Sanding braces in a radius dish.

[Dominic]

Sanding braces in a radius dish. I was recently reminded of the debate about sanding braces in a radius dish so I thought I would give my take on it.

When sanding your back braces in the radius dish it does not matter where in the dish you sand, the radius will be the same. And if you keep the brace perpendicular to the tangent of the part of the dish where you sanding you end up with a bottom edge that is square to the sides of the brace. When you glue on the braces they will be perpendicular to the back and once the back is glued to the sides bending the back longitudinally they will point to an imaginary axis 15 feet away rather than being parallel. The reason to do this is because the force the brace is exerting on the back is most effective when it is at 90 degrees to the force the back exerts on the brace as it tries to flatten out. And the stiffness of your perfectly quarter sawn brace stock will be maximized. It will also mean that the braces are perpendicular to the bottom edge of the side where they meet the kerfing rather than being slight off square.

I have seen it argued that you should sand your back braces on the dish in about the place they are on the back because the radius is different as you move out from the centre of the dish. This not correct.

First, the radius on a radius dish is the same anywhere on the dish. If it were not the same it would not be a radius dish. You can sand anywhere on the dish and as long as your dish is accurate the radius will be the same.

I have also seen it argued that you should do it like this because the back curves in both directions. Well a radius dish curves in both directions already so there is no need to compensate for this. But if you sand your braces on different parts of the dish and keep them at 90 degrees to the floor and not the dish so the bottom of the outside braces are not square you are effectively tipping your brace away from 90 degrees once glued to the back. This renders your perfectly quartered brace stock as not quite so perfectly quarter sawn which defeats the purpose.

The only reason to sand your braces on different parts of the dish is to get to fresh sand paper.

Cheers
Dom

[Tod Gilding]

G'day Dom,
I think your correct in what your saying,but the way you have said it, I think will generate a lot of debate. Your point can be shown by cutting a piece of paper into a circle and then cutting it like a pizza with each piece being a different size and then checking the radius of all the pieces, they will match, so you are right, but my post of march 25th (soundboard radius) showed that not all builders dome their tops and backs as a dish ,some only radius across the bouts and waist and some don't radius at all, so this might explain why some use a sanding dish in different ways, maybe, anyway I will sit back and expect some debate

[needsmorecowbel]

Where do you get/ how do you make the dishes?

[Tod Gilding]

Where do you get/ how do you make the dishes?

This One's worth a look http://www.youtube.com/watch?v=tvGemvizrz4

[Kim]

When I read Dom's post I immediately thought of this thread which was thrashed out at the OLF a few years back: http://www.luthiersforum.com/forum/view ... =0&sk=t&sd Note: As you read through the thread at the attached link, you will see where one member's input appears to have been deleted. Quite sad when that happens but fortunately the meat and bones of the topic remain so the attempted sabotage becomes no more off putting than a few momentary losses of transmission on the teli.

I think the bottom line is that it simply does not matter that much.

Dom's point about loss of structural integrity if you 'do' position the brace in the dish relative to its position on the guitar and sand with it held vertical relative to gravity is 'technically' true...(that is if you follow the belief that quarter sawn wood is stiffer/stronger than flat or skew sawn wood.) However the reality is that any loss would be so small it most probably could not be measured.

But the same is true for the opposing argument which states that the braces would tilt and 'look' wrong if you do 'not' sand them in position on the dish, vertical relative to gravity. Yes the tilt is there, but when you do the numbers bases upon the height and width of the brace, the tilt would be so minute that your really not going to notice it.

Reading through those four pages, I came away feeling that in 'technical terms' Billy Thomas nailed what is actually going on and has some effective drawings to explain why folks like me 'feel' that the brace should be sanded in position on the dish whilst held vertical relative to gravity. I think Tony Karol nails why being 'technically' correct does not always reap any benefit, and our own Craig Lawrence explains a 'practical' reason why he sands the braces vertical relative to gravity.

All in all it is an interesting question that seems to leave most parties taking stock of their initial assessment...I guess that's why the topic was memorable...For the records I will continue to hold the braces vertical in relation to gravity just because it feels so good.

Cheers

Kim

[kiwigeo]

How much tilt you get on your braces depends on radius of the dish doesnt it? I definitely notice tilt when I do my back braces in a 15' dish and shape them at dish centre.

Anyway Im completely anal about such things and even if it doesn't look crooked I KNOW it's not vertical...

[Taffy Evans]

I radius as shown in the photo with the added support to make sure the pressure I'm putting down is more even along the brace's length. You might just see the wooden adjustable end "clamps" that holds the brace in position. Works for me anyway.

IMGP4300 (Small).JPG (30.84 KiB) Viewed 14067 times

[liam_fnq]

Where do you get/ how do you make the dishes?

What ever happened to that old tutorial by Graham Mcdonald?

Thats how i did mine. Thinkin'[ about making a smaller, more manageable, uke sized dish using the same method.

[Steve]

Where do you get/ how do you make the dishes?

What ever happened to that old tutorial by Graham Mcdonald?
Thats how i did mine. Thinkin'[ about making a smaller, more manageable, uke sized dish using the same method.

Me too. It's still there, but it looks like a lot of the tutorial is missing...viewtopic.php?f=24&t=231

[Kim]

How much tilt you get on your braces depends on radius of the dish doesnt it? I definitely notice tilt when I do my back braces in a 15' dish and shape them at dish centre.

Anyway Im completely anal about such things and even if it doesn't look crooked I KNOW it's not vertical...

That's the point made by those whom advocate sanding with the brace held vertical relative to gravity at a place in the dish relative to its intended location on the guitar, there will be no tilt. I copied Billy T's post with images from the OLF topic mentioned above (Hope you don't mind Billy). For me this gives a good visual of what is going on.

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.

_________________
Billy Dean Thomas
Covina, CA

Must say that I really like Taffy's brace sanding support...gonna build meself one of those suckers today...THANKS TAFF!!

Cheers

Kim

[Dominic]

Yes, someone mentioned the topic to me the other day concerned about how to make this adjustment when building without a dish and I remembered the thread Kim has put up. The main point I wanted to make particularly for those new to building was that you don't have to sand in position in the dish to get the correct radius because it is the same everywhere.

I’m surprised the thread went on for 4 pages and didn’t seem to get resolved. The point people in the thread made that there is some difference in radius is a measurement error because looking straight down on a brace sanded in position near the edge of the dish you will see that it no longer forms a straight line but is slightly curved. If you measure the radius on any straight line on the brace it will be 15 foot.

But its personal choice if you want to align your braces at 90 degrees to the back so they are square where they meet the kerfing or vertical. If the latter the planes that the braces are on are still not square to the top because of the tapper of the sides. Visually you would not tell the difference through the soundhole anyway. So I wonder why, given the complications and compromises anyone would do it this way. But hey, each to his own.
Cheers
Dom

[Lillian]

Taffy, could we get some more pictures of your brace. Its really hard for me to make out the details from here.

[kiwigeo]

Craig Lawrence explains a 'practical' reason why he sands the braces vertical relative to gravity.

Kim

Um I don't think gravity always acts vertically does it?? What if youre building your guitar next to a large granite mountain? Surely the large mass of granite would complicate matters?

[Kim]

Only if ur a geologist Marty.