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K folks. Luckily, I was smart enough to "dry run" my back brace glueup. Check out the pics below and you will see my problem. Brace 2 is "out of whack". I checked the radii of the braces against each other and they all look right. The issue comes when I put the clamps on brace #3. That's the one that's making the gap in #2. If I remove brace #3 bars then brace #2 looks fine. So, what did I do wrong?

Brace gap.


Brace gap again.

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nay

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Nathan - I guess the first thing that I'd check is that the radius on brace 2 is correct. I'd be wondering why one or more of the braceds didn't sit firmly in the dish.

If you remove brace 3, the back springs up and contacts brace 2 and you don't see the gap?

I'm rambling. I'd check to see that all the braces sit comfortably flush to the radius dish, without pressure.

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Jim Kirby
kirby@udel.edu

Looks like it's hanging up on the back graft. If not that, then it may need reshaping.

If your dish is perfect, and your braces are profiled, they should work. You'll figure it out. Matter of fact, do you have that thing glued yet? grin.

My old mechanic teacher, always shouted, "Get a bigger hammer!"   

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[Quote]If you remove brace 3, the back springs up and contacts brace 2 and you don't see the gap? [/Quote]
Correct!

Bruce, it's not hanging on the back graft. It does look like that in the pic, but it's clear of the back graft.

Hesh, you're right. I screwed up the "dish". Basically, I don't have a dish, but I do have a workboard ala Cumpiano. The soundboard X braces are glued using the workboard and a flexible "slat". So, I thought I'd use the same method for the back. I forgot the slat. I'm thinking of not using the workboard to glue up the braces. I'm thinking of creating 4 cauls with the proper radius and glueing up that way. To me, that sounds like a better solution. I've been trying to avoid a radius dish, but it looks like I'm going to have to break down and buy a couple. I'm just not wild about making my own.

Thanks to Robbie's DVD I was smart enough to do a dry run before this became extremely disasterous.

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Nathan,

This might get you through this step.

Kenneth Michael Guitars

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now known around here as Pat Foster
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http://www.patfosterguitars.com

   Your brace isn't matching as you know. It really doesn't matter on where you shape it but that you shape it correctly.
    If you made your own radius disk , check that first and also check that brace to the others. You know your problem now you need to find your solution.
     If your disk and brace are true then it is the back holding off the fit.
Good luck and have fun
john hall
blues creek guitars

Thanks for the replies. I'll let post the results once I decide on what to do.

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Nathan,

As you haven't got the back in a radiused dish it could be that you have the back in a bigger arc than your brace profile and brace #3 is being pushed down by the go bars - i.e the top of brace #3 is not a straight line and is slightly concave. This is more likely with the longer braces that can flex more. I would be inclined to make a couple of card templates as per Pat's suggested link and glue the back braces on one at a time shortest to longest using the card templates under the back on each side of the brace. So that would be brace #2 first followed by #1, #3 and #4.

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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

hesh, i am puzzled by you comments on the relevance of sanding in a particular location on the dish.

the surface of the dish, provided it is made accurately, is spherical, that is to say, it has a uniform curvature in all direction anywhere on its surface.

would you also say that it is important to sand on one part or another of a surface plate to get a flat surface?

to say it is important to sand in one location or another to get a good fit is failing to recognize the nature of the beast. just as sanding anywhere on a surface plate will give you flat, sanding anywhere on a radiused dish will result in the same curvature. that is of course, provided in both cases that your sanding technique is sound.

Well maybe I see it a bit differently. When I look at the photos I see the brace sitting on top of the back graft. It appears that the brace need to be inlet to go over the back graft. If the brace is already inlet enough I just can't see it.

Philip

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Michael,

You are correct that the dish has the same radius everywhere, relative to it's center. And it's true for a brace, if the brace is sitting perpendicular to the dish at it's lowest point. If you take the brace, hold it's sides vertical, and move it away from the bottom position, the brace has to take on a compound curvature to fit firmly against the dish. It's forcing the brace to stay vertical, where it is no longer pointing at the center of the sphere, that causes the compound curvature. I also always sand with the braces vertical and in the dish at the same point they will be when glued up.

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Gene

Politicians and diapers must be changed often, and for the same reason- Mark Twain

the brace will have a compound curvature wherever on the dish you sand it. ceterus paribus, the gluing surface will have the same curvature wherever on the dish you sand it.

if it makes you feel better to do it in one place or another thats fine too, since it matters naught.

Phillip, the image does look as if brace #2 is sitting on the back graft. I promise it's not. There is room for the brace to fit, actually, it's a little loose.

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Dang no edit.

Dave, good point. I may be putting too much pressure on brace #3 and actually "flexing" the brace more. I'll have to check on that when I get home.

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[QUOTE=gozierdt] Michael,

You are correct that the dish has the same radius everywhere, relative to it's center. And it's true for a brace, if the brace is sitting perpendicular to the dish at it's lowest point. If you take the brace, hold it's sides vertical, and move it away from the bottom position, the brace has to take on a compound curvature to fit firmly against the dish. It's forcing the brace to stay vertical, where it is no longer pointing at the center of the sphere, that causes the compound curvature. I also always sand with the braces vertical and in the dish at the same point they will be when glued up.[/QUOTE]

I just love these "theoretical" debates - they can go on for ever

Gene,

Michael is technically correct methinks. Any two points that are on the surface of the sphere that are connected in a straight line along the surface of the sphere form an arc that has the radius of the sphere - no compound curvatures. There is no "high point" as such on a sphere - think of a ball. If a point is not a 2high point" than rotate the ball slightly and voila it is a "high point".

Actually sanding the braces across the middle point of the sanding dish probably helps you keep the brace vertical in relation to the tangent of the spherical surface better - although once the braces are carved this can easily be corrected for. Even though the brace top may "tilt" in relation to the tangent of the surface, the bottom profile will still be a spherical section in each and every direction and not a compound curve.

That said - sand wherever and in whatever way it works for you.

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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

I've seen this discussion pop up the MIMF and other places from time to time. It has to do with how you orient the braces when you sand; perpendicular to the surface of the sphere or perpendicular to a section of the sphere. In other words, like the difference between the angle of the "short" and "long" sides of a piece of cake that is cut the normal way and one where the cake is cut by a series of parallel lines. A piece cut from a normal way will have identical pieces (well, theoretically), the pieces cut from the other cake will all have varying angles.

I like to have the same curvature on all my braces, and I cut them with a template on the router table, but that's because I don't like to glue sanded surfaces if I can avoid it.

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Rian Gitar og Mandolin

Guys: A simple curve is the arc of a circle (with a single radius). This is what our dishes have.

A compound curve has more than one simple curve.

With respect to braces see this diagram:



The dish is shown in a sectional view with a small radius to illustrate the following:

Brace 1 is a brace at the center of the dish which may also be considered the guitar back in cross section along the long axis of the instrument. It has a simple radius corresponding to the simple radius of the dish. The brace is normal (right angles) to the dish surface and the flat reference below the dish. When glued to a back it will rise at a right angle to the back. This brace will tilt inwardly relative to a flat reference if used at the neck or tail end because the guitar back has an arc like that of the dish (see brace 2).

The brace at "2" is being formed or positioned nearer to the edge of the dish and at right angles to the dish surface but at an angle compared to the flat work surface under the dish. This brace will have a simple radius exactly like brace 1. It will also lie at right angles to the back and have a slight inward tilt if used at the neck or tail relative to the reference.

Brace 3 is being formed at the same location as brace 2 (off center) but at right angles to the reference surface NOT the dish surface. It will have a compound radius, not a simple one (1 curve along the width of the brace and 1 curve along the length). Thus, this brace will not tilt compared to the reference surface if used in the end positions of the guitar back because it has a compound radius built in corresponding to the arc of the back.

So, if you want your brace sides to be parallel to each other, in the height (Y) axis, you should create the radius by holding them at right angles to a flat reference, and at the position in the dish corresponding to their position on the back.

If you want each of the braces to rise from the back at right angles to the back itself, but not parallel to each other, you make your braces like 1 or 2.

Now, having said that, maybe it don't matter at all if they are parallel or not, I have no idea.   

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Now I am sorry I posted this as it looks like everyone gets it. Oh dear.

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"Building guitars looks hard, but it's actually much harder than it looks." Tom Buck

Nathan, it seems clear to me that your back is being forced into a tighter radius at the waist by your caul. Since the distance between the inner edges of your caul is least at the waist, the back has to travel a tighter arc to accomodate. If your back were rectangular and the caul rectangular the back would bend the same radius along its entire length. Because the back and caul are an irregular shape, the back must assume different amounts of bend.

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"Building guitars looks hard, but it's actually much harder than it looks." Tom Buck

So imagine a triangular back. And you are pushing the back down 1 inch in the center. Where the triangle is wide the back deforms in gentle wide arc. Where the triangle narrows to a point, the back must bend more and more radically to bend the inch. That is what is happening at your waist.

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"Building guitars looks hard, but it's actually much harder than it looks." Tom Buck

Steve,

That's a great diagram that shows what I was trying to say - badly - in words. One point that a don't agree with though is that brace 3 does not have a compound radius on it's bottom. It has the same spherical surface as braces 1 and 2. All that is different between brace 2 and brace 3 is the angle in relation to the plane of the top of the brace is tilted rather than perpendicular. If you took brace 3 and put it where brace 1 is it will still fit the spherical surface but the top surface of the brace will be at a tilt. Another way to think of it is to take brace 1 and angle it to rhe right slightly (so that the top surface of the brace is no longer perpendicular to your flat surface) and sand the profile on the bottom. The bottom shape is the same as brace 1 and if you move this brace up to where brace 3 is in your diagram it becomes identical with brace 3.

A further way to think of it is to tilt your dish so that brace 3 moves to where brace 1 is in relation to the flat surface. This becomes the same as tilting brace 1 and sanding as I described above.

Compound radii surfaces come from shapes like cones not spheres - think of a compound radii fretboard.

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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 - you are completely right - I stand corrected. Thanks for clearing that up!

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"Building guitars looks hard, but it's actually much harder than it looks." Tom Buck