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| Is steam bending a good idea http://mowrystrings.luthiersforum.com/forum/viewtopic.php?f=10101&t=51396 |
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| Author: | 6string [ Mon Dec 31, 2018 7:03 pm ] |
| Post subject: | Is steam bending a good idea |
I already have a steam box for bending wood and was wondering if anyone uses this method with a bending jig as opposed to a heat blanket with a jig. I didn’t really want to spend the money on a blanket if it’s not necessary. I’m not really concerned about time savings on the actual bend. Just quality of the end product. |
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| Author: | Clay S. [ Mon Dec 31, 2018 8:32 pm ] |
| Post subject: | Re: Is steam bending a good idea |
Steam bending can make thin pieces of wood warp, so isn't usually suggested for guitar sides. A piece of pipe and a propane torch can make a cheap bending iron. Bending with a blanket is a safer bet, so if the woods you are working with are expensive then using a blanket might save money in the long run. |
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| Author: | Goat Rock Ukulele [ Mon Dec 31, 2018 10:16 pm ] |
| Post subject: | Re: Is steam bending a good idea |
Yes it can be if used along with a heating blanket and form. Or maybe with a pipe, I haven't tried it with a bending iron. I used it with a blanket and form. It was the only way I got a set of leopard wood to make the tight bends of a ukulele. Here is a link to a Robbie O'brien video blob:https://www.youtube.com/1730e907-24bb-4cae-8787-4322782366d6 It does work. |
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| Author: | Freeman [ Tue Jan 01, 2019 12:17 pm ] |
| Post subject: | Re: Is steam bending a good idea |
I question whether steam bending can bring the wood to the required temperature. While water is undergoing the phase transformation to steam it stays at 212F. When I bend on a hot pipe or blanket (Fox bender) the temperatures are between 300 and 400 F. There is a thread going on at a different forum where a guy first tried bending some practice pieces with steam, it failed completely. He then moved to a hot pipe and is have better luck. He is also trying to bend a tight double cutaway for his first attempt - certainly a difficult way to start http://www.tdpri.com/threads/buiding-a- ... ar.896313/ |
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| Author: | Alan Carruth [ Tue Jan 01, 2019 1:08 pm ] |
| Post subject: | Re: Is steam bending a good idea |
The objective with bending is to get the wood up to the temperature of the glass transition of the lignin 'glue' that holds everything together, and then bend it to shape before it cools off. The actual temperature you need is somewhere around 180F, but the exact temperature depends on a lot of things. One of those seems to be the amount of moisture in the wood: more moisture seems to lower the lignin softening temperature. It also weakens the bonds between fibers, making the wood more likely to peel on the outside of the bend, or crush on the inside, either of which is a problem. Steam bending works by saturating the wood with moisture at 212F, which is comfortably above the lignin softening temperature. The problem is that you have to get the wood out of the steam box and bent before it cools off. It works well for things like oak 2x2s used for ribs in boat building, so long as you move fast: all of that moisture evaporating from the wood can cool it off in a hurry. I doubt you could transfer a guitar side to the form and bend it fast enough. Some folks have used steaming as a 'pre-heat' stage, to get the wood up to temperature all the way through without drying it out. Then you use a blanket to maintain that while you bend it. A lot of moisture can cause a lot of cupping in wood that's not very well quartered, and ripple across the grain in striped wood, such as mahogany. Using a very hot iron or bending blanket seems to work by speeding up the rate of heating, so that you can get up to the bending temperature before you've driven out all of the moisture. Timing is everything. A bit of moisture on the inside of the bend can help to carry the heat through the piece without reducing the lignin bond strength so much as to promote peeling on the outside. Different woods scorch or discolor at different temperatures, and time at temperature is a variable. Students bending by hand tend to move too slowly, driving off the moisture, and scorching the wood before they get it bent. Once it has gotten scorched you have to wait until it has re-hydrated fully before you can try again, and even then... It takes some practice to get the feel for when the wood is ready to bend, and develop the judgment to know just how far and how fast you can go. After 45 or so years, I'm still learning. A 'back strap' helps to hold in the heat and moisture, facilitating things in that way, as well as reducing peeling on the outside of the bend. Figured woods need more heat and less moisture, along with the back strap: well quartered curly maple can be a challenge. Softer woods, such as some mahogany, spruce, butternut, cedro, and sapele, tend to fail by crushing on the inside of the bend; with those you need to avoid too much moisture on the inside. |
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| Author: | Joe Beaver [ Tue Jan 01, 2019 2:28 pm ] |
| Post subject: | Re: Is steam bending a good idea |
Alan, Well thought out and very informative. It confirms my suspicions as to why I have had occasional problems bending wood like sapele and black limba. At first I thought I was bending to soon and needed more heat. That made it even worse. I have since come to the conclusion that getting non-oily wood to hot, drying it out, then bending was not the way to go. I now bend as soon as it reaches the stream stage and complete it within 3-4 minutes per bout. And I keep the blanket away from the bout I am not bending until I am ready to go on that side. It works for me. |
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