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when i was at benning it was hot enough that i ddn't need any added outside moisture to look look like a vanilla lamington!

[QUOTE=Hesh1956] This is a topic that I have gotten in trouble before when discussing.....but that never stopped me any way....



There are other builders who do this as well and I learned it from someone but can't remember who. It might have been Frank Finichio since I have his very fine DVD set.

AND..... Bolix welcome to the OLF!!! [/QUOTE]

Hesh - That's a good idea and I think I'll try it next time.

Actually, I'm not sure what's on Frank's DVD, but in his class the build is done without dishes at all - braces are glued to plates with cam clamps, after pre-radiusing on a router jig and radiused sanding block.

Welcome Bolix!

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Jim Kirby
kirby@udel.edu

Structurally the only thing you will achieve by tilting your braces away from the centre of the sphere is to change your perfectly quarter sawn brace wood into not quite so perfectly quarter sawn brace wood.

That is, the most mechanically efficient way to get your braces as light and strong as possible is to place the brace with its stiffest grain direction perpendiular to the load it is supporting which in this case is the radius it is forcing the back into. If you tilt your braces you are moving away from this most efficient relationship which means you need to make the braces heavier to achieve your desired stiffness.
Dom

very apt analysis dom.

   These are a few figures that give an exagerated conceptual difference to sanding braces at different angles to the center line(green CL) of the sphere. This is to keep the brace CL(red CL) perpendicular to the top and not to the CL of the spherical form.

    The first figure shows, though the spherical radius is the same, the spherical segment(purple) generated on the brace is not, again to keep the CL of the brace pointing "straight up" in relationship to the finished guitar.



   This figure shows that one can, in fact, as Rod said sand in any place in the spherical sanding dish and generate the same form simply by angling(blue line) the brace CL{red} to the spherical CL(green).



   One can do the opposite by sanding offcenter to the perpendicular CL of the spherical radius and still obtain the proper spherical segment formed on the brace.



   By not sanding as Hesh does what one really gets is a toroidal segment(the outside portion of a donut shape) not conical. This causes the braces CL's to be lined up at angles to the horizontal plane of the guitar, the top facing up.



    Yes the spherical radius is the same, the spherical segment, again, is not! Considering the size of the radius in real terms does it make a difference.... depends on how much you want to put into the guitar. I can do the math in numbers but getting into spherical radius points gets hairy if you don't do it often and I'd have to pull out length and width measurements of the braces themselves.

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Billy Dean Thomas
Covina, CA

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

Billy - Very helpful drawings here, but one correction. As also pointed out by Rob in his also very helpful drawing, the radii of the braces done off of the low spot are indeed smaller. Think of a globe with latitude lines above and below the equator. The braces away from the low spot above are being oriented in the direction of constant latitude, and each circle representing a constant latitude gets smaller (i.e., lower radius) as you move away from the equator. So, these braces, which are oriented along latitude rather than great circle lines, do get a tighter radius as indicated in Rob's drawing.

I'll see if I can crank up matlab and do a table of results for a range of dish sizes. It would be good to have a table available.

Jim

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Jim Kirby
kirby@udel.edu

I was involved in this same discussion a fair while back . Like Hesh ,I radius the braces at their respective areas on the dish .
I was shot down in flames by the other guys so much ,I wish I hadn't mentioned it    

I have given the subject a great deal of thought ,and the only difference is that Hesh's ( and mine) have braces that stand upright ,rather than lean into the centre of the sphere . That is the only difference I can come up with . I wouldn't think that would make them appreciably stronger , but I feel better about it , having them this way .
I'll continue the same method too, but won't mention it again. ,,,,,unless I'm talking to Hesh of course

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CRAIG LAWRENCE of AUSTRALIA
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So there still are 2 considerations:

1) "Leaning" braces...where the centerline of the brace is perpendicular to a tangent line on the curve.

2) "Straight" braces...where the centerline of the brace is perpendicular to the horizon.

And I'll repeat what I asked about 40 posts ago...is there a structural advantage of either or is it just an aesthetic consideration? How would a structural engineer optimize the the support of a dome?

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JJ
Napa, CA
http://www.DonohueGuitars.com

In an 18 foot radius bowl, the curvature (height of the arc) on the bottom of a .4 inch wide brace is a measely .00037 of an inch - thats 3.7/10,000 (and thus even less for a 1/4 inch wide brace). IMO you are wasting your time sanding a curve onto the bottom a brace - the glue joint is thicker than this ... plus, if you plane the bottom of your braces, you get a better strength glue joint than one that has been sanded. Chances are you are rounding you brace bottoms more than required while trying to hold them straight and sand them in a bowl.

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

[QUOTE=KiwiCraig]
   I was shot down in flames by the other guys so much ,I wish I hadn't mentioned it    
[/QUOTE]
This has turned out to be quite interesting. I was the one who initially brought up the practice of keeping braces vertical to the earth plane. Unfortunately, I don't have the experience to back up the theory, so the worms remain out of the can.

It all began long, long ago in a faraway place known as Paw Paw, Michigan, in the shop of the powerful lutherie wizard, Abe Wechter. He was allowing me to observe some of his building process as he finished his last orders in his one-man shop before switching over to his small "factory" concept, and his new Pathmaker design. Abe mentioned the idea of keeping the braces vertical so matter-of-factly that I assumed it was something all the "pros" do. It was probably something he learned from his mentor. I didn't know enough about building at the time to ask "why ?", so I don't know if it offers a structural advantage, or if it is just one of those "tweaks," like mitered corners and rounded edges, that is considered a mark of good craftsmanship.

I suspect the practice still has supporters and detractors among the big boys and girls, too.

Just another ingredient to think about, when you sand braces in a dish, by hand, you will inevitably tilt them side to side, its just not possible to NOT tilt them as you sand forward and back free hand. What I have found is that this tends to round off the sharp edge of the gluing surface and it makes the join between the brace and the back not as clean, whether this has any effect on strength or not, I have no idea. I also think that the way Hesh does (stacking braces) may reduce some if not all of what I'm taking about.

For what its worth, I radius my braces on a jointer using Tracys slick jig, and go stright to glue.

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Lance@LuthiersForum.com

      The shape generated in the back is exactly the same the only difference is where the braces point. I knew it would probably be as small as a few thousands. Tony's numbers show it even smaller, I'll have to check them out just to confirm.

       But I don't think he's taking into consideration the error generated at the bias at the corner of the braces that can extend to 9"(225mm) plus the .400(10mm)! It's more complicated than computing in one axis.

     My head hurts! I need to go watch "JJ the Jet Plane", where's my aqbada and buba!!!

     Surprizingly nobody caught the toroid is a sphere when the looping radius is the same as the loop itself! Shame on you guys! I guess we all went brain dead when we saw the word donut! OOOOh!!! DONUTS!!!


[quote=Heshasaurus]Billy GREAT job my friend and giving you something helpful to draw out helps keep me not looking like a purple dinosaur.....    I hope..... [/quote]

    Hey! I thought you guys where extinct!!!! (things to remember.... get a telescope)



      

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Billy Dean Thomas
Covina, CA

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

You are right Billy - there would be an error as you move away from the central axis - but we are talking about thin pieces of wood here, not structural steel - it will flex/twist enough to conform to the required shape. Remember there are guys out there who used to brace with completely flat pieces of stock, and then clamp it onto the top or back in a concave form, flexing it into shape ... this is one up from that at least.

That same error is actually being averaged by those who sand at the outer regions of the bowl ala Hesh - moving the brace back and forth causes the error - because the shape in each part of the bowl for each brace not going thru the bowls centre is different. - take a look way up at those slices thru the sphere ...

In general, we are way over-complicating this ... the wood flexes as you clamp it down ...

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

More thought here Billy ....

If you plane the bottoms of your braces flat, and then glue them on at an angle (but perpendicular to the tangent of the bowl at the spot which the brace is glued, pointing towards the centre of the sphere, unlike Hesh's method), then there is no bias error - each brace is glued in its own plane, and only the curvature of the back surface under the brace width comes into play.

And for an 18 foot bowl, if the brace is glued 6 inches out from the bowl centre, then the lean-in is a meager 1.6 degrees. For those who are concerned about keeping the brace absolutely quartersawn, I would say that the loss of strength due to 1.6 degrees off quarter is negligible.

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

When I radius the braces on the respective dish position , I have the top of the brace double taped to a firm straight piece of wood (stiffenner), which won't let the brace bend . I also have a straight wooden fence straddled across the top of the dish to keep the brace and it's stiffener at it's upright angle , so it won't tilt while being radiused.

Another big consideration ( for me anyway) , is when I install the Lowden type A Frame braces . I use a rather heavy A frame , so the soundhole braces are notched , and the upper transverse is also notched to receive them . The A frame set up is also let into the headblock. With all this going on , The braces have to be at a predictable angle for all notches and pockets to line up properly for a good fit.

Now ,,,Brace yourself.....





I'll bet it's just not you and me , Hesh ,Bro, Mate

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CRAIG LAWRENCE of AUSTRALIA
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I was just trying to show how I keep the braces upright on the dish ,and the reason why I do it.

That.........and to show off my latest ONCE AGAIN !

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CRAIG LAWRENCE of AUSTRALIA
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I'm staying out of this discussion except to tell Craig that he does some good lookin work !

 

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Reguards,

Ken H

[quote=TonyKarol]it will flex/twist enough to conform to the required shape. [/quote]

   Yup! And another thing, the braces are being glued to a flat piece, the back of the guitar, which is going to bend the brace somewhat back down!!! A la brace cutting jigs that reverse the brace and cut, to give a radius in the relaxed position. The forgiving nature of wood!

     I like thinking these things through because of all the years I spent as a mold maker, if you missed a spacial point you could mess up bad, and sometimes other things didn't add up to anything.

      I did it so much I actually started to like it. Like hitting yourself in the mouth with a hammer- it feels so good when you stop!

    It's good talking to somebody who understands these things, cause it brings out points to look for in the future.

_________________
Billy Dean Thomas
Covina, CA

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

[quote=jtkirby]Billy - Very helpful drawings here, but one correction. As also pointed out by Rob in his also very helpful drawing, the radii of the braces done off of the low spot are indeed smaller. Think of a globe with latitude lines above and below the equator.[/quote]

    Jim- Sorry about overlooking your post... Yes, but angling the braces themselves makes this more consistant to longitudnal lines that are the same around the sphere/globe. I understand what your saying but considering spherical segments instead of lines makes things much more complicated.

    The radius, is in fact, exponentially smaller in plane segment perpendicular away from sphere centerline(Hesh mode), as you pointed out. This is what's necessary to form the sphere shape on the back. Gluing as Tony mentions changes to a longitudinal reference, bypassing this form.

      Yours, is good comparative example to bring up, latitude, compared to longitude, and they both configure the sphere.

Looks like the poor kitty gets skinned again!!

_________________
Billy Dean Thomas
Covina, CA

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

Very Interesting thread guys.

Craig, I am a little disappointed to see that you have begun to introduced shortcuts into your work and have chosen to utilise only a singular species to cap your kerfed linings. Given your usual standards, I fully expected that you would have taken just a little bit more time to go the extra yard by layering up a 5 piece laminate decorated with a relief carved floral motif for this chore.

Very tardy my friend, very tardy indeed.


Just yank'in ya chain

Very impressive work as always mate, well done

Cheers

Kim

Thanks Kim , Hesh and Hodges Guitars. I haven't meant to hijack the thread . I just wanted to make the point that this type of brace work needs to have all braces at the same upright angle . To have them all angled towards the middle of a sphere would mean compound angles on the notches and headblock pockets ,,,,( not good )....

I don't think maths is needed here . As I said above :

Sanded,like mine ,Hesh's and I think Tracy's = braces of similar upright angles.


Sanded in their unrespected dish areas = braces ,all tilted towards the middle of the sphere.

Not a,lot of difference,,,,,, except when your installing bracework like mine above.

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CRAIG LAWRENCE of AUSTRALIA
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Bolix, I had exactly the same problem with my last classical while fitting back braces withe back sitting in the 15' radius dish. It turned out the back was a fraction thick along the centre seam. Took me a while to work it out. I ended up sanding the central area of the braces to fit the bulge. Next time Ill be more careful checking back thickness before bracing up.

It may not be the cause of your problem but it might be worth running the caliper over your back and seeing if the back is a bit "lumpy' along the centre seam.

[QUOTE=KiwiCraig]
Thanks Kim , Hesh and Hodges Guitars. I haven't meant to hijack the thread . I just wanted to make the point that this type of brace work needs to have all braces at the same upright angle . To have them all angled towards the middle of a sphere would mean compound angles on the notches and headblock pockets ,,,,( not good )....
[/QUOTE]

Uh, not really, no. I do a similar kind of joint in the headblock pocket, and it hasn't been a problem at all. We're working wood, and gluing thin sticks to even thinner plates that have their own inertia and bending modes (seriously, the wood isn't forming a perfect exact sphere anyway), and the amount of tilt or shift involved in braces that are no higher than 2 to 6mm, tops (based on my own) when inletted is something that really doesn't require consideration.

Basically, I'm with Tony and co. Sand in the middle of the dish, clean up with a squared scraper blade (sanding at 80 grit doesn't give me the glue surface I want), glue, quit worrying about the rests. I fit the slots/inlets to the braces, not the other way around.