean that the central brace would be ever so slightly thicker than the more perhipery braces. That would allow the brace wood to maintain it’s unbent strongest configuration.

Or to put it another way, stick all the flat tops of the unfinished but radiused braces onto a piece of perfectly flat material in their exact final layout and put an acrylic dish with the correct radius over the top of them and it should contact ALL of the gluing surfaces.

Right, so now comes the actual point of this whole diatribe…

I have seen a fair few pictures of people radiussing braces and they all seem to sand them in a dish intersecting the centre point of the dish.

Now if none of your braces on the top intersected each other in the final layout, this wouldn’t be an issue. The back of each brace would simply describe a different ’plane’ intersecting the massive sphere. However, as braces *do* intersect, surely there will be additional stresses placed upon the surface of the top in certain areas due to the nature of a flat join?

Now, clearly I’ve no experience yet of doing this, which is why I’m asking the encyclopaedic experts here. It strikes me that these areas of increased stress would have a compound effect on the eventual tone of the top? I realise the differences can be measured in the thousandths of an inch, but given the fact that sound has wavelengths within this range, I can only see it having an effect on the resonance, if not vibration of the braced top.

Am I going mad?

Are the pictures I’ve seen not indicative of how most people do radiussing?

Has anyone used ‘true’ radiussing and found the difference to be not worth the hassle?

Please note if this is a dumb question and/ or it’s already in the archive somewhere beyond the 70 pages I’ve read so far, feel free to just tell me flat. I make no claims to any degree of experience here so it’s just something which struck me as I was thinking the process through…

With the greatest of respect for the more knowledgeable,

--Dave

In a word dave, it just doesnt matter - we went thru this a while back, and while you explanation of hte geometry is in fact true, with the talerances and gaps we are talking about here, the numbers are so small as to make them indifferent. If you do the math, what is the height of the arc above a chord (again see previous post or wikipedia as to what an arc and a chord are) for a 1/4 inch wide brace if the bottom of said brace had a 28' radius on it - its almost nothing - the glue line is probably thicker. Nobody except Hesh worries about it IMO ... I certainly dont - infact I dont sand my braces at all - they are done in a radius jig with a plane to get the best possible prepped gluing surface. I would think sanding them in the bowl might actually make it worse (ie rounding the edge slightly, giving less glue surface contact).

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

too funny Hesh .. we were composing at the same time

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

yeah, then take some pictures ... cleanliness is next to ....

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Uh .. you mean you went to bed last night with DIRTY shop .. shame on you .. must have been a restless night

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

Thanks for the quick responses guys, I did wonder whether this had been gone into before as it seemed just a little too obvious not to have been looked at by an industry who work to the tolerances you do. (can hardly wait until I can use the term ‘we’ instead of ‘you’).

Ok, I can see both points of view here. I am still erring slightly towards hesh’s point of view SOLELY from a player’s perspective (one subject I can at least pretend to have some authority on…)

You see, I am thinking here about what tony is saying about the difference in distance being less than the thickness of the glue, and I have no doubt he’s right. However, there’s two points here of note as I see it.

1) I’ve seen ‘reviews’ of different kinds of glues and their effectiveness for luthierie somewhere on the web, and it’s interesting to note that tone can in fact be effected by the glue used. Obvious really, I suppose, but the glues which make a strong bond with the final glues wood block less distinguishable from an unjoined solid piece of wood appear to be the ones which people said were most effective. So I would *guess* this means that density of the final glue is pretty similar to the woods being glued. This would suggest to me that (again speaking out of total newbie inexperience here) it would have less effect upon the vibration and resonance of the material than, say, a stress-induced compression of the brace material would.

2) Ok here’s my maths/physics background coming back to haunt me. So let’s take a concert pitch of 440Hz for the low A. That means that, with the speed of sound being ~340m/s we have one fundamental compression wave every 3/4 of a centimetre or so right? Ok, so we know wavelength halves itself every octave up (or rather frequency doubles, but it amounts to the same thing). So the middle A note is one compression wave every third of a centimetre, the next A is a little over a millimetre. These are just fundamental notes. So now think of the harmonic overtones which give great guitars their ‘zing’. They’re going to be even higher, particularly as you move up the neck.

So, although I don’t know what effect increased density (compressed wood) on a scale that small would have upon the overtone characteristics of a guitar, I can’t help but have this nagging feeling it would have some sort of effect.

On the other hand - planing the wood in a radiussing jig sounds like the sort of thing I’m less likely to mess up on my first build, so meh, I guess the more complex dish sanding is something for me to experiment with when I get a few builds under my belt… ;)

Thanks again for sharing the knowledge 

--Dave B

for hest a dirty shop means a .01 micron dust particle on the under side of his workbench top.

yes the subject has been waffled on about all too extensively.

i route my radiused surface onto my braces, and only use the dish to remove any routing marks.

i fall in the category of those who know that it makes no difference where you sand on the dish. it is how you hold the brace in relation to the dish surface whilst sanding that is important.

What the others said, it makes no difference where you sand the braces in the dish. Both the dish and the top or back radius your bracing are a section of a sphere, so it makes more sense to me, structurally anyway, for the braces to converge on the centre of that sphere along a radius of the sphere rather than to carry on parallel to infinity.

But really it doesn't matter.

Colin

_________________
I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

[QUOTE=Hesh1956] BTW Frank Finichio shows this and teaches this in his DVD series.[/QUOTE]

Actually... (and yes Hesh, I know I am going to pour gas on this fire by saying this.    )

Frank has mahogany block made up that has a straight 16' (and 28') radius cut into it. So it is not a dome, but a cylinder.

I am not sure what he does in the DVD, but in the tapes and the way he taught me was with this tool.

_________________
Brock Poling
Columbus, Ohio
http://www.polingguitars.com

It only boils down to "If you want your braces vertical plane to be tangent and perpendicular to the dome or not.

if you take a given radius brace and attach it at any point and in any orientation on a matching dome then the cross section of that brace will be tangent and perpendicular to the DOME.

Now if you want all your braces cross section appear to be coplanar with each other then you would have to sand them to shape in their relative position on the dome while hold and maintaining them in a theoretical vertical plane. this will impart the radius of the brace at a non-perpendicular angle in relationship to the cross section of the brace. but this can make all the braces coplanar in a theoretical vertical plane.

So not worth the time it is unreal. You will drive your self crazy with this

This is an over exaduration but if you sand in place maintaining a vertical plane then you end up with a cross section like this

Here is a comparison between a UTV brace sanded tangent to the dome. and one sanded in its relative glue up position maintaining the vertical plane of the brace to theoretical vertical.



Hardly seems worth the trouble to me

[QUOTE=MichaelP] It only boils down to "If you want your braces vertical plane to be tangent and perpendicular to the dome or not.

if you take a given radius brace and attach it at any point and in any orientation on a matching dome then the cross section of that brace will be tangent and perpendicular to the DOME.

Now if you want all your braces cross section appear to be coplanar with each other then you would have to sand them to shape in their relative position on the dome while hold and maintaining them in a theoretical vertical plane. this will impart the radius of the brace at a non-perpendicular angle in relationship to the cross section of the brace. but this can make all the braces coplanar in a theoretical vertical plane.
[/QUOTE]

Excellent explanation Michael, and diagram. When this subject comes up next time, the poster should be directed to this thread.

"Imagine yourself trapped within a 28 foot sphere whose interior is cover with sandpaper. In your hand is a parabolic brace. Now, where would you like the top of the brace to point to?"

Dennis

_________________
Dennis Leahy
Duluth, MN, USA
7th Sense Multimedia

Well...even though guitar tops and backs may be rounded, they are not spherical. They can't be due to the fact that guitars are not round.(?)

_________________
http://www.PeakeGuitars.com

The top is most certainly is an irregular shaped segment of a sphere The fact that the shape (top view profile) of the guitar is not round does not change the fact that the top is a segment of a sphere.

I started laughing as soon as I read the subject.

_________________
Say what you do, Do what you say.

The music of the spheres or a lot of "round things"?

_________________
Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

Dave Bamber:

Congratulations on your ability to visualize in three dimensions. It will serve you well as a guitar maker. But why would you think that in order to have an "actually" radiused top, the back of each (unshaped) brace ought to be coplanar? For what the difference is worth (not much) I would think it is more important for the back of each brace to be in a plane that is perpendicular to a radius of the sphere--in other words, oriented so that if the brace is perfectly quartersawn, the annular rings are perfectly perpendicular to the top of the guitar along its length.

At any rate, you are clearly way past the overthinking stage, and should begin building your guitar (please don't build a build) forthwith.

_________________
Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

[QUOTE=MichaelP] The top is most certainly is an irregular shaped segment of a sphere The fact that the shape (top view profile) of the guitar is not round does not change the fact that the top is a segment of a sphere.[/QUOTE]

Let me clarify a little...I was really thinking that even though the profile of each of the braces may be circular, the surface of the top is not spherical. If it was, then the binding would not be straight.   

I've been taught (perhaps incorrectly?) that radiusing the braces was done simply to "pop" the top out as opposed to building it flat and causing it to sink. I've also been taught that the functional purpose is to orient the fingerboard at about a 2 degree angle.

_________________
http://www.PeakeGuitars.com

I am tempted to just eye ball a radius and make a plastic pattern to follow just to be heretical.

Actually I want to experiment with having different radius for the bass and treble sides, T 25'-28', B 35-45' just for fun.

I think the important issue is that that the braces be well fit to the top (no dips and bumps ).

With that said I also see the importance of the precision and repeatability given by the dishes.

I like to think of glues in terms of if they seem to have conductive or insulative propeties.

This comes from a very amuture dulcimer makers point of view. I listen very carefully to the tried and true, then do my own thing.

My point is that each piece of wood is different. That is a larger difference than where in the dish the brace is sanded in my mind.

_________________
"It's a Tone Faerie thing"
"Da goal is to sharpen ur wit as well as ye Sgian Dubh"
"Sippin Loch Dhu @Black lake" ,Kirby O...

Hesh,

Your tutorials are always top notch ...how about a shop cleaning tutorial . Lord knows I could use a few tips.