Back bending

I'm struggling with the geometry of having the back radiused on the braces and also curved on the sides - if you know what I mean.

I hadn't really given it much thought but when I got ready to do a dry run with spool clamps and actually had the upper bout in place I realized that the radius across the lower bout and the curve towards the tail end were going to be at odds with each other, at least if there wasn't quite a bit of heat and/or moisture involved.

Anyone know a trick to keep the radius nicely curved when you pull the tail end down?

Are you using a radius dish to get your radiuses? Also use the radius dish with a gobar deck to build back and top.

No radius dish, unfortunately, and no Go bar arrangements available. I made a radius template and hand radiused the braces as shown by Cumpiano, braced the back with a flexible slat and everything turned out fine with a nice even radius to the back. But, as I said, when I pull the tail end down over the breaking point on the sides the cross-brace radius flattens out in favor of the curve over the sides. The waist and upper bout are fine, just the lower bout goes flat.

Maybe I'm not getting what you're saying Erich, but is the tail block radiused too? If it's not maybe that's the problem.
Doug

I shape braces in the radius dish before glueing them to the back which is done by pressing them into the back laying in the radius dish using go bars. The back of the instrument is also shaped in the radius dish so when it's time to do glue up they fit each other very well.

I'm having a hard time envisioning what you are describing though. Perhaps a picture.

I think I know what Erich means - the side profile is radiused front to back, and the braces are radiused side to side. In theory, once the back goes on it domes to accommodate both radii.

I've seen this in an old film of the Nunes factory, and it worked for them.

Tried it once and the back split - so I flattened the braces and went with a back to front radius only.

I suspect the heel block, if flat, is a large part of the problem. It will pull down the back at the heel and tend to flatten it.

Maybe you also need a radiused brace across the widest part of the back? I think the Nunes had this.

In the Cumpiano book they don't use a radius dish per se, but they do assemble the guitars on a workboard which has a 1/8" think ring of cork on its outer edge. The effectively makes it a crude radius dish. (See page 37), which the top and back are glued to using a ton of rope in place of a go bar.

The sides aren't really radiused front to back - they are straight from the neck block down to about the lower cross brace and then turn down (over a very slight curve) towards the tail block.



Maybe it would help if I evened out the curve a bit more. And yes, radiusing the tail block might help as well, although in this case it is really tiny - just 12 x 27 mm.

I'll try and take of few shots for you later.

erich@muttcrew.net said:

The sides aren't really radiused front to back - they are straight from the neck block down to about the lower cross brace and then turn down (over a very slight curve) towards the tail block.

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I think that's the problem. I've also noticed this and never understood how it could work. Some uke plans show the profile of the sides as 2 straight lines at an angle which, to me, is all wrong.

If you even out the curve from one end to the other it should work much better, but the best solution is to use a radius dish in the way that Allen says above. If you shape the braces in a radius dish (with sandpaper stuck to the dish) then glue them to the back in the dish, you're making the back take a spherical shape. Take a tip from Pete and clamp your back braces together side-by-side and you'll not struggle to keep them upright whist sanding. Then if you sand the assembled sides, end blocks and linings in the radius dish you're creating another spherical surface to which to glue the back. Problem solved.

Making a radius dish doesn't have to be a long messy job. Have a look here:
http://www.ukuleleunderground.com/forum/showthread.php?30203-How-to-make-a-Radius-Dish

ProfChris said:

I suspect the heel block, if flat, is a large part of the problem. It will pull down the back at the heel and tend to flatten it.

Click to expand...

That is what happened to my first two build. They are from StewMac Soprano kit.
The back is radious, but the heel block is not. I always observed that the back closed to the neck is very flat.
Now I know why that happened...thanks ProChris