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joe, not if, as keith points out, and i was not clear on(my reference to proper technique)it is kept perpendicular to a plane tangent to the surface of the sphere. this can be anywhere on the dish. it will be sanded on the gluing surface only.

i assume kiwicraig's reference surface board rests on the edges of his dish. if so it will only be correct if it keeps the works centerline on a centerline of the dish. at any other position the work will not be perpendicular to the tangential plane. whilst it is theoretically possible to provide a reference surface to give the perpendicular orientation anywhere on the dish, the calculation of the angles would be beyond the abilities of most, and the necessary apparatus cumbersome. eyeballs and skilled hands work fine.

IMHO we are trying to maintain the spherical shape of the top which is were we get strong and light. Bracing to oppose some perceived some vertical force will result in a weaker sphere unless the braces are made heavier.

i don't understand your last post keith.

just the X and tone bar's. John, John Slobod is going to HB as a spectator

[QUOTE=crazymanmichael] i don't understand your last post keith.[/QUOTE]
I guess I can see why. I think what kiwi is saying is that you should brace perpendicular to the chord formed by the top of the sides and the sphere. In my mind this will put the brace at an angle to the tangent of the sphere and so not support the shape of the sphere as well. Unless the brace is made taller and heavier.

i hope he responds to your thought. i don't think he was saying that however. i think we are all on the same page about how we want our bracing to sit relative to the top. there just seems to be disagreement as to how the dish can be used to sand to achieve that result.

Whew! This is exciting. We are all correct, we're just visualizing a different deal. I've been experimenting and agree that the same curvature will work. But here is why:
If you place a radiused brace on your dish at the center, say oriented north and south; now move that brace (east or west) 8 or ten inches (KEEPING IT VERTICAL) and keeping it north and south,it doesn't fit because it's radius is not the same. BUTTTTT! When the top (or back) is domed end to end, those off center braces are now tilted toward the center and the center of those arcs will pass through the center of the sphere!    I see! I see!

Ron

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OLD MAN formerly (and formally) known as:

Ron Wisdom

Somewhere in the middle of Arkansas......

It doesn't matter where you sand as long as the brace is flat on the surface. If you sand off center and try to keep the brace vertical (Which I think is what Kiwi is saying) it will impart an angle on the brace bottom that will be TOO much when the top or back is domed end to end.
As long as you sand with the brace pointed toward the center of the dish or through that center, it will work.

Ron

_________________
OLD MAN formerly (and formally) known as:

Ron Wisdom

Somewhere in the middle of Arkansas......

C'mon guys -- use a little math and logic. If you use inappropriately fine
tolerances in your construction, you're not making a better guitar, you're
wasting time. There are places where fine tolerances are appropriate (fret
location, jointing plates) and places where less precision is just fine. While
building a superior guitar is incredibly complex, these particular choices
are basically simple engineering problems.

Why flatten the upper brace? It resists a downward pressure and an
arched brace will do that a lot better than a straight one. In fact, the flat
one will necessarily flex downward as soon as any force is exerted on it -
you've accelerated the distortion of the body by using a flat brace.

For a fingerboard of normal width, approximately 2 1/8" wide at the
body, the curve of the soundboard (using a 25' radius) introduces less
than .002" of arc under the fingerboard -- hardly enough to weaken the
glue joint (which is under compression anyway) and less than the depth
of the sanding scratches you leave on the top. A couple of passes of 120
grit across the top of the soundboard will remove that small amount of
arch if you feel the need.

Similarly with the bridge plate. The arc under the plate is around .006", a
gap easily closed with very little pressure unless your bridge plate is
absurdly stiff. The bridge, being hundreds of times stiffer than the bridge
plate, will determine and maintain the radius of the soundboard.
Radiusing the plate is a waste of time. (Actually, I bet that hand sanding it
on a radiused form introduces more inconsistencies in the gluing surface
than leaving it machined flat.)

And those finger braces -- figure out the difference between a straight
line and your preferred radius over the length of the brace, compare that
result to your operating tolerance and the stiffness of the brace, and
you'll know whether or not to radius. Having a good gluing surface is
important; whether you achieve that by sanding on a radiused form or
planing the brace is up to you.

Rick I also use, as many guys do, a plywood, or some other wood block
underneath the fretboard extending from the edge of the rim to the edge
of the transverse brace. The top of the block is flat just like the brace,
and I can garuntee this block is as stiff as any curved transverse brace, if
not way more rigid. I don't have to worry about any flex there at all.

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John Mayes
http://www.mayesluthier.com

ron, it will work no matter where you point it. if it is on the surface of a sphere it is always on a great circle on the sphere. take a ball and a short piece of string. place the string on the surface of the ball. that string will always lie on a circle going around the ball, that circle being of diameter = to the diameter of the sphere.

nuf sed.

I am still using the flat transverse brace because that is what I learned in Frank Finnochio's class.

If I ever get smart enough to figure out what all is going on I may make a decision to change some of the things I am doing.

For noe, it seems to be working just fine for me.

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Dave Rector
Rector Guitars

A great circle is only a great circle if its center is the center of the sphere. Make a 10" radius from the center of your dish and place a brace perpendicular to that radius. That radius is only on a great circle if it is sitting flat on the dish. When it is vertical it is not part of a great circle. Am I wrong?

Ron

_________________
OLD MAN formerly (and formally) known as:

Ron Wisdom

Somewhere in the middle of Arkansas......

I radius all braces, transverse included, for the reasons Rick stated. The brace is much stronger being arched (25' radius for me), and better handles the load coming in from the fbe.
As for reducing stress in the top, the entire top is stressed when pulled into a dome by arched braces. Stresses are not quite the enemy some think. (I wonder if Arthur Overholtzer is to be thanked for that notion?)

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From Nacogdoches...the oldest town in Texas.

http://www.stephenkinnaird.com

Personaly I don't want to argue over the rights and wrongs of this topic because its so much like the age old argument whats better (Mortise and Tenon or Dovetail).
Ill just say, there are a LOT of BIG DOGS doing it BOTH ways. Off the top of my head I can think of 3 builders that go flat above the sound hole, and are nationally recognized as at the top of the field.
There is no right way or wrong way, so long as your guitars stay together, and sound good to SOMEONE - cuz ain't everyone gonna like'em.

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Lance@LuthiersForum.com

I think I led folks away from my real point by mentioning the stiffness
(not strength) of a curved upper brace. I certainly didn't mean that one
way is "right" and the other "wrong."

John, I didn't mean that a straight brace would lead to disaster, only that
a curved brace is inherently stiffer in this case. I stand by that: a straight
brace must be infinitely stiff not to deflect. Even plywood is not that stiff!
All straight braces deflect, period. So do arched braces, but a lot less --
and, properly sized, they'll won't go concave as the straight brace will. If
you want to reduce the mass of the complete soundboard while keeping it
stiff, an arched upper brace is the most efficient.

But the question is: does it matter? If you choose to use a straight brace,
for whatever reason, is it flexible enough to create problems? The
theoretical discussion is irrelevant if the deflection is so slight as not to
matter. That's what I was trying to say in regards to the curve of the
soundboard under the fingerboard extension -- it's there, but too little to
matter. My point is simply to apply the appropriate measurements to each
part of the instrument.

Just for a little perspective: the top plate, without bracing, for a typical steel string guitar will probably weigh somewhere between 120-150 grams. All of the bracing, including the bridge plate, taken together, will probably weigh about 30 grams: just about the same as an ebony bridge. The difference in weight between a 'heavy' straight braced top and one that has been scalloped to about the limit will be, maybe, 5 grams. The difference in weighton a brace that will be needed to compensate for a small departure from the 'perfect' angle (whatever that is) has got to be pretty damned small. The place to save weight is in the top.

It has been shown that planed surfaces glue much better than sanded ones. I use a shim on the sole of my plane to put the curvature into the brace surfaces. This results in the smoothest possible gluing surface. I also find it just about impossible to sand a surface witout rounding the corners unacceptabally. I try to plane them just before gluing, as it has also been found the result in noticably stronger joints.

alan, could you post a photo of your plane shim in action?