Radius dish questions.

I'm planning on beginning construction of my first ukulele shortly. I own the Hana Lima & Ukulele Construction/Design books, & neither say much of anything about radiusing the back & top. I want to build the best ukes I can, & it seems all the best ukes are radiused. I'm not sure of the point of radiusing, other than it maybe adding strength. So my first question is: Whats the purpose of radiusing the back & front?
Also I'm not sure exactly how to use a radius dish. The books I have don't mention them. Do you radius the entire back & front, or just the lower or upper bout area? It seems like a dumb question & that you'd want to radius the entirety of the tops/backs, but like I said I know just about nothing about radiusing.
I read the thread on builders preferences in size for their dishes, but are those sizes used for every size uke they make? For instance Pete uses 22 for fronts & 12 for back. Is that for soprano, baritone, & everything inbetween? Or does it vary by uke size? My uke will be a tenor.
I have access to a cnc, so making them shouldn't be too difficult. Do you think MDF would be good material to use? Or would you suggest something else?
Sorry for asking so many questions. Any input would be very much appreciated.

The simple answer (about all I'm capable of on a Saturday morning) is strength.
Hana Lima doesn't mention it because it is a bit of an advanced technique. As you mentioned, it is found on some of the better instruments.
The primarily reason for radiiusing the tops and backs is being able to build lighter. A curved surface is much stronger than a flat one, therefore your bracing can be lighter and the wood plates themselves can also be a bit thinner. This all allows the ukulele to vibrate more while keeping the mass of the materials to a minimum. Keep in mind that the strength and stiffness of the wood used, the plate thickness, the degree of radiius, and the type and the mass of your bracing all work together and have to be thought of a such. A change in one element will affect all of the other factors. Bracing dampens a sound board, so as a general rule, lighter is better. I use 15'/25' for all sizes I build.

Putting a radius on the top and back does several things, but one is it pre-stress's / tensions the plates. This allows them to be built just a little bit thinner while still having their strength. Changes the geometry of the top though, so depending on how much radius you put in, and the size of the instrument, you may need to take some design elements into consideration when building.

You don't need to get pedantic about how much radius you use. It's not a silver bullet. It's traditional to have less on the top than on the back, but they can be the same.

I build my ukes on a solera and have just put a radius in the lower bout only. Keeping everything above the waist flat.

For instance on acoustic guitars, you need to have the neck set back just a bit to get appropriate action on the strings. On my ukes I need to do the same thing.

You can use MDF without any worries. I made mine for the guitars at 15' and 25' radius with 2 layers of 19mm for strength, and be sure to seal them on all sides with at least a couple coats of varnish, lacquer, polyester or similar.

The most common way to use a radius dish is in a go-bar deck. Simple apparatus that allows you to place go-bars (wood, bamboo or fibreglass rods) between the brace that you are gluing and something above to push against. If you do a search on this you are sure to see dozens of examples.

And finally, you don't have to use a round radius like a section of a ball. You can also use an arch like one that tensions the plate from side to side (parallel to the bridge) or one that goes from neck to tail block. Reasonably common methods in the guitar world, and it may produces a different voice depending on how far you develop the design.

One thing I'd recommend is that you draw out the instrument geometry with what you are planning on doing full scale just to give you and idea of what you might be up against as far as bridge / saddle hight is concerned, as well as a possible set to the neck.

The other advantage to adding a radius to the top and back besides strength, is it is also protective from sudden drops in humidity. The slight dome allows the top and back to flatten slightly if they dry out rather than cracking.

Brad

Bradford said:

The other advantage to adding a radius to the top and back besides strength, is it is also protective from sudden drops in humidity. The slight dome allows the top and back to flatten slightly if they dry out rather than cracking.

Brad

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Good point Brad.

Thanks for all the info guys.
I'm thinking of picking up the book "Guitarmaking: tradition & technology". Anyone have it? In 392 pages I know it'll have loads of information. Some of which applicable to ukuleles. Does it mention radiusing? Or designing & how certain shapes, depths etc. shape the tone? If no, do you know of any books that do?

Looks like all of the right points on why to radius have been made. I'd just like to add that I personally believe radiused tops and backs are more aesthetically pleasing. I knew right off the bat I couldn't go for a flat top, beauty and strength... the name of the game is to build as light as possible without compromising strength. if you want cannons... you just can't as far without the radius IMO. I used Dominator's technique making my radius dishes... he has a great youtube vid showing just how to. Good luck and happy building!

I used Dominator's technique making my radius dishes... he has a great youtube vid showing just how to.

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Link? I couldn't find it on Dominator's YouTube channel.

Perhaps this is the one Vic D spoke of


http://uketalk.com/luthier-tutorials/domed-dish.htm

mzuch said:

Link? I couldn't find it on Dominator's YouTube channel.

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I never did a video (That would be one messy video ). Just the writeup at Uketalk in the link above.

Right on, my bad... good tutorial anyway. There is a vid on youtube showing that method but I dunno who's. No joke about the dust though... definitely want to rig up a shop vac or something.

Keshalalalala said:

Thanks for all the info guys.
I'm thinking of picking up the book "Guitarmaking: tradition & technology". Anyone have it? In 392 pages I know it'll have loads of information. Some of which applicable to ukuleles. Does it mention radiusing? Or designing & how certain shapes, depths etc. shape the tone? If no, do you know of any books that do?

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"Guitarmaking: tradition & technology" is an excellent book--Knowledge is power! I read everything I can find…