View Full Version : Thicknessing for Stiffness

09-10-2014, 04:43 AM
I'm getting ready to attempt my second ukulele and would like to do a better job on the top. My first build makes sound, but I may have left the top a bit thick at 2.5mm. That being said I've noticed that the overwhelming answer to "How thick?" seems to be "depends on the stiffness." Being completely new to this I know I don't have a feel for the right stiffness and hope that there might be a way to measure it.

With that in mind, does anyone know what a good stiffness would be for a tenor top without bracing? And how that measurement was made?

I plan to make up a little three point bending jig with a dial indicator and a weight to try and record deflection of the top for my records. I was hoping someone might be able to provide a starting point.

If this has already been discussed I apologize, I did a search of the forum but did not find it.

09-10-2014, 05:38 AM
This page has some info on guitar top deflection http://www.wrenguitarworks.com/Workshop/workshopIndex.html
and the thickness topic comes up here and there http://forum.ukuleleunderground.com/showthread.php?100271-How-thin-can-you-go
You Tube also, I get lost in videos but heres a start then check the side panel for more related videos http://www.youtube.com/watch?v=JHs7s1_pbAg
Good Luck with the build.

09-10-2014, 06:30 AM
I'm currently around 30 builds, and am now consistently getting a tone and volume that I like.

What i want to end up with is a top which is light but stiff, so I start with wood which I think has a chance of achieving that. I thin it until it starts to have some flexibility, then work it down until (a) I can flex it a small amount along the grain using light finger pressure, but (b) it doesn't flex excessively across the grain (it will always flex more cross grain). For a soprano top I might get a curve about 4 or 5mm deep along the grain, and twice that or a little more across. Mahogany tends to end up around 1.6 mm, spruce 1.9, but each piece of wood is different.

For a spruce tenor I'd start at 2.5 mm and expect to end up nearer 2. Mahogany would probably be 2mm or less.

Of course, I can't tell you how much finger pressure I'm using, but if it isn't easy to get some flex longitudinally then I think the top is too thick.

09-10-2014, 08:08 AM
Thank you both for your replies.

My first build I tried to copy the flopping idea in the youtube video, but not really being experienced I think I missed on the high side. I know it is a combination of techniques (all of which I lack) and not just the top thickness, but my first Ukulele seems to lack sustain on everything but the C string. So I'm thinking I probably went too thick, and maybe too heavy on my top.

It could be the bracing, it could be the top, but I'm aiming to get one thing "right" so I can figure out what I might be doing wrong in other areas. My plan is to build a little jig similar to the one in the wren guitar works link. I had drawn up a jig that used two rails 9 inches apart (about the same width as the lower bout on the plans I have) and a rod of a given weight centered between the two to test the deflection. This way I could measure the deflection with and against the grain on a given top.

I have enough engelmann spruce for 9 tops (I purchased anticipating disaster on my first uke) so I was planning on gluing up all nine and then thicknessing to about 3mm to compare them all side by side for stiffness to weight. My biggest problem, like I've outlined above, is trying to figure out where I go from there.

09-10-2014, 08:54 AM
AdamH, Here is a link for David Hurd's website that has lots of building information. Really helpful and quite a bit about deflection testing including a jig to do the testing


Matt Clara
09-10-2014, 09:06 AM
As a fellow newbie, I've stuck with 1.8 to 2 mm, and haven't gone wrong yet. By all means, read about it all you can, but I think this is one of those things that only experience is going to serve to teach you the finer points.

09-10-2014, 09:31 AM
As a fellow newbie, I've stuck with 1.8 to 2 mm, and haven't gone wrong yet. By all means, read about it all you can, but I think this is one of those things that only experience is going to serve to teach you the finer points.
The last 2 I completed were at 2mm and so is the 3rd one Im finishing up. 1st was Cedar, 2nd was E. Spruce and the 3rd is Afro Mahogany (Tops). Im not sure what they end up being after final sanding but they sound good, so Ive been told, and thats good enough for me for now. Maybe when I get a bunch more built I will get into deflection but for now I just want to produce a Uke with no issues. One that sounds great and looks clean and straight. Matt says it short and sweet with one word that stands out and thats experience.

Mark Roberts Ukuleles
09-17-2014, 01:12 PM
Hi Adam,
I would urge you to great a simple method for measuring deflection of various varieties of wood.
The effectiveness and characteristic of a top, or back, if relative to its flex/deflection, weight and density.
Thickness is only a generality, and does not relate to the characteristic of the wood that directly.
Every piece of wood is different. No two pieces of Koa, Walnut, or Rosewood are the same.
Keep a record. Cut all your tops or back to a similar size for consistency or measurement. Weigh them.
You can set up a simple method of measuring deflection, by supporting two opposite edges on a support. I use some 1" angle aluminum strips @ 14" long.
Add a micrometer beneath the plate and set it to zero.
Add a consistent weight on top of the center of the plate. I use an aluminum pipe @ 14" long.
You can see examples here:

Measure the deflection for that top or back. Write it down. This is for reference purposes.
As you build more, with this record, you will become to understand what worked well and what didn't.
You will begin to adjust for flex and deflection of the top...the most important aspect for tonal generation.
The instrument is an air pump. Make it work efficiently.
As you do this testing and recording, also take the time to flex the plates in your hands on both axis, tap the plates for tone, and observe the wood fibers characteristics.
With time, you will eliminate the mechanical testing for deflection and density and rely on your hands, ears, and eyes...but this process will speed up your learning curve.

Best of luck to you.

09-17-2014, 06:07 PM
Same here, 2mm and lower with or without tonebars.

QUOTE=Matt Clara;1576088]As a fellow newbie, I've stuck with 1.8 to 2 mm, and haven't gone wrong yet. By all means, read about it all you can, but I think this is one of those things that only experience is going to serve to teach you the finer points.[/QUOTE]

09-20-2014, 12:55 PM
and is that thickness and deflection only correct for a flat top? what if you build with a radius? do you go thinner since it's built pre-stressed so to speak?