Soprano Back Bender

My noo camera arrived this morning, so now I can show some pic's of my latest project.
it's a "Timbuck Back Bender"..I was'nt happy with the form of some of my backs as they came off the Go-Bar Deck..My Soprano's only have two back braces and the spring back in the material causes most of the back arch to flatten out again..sanding the sides to a 10' radius dosn't seem to help much either..So I've made this device to heat form the back radius, prior to fitting the braces...I made the former from a "Drip tray" removed from a scrapped Electric cooker...I wanted to use a roof section cut out of a VW Beetle for this job ..but I couldn't locate one..the "first off" back piece I made with this method has turned out quite good..and it is now on a uke and being french polished...*Note*This is no good for bookmatched sets co's it would melt the glue seam.
Here is a slideshow of the build so far..just click on the pic.

Very Cool....I admire builders so much. Thanks for the slideshow Timbuck!

Very cool. If I ever build a ukulele factory, I am calling you to tool it up.

p.s. I bet that would steam some mean salmon too.

That's a clever jig for sure but I can't understand why you're getting ANY springback especially since the sides are radiused.... Two back braces are plenty for a soprano. I wonder if your trouble is climate related. You really shouldn't be having to go through such measures to maintain your radius. I'm not doubting you, I'm just confused. (My natural state of being these days.)

Very impressive! Nice work Ken

Moore Bettah Ukuleles said:

I can't understand why you're getting ANY springback especially since the sides are radiused.... Two back braces are plenty for a soprano. I wonder if your trouble is climate related. You really shouldn't be having to go through such measures to maintain your radius. I'm not doubting you, I'm just confused. (My natural state of being these days.)

Click to expand...

Yes! Chuck ..I think it could be the climate/damp/cold thats got a lot to do with it..it dosn't happen on all of them...Koa gives me no problems like that at all..it's just with Mahogany...Maybe I sand it too thin sometimes "working to close dimensions" and not taking the "feel of material" into consideration...I'm still learning that skill

Cool idea, and the results speak for themselves.

I'm as surprised as Chuck though that the backs are flattening out on some instruments. The only time I see that is when there isn't adequate humidity control during bracing and closing up the box.

Allen said:

Cool idea, and the results speak for themselves.

I'm as surprised as Chuck though that the backs are flattening out on some instruments. The only time I see that is when there isn't adequate humidity control during bracing and closing up the box.

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That explains it..I have zero humidity controll.

I used to go from one side of the island to the other doing uke building demonstrations. The humidity range was from between 25% and 95% depending upon where I was. Those demo components were a mess, all out of whack. I'd get them back to the shop and they would change some more.
Consistent climate control is essential.

The way I see it is... that with this method the back is not under stress and evenly arched..a bit like plastic back ukes "Flea's Ovation's etc:" ...with a go-bar deck you are forcing the back into shape..held there with the glue of the bracing and linings..areas that do not have bracing will spring back ..the way to find out..is to go over the area with a profile comb set to the radius required..anyway I'm just experimenting at the mo'
time will tell if it works...it's not a new idea there are a lot of guitars out there with heat/steam pressed backs and fronts.

The way I learnt about humidity control was bracing up my first guitar back in December during the monsoons. Temp and humidity out of this world. Out of the go-bar deck they looked beaut. But overnight they turned inside out. I was devastated, and combed the forums trying to find the cause.

I watched day after day the back go from one extreme to the other. From being convex to concave, just hoping for the right time to get a chance to glue it to the rims while it was in the shape that I wanted.....I've learnt a lot from that time.

Now all wood goes into a humidity controlled room a couple of weeks prior to building. All bracing is done in that room and the box doesn't come out until it's closed up. Once that point is reached it comes out into the "real world" and learns to fend for itself.

I still like your idea. Especially with some complex and extreme bends like you would see on arch tops. Might just be able to make a "faux" archie this way without laminations.

It's humidity Ken - we all come to it one way or another. Glue up all components in between 45% and 55%...

What Ken Timms is saying in terms of built in tension makes sense to me, but I am not a luthier, so take it all with a grain of salt. Everything I read says that cold bending is not as permanent as hot bending because hot bending breaks down the bonds somehow and permanently changes the wood, so it stays pretty much like you bent it after it dries out.

I can't see why a top would be any different than a side with respect to how permanent the bend is with heat or not. Dave G has made some cold bent instruments, which held up well, but he seems to favor Spanish necks, which pretty much ensure that the sides can't go anywhere once they are assembled into the neck and glued to the tail block. Tops and backs are kind of the same way - once they are glued to the braces and then to the sides they really can't go anywhere unless the glue joint breaks.

But the real question is whether hot bending a back is necessary. Luthiers that use arched backs have been building guitars and ukes for a long time simply by cold bending them on a solera and gluing them up with braces. It might even be that luthiers would not even use heat on the sides if they used a different method for holding it all together (such as the Spanish neck) and it did not speed things up and help prevent cracking the wood as it is bent.

Interesting.

Another method of getting a plate to take a radius and is used by several luthiers I know is to over dry the plate in an oven at around 100 - 110 F. After being in the oven for about 1/2 hour the wood is super dry and they then brace it on a flat board. Afterwards the top or back sucks up ambient moisture and takes on a radius. I've heard that they will use braces that have either some radius shaped on to them, or none at all.

I'm very happy with the way the the first off from the experiment turned out.
I wanted to know what the end result would be? after Water based stain was applied and several coats of French Polish...The results are in the pic's below..It looks like I'll be doing all my sopranos this way in the future.
The main problem now is that, I feel the uke fronts could do with a similar treatment......*EDIT * i've bought a Hygrometer thingy as well (just to cover my arse.)

THE UPPER BOUT

THE WAIST AREA

THE LOWER BOUT

LENGTHWAYS

I've been reading this thread with interest and a couple of thoughts come to mind.
I have the same problem with humidity, living in the northwest of England it's not like
Seville so I keep all my wood in the same place in the same room all the time.
I take a piece of wood to my workshop when I need to work on it, when that
job is done it goes back on its shelf. I was planing up a top and back the other day
an in the time I planed the top the back had a good 5mm curve on it.
This dosn't completly cure the problems but goes a long way to help.
The Soprano I finished a few weeks back still has a domed back but not as much
as when I glued it up. It's settled down now.
So to get to my points, moulding the back will help the issue because like bending the sides
the process softens the wood fibres and so it will stay pretty much where its moulded to,
and to my second point and this applies to the sound more so,
if you mould a back or sound board do you not take the tension out of it.
The way I learnt how to make classical sound boards using a solera all the tone bars/braces
are made straight and then glued to the soundboard on the solera forming a domed soundboard
that is under tension producing a stronger more resonant sound board.
...........my 2p.

narrowuke said:

The way I learnt how to make classical sound boards using a solera all the tone bars/braces
are made straight and then glued to the soundboard on the solera forming a domed soundboard
that is under tension producing a stronger more resonant sound board.
...........my 2p.

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It is an interesting topic indeed, narrowuke. I do not believe that it is the tension in a cold-bent arched back that makes it stronger - it is the structural effect of it being an arch that transfers vertical load down the arch to the sides making them partially horizontal in such a way to keep the arch from bending as much. Here is a very short video that illustrates well how much stronger an arch is to vertical loads than a flat span.



None of those old Roman arches had any steel reinforcement, let alone steel bars tensioned in the opposite direction to the expected load. Ignoring the humidity question for a moment, about the only mechanical load we have to worry about on a ukulele back is something pushing from the outside and tending to push it in towards the body. If the top is too weak in the vertical direction the top would become concave and sag in at the middle. With an arched back, the load is transfered to the sides tending to push them out, but they are strong enough to hopefully not split apart. Violins are no slouches in the sound department, and their arched tops and backs are not pre-tensioned. Instead they are carved into the shape they need to be when assembled.

There are such things as pre-tensioned concrete beams, and indeed they are stronger if the tensioning steel rods are in the right place. If you think about the example in the video we would want to pre-tension the arch in such a way as to resist flattening out when we put a weight on it. That would mean that we would want to have the top compression and the bottom in tension, since the weight would impart the opposite forces. When we bend a top cold in fact the forces we put on it are just the opposite of what we want - i.e. the top is always trying to flatten itself out.

In short, I believe that Timbuck's method gives us a strong arch, without imparting a pre-tension which is fighting against us.

All the science aside, for a bit anyway, we really need to look at the function of the arch on the instrument too. As I understand it, the purpose of the arch isn't structural but either to accent the tone or simply because its traditional. Whichever purpose you choose may influence your choice on how you create your arch. I would think that creating an arch that is pre-tensioned, as in the example with the solero, then I would think the back wouldn't "pump". Honestly, I only put an arch in my backs because it has more intrinsic value than a flat back.

I wonder about doing a carbon fiber laminate on the braces to help stiffen them.

An arch does more than just give a better look to the instrument.

An arch give some room for movement to the plate due to humidity fluctuations. Will raise or lower without too much adverse effects to the instrument. A flat one has a much greater likely hood of developing a crack.

A arch by it's nature gives us a stronger structure for a given thickness. Hence we can build lighter (more responsive) by building in an arch.

By changing the shape of the arch and bracing we can affect the sound from the instrument. Though in small instruments like ukes it may be less evident. On guitars it becomes quite apparent. We can shape these to give an intimate enveloping sound that surrounds the player but doesn't carry that far, or one that throughs sound to the back of the concert hall, but doesn't sound all that loud to the player.

I saw and heard some great demonstrations of this at a luthiers conference with Greg Smallman a couple years ago.

Allen said:

An arch does more than just give a better look to the instrument.

An arch give some room for movement to the plate due to humidity fluctuations. Will raise or lower without too much adverse effects to the instrument. A flat one has a much greater likely hood of developing a crack.

A arch by it's nature gives us a stronger structure for a given thickness. Hence we can build lighter (more responsive) by building in an arch.

By changing the shape of the arch and bracing we can affect the sound from the instrument.

Click to expand...

EXACTLY. Thank you Allen.

And Ken, I know what your problem is now. You have a radius on your straight edge!