Question: Back Seam Reinforcement

sequoia

sequoia

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I don't even know what these things are called so I'm going to call them back seam reinforcement patches. I decided I probably needed some insurance on this radioused back seam since it just seemed as Pete says "good lutherie practice". It couldn't hurt (the sound that is), so why not?

Since I've never done this before, I just kinda went by gut feeling. The only thing I was sure of is that the patch grain has to be perpendicular to the grain direction of the back. So I grabbed some spruce scraps and sanded down to the approximate thickness of the back (~0.075 or maybe 1.8mm). This resulted in a very fragile strip. I made them 10mm wide, glued them in. Now my question for you pros:

- Are these things even needed in an ukulele?

- Are my dimensions correct?

Thanks

DSCN7081.jpg
 
Out of the box take, as usual. A couple of years ago I had this thoroughly trashed laminated soprano. I needed some purfling, so, as the top, back and neck were history I put the body on the table saw and used a fence to cut out the purfling. Kept the same width and cut the rest of the laminated body into strips. I was going to double or triple them up for more purling. Well I ran across them the other day while looking for strips to reinforce a bookmark seam. The grain is a right angle to the top and the thin outside looks reasonable should keep the seam from separating and still allow flexibility. Never seen it done before but I will.
 
Its probably not NEEDED....but good insurance. Did you fit the braces over the patch or through it? I fit my braces through it.
Ditto on feathering the edges. Just so it looks right through the soundhole
 
Braces are through it... Yes, feathering will be done. But is this necessary was the question? I'm thinking 30 years down the line? Maybe. Maybe not. Who cares? But are we not supposed to care?
 
First... Why Spruce? Why not match to your backwood? I leave enough length on my back to use the left over strips for this purpose.
On another note....I couldn't help but notice your back bracing ..... They seem a little on the heavy side. If you get a chance to read the book "Left Brain Lutherier"
https://www.amazon.com/Left-Brain-L...=c&hvqmt=e&ref=pd_sl_12r4hxektx_e&tag=mh0b-20
you might consider toning them down a bit and his math shows why a triangular cross section might be a good way to go.
Every little thing we do in the construction adds to the quality of the end product... both good and bad.
 
One question I've not seen in relation to this back strip, is whether it needs to be radiussed to form a good glue joint, rather than glue a flat piece to a radiussed back piece. I would imagine the wider the strip, the more it would want to straighten out the radius of the back when glued up. Or would this be overkill?
 
It doesn't have to be radiused. That's really done just to make things look neat. Spruce crossgrain strip is traditional irrespective of the wood type used for the actual back. You can use virtually any wood though. Grain is sometimes placed running the same direction as the back grain. Both are more than adequate to reinforce the back centre joint, providing the centre joint is well done in the first place.
As for the back braces and their heft. Who knows? There are backs that are more (or less) lively than others, by design. Some makers try to minimise the vibration of the back as much as they can, others want the complete opposite. I'm sure there are various shades between the two.
 
tparse said:
First... Why Spruce? Why not match to your backwood? I leave enough length on my back to use the left over strips for this purpose.
On another note....I couldn't help but notice your back bracing ..... They seem a little on the heavy side. If you get a chance to read the book "Left Brain Lutherier"
https://www.amazon.com/Left-Brain-L...=c&hvqmt=e&ref=pd_sl_12r4hxektx_e&tag=mh0b-20
you might consider toning them down a bit and his math shows why a triangular cross section might be a good way to go.
Every little thing we do in the construction adds to the quality of the end product... both good and bad.
Click to expand...

All good observations and relevant. Let me respond:

1) Why spruce and not a matched backwood? Good question and thought about that. Why not? Because when it comes to weight to strength it is hard to beat spruce and that is what you want here. I admit that spruce looks kind of ugly there, but that is the best wood for the job.

2) Heaviness of back bracing: Two things going on here. One is that the picture might make them look a little bigger than they are in reality and two, there is a pretty radical radius on that back and you need a good footprint to glue to. And three, who cares, because I'm not a big believer in the acoustic flexibility of the back. I want it stable. This is not a sound board. However, your point is well taken and perhaps they should be taken down a bit. Maybe I was a little lazy there.

3) Mikey's question: Do the strips need to be radiused to the back? Good question, but the answer is no. The thin strips conform just fine... Oh and putting in strips with the grain parallel would defeat the whole purpose of the exercise if you ask me. A laminate ply needs to be perpendicular. Think of plywood with parallel grain. Not gonna work.
 
I use Spruce for my back seam reinforcement. Grain runs with the back. Not across. This is more than enough to reinforce any back seam that has been glued up correctly in a uke size instrument. And yes, I believe it's necessary after having to repair some store bought ukes over the years that didn't have them installed.

Mine are 12mm wide and 2.3mm tall. Edges are chamfered with a block plane. Braces cut through it.
 
sequoia said:
All good observations and relevant. Let me respond:


3) Mikey's question: Do the strips need to be radiused to the back? Good question, but the answer is no. The thin strips conform just fine... Oh and putting in strips with the grain parallel would defeat the whole purpose of the exercise if you ask me. A laminate ply needs to be perpendicular. Think of plywood with parallel grain. Not gonna work.
Click to expand...

The ply strip I use is 2 layers, the thicker one being glued to the boards is perpendicular the thinner decorative is parallel. Will post pictures soon.
 
Last edited:
sequoia said:
All good observations and relevant. Let me respond:

1) Why spruce and not a matched backwood? Good question and thought about that. Why not? Because when it comes to weight to strength it is hard to beat spruce and that is what you want here. I admit that spruce looks kind of ugly there, but that is the best wood for the job.

2) Heaviness of back bracing: Two things going on here. One is that the picture might make them look a little bigger than they are in reality and two, there is a pretty radical radius on that back and you need a good footprint to glue to. And three, who cares, because I'm not a big believer in the acoustic flexibility of the back. I want it stable. This is not a sound board. However, your point is well taken and perhaps they should be taken down a bit. Maybe I was a little lazy there.

3) Mikey's question: Do the strips need to be radiused to the back? Good question, but the answer is no. The thin strips conform just fine... Oh and putting in strips with the grain parallel would defeat the whole purpose of the exercise if you ask me. A laminate ply needs to be perpendicular. Think of plywood with parallel grain. Not gonna work.
Click to expand...

I don't think you are correct on point 3. I've seen many guitars with the reinforcing strip running parallel to the back wood grain direction. In fact Jose Romanillos throughout his whole long career did nothing but that. He wasn't the only one. There are two factors at play. Firstly, it doubles the thickness of the amount of wood in that area. Secondly, even if the grain runs the same direction it is highly unlikely that Spring and Summer grain lines of the reinforcing strip and the back wood are going to align. It's almost certain that they will cross even if they do align at the odd point. I'm not saying that it reinforces it to the same degree but it is certainly more than adequate, especially for instruments that aren't very long. I don't make all of my instruments with the parallel grain, roughly half have been and I've not heard of one single report of back seam separation.
 
I use spruce cross grain mostly but haver matched it with the back wood as well. As for the braces... I only use 2 with the lower one very wide and low. The back is also thicker than received wisdom would dictate and my instruments are always loud and sweet, or so I am told!
 
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