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Fritz
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[*] posted on 11-8-2014 at 01:58 PM


Quote: Originally posted by faggiuols  
the ribs at the point of rupture. is the same for both. obviously there is a weak point along the rib. you think it is possible?


This is the result of bending too fast and on a too small diameter ! A very small area is heated and the further areas aren´t ! So the wood will break if too many stress while bending.

Use a more oval tube with enough diameter and make the wood wet enough.

Allways keep a small sponge beside to re-wet the wood when it becomes too dry.




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faggiuols
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[*] posted on 11-8-2014 at 03:13 PM


thanks Fritz
I am happy that you have come to give me advice.
I need it as you can see!
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[*] posted on 11-8-2014 at 03:24 PM



Quote:

Sometimes its a change of the direction of the grain what creates some probs bending the rib in the right curve. So you have to adapt the stress to the rib on the grain and the abilitư of the wood to be bent. The wood "says" if it will bend more wet or dry, more in a bigger area or in small steps...

Unfortunately I do not have many cartridges in my gun. if I break a lot of ribs will be a big problem.
first of all, I organize a hot iron better than mine and I see if it improves the work.
when I made the guitar I have bent the wood with a liuther's hot iron and the work was not that complicated.
with my hot iron made by me I felt it was much more difficult.
I'll try with better equipment.
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faggiuols
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[*] posted on 11-8-2014 at 03:36 PM



Quote:

This is the result of bending too fast and on a too small diameter ! A very small area is heated and the further areas aren´t ! So the wood will break if too many stress while bending.

surely, as I noted Jdowning, also the temperature of the iron was not sufficient even.

for this I may have forced too much wood, breaking it.
However, the fact that the break is in the same position in both ribs suggests a weak point in all ribs.

I thought I would find the point on the ribs before folding and mark it with a sign to increase attention during bending.

What do you think?
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jdowning
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[*] posted on 11-8-2014 at 04:55 PM


The temperature of the upper surface of the bending iron tube is infinitely variable - dictated by where the flame is applied on the underside of the tube and the intensity of the flame. The uniformity of the surface temperature also depends upon the heat capacity of the tube (greater wall thickness = greater heat capacity).
The tube MUST be open at both ends as a propane torch requires air for combustion. Do not attempt to close the end of the tube. At best this will cause 'flame out' due to lack of oxygen or worse, flame 'blow back' that could cause personal injury.

A woodworker must learn to 'read' the grain of a wood in order to work it effectively - not only when planing wood but when bending it. The wood will tell you how it can best be fashioned.
If you have identified a weak spot in your ribs 'the hard way' (by breaking a few) then by all means mark the weak spot on the remainder of the ribs to take greater care when bending in that location.
You may find that with a hotter and larger diameter iron the breakage problem will disappear. You may even be able to recover those ribs that are intact but uneven in curvature by re-working them on the new bending iron.
I would suggest that, once you have made a new bending iron, to trial bend the broken rib pieces or non uniform ribs to gain experience with finding the best temperature and the method before working with the rest of the ribs.

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[*] posted on 11-8-2014 at 11:34 PM


thanks Jdowning
the procedure you described is exactly what I had in my mind.
well.
Now I have to do a hot iron.
I tried to buy a tube of 3 mm aluminum but they sell only 6 meters long!
I have to turn some places.
do you think I can recover broken ribs putting some glue in the breaking point? or are they lost forever?
thanks
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[*] posted on 11-9-2014 at 05:39 AM


Copper makes the best bending iron. Aluminium is OK but has only about 60% of the heat conducting capacity of copper. Iron tubes were used by the old Spanish luthiers - mounted vertically over a charcoal fire - but iron has only about 10% of the heat conducting capacity of copper so plenty of heat required but iron (steel) might work with a strong propane flame.

Some of the old lutes seen in paintings had two piece ribs of contrasting woods (i.e. with a horizontal joint at the mid point of each rib) so a cracked and repaired rib would be structurally viable - although it would be practically difficult if not impossible to make a crack invisible. You might also have problems re-bending the ribs to a smooth curve across the cracked area. One approach might be to glue strong paper reinforcement over the cracks (with hot hide glue scorched in place) - on the outside surface of the rib, of course. This might be sufficient to hold everything together during bending - the paper can then be removed with hot water and scraping at a later time. Only one way to find out - try it and see.
Another alternative, if you do not have enough rosewood ribs remaining, would be to make up extra ribs in a contrasting colour wood so the ribs of the bowl would be arranged as alternating dark/light colour. Be aware that the oils in rosewood can stain lighter woods - particularly during finishing work on the bowl.
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[*] posted on 11-9-2014 at 09:49 AM


thanks Jdowning
your posts are always very interesting. I reread some of them many times to store them.
I have several pieces of rosewood, so I think I have enough wood, unless I keep breaking them.

So I hope I haven't the need to repair the broken ribs, at least I hope so. but if I had to break them all maybe it's better if I stop doing oud and start working to make jerseys and socks!!:D
But what you write is very interesting and important. repair errors is an art, perhaps making strengths.
so thanks again
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[*] posted on 11-9-2014 at 04:08 PM


With experience and skill an oud may be created with some of the most basic tools. The simplest possible rib bending apparatus and procedure is shown here on the video at 0.45 to 0.49 minutes. Sufficient heat is all important.

http://www.youtube.com/watch?v=kpr9pXEni3I

Just for information!
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[*] posted on 11-10-2014 at 12:20 AM


I saw this technique on youtube and Samir suggested it to avoid the hot iron.
it seems more difficult then hot iron.
But I can try.
I'll tell you later.
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[*] posted on 11-10-2014 at 12:27 AM


practically this technique is that of this video ..

https://www.youtube.com/watch?v=rvsdS9sbuq0

I have an identical gun!
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[*] posted on 11-10-2014 at 05:08 AM


Your choice faggiuols - whatever works for you.

When I started making lutes over 40 years ago some lute makers simply held the ribs in front of a radiant heat electric fire to bend them - but then lute ribs are usually only 1.5 mm or less in thickness so are easier to bend uniformly than thicker ribs with such a heat source.

I suspect that the preferred method among most luthiers is a bending iron as it provides greater control over the bending process as well as providing support of a rib to minimise risk of breakage during bending. This is particularly important in fabricating a lute bowl as the rib profiles must match each other more precisely than the ribs of an oud bowl - the ribs of an oud bowl having extra thickness to allow the exterior (and sometimes the interior) surface of the bowl to be shaved down to a finished smooth profile.

Richard's video was only a quick demonstration of the principle behind using a hot air gun as a heat source but the bent rib blank looks a bit non uniform in its curvature?

I doubt if a hot air gun would work very well with the wider and relatively thick ribs of a guitar - but that's another story.
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[*] posted on 11-11-2014 at 01:00 AM


Hello Jdowning and everyone
I tried to bend the ribs with just a heat gun and I must say I am very happy because the work I was very easy. I tried without water. when the wood is very hot it becomes plastic and workable.
before using rosewood I tried to use the mahogany that I had. Mahogany is easier and above all it does not have that damn weak point that my ribs of rosewood.
in any case it seems feasible in this way.

I inquired to find a piece of copper tube as which ones you suggested me. The price of copper is very high. if you add the rest of the components and the time to build it I almost should buy a bending iron professional!
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[*] posted on 11-11-2014 at 01:01 AM


here's the first rib bent.
the second that has been worked much better but I forgot to photograph it.
has some flaws, but in the second I easily corrected these flaws.
I think the next will be in a satisfactory manner.
[file]33281[/file]
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[*] posted on 11-11-2014 at 01:08 AM


Now I let them fit in the mold.
I saw that with time the bending is released and the rib loses curvature.

those bent yesterday with pins white!
the work seems good enough.
improvement but good!

[file]33283[/file] [file]33285[/file] [file]33287[/file]
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[*] posted on 11-11-2014 at 02:18 AM


:xtreme:

Yes keep going ragazzi! Make sure the bend is perfect against your template. Also it's a good idea to bend each rib again to the final shape before you glue it.




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[*] posted on 11-11-2014 at 08:00 AM


Quote: Originally posted by SamirCanada  
:xtreme:

Yes keep going ragazzi! Make sure the bend is perfect against your template. Also it's a good idea to bend each rib again to the final shape before you glue it.

Samir, thanks for sharing.
do you think it would be better to use the water too, or is it better to fold in the dry?
I think I'll try to get wet ...
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[*] posted on 11-12-2014 at 12:17 AM


another bit of work done.
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[*] posted on 11-12-2014 at 12:20 AM


I leave the ribs to take shape about 24 hours, then I take off them from the mold and I keep them that way. if you look closely you can see differences in the curvature.
those differences can be a complication later during bonding?
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[*] posted on 11-12-2014 at 12:25 AM


as the weak point of the ribs I still had problems, I developed a reinforcement to be used during the bending of the ribs.
with a small vise block the weak point and do not submit it to bend.
I hope that it bent during installation in the mold.
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[*] posted on 11-12-2014 at 12:29 AM


I tried to soak the wood during bending but did not find a difference.
in any case it is necessary to proceed very slowly and carefully. if you try to speed up the work the wood rebels and the job is bad.
under the image of the heat gun.

[file]33327[/file]
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[*] posted on 11-12-2014 at 05:57 AM


Your current difficulties are due primarily to insufficient heat in bending. Wood takes a permanent bend due to softening of the 'glue' (Lignin) that binds the wood cells together. If the heat is insufficient and so does not penetrate the full depth of a rib, the Lignin in the outer layers will only be partially softened or not softened at all so may cause difficulties in bending a rib or result in significant 'spring back' of the rib curvature on cooling. Bending a rib with heat should not be a slow process or require application of significant force - as you report - if heat is sufficient. Spring back should be minimal or non existent.

The irregularities currently visible in your bent ribs may well cause problems when you come to assemble and fit the ribs on the mold due to mismatched rib profiles along the glue joints. The ribs should match the mold longitudinal contours with some precision without forcing a rib to fit the mold as you are proposing. That is not the function of a mold as some may assume.

You may be able to literally iron out the local irregularities in your ribs (or even bend the ribs entirely) with a standard electric household iron (used for ironing clothes) set at high heat - ironing the rib over a piece of suitably curved wooden mold held in a vice. This no doubt will also give you better control over the bending process.

To reinforce the weak spot in your ribs you might try gluing paper reinforcement over the area with hot hide glue as suggested earlier in this thread. The glue soaked paper on the rib should then be heated to immediately form a hard crust (scorched) using a hot iron. A household electric iron will do the job but use an old iron as hardened glue residues will stick to the sole of the iron. These residues can be removed with hot water (first unplug the iron from the electrical power source!) but it is advisable not use your wife's best household appliances for luthier work.

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[*] posted on 11-12-2014 at 06:26 AM


Quote: Originally posted by jdowning  
You may be able to literally iron out the local irregularities in your ribs (or even bend the ribs entirely) with a standard electric household iron (used for ironing clothes) set at high heat - ironing the rib over a piece of suitably curved wooden mold held in a vice. This no doubt will also give you better control over the bending process.


this is great advice... I have done this before and it works to get you close, you can then use the heat gun to get you close.




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[*] posted on 11-12-2014 at 06:56 AM


thanks Jdowning
your posts are always interesting and instructive.
I did not know of lignin!
very good idea to iron the rib before bonding.
will definitely help when it alongside the ribs.
of course this work will be done before working on the ribs to finish, otherwise you will not be able to see when the ribs coincide perfectly! I got it right?
Instead I've not understood well the reinforcement rib paper with glue. The glue when I switch the iron on the rib should return to liquid preventing the completion of the bent! I can not understand how the iron would "burn" the glue does not melt glue, or when the card is cemented?
I have seen the hot glue burn, but when subjected to great heat as the heat gun!
if I remember correctly I have an iron at home that I could use!

I'll take it in secret from my wife !!
then I will tell her that the economic crisis is turning a lot of people thief, and I doubt the next-door neighbor !!
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[*] posted on 11-12-2014 at 06:58 AM


Quote: Originally posted by SamirCanada  
Quote: Originally posted by jdowning  
You may be able to literally iron out the local irregularities in your ribs (or even bend the ribs entirely) with a standard electric household iron (used for ironing clothes) set at high heat - ironing the rib over a piece of suitably curved wooden mold held in a vice. This no doubt will also give you better control over the bending process.


this is great advice... I have done this before and it works to get you close, you can then use the heat gun to get you close.


thanks Samir

I will try this way. I update you on the results!

... If my wife does not notice anything ....
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