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Radius Dish Sizes - Dumb Question
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Post
Craig

Total Topics: 25
Total Posts: 58
I'm doing my homework before my Stu Mac kit gets here, and having previously ordered the Robert O'Brien DVD (which is excellent by the way!) and read a bunch of stuff....how do you determine the size of the radius dish(es) you want?

Craig

Aug 11, 06 | 3:19 pm
Bill Cory

Total Topics: 158
Total Posts: 3584
Not at all a dumb question. John Hall or Tracy Leveque should chime in. Tracy (www.luthiersuppliers.com) makes 'em. John Hall (www.bluescreekguitars.com) does the same. I think the diameter needs to at least equal the length of the body, as Robbie's DVD shows.

Aug 11, 06 | 3:31 pm
jhowell

Total Topics: 37
Total Posts: 676
Craig--

Physically, most of the dishes are 24" in diameter. If your question is refering to the radius of the dish (15', 28', etc.) there are multiple schools of thought. I went with 15' for the bottom and 25' for the top, only because that seemed to be what most folks do. My personal level of ignorance in this area is way too high to even think about discussing the merits of one radius vs another.

Talk to John or Tracy -- they can fill you in and sell you what you need based upon what you want or expect to be building.

--Jim


Aug 13, 06 | 7:56 am
Ted

Total Topics: 22
Total Posts: 158
That's not a dumb question Craig - this is... What does a radius dish actually do? Does it replace the sanding board that I am currently using on my StewMac OOO? Or does it radius the actual board?

Aug 13, 06 | 8:02 am
jhowell

Total Topics: 37
Total Posts: 676
Ted--

Not a dumb question either! This is my understanding -- radius dishes are used to construct tops and bottoms to a spherical curve rather than flat. My 'in a nutshell' understanding is that this gives the tos and bottoms more strength and also some wiggle room when the humidity drops. It means the tops a nd bottoms are less prone to cracking. The bottoms of the bracing are sanded to these radii then the top/bottoms are glued to the bracing. They are also used to contour the edges of the rims so that there is a good glue line and the height of the sides are good at the waist. Some weird geometry going on with the spherical thing -- it took me a while to visualize it, but once I had it, it makes so much sense.

Someone smarter than me please chime in!

--Jim

Aug 13, 06 | 10:12 am
Luthier Suppliers

Total Topics: 2
Total Posts: 68
Jim,
Good answer, and there are no dumb questions! I didn't know what a go-bar deck was when I started building and for sure I didn't know what a radius dish was! But Jim summed it up well. The radius adds strength to the guitar top and back when the wood takes on humidity. It also raises the pitch of the guitar slightly. So a flatter radius like 42' will produce more bass than a 25' radius. But 25' radius on the top will give you a bit more wiggle room in extreme humidity changes. Just don't go much below 25' radius on your top, or you will have some serious neck angle problems. This is no big deal for an experienced luthier, but very difficult for a beginner.

There are many things that you can do with a radius dish including the following:
1. Sanding your braces to profile them.
2. Profiling the rims to the correct radius(if you don't cut your sides to the profile before bending, then you must plane them down to the right size, and the dish helps)
3. Once the Kerfed lining is on the rims, you can sand them to get the proper sphere/radius.
4. When gluing your tops and backs to your rims in a go-bar deck, you place the top/back down in the radius dish, and then glue your radiused braces using go bar sticks. A common misconception is that you must have a radius dish with sandpaper to do the braces and the rims, and another without sandpaper to glue in the go-bar deck. This is not true, just 2 sheets of newspaper over the sandpaper is enough to protect the top from getting marred by the sandpaper. Trust me, it works.

Hope this helps explain. After you sand your braces a few times in the radius dish, you will be begging to buy my brace maker jig. Just clamp the brace in and pass over a jointer or a table saw, and in about 5 seconds, you have a perfectly radiused brace. Then just rub in the radius dish a few times, and you have a perfectly radiused and spherical bottom on your braces. Ok, end of sales pitch!

Good luck my friend, and please ask dumb questions, we were all there too at one time.

Aug 13, 06 | 7:05 pm
Craig

Total Topics: 25
Total Posts: 58
Great explanations - thanks!!! Now what is the reason for using the radius dish in the go bar deck? Does it just help preserve it for the top and bottom while the glue is drying?

Craig

Aug 14, 06 | 7:56 am
Luthier Suppliers

Total Topics: 2
Total Posts: 68
Craig,
Imagine this...you just put a curve on the bottom of your brace and now you want to glue that brace to the inside of your top. If you put the top face down in the go-bar deck on a flat surface and then place your brace against the back of the top, the ends of the braces will not contact the top. If you clamp it using your go-bar sticks, it will just force the brace down flat again. So the purpose of the concave surface of the dish is to allow the brace to be clamped at a curve. So imagine you have you have a sandwich of cardboard, and the 2 outer pieces are curved to the same radius and the inner piece is flat. If you smash all 3 pieces together, the middle piece will conform to the 2 outer pieces. This is the same concept of a radius dish clamping in a go-bar deck. Hope that help!
Tracy

Aug 14, 06 | 8:21 am
Craig

Total Topics: 25
Total Posts: 58
I get it now - many thanks!!!

Craig

Aug 14, 06 | 12:28 pm
Luthier Suppliers

Total Topics: 2
Total Posts: 68
Just in case there is any confusion...you can see in this pic that there is a dish on the bottom, then the top, then the curved braces being glued in the go-bar:


Aug 14, 06 | 1:21 pm
Ted

Total Topics: 22
Total Posts: 158
So is there a difference between this method and the curved caul/brace combined with angled sanding as in the StewMac kit I am making? Are radius dishes mainly for other kits or builds from a blank?

Aug 15, 06 | 6:48 am
Luthier Suppliers

Total Topics: 2
Total Posts: 68
Ted,
There are about a 1000 different ways to build a guitar. The go bar deck with go-bar rods is a cheap and easy way to glue that allows you to get to the glue squeeze out with minimal effort, and the dish just saves you from having to buy a bunch of cam clamps. But it can be done either way. The added benefit of the dish is to radius and/or contour your rims before gluing the top and back on. It just makes it really easy to sand the whole rim at once getting everything very consistant.
Tracy
http://www.luthiersuppliers.com

Aug 15, 06 | 7:39 am
Craig

Total Topics: 25
Total Posts: 58
Ok, I'm stuck on the radius issue again. I get it for the top, back, sides and braces. Now what about the fingerboard? The fingerboard is radiused but how do you determine which radius you want on it?

(I think some of these things are so obvious to folks who are experienced woodworkders - but I've gone through a bunch of forums and the Cumpiano book and, while there is a ton of instructions to radius - and all the jigs you can make - I can't seem to find the reason why nor how do you determine the amount of radius).

Aug 21, 06 | 5:40 am



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