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radius dish, braces and deflection
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Author:  Chuck [ Sat Oct 20, 2012 2:05 pm ]
Post subject:  radius dish, braces and deflection
I am building my first guitar from a plan I found on a 1934 Gibson L-00. The plan states the radius on the back and top as 15' and 28'. I understand how to build the dishes and will be starting them next. What I don't understand is the plan shows all the braces, top and bottom baces, as flat on the bottom so I now I must radius the bottom of the braces somehow. However I don't understand how I know where to place each brace in the two dishes to sand to get the correct brace deflection as it doesn't make sense to me to place them all in the widest part of the dish. There is nothing on the plan to give me the amount of deflectiion or arc as each 15' radius moves toward the heal or the neck of the body.

So my question is how do I determine where on the radiusing board do I place each bottom and top brace to know I will end up with the correct amount of curve for the back and the top?

Thanks for any help you can give me. I did try a search but I did not find anything that I thought fit for what I am trying to figure out. I am sorry for having to ask such a beginner question but I have been stumped now for a few days trying to figure it out on my own.
Chuck
Author:  PeterF [ Sat Oct 20, 2012 2:10 pm ]
Post subject:  Re: radius dish, braces and deflection
A radius dish is like a slice through a huge sphere, so the radius will be the same over the whole dish. That means you can sand the braces wherever you like as long as you keep them roughly perpendicular to the face.
Author:  ChuckB [ Sat Oct 20, 2012 2:15 pm ]
Post subject:  Re: radius dish, braces and deflection
Chuck, the dish is the inside of a portion of a sphere. So, it does not matter what area of the dish you use. The radius is constant in all directions.

You might consider radiusing the braces prior to shaping.

Chuck
Author:  meddlingfool [ Sat Oct 20, 2012 2:18 pm ]
Post subject:  Re: radius dish, braces and deflection
I draw the radius on the brace bottoms and get it real close on the edge sander, then final sand in the dish...
Author:  Chuck [ Sat Oct 20, 2012 11:17 pm ]
Post subject:  Re: radius dish, braces and deflection
Thank you all for the answer. I get it now! Now to make them. Onward and upward I hope.
Chuck
Author:  bluescreek [ Sun Oct 21, 2012 7:53 am ]
Post subject:  Re: radius dish, braces and deflection
Sometime we just over think the solution. Remember " You don't know what you don't know till you know it".
You are well on your way just have fun.
Author:  Bob Menzel [ Sun Oct 21, 2012 12:43 pm ]
Post subject:  Re: radius dish, braces and deflection
Quote:
Sometime we just over think the solution. Remember " You don't know what you don't know till you know it".


Boy, aint that the truth John. Recently went down that stretch of road. Braces weren't a problem, but putting a radius on the bridge plate really perplexed me because I was overthinking it. Once I put it in the dish, it was a piece of cake. On a 28' radius, you're not taking off very much wood.

Chuck: After marking the radius on your braces, laying them on their side and hogging off most of the waste with plane will speed up the process and create a lot less dust.
Author:  charliewood [ Sun Oct 21, 2012 1:13 pm ]
Post subject:  Re: radius dish, braces and deflection
bluescreek wrote:
Sometime we just over think the solution. Remember " You don't know what you don't know till you know it".
You are well on your way just have fun.


Conversely - Its not "what you dont know" that messes you up, Its "what you dont know you dont know"...
Cheers
Charlie
Author:  John Coloccia [ Sun Oct 21, 2012 3:01 pm ]
Post subject:  Re: radius dish, braces and deflection
FWIW, I calculated the deflection of 2', stuck some pins in a board, bent a piece of steel to conform and drew a line. I used my shooting board to plane to the line. That's my pattern. I use the pattern mark my braces. Then I toss them on the shooting board and plane to the line. It's sooo much easier and quicker than sanding them on a radius dish.

Not a suggestion. It's just what I do.
Author:  Alan Carruth [ Mon Oct 22, 2012 11:37 am ]
Post subject:  Re: radius dish, braces and deflection
A thin shim super glued to the sole of the plane behind the cutter will cause it to produce a uniform radius. You have to work out the correct distance based on the shim thickness, and it takes a bit of trial and error to get it right, but once you do you can scribe a line on the plane and get it back in the same place every time. These days I use a small shooting board for this, and make three or four braces at the same time. This helps ensure that the planed surfaces are perpendicular to the braces, and not twisted.

Mario Proulx likes to plane them freehand. If you've got his tool chops, and never have a bad day, that's a good way to do it. My way is more reliable, and about as fast. Planing has the advantage of leaving a better gluing surface than sanding.
Author:  PeterF [ Mon Oct 22, 2012 3:52 pm ]
Post subject:  Re: radius dish, braces and deflection
John Coloccia wrote:
FWIW, I calculated the deflection of 2', stuck some pins in a board, bent a piece of steel to conform and drew a line. I used my shooting board to plane to the line. That's my pattern. I use the pattern mark my braces. Then I toss them on the shooting board and plane to the line. It's sooo much easier and quicker than sanding them on a radius dish.

Not a suggestion. It's just what I do.


A batten bent through a single point will not actually produce an arc - it will be a parabola, so it won't fit properly in a radius dish.
Author:  Bryan Bear [ Mon Oct 22, 2012 4:23 pm ]
Post subject:  Re: radius dish, braces and deflection
PeterF wrote:
John Coloccia wrote:
FWIW, I calculated the deflection of 2', stuck some pins in a board, bent a piece of steel to conform and drew a line. I used my shooting board to plane to the line. That's my pattern. I use the pattern mark my braces. Then I toss them on the shooting board and plane to the line. It's sooo much easier and quicker than sanding them on a radius dish.

Not a suggestion. It's just what I do.


A batten bent through a single point will not actually produce an arc - it will be a parabola, so it won't fit properly in a radius dish.


I doubt the difference is of any real world significance. After all, the radius of the dish will not be the same as the radius of the inside of the plate forced into the dish (having to account for the thickness of the plate, the inner radius will be tighter than the radius sanded into the brace if done in the dish) but this seems to work just fine. . .
Author:  John Coloccia [ Mon Oct 22, 2012 5:48 pm ]
Post subject:  Re: radius dish, braces and deflection
PeterF wrote:
John Coloccia wrote:
FWIW, I calculated the deflection of 2', stuck some pins in a board, bent a piece of steel to conform and drew a line. I used my shooting board to plane to the line. That's my pattern. I use the pattern mark my braces. Then I toss them on the shooting board and plane to the line. It's sooo much easier and quicker than sanding them on a radius dish.

Not a suggestion. It's just what I do.


A batten bent through a single point will not actually produce an arc - it will be a parabola, so it won't fit properly in a radius dish.


If I wanted a true circle I would simply drive 2 tacks at the end points, and use two sticks to make a long compass, and draw a perfect circle like that. I don't use a radius dish for anything, so in my case it doesn't really matter. I also suspect that the catenary/parabola (and a stressed, thin rod will be very close to these shapes, which are generally close to each other) is probably a more ideal shape anyhow, though I doubt it matters.

Incidentally, if you shape your braces in a radius dish, the braces will not fit properly once they are placed on the plate, which is sitting in the radius dish. The braces will be radiused .100" (or whatever the plate thickness is) larger radius than the inside of the plate.

Same problem if you radius your neck with a radius block, and then use the radius block to radius the top of the frets. You're off by the thickness of the frets.

edit: just noticed Bryan made the same point about the radius mismatch
Author:  John Arnold [ Tue Oct 23, 2012 11:33 am ]
Post subject:  Re: radius dish, braces and deflection
Quote:
The braces will be radiused .100" (or whatever the plate thickness is) larger radius than the inside of the plate.

That amounts to a 0.056% difference with a 15 foot radius, and a 0.033% difference with a 25 foot radius.
Considering the flexibility of the parts (and their movements with moisture changes), I hardly think that matters at all.
Worrying about this is akin to considering the crosswise radius of a 5/16" wide brace. If you cut the lengthwise radius with a plane, it won't have the crosswise curvature that it has when sanding on a radius dish. However, the difference is extremely small.
In woodworking, a worthy goal is glue joints that are 0.001" or less. In practice, it is very difficult to achieve.
The two examples above are an order of magnitude less than that.
By its nature, sanding tends to round the edges, so when shaping the bottom of the braces, I try to keep the sanding to a minimum.
Author:  Goodin [ Tue Oct 23, 2012 12:08 pm ]
Post subject:  Re: radius dish, braces and deflection
Bob Menzel wrote:
Quote:
Chuck: After marking the radius on your braces, laying them on their side and hogging off most of the waste with plane will speed up the process and create a lot less dust.


Here's what I do to get the radius on a brace. I learned this trick from A. Manson. I made up a jig which clamps the brace in the center, then two wedges are pushed in at each end of the brace to bend the wood up to match the radius (I used a scrap piece of thin particle board sanded on one edge in the radius dish as a guide to achieve the radius). Then use a sharp well tuned hand plane to plane off the ends until flat (usually 5-15 strokes depending on how deep your blade is set), release the clamp and the brace springs back into a perfect match of your radius. Works for me every time, is quick, and doesn't create any dust. And no noisy machines. I have started doing two braces at once. See pic...
Attachment:
529169_10150761137104696_1618453276_n.jpg
Author:  John Coloccia [ Tue Oct 23, 2012 2:54 pm ]
Post subject:  Re: radius dish, braces and deflection
John Arnold wrote:
Quote:
The braces will be radiused .100" (or whatever the plate thickness is) larger radius than the inside of the plate.

That amounts to a 0.056% difference with a 15 foot radius, and a 0.033% difference with a 25 foot radius.
Considering the flexibility of the parts (and their movements with moisture changes), I hardly think that matters at all.
Worrying about this is akin to considering the crosswise radius of a 5/16" wide brace. If you cut the lengthwise radius with a plane, it won't have the crosswise curvature that it has when sanding on a radius dish. However, the difference is extremely small.
In woodworking, a worthy goal is glue joints that are 0.001" or less. In practice, it is very difficult to achieve.
The two examples above are an order of magnitude less than that.
By its nature, sanding tends to round the edges, so when shaping the bottom of the braces, I try to keep the sanding to a minimum.


Hi John. My point wasn't that we SHOULD worry about it, but that I no more worry about my arc being catenaries than people worry that their circular braces being under radiused. :)
Author:  Bryan Bear [ Tue Oct 23, 2012 9:54 pm ]
Post subject:  Re: radius dish, braces and deflection
Gil, I suppose had you learned that trick from a cobbler the wedges would be supported by a pair of shoes. . . laughing6-hehe
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