Official Luthiers Forum!

BTW, if anyone doesn't know how to make a "long compass," here is a very helpful page from Jon Sevy. This is a nifty and simple jig for drawing large radii.

http://gicl.cs.drexel.edu/people/sevy/luthierie/compass/Long _compass.html

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Howard Klepper
http://www.klepperguitars.com

When all else fails, clean the shop.

[QUOTE=Parser] [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.[/QUOTE]

The bindings are parallel to the sides and intersect the spherical radius of the top. If you look at the cross section of the cut your router makes into the top you would see it is at an angle not perpendicular to the top but is parallel to the sides

By your definition if you took a guitar shaped cookie cuter and cut straight down trough a sphere the piece inside the cookie cutter would no longer be a segment of the sphere. You would be wrong. The irregular non-circular shape of the cookie cutter mearly cut an irregular shape spherical segment. with out trying to be a turd here. and portion of a sphere is and always will be a spherical segment. The op formed by radial bracing glued up in a domed dish forms a spherical segment. period. Geometric fact.

My top/back is indeed spherical, and the bindings, although starting out straight/flat, need to be coerced into the cookie cutter shape that Michael speaks of as you go around taping them into place - thats why the waist area is such a b.... to get tight when building this way.

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

You're right. I was thinking flat tops and talking out of the wrong part of my anatomy.

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http://www.PeakeGuitars.com

I guess what some of the flat top builders do is kind of in the middle somewhere. I believe Collings only radiuses the fingerboard brace and the rest of the bracing is left flat. One of my buddies was talking to Bill C and I believe this is what he said.

Maybe this kicks up the neck angle adequately and doesn't cause the weird binding problems at the waist as Tony K mentioned..

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With what you said in you last two posts, it accrued to me that you may not be sanding the spherical dome shape into your linings and blocks prior to attaching the top, but rather leaving them flat. If this is the case, when you clamp and glue up, you are forcing the perimeter of the top in to a plane that it is trying to resist and adding un-needed stress to the top/lining-block glue joint.

If this is the case then you would have a compound contour in your top. A flat followed by a small fileting radius leading into the the spherical shape. However the majority of the top will assume a spherical shape. Only the perimeter and the area affected by this deformation would not be spherical.

I know of some that do this, but I could not recommend it. Like I said, doing so you are just adding un-due stress and a possible path of failure for the glue joint.

You have found me out! I do not sand the spherical dome shape into the linings. I'm not against it, I have just never done it. I could cut the form pretty easily on the CNC...just haven't convinced myself it is necessary.

I've been kicking around the idea of making an arched form specifically for the shape of my acoustic. I think it would be pretty slick if I could set it up so that the binding remained straight, but the top was still arched. Then I could setup bracing specifically for that and cut it on the CNC. I've designed and carved backs on my CNC..so it is no problem at all to design an arched form.

I think I like what Collings does (see my previous post). I'm going to try to setup the current guitar I'm making based on this idea.

Sorry for the confusion, and thanks again for your perspective.

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http://www.PeakeGuitars.com

I suspect most people's unexpected issue when binding is not understanding that the elevation profile of the domed top is different all the way around the guitar.

If you could take 3 MRIs of the guitar, one at apex of the Lower bout, Upper bout and Waist valley this change of elevation is evident



Dealing with this some times make one over think. Truth is the binding gives this much with no issue.

However if you router is indexing the top and not the sides then the angle of the channel can change dramatically as you go around the guitar. That is why you want the binding channel indexing off the sides for angle of cut and only off the nearest possible point to the channel on the top for depth only.

Wow! Can open - worms EVERYWHERE!

Ok, just to clarify a little here. This isn't something which I've given an enourmous amount of thought to. I'm in that transitional period of having the desire to get building but also having to actually have a place and tools to do so (which is coming together one weekend at a time).

So, having read through 75 pages of archive on here so far (and either missing the original thread about this or perhaps it's still waiting as I work my way through), I'd seen lots of references to the top/back taking the form of th edge of a sphere, and the immediate thought which sprung to mind was about how the surfaces of the braces should, in an ideal world, match this exactly and yada yada yada - you people clearly know the rest =)

It's my math and IT background creeping in where even a single digit out of the billion or so making up a file can make the difference between it working or not. I'm getting the distinct impression I will feel different about the issue when I finally get to get started properly.

Hope that answers your concerns @Howard - I fully intend to get started the moment I have enough tools and workspace not to have to stop and wait for things after every little stage. I am trying not to overthink it, it's just my math/it background which jumped, unchecked, in with a perspective, and my knowledge of working with wood has not yet caught up enough to draw the conclusion you did (which obviously also makes a lot of sense, structurally, now you come to mention it).

So, erm, sorry for stirring up the hornet's nest and everything. Still, in a way I'm glad I did; as those diagrams are just brilliant, and will serve other people with the same idea as me well by way of explanation.

While I'm on controversial subjects where's the edit but-- eek *ducks and runs*

Don't be sorry this is exactly the kind of discussions that moves the Craft forward.

Ok, ok, ok...let me get this straight. You people sand some sort of radius onto the braces????

Dang this is some complex stuff I've gotten into.

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Actually I hav a jug that I use to form my brace radius on my router table and only sand to fix any irregulartities .

But the answer is yes! The vast majority of guitars have a dome inmaprted to tops and back via radiused bracing to give them better strength and resistance to sting pull.

Yes I have a Jug

Jug with a complex radius?  

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

If it was my jug it would just have a complex.

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For any of you that leave your top un-profiled to match the proper intersection of the dome. you are having to pull the top out of shape a bit more than 1/16" at the waist to make contact with your linings. That is of course if you keep the rim edge and linings all in one flat plane. If you pre conture the top of the rim to account for the arch then it is not as big of an issue. but if you dont sand it in then how do you map it out?

If all of the top rim and lining is in on single horizontal plane you are spring loading the braces a good bit and building in tension that is constantly trying to relive it self.

[QUOTE=MichaelP] With what you said in you last two posts, it accrued to me that you may not be sanding the spherical dome shape into your linings and blocks prior to attaching the top, but rather leaving them flat. If this is the case, when you clamp and glue up, you are forcing the perimeter of the top in to a plane that it is trying to resist and adding un-needed stress to the top/lining-block glue joint.

If this is the case then you would have a compound contour in your top. A flat followed by a small fileting radius leading into the the spherical shape. However the majority of the top will assume a spherical shape. Only the perimeter and the area affected by this deformation would not be spherical.

I know of some that do this, but I could not recommend it. Like I said, doing so you are just adding un-due stress and a possible path of failure for the glue joint.[/QUOTE]

I don't know Michael, some pretty big names in the industry are doing it that way. I suspect they would not continue to do so if their guitars kept coming back for warranty repairs because of it. And as this discussion shows, it is another aspect of the construction that can successfully be dealt with in a number of ways; more interesting to me is how does it affect sound? I have built both domed tops/contoured sides and domed tops/flat sides, but not enough instruments to be sure of what the tonal effects of the different construction principles are. Here is an excerpt from what Huss and Dalton say about it (from their web site, under "Technical info":

Huss & Dalton guitars feature two distinct construction styles. On all of our standard series models, we employ the use of a 25' radius built into the guitar top, which is achieved by milling an arch into the braces and preparing the sides with the same radius to accept the soundboard. A positive by-product of the radiused soundboard design, besides it’s load-bearing properties, is a boost in the mid-range. This helps our guitars to have a more balanced tonal quality than traditional designs. Our Traditional series features the same 25' radius prepared into the braces, but the sides are left flat for a more traditional build style. This build style tends to have a more traditional tonal character, emphasizing a bit more bass.

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Rian Gitar og Mandolin

I sand my braces in my radius dish in the approximate area that they will be glued into the top for no other reason than do try and wear the sandpaper evenly.

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Allen R. McFarlen
Barron River Guitars & Ukuleles
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Cairns, Australia

Guys I never said it was a guaranteed fail. I only said the geometry shows that by doing a straight top rim with a dome top that the builder imparts built in stress. on the glue joint.

This discussion has inspired me to design a form specifically for the shape I'm building right now. I'm going to shoot for a rounded top with straight bindings. I'll design the surface in CAD...cut the bracing specifically for this surface...and carve out an MDF mold specifically for this "arched" surface. Seems like it is feasible..I'll just have to make it so that the curvature of the top changes in unison with the curvature of the profile so that no weird stresses are caused by gluing the top to the sides. I'll post the results after I glue it up. It will probably be a week or so...I just glued together a top last night.

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   I dont realy know BUT it seems to me , if it is truly  a radius dish , all areas of the dish will have the same radius, no matter where you sand .   jODY

[quote=Howard Klepper]BTW, if anyone doesn't know how to make a "long compass," here is a very helpful page from Jon Sevy. This is a nifty and simple jig for drawing large radii. [/quote]

Long Compass
Pretty Dang Cool! I assume one just sets the centerline of the pencil to the chord height from the centerline of the pins. You could do any radius you like! Never seen it before! Thanks Howard!

Look like there's a jig to be made!

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Billy Dean Thomas
Covina, CA

"Multi famam, conscientiam, pauci verentur."
(Many fear their reputation, few their conscience)

The dome is one constant radius or sphereical segment, but because the shape of the body is not one common width or the same distance from the center of the dome everywhere, the intersection of the body with the dome has an ever changing elevation. The closer to the center of the dome the taller the side profile. The further from the center of the dome the shorter the side profile.