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Thread: rib bending

  1. #1
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    I've been building for nearly a year now, and have completed two "F" body mandolins, with three more currently under construction, Untill now, I have yet to assemble a rim set that didn't have some grain lifting to deal with, except for the most recent attempt. I've used a steam chamber, torch fired bending iron, and most recently an electric bending iron, (the later seems to work the best for me).

    My question to the builders on here is, how many of you still have issues when bending your ribs? Do you start over with a new piece of stock or repair the lift? I'd appreciate any advice from anyone.

    I also need to ask if any of you have used amonia in the water that you soak your ribs in prior to bending. I was given that "tip" from a wood supplier. I used wood from the same tree on two different rim sets. One I bent without soaking in amonia water, one with. Guess what,...the one soaked in amonia water was bent without any grain lift whatsoever,....first time for me! What I learned......the water used to soak the rim slats turned a tea consistancy, (brown), and after the wood dried, it was darker, but the grain seemed to stand out much nicer. I like the look!

    Now, am I on the right track? is the amonia detremental to the wood?

    any comments are welcome...thaks

  2. #2
    Registered User sunburst's Avatar
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    Are you using a bending strap?
    That usually prevents most grain lifting. It's a thin piece of stainless steel, copper, brass, something that won't stain the wood, held tightly against the outside of the bend.

    Soaking for too long can make the wood grain more likely to separate. Soaking is generally not necessary. Just the moisture in the wood is usually enough for me, though I keep a damp sponge or a spritzer bottle near by while bending sides.

    I don't know about the amonia in water. I've never heard of that. I've read a little about amonia plasticizing of wood, but that's amonia gas. It requires expensive, sophisticated equipment, and it's very dangerous if the expensive sophisticated equipment goes wrong.
    My first thought is; the small amount of amonia in the water couldn't make any difference, and the guy who told you to try it doesn't know what he's talking about, but maybe he's right, and I'm the one who doesn't know what he's talking about.
    The amonia plasticized wood that I've seen had a dark, sort of greenish-grey-brown color.
    Maybe someone will post and shed some light on the amonia water, and what it does.




  3. #3
    Adrian Minarovic
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    Ammonia is a base, and may react with your stains, which are often slightly acidic... It may take years, but some of your color may turn into green...
    Use care with chemicals.
    Adrian



    Adrian

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    I started bending dry a while back and have much better success. It takes a little longer, but I don't get as much scorching and much less grain lifting. Additionally, if your sides are too wet they seem to want to cup a bit when they fully dry. In really tight bends I will sometimes thin the wood a little more. (this usually coincides with locations of the blocks so I don't worry about strength) If you think mandos have tight corners, try a violin c-bout! (much thinner wood though...)
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    Registered User sunburst's Avatar
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    One other thing that I just thought of. I learned this from someone here on the cafe.
    Slow down! In the tight bends, if it feels like it's just stopped bending, leave it alone, let it cool, wait a little while and try it again. It will bend more when you come back to it. If you have to, leave it and come back to it a couple of times. If you keep trying to get the wood to bend without letting is "rest", it's more likely to crack, break, or separate.

    I wish I could remember who posted that advice originally, because it's helped me. I don't recommend this to others, but I finished bending a rim for an F one day, and realized I forgot to use my bending strap. No cracks, no lifted grain, no problems. I don't use the strap anymore, since I learned to slow down.

  6. #6

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    I use Docys heat blanket bending system. I sold my bending iron after I got this. Quarter sawn is no problem now.
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    Chris

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    Thanks for all of the imput! Yes I do use a backing strip, 6061 t6 heat treated aluminum sheet. As for the amonia tip, you guys would flip if you knew who this information came from, ( a highly respected builder, but still second hand info). After further review, the patience advice is probably the best yet., (not one of my best abilities). Man, do I like the heating blanket system! tell me more about it!...Thanks

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    Registered User sunburst's Avatar
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    Quote Originally Posted by (junksuph @ Dec. 07 2005, 12:47)
    ...heating blanket system! tell me more about it!...
    They work fine, but if you want to make a custom shape, a different size, or a different instrument, you might want to keep your bending iron.

  9. #9
    Registered User barry k's Avatar
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    I have been told that IF, you soak your sides, that some guys use fabric softner in the water? I just use an atomizer and "spritz" as I go. Good advice to slow down,even Oak will bend with patience.

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    HoGo is right that ammonia is a chemical base, and will react with acids (the tannins in the wood). The more tannins the darker the color change.

    Ammonia is not naturally a liquid, but a gas, so you can use household ammonia but don't soak the wood in it. Household ammonia is only about 5% in liquid form but will concentrate as a gas. Place the wood just above the liquid in an enclosed container, so the ammonia can gass off and still be contained. It is the gas that effects the wood. It might take a day or two to make the wood soft, but when it becomes like stiff leather it is time to bend it.

    Strong ammonia is used to Mercerize cotton thread. This process subjects the fibers to the base along with dye. The dye is activated by the ammonia and the fibers are softened and pick up the dye. The thread is put under a tensile stress and dried, which makes the thread both stronger and more lusterous.

    In short, ammonia won't hurt the wood but can change it's color. Ammonia can also cause the wood to shrink across the grain.

    I have also bent wood with lye. CAUTION !! LYE IS DANGEROUS!!! It can cause chemical burns. Course, so can really strong ammonia.

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    I suppose that if you were going to do an oil finish and did not use any stain that you could soak the side wood in vinegar. I have seen bones soaked in vinegar that were tied in knots. I don't think that stain and vinegar would work but on a blond finished with oil, it might. I am not sure because I have never tried it.
    "If at first you don't succeed, then keep on suckin' till you do succeed."

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    Registered User sunburst's Avatar
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    Vinegar (an acid) dissolves the calcium in bones and leaves the flexible tissue. Wood doesn't contain calcium, (normally) so unless you're building mandolins out of bone, you're not likely to be able to bend sides with vinegar.

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    I've found that a few thousands makes a huge difference in bendability. In my first efforts I thinned my ribs to .115 and found myself breaking alot of wood. After talking around with other builders I reduced that dimension to .090, which made a world of difference. I use a spray bottle to add a little moisture as I go along. With a good strap held as tightly as I can muster I have gotten pretty confident in bending without breaking or lifting.

    Ragman

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