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Fritz
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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?
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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.
Music is the food for the soul
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faggiuols
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thanks Fritz
I am happy that you have come to give me advice.
I need it as you can see!
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faggiuols
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| 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 abilitý 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...
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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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| 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.
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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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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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faggiuols
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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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jdowning
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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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faggiuols
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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!!
But what you write is very interesting and important. repair errors is an art, perhaps making strengths.
so thanks again
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jdowning
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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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faggiuols
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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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faggiuols
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practically this technique is that of this video ..
https://www.youtube.com/watch?v=rvsdS9sbuq0
I have an identical gun!
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jdowning
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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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faggiuols
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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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faggiuols
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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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faggiuols
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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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SamirCanada
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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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faggiuols
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| Quote: Originally posted by SamirCanada |
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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faggiuols
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another bit of work done.
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faggiuols
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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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faggiuols
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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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faggiuols
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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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jdowning
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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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SamirCanada
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| 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.
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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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faggiuols
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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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faggiuols
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| 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.
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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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thanks Samir
I will try this way. I update you on the results!
... If my wife does not notice anything ....
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