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Thread: side bending - cracking

  1. #26
    Registered User sunburst's Avatar
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    Default Re: side bending - cracking

    Are you using a bending strap?

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  3. #27
    Registered User bennyb's Avatar
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    Default Re: side bending - cracking

    First of all, listen to John(Sunburst). But even allowing for unknown magnification, angle and "artifacts" that if very thick looking for .075".

    benny

  4. #28

    Default Re: side bending - cracking

    Sunburst -- I've tried both with and without a bending strap. Cracked both ways. Benny -- I think it's a perspective oddity, I've measured thickness at 075 with a digital dial indicator. I managed to bend one other piece of this material but it took many hours and it's still not quite where I want it. Anyway, the question still stands -- can maple be too dried out or brittle to bend successfully?

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  6. #29

    Default Re: side bending - cracking

    Sounds like your iron may not be hot enough. Water should bounce off, not simmer off. Use a strap. Wet the side going against the iron. Use a fluid back and forth motion as you bend the ribs.

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  8. #30
    Registered User John Kelly's Avatar
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    Default Re: side bending - cracking

    I use a very thin nickel silver sheet that I hold against the side, sandwiching the side between this and the bending iron. From your photo it seems that you are concentrating on bending the side at a very narrow point - both sides beyond the crack seem very straight, and it may be that you are applying bending pressure too much on that one point. Does the wood show scorch marks from the bending iron? Since you have tried varying the amount of water you are using, from dry to soaking, it may well be technique rather than moisture content in the maple, but I await responses from others. I am not acquaint with hot SW desert conditions as I live in the cool/cold and damp Scottish Highlands!
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  9. #31
    Registered User j. condino's Avatar
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    Default Re: side bending - cracking

    So we've taken this thread 12 years to accomplish a 4 minute task.....

    No scorch marks means your pipe is too low temperature.

    Turn up that bending pipe until it is sizzling and take one of your broken pieces to use as a test piece and heat it up until it burns. "Feel the burn...". You should be able to make an F5 scroll bend in under 30 seconds. Once you have it working with the test piece, turn the temp down little and bend a few more curves.

    The only way to get good at bending wood is to bend lots of wood. Take all of your scrap shop woods and break and burn them so you can figure it out. Try some thin veneers as they bend almost instantly with no water. Before I let any students bend their actual ribs, they practice on a half dozen test pieces.

    Then use your nice wood.

    Some people have a more number oriented approach. Most of the rosewoods will not budge until you get the pipe approx. 275 degrees F. At that point, they plasticize and bend like hot taffy. Maple bends at a slightly lower temperature. In an enclosed system with heat blankets, the temp can get higher because their is no way to release the heat. Maple torrefication happens over a longer time at 190-240 degrees C (374 F-464F).

    Keep at it, you'll be pleased with the results.

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  11. #32
    Registered User sunburst's Avatar
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    Default Re: side bending - cracking

    Quote Originally Posted by Tom R View Post
    ...can maple be too dried out or brittle to bend successfully?
    I don't think so. I believe more seasoned wood bends differently from newer wood, with newer wood being easier, but that is anecdotal. I have no evidence one way or the other. Wood is wood no matter how old; mostly lignin and cellulose, but perhaps it is more difficult to bend when it is older. As far a drying out, even very old wood can become saturated with water under the right (wrong?) conditions.

  12. #33

    Default Re: side bending - cracking

    Thanks for all your suggestions. Much appreciated. The bending iron is at about 400F according to surface temp gauge. I have been practicing with other material and it bends relatively easily. The maple in question is quite different.
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    With the upper piece, I was pretty aggressive and it bent quickly. With the lower piece, I went much more gingerly yet it still cracked. You can see that it has some figure to it whereas the easily bent piece does not. This could be the main difference, eh?

  13. #34
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    Default Re: side bending - cracking

    I think you need to have a better understanding of wood grain and what is going on here. Imagine that the wood is a bunch of little straws all bundled together, because in a sense that's what it is. With the maple without the figure, all the little straws are long and straight. Grab a bunch of soda straws in your hands. You can bend them quite easily. When you get into figured wood, like curly maple, those little straws are all "buckled", going up and down, wavy like they look. What you see in the dark bands in curly maple is actually the shadow of the ends of the little maple straws. Light color- sides of the straws. Dark color- ends of little straws. So imagine that handful of straws in our first example is now a bunch of little short straws. Now imagine what it's like to try to bend that bunch of little short straws so that they don't bust apart. You're relying on the "glue" that holds them together rather that the straws' internal strength. It's a whole different animal. See how that piece of curly broke right across at one of the curls? That's the end of the little straws. A bending strap really helps, for obvious reasons.

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  15. #35

    Default Re: side bending - cracking

    Thanks Dale. Very informative. I have used a strap with same results. The one success I had with this wood took several hours to get the bend I needed. So I guess patience is the other necessary ingredient.

  16. #36
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    Default Re: side bending - cracking

    Tom, I bend my sides at about .080. Anything much thicker is just a pain. Patience is a virtue. You didn't show the back sides of those pieces. Was there some scorching? If there isn't, you're not hot enough. Practice on the easy non figured stuff. Just relax, constant light pressure, constant rocking back and forth like rocking a little baby. When the wood gets to the right place, you will feel it. It's magic. It becomes plasticy. Like James said, break some pieces and pay attention to what you were doing, what the wood was doing, and all such. No rushing and complete concentration. After you bust a few pieces of the easy stuff, you'll know when you're trying to force it. Then you don't do that again. It's like trying to coax someone into doing something that they don't think is a good idea. Or cooking a frog slowly....

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  18. #37

    Default Re: side bending - cracking

    My experiences mirror Dale's. 0.080" max thickness. I've seen some builders go much thinner in the scroll area. The wavier the grain, the slower you have to go, the tighter you hold the strap down, and more misting the wood. I'll add that I have the mold handy and clamp the sides inside it when I've got them bent to where I want them. They stay in there overnight, less springback that way.

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  20. #38

    Default Re: side bending - cracking

    Back when I was building mandos years ago, I was using big leaf maple for sides and backs and resawing my own wood. For heavily figured wood, I found that if I sawed it about 20 degrees off of perfectly quartered, you could tie it in knots on the side bender. Only made a very slight difference in appearance.
    Brad

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  22. #39

    Default Re: side bending - cracking

    I learn so much here. Thanks to those who ask questions, and the fine folk providing free information. I am new to building, and made a mold and bought a silicone heating blanket($35.00). The, lightly curled maple .070 side was dampened lightly, wrapped with wet paper towel, then covered by aluminum foil. It was then set inside the heat blanket and set loose in the mold, then heated up, wait a few minutes, start clamping, wait a minute, finish clamping, wait a minute, turn off heat.

    I feel like I'm cheating, but have yet to find out how well the wrap and heat blanket method works with very curly maple.

    My first set of sides were 'on the quarter', is this typical or a requirement?

  23. #40
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    Default Re: side bending - cracking

    Looking through MC to be sure Ive covered everything for fourth edition of The Ultimate Blugrass Mandolin Construction Manual (to be released in early 2021) and I noticed this post on rib bending. Id like to share a few things:

    1) Ideally the ribs should be between .100˝ to .120˝ for bending - and probably finish to about .080˝ after final sanding.

    2) Unless I missed it - which it totally possible - I dont see any comments in this post about grain direction, but its very important. The diagram below shows the ideal orientation of the annular rings through the ribs. Very difficult to bend wood that has perpendicular grain as in A. The wood in B will bend very easily, but the long-term stability of flat grain is questionable. Ideally, the annular rings should go through the ribs similar to what is shown in C."

    3) We bent our ribs using steam. We had a rather large steam generator, and we soaked our ribs for about 20 or 30 minutes in medium warm water. Then we put the ribs into a fixture that had two long aluminum straps and basically rolled it up onto a cylinder for the scroll area, and then bend it sideway for the reverse curve below the scroll, and then wrapped it around a form for the rest of the body. We applied a high blast of steam to the wood as it was being bent. The heat and steam quickly softened the wetted wood and it virtually bent like spaghetti. We could bend the long rib piece in about 20-30 seconds. We had separate fixtures for the W piece and for the piece from the headblock to the first body point. As soon as the wood was bent, and we shut off the steam, we removed the wood quickly from the fixture and clamped it into drying forms. Out of 10 ribs per bending session, we had maybe one piece break every other session - so about one out of 20 pcs broke. I can dig up photos if folks want to see more.

    4) For hot pipe bending, using aluminum straps and keeping the wood wetted between bends is really important.

    R
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  25. #41

    Default Re: side bending - cracking

    Thanks, Roger: all illuminating points. Grain has been mentioned, but just in reference to highly figured wood where grain ends intersect the surface all over. General woodworking steam bending usually specifies B - which you can easily see in furniture. The mention of two straps is important, as pipe bending with one strap constrains the outside, tension part, which is helpful, but the inside, against the pipe, has very non-uniform pressure and temperature. Your method is similar to bending in a press, where the wood is captive between dies, and heated uniformly. Not so easy in a minimal shop environment, so more breakage. If warping after bending or resistance to shock and stress are issues, as in, say, a bentwood chair, B is probably best, but for the shape and constraints in a rib, C should be fine, and most decorative. No, I’ve never bent, successfully, a piece of wood.

  26. #42

    Default Re: side bending - cracking

    I've been using a simple aluminum strap on the outside only. So I should be using one on the inside as well? I'll give that a try. I'm using a propane heated brass pipe. I was using black iron but that seemed to cause discoloration of the cherry rims I bent last time.

  27. #43
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    Default Re: side bending - cracking

    Parker... I think you'll find having straps on both side of the rib piece to be very helpful. It will help distribute the heat and reduce surface burning or charing of the wood. The problem comes in being able to hold and manage two pieces of aluminum strapping plus the rib piece.

    Key to getting the straps to do their job is to not just pull down the straps down onto the bending pipe, but to put them under tension - pulling outward - so their lateral force against the pipe is distributed over a short distance rather than at the immediate bending point. (I hope I've described this clearly - don't hesitate to ask if you need more clarification.)

    Here's a photo of the fixture we used to steam bend the "S" piece that goes from the headblock to the first point. I hope you can see that there are two straps and that each strap is individually spring loaded and placed under tension. The rib goes between the two straps. As steam is applied, the two center bending posts are mounted on a plate and rotated about 100 (the black knob to right of center is the handle that rotated the posts). The fixture is then locked in place, the rib piece is allowed to cool for two or three minutes, and then removed and placed in a drying form. When steam is applied to the ribs - which also heats the posts - the ribs are easily bend into an S shape in about 20 or 30 seconds, maybe slightly more. (I never tried it, but I'm confident we could have heated the two bending posts, wetted the wood, and then bent the wood around them without using steam. The key is the straps being placed under tension.)

    The more outward pull you have on the straps before you bend, the better your bend will be.

    R
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  28. #44

    Default Re: side bending - cracking

    Aha steam! I've seen some mandolin ribs that were 0.100+" and wondered how they got them bent with no scorching, now I know. I also like the idea of inner and outer straps on the hot pipe. I'll have to give that a try. I imagine it requires gloves to keep from burning your fingers.

  29. #45
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    Default Re: side bending - cracking

    Rob Roy...
    Yes, steam is a wonderful bending agent; • softens the wood, • doesn't scorch it, • literally bends the wood's cells rather than crazing them, etc. Absolutely wear gloves; while water boils at 212, steam under pressure can be as high as 220-230 and can give you a very nasty burn.

    You can get a Wagner #915 steam generator at Lowes for about $130 or so. Super for softening hide glue or Titebond when resetting necks and can provide what's needed for bending ribs under .090˝ (thicker than that needs a bit more steam than the Wagner can provide).

    R

  30. #46
    Adrian Minarovic
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    Default Re: side bending - cracking

    One should keep in mind that pure steam bending and bending around hot iron (or heated forms/ counter-forms) are two different things. Wood structure softens by action of heat and water on lignin. I remember it is most plastic at temperatures between 130 and 180 degrees CELSIUS depending on species.

    In steam bending the wood is completely saturated with water steam preferrably at temperatures above boiling point (boiling at elevated pressure in a steam chamber) this will make the bonds between wood cells/fibers very plastic and the wood will bend easily. You have to prevent uneven bending or unwanted elongation of outside surface by use of bending strap. For thicker pieces the strap is on outside and anchored to both ends to prevent any stretching during bends. For thinner piece well tensioned strap held tightly against wood is often enough to support the bend evenly without breaking.
    If you are bending wood around iron the heating is not even through the thickness of the wood and best practice is only wetting inside surface (often no wetting at all) and bending slowly so the outside of the bend is not softened too much and inside will heat and be able to compress. If you heat it too long the outside will soften too much and will be prone to cracking. On curly wood this is critical as any moisture will travel through the curly grain and transfer heat to the other side much quicker right at the spots where the endgrain is exposed. Those are the weakest spots and they get even weaker with heat/ steam passing and the wood will just snap. To prevent this you want the iron at very high temperature (at the very limit of scorching the wood during bending) use no or very little water and use bending strap. The strap (held at tension) will allow smooth even bending without too much stress at one spot and the hot iron will make the inside of bend softer than outside and make it possible to bend the wood without fracture. For very complicated pieces of wood it is best not try bend it to finished radius at once but in few bend / let cool cycles (so the outside doesn't become too soft in one prolonged bending session).
    Adrian

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  32. #47

    Default Re: side bending - cracking

    Thanks HoGo for the thorough explanation. I love the look of curly maple in the rims and have been using an outside strap over a hot pipe. Of course, I have experienced the wood cracking along the grain as I learned the art of bending. There's no better teacher than experience. Thanks for sharing your expertise.

  33. #48

    Default Re: side bending - cracking

    Quote Originally Posted by siminoff View Post
    Parker... I think you'll find having straps on both side of the rib piece to be very helpful. It will help distribute the heat and reduce surface burning or charing of the wood. The problem comes in being able to hold and manage two pieces of aluminum strapping plus the rib piece.

    Key to getting the straps to do their job is to not just pull down the straps down onto the bending pipe, but to put them under tension - pulling outward - so their lateral force against the pipe is distributed over a short distance rather than at the immediate bending point. (I hope I've described this clearly - don't hesitate to ask if you need more clarification.)

    Here's a photo of the fixture we used to steam bend the "S" piece that goes from the headblock to the first point. I hope you can see that there are two straps and that each strap is individually spring loaded and placed under tension. The rib goes between the two straps. As steam is applied, the two center bending posts are mounted on a plate and rotated about 100 (the black knob to right of center is the handle that rotated the posts). The fixture is then locked in place, the rib piece is allowed to cool for two or three minutes, and then removed and placed in a drying form. When steam is applied to the ribs - which also heats the posts - the ribs are easily bend into an S shape in about 20 or 30 seconds, maybe slightly more. (I never tried it, but I'm confident we could have heated the two bending posts, wetted the wood, and then bent the wood around them without using steam. The key is the straps being placed under tension.)

    The more outward pull you have on the straps before you bend, the better your bend will be.

    R
    Hi Roger,

    I meant to reply before this. I'm bending just A-style rims, so thankfully I don't have quite the issue with tight bends. I will certainly try two straps next time and try to hold them in tension. I've been doing that with just the outer strap when I can get my thumbs inside the wood to apply tension to both the strap and the wood, or at least that's what it feels like. How much actual tension I'm applying, I really don't know. I'm not sure how that's going to work with the inner strap, but I'll give it a try. I guess your setup applies tension only to the straps, so perhaps I don't need to worry about tensioning the wood in between.

    Right now, I'm learning to carve my first plate, so it will be a little while before I fire up the hot pipe again.

    Thanks for the thorough explanation of your process.

    Parker

  34. #49

    Default Re: side bending - cracking

    I only have had problems in cracking right at the scroll curve. It took me a few years of braking a lot of scrap wood until I got the hang of it.
    I have found that pre-soaking the sides for 30 min helps a lot, but patience and a strong backing strap is the key to getting a smooth crackless bend.
    Curly maple is the hardest for me to get a good bend with out separation especially at the edges. I keep my steam box handy and sometimes flood the area I'm bending with steam to help coax it along. It's kind of an art. Just keep practicing. As far as thinning the sides where the bends are, it never worked for me. The thin spots, especially where I may have watered them down a bit, started to cup which wasn't all that pretty. Stay with the specs and take your time.
    P.S. If anyone has been able to bend torrified sides without them splitting like a soda cracker, please let me in on the secret.

  35. #50
    Adrian Minarovic
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    Default Re: side bending - cracking

    Soaking is not always good. Hydraulic effects within wood cells filled with water can damage the structure during bending. Scientific papers on plastification of wood I read suggest optimal moisture content at about 20% and temperature around 160 Celsius making lignin most plastic. Steaming wood held above water level in pressurized chanber gets you close to this and is vital for bending thick pieces.
    There are many kinds of torrefied wood (different processes) so it is impossible to tell what's wrong with your wood. Certainly that wood is more prone to cracking and holding the wood on compression (strap anchored on both ends to strap like on bending thick chair pieces) would be optimal solution. Some violin makers had good luck using strong sanding belt, the grit holds the wood and lowers the tendency to stretch the outside surface, which is the critical point here. Bending it thinner would help as well.
    Adrian

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