Back & Bracing?

I built a StewMac Concert Ukulele one time from a Kit, the back had a slight arch from side to side. Now I'm going to build one from scratch, can the back be made flat, or is necessary to arch it like the StewMac kit?

Thanks very much!

A slight arch is good insurance, even for composites and laminates.
Many builders regard it as mandatory for solid back plates, to allow a safety margin for expansion and contraction of the plate with moisture changes.
It takes only a few minutes to pencil the desired profile onto the braces, and hand/machine sand to the line.
For very slight arching, the build is the same as a flat plate build, although you may consider tapering the linings (laterally) to ensure perfect contact of the gluing surfaces.
I have built with flat plates/braces with no adverse consequence, but now prefer to use a slight arch.

An arch is definately desirable and I dare say a requirement. It will raise or relax with changes in humidity, so makes the instrument less prone to having the back split. Similar would be said for the soundboard.

Also if it's built flat they tend to look hollow even when they aren't.

Arching (or radiousing) is always a challenge in building. Here is a quick and dirty way to do it: Take a string and a pencil and make the string as long as you want your radius. Take a piece of paper and scribe the arc. It really doesn't matter and you can pull the figure out of the air for the arc. I use a radical 17" radius on my backs, but you might want to do something a little less like 2 or 3 feet or even more. Carefully cut out the paper with an exacto-knife along the line and glue it to your piece of bracing material. Then using a sanding spindle, (or whatever), sand the brace to match the line on the paper being careful to not get any flat spots. Now you have your arched brace. Glue the arched braces to your back giving the back an arch. Ideally, you will have an arched sanding disk to arch your sides before gluing on the back but you can get away with just flattening the edges and gluing down especially with your more subtle arches.

Having an arched back not only is structurally superior to a flat back as bazuku pointed out, but also improves volume and projection. Think of the back as a parabolic mirror focusing vibration onto and through the top.

If the back has 3 braces does all 3 have the same radius?

It would depend on if your back is designed with a constant radius, or some other design. But don't get too caught up in trying to make them spot on. There is some wiggle room in this and timber absolutely will move with fluctuations in RH.

It's important to do your cross grain glue ups with some sort of control of the RH, or at the very least to be observant of what it is. Otherwise you stand the risk of that carefully made back or soundboard turning into a potato chip.

Sawdust said:

If the back has 3 braces does all 3 have the same radius?

Click to expand...

I just use 2 radiused back braces and both are south of the soundhole. Above the soundhole the back is flat so I suppose I get a compound radius. I always thought 3 three braces on the the back of an ukulele was overkill, but it can't hurt.

I want to thank everyone for your replies.

The number of back braces for 'standard' sizes can vary a little depending on the extent of arching, wood type / thickness and who is building.
I know that it can be dangerous to deal in generalisations but most of the popular plans show 0 or 1 for Soprano, 2 for Concert, 2 or 3 for Tenor and 3 for Baritone.