Primitive Archer
Main Discussion Area => Bows => Topic started by: Badger on November 02, 2008, 10:54:55 pm
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The microwave topic got me thinking about my steam gun project which has been on the drawing board for so many years now. I have always wanted to build a closed container like a large pipe filled with water and equiped with at least two pressure reliefe valve for safety and a very small hose comming off it for spot steaming bows, making curves etc. Not sure how hot the steam would be if under about 60# pressure but would be easy to look up and probably act a lot like dry heat but no scorching. A long pipe turned into a pressure cooker is another thinh I have wanted to work on, these things can be very dangerous if precautions are not used though. Steve
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I have plans to make a long pipe pressure cooker using pressure regulating parts from an actual pressure cooker. I will also put a valve that I can quick release the pressure. Once the pressure is out I hope I can remove the lid safe and quickly so I can pull the wood out and bend it while still hot. I have tried to use my wife's steam cleaner but it doesn't produce the kind of volume we need. Justin
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My pressure cooker runs with up to 20 lbs pressure , I would stay way back, WAY BACK, from anything with 60 lbs unless a certified boiler inspector had seen it first and told me to fire it up. Seriously.
Men are cut in half with steam leaks at high pressures.
Microwave: ends only.......no pressure.
But, I agree, if you could heat the whole stave to the temps that 60# will give you it would be a noodle when you took it out. You'd have 45-seconds to get it into the form, maybe a bit more, and drop all the clamps on it and man, you could write your name with a 2x4! ( if you have a short name)
piper
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60 pound Steam would be approximately 307 degrees F........we run 300 pound(421deg) and 600 pound (488 deg)Steam in the Refinery I work in....we use the Low Pressure Steam in a 55 gallon Barrel full of water to boil Oyur Deer Heads and other Trophies.....as long as the Company dont find them..... >:D........I wonder what a piece of Osage would look like after a couple Minutes in 600 pound Steam at 488 degrees............
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I gave my handbook away when I moved south so I can't look up the numbers any more. I thought my 20# pressure cooker ran at 280 F or so. Seems like 60 would be more. I'm still pretty careful around steam at most any pressure. Lots more energy than boiling water.
To paraphrase an old saw: there's old bowyers and there's bold bowyers, but there's dam few OLD BOLD bowyers.
piper
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http://www.indpipe.com/images/PDF/steam_temperature_pressure_table.pdf
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Hi Steve
60 psi is definitely 307 deg F.
The problem with using 300deg F to steam bend wood is that the max temp wood can stand without any detremental change is 150 deg F and it is considered that at 195 deg F even for short periods that you get cellular damage and significantly reduced strength ( Bootle et al. 2005)
I guess what you need to keep in mind is that the lignin that you want to soften so you can bend the wood becomes plastic at about 140 deg F and the biggest difficulty is getting the core of the wood to the same temp as the outside because it is good insulator -- so my money would be steam at about 150deg F and enough time to let it heat soak evenly which means the outside can not get over the 150 which is considered safe
Graeme
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My pressure cooker runs with up to 20 lbs pressure , I would stay way back, WAY BACK, from anything with 60 lbs unless a certified boiler inspector had seen it first and told me to fire it up. Seriously.
piper
Your pressure cooker is also aluminum. I have no intention of using aluminum. ;) Justin
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Graeme, that is good info! I had no idea those temperatures damaged the wood. Could dry heat possibly harden the lignin back up? Steve
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I've thought a little about this type of device, and I've concluded that perhaps it is better to build an oven for baking the bow. If I could fit my wood in our oven, set the temp at 200F, 220F or whatever, and keep it in long enough for temperature to be that high in the middle of the wood, I'd be happy. Wouldn't a nice long skinny oven be a good way to go? Dave
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David baking belly lams is something I think would be worthwhile. Heat applied to a backing does more harm than good. I think I would bake at about 3oo till wood had a nice light brown look to it. Steve
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I don't like the idea of using steam on dry wood without sealing the wood from the moisture first. I know shellac will work as a sealer even when using steam. I personally would rather use dry heat(heat gun) and protect the wood with veg oil. I also think the oil helps distribute the heat evenly and help to hold the heat longer...and the heating process takes less time with a heat gun. Pat
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Pat, I tend to agree with you for the most part, I haven't used steam in over 5 years now. But after I retire next year money will be tighter and I am thinking I would have more staves to choose from if I wasn't so fussy. I am mostly thinking about straightening stves that would otherwise be junk. Steve
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Steve, That's about all I have to work with anyway...staves that would otherwise be junk! ;D...and my heat gun cost $5 at a garage sale many years ago. I can usually get tips hot enough to straighten in about 5 minutes and if I need to work the entire stave(near bow size) it usually takes about 30 minutes. After I have the stave relatively straight and clamped to the caul I re-heat the entire stave and let it rest until tomorrow. Sometimes I have to make slight adjustments after that but not too often.
When I was steaming wood it would take 30 to 45 minutes to get the wood hot enough but it would cool down quickly so I could only do a small section at a time. I have spent a few days with steam when I can get the same results with a heat gun in an hour or a bit more...and I don't have to let the wood dry out for a week or more. Pat
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Hi Steve
The way it works is that lignin is an organic polymer and when you heat it a little you make it plastic by releasing some of the bonds which reestablish when cooled. If you heat it a lot like when you heat treat a belly you do a few things - firstly you drive off organic volatiles that would normally not be removed except over decades of aging secondly you cause cross linking of molecules within the lignin similar to decades of aging and some that would just not happen ever at ambient temperature and thirdly you cause oxidation reactions which can be seen externally as darkening of the wood or charring. All these tend to make the wood more brittle and less elastic but stronger in compression and as long as you only have a thin layer on the belly and the rest of the bow is still unaffected and elastic the you generally improve the performance.
It I was intending to fix a bunch of staves I would build a skinny oven and use dry heat at about 150 deg F - the heat soak time is the important thing and my guess is it probably take the best part of an hour for a largish stave to be heated right through -- you would need to do some testing with a electric thermometer like you can get for baking which you could put the probe in the center of a stave by drilling a hole to check the times. I can not see any advantage to using steam as the heating medium unless you are working with wet wood where you are trying to keep the humidity high to regulate the drying rate. With dry wood it really just causes problems by raising the MC in the outer layers and causing the grain to open and check -- also it is more difficult to build and potentially dangerous.
Graem
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Graem, Thanks for that explanation.
I guess a typical hot box will work for a stave oven. My box has a thermostat that maxes at 168deg. It takes 4 light bulbs, anywhere from 60w to 100w. I regulate the temp by changing bulbs or just unscrewing a bulb or 2...or 3. One 100w bulbs will keep the box at about 90degs good for storing wood and maintaining lower M/C. I sometime pre heat a stave or almost bow in the hot box for a while before straightening. Pat
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Good info Graeme, I have seen staves become checked while recrving with steam, presently I do it just as Pat described but have found some white woods that just don't seem to respond to dry heat as well as I would like. Steve
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The problem with dry heat is that some woods like Ipe tend to burn before you can heat them enough to bend. I imagine an oven set to 150* would heat it throughout, but it might dry it so bad that bending would cause fractures and splinters. Not to many woods like 2% MC. I know that heat makes some changes in the wood, but many self bows have been built from kiln dried wood that has been heated well beyond 150*. Many kilns now rapidly raise the temp to 400* to dry. By keeping the moisture high inside the kiln until the temp is reached it reduces checking. I think it is worth a try. Justin
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Many years ago I had fashioned a solar heater where I used black plastic pipe and a little moisture inside with wet rags. I would use rubber or some other spring clamp type thing to lash a bow into the shape I wanted and then leave it in the sun for a week or two until the shape held. It actually worked pretty well. Steve
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Well, what Graem has said explains a lot of ruined hickory stave's for me. I made a boiler out of a drive shaft that I could put on my propane turkey cooker. I could boil a whole limb at once. I had so many failures with the limb just collapsing that I quit using it. I didn't know I was destroying the wood getting it that hot.
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Some good research on how steam and heat affect wood might be a good addittion the next time bowyers bibles come out. I know the heat treating Mark St Louis made popular has had a major infuence on todays bows. Interesting how too much heat can severly damage the wood if not applied right.
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Wow, I could probally write that one. I've destroyed some wood trying to use my boiling tube. :o
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Hi Justin
The temperature thy run drying kilns is very high but you can only get away with this while the timber has a high MC and if you can control the the humidity in the kiln because this stops the timber actually getting to the temperature of the kiln. As the MC of the wood drops usually the heat programed for the kiln does too or you get cellular collapse.
I have not worked with Ipe but in general it is considered that wood can withstand 65 deg C (about 150 deg F ) for periods of days without measurable changes to physical properties after the timber has regained its appropriate moisture content. I have found this to be true with the woods I have worked with and I think there are some misconceptions about the affects of heat on wood that are result of people being unaware of just how hot they are getting the timber with localized heating and how long it takes to heat evenly all the way through. If the surface of the wood is going dark brown there is a good chance that the temperature is in the of 300 to 500 deg F region and that is really way too hot ( the Indians reckoned if you discoloured the wood while heat treating it was a sign of a lack of skill ) The other part is that I doubt that many people ever get the core of the timber to suitably hot or at least do it without getting the outside too hot so I think the concept of what may be possible with controlled oven heating is biased by this.
If I had a little more time I would do some definitive testing so we had accurate data to work with -- I will put it on my to do list but is depressingly long at the moment.
Graeme
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What about sealing the entire bow with shellac before putting into an 150 deg oven? Or maybe use a heat blanket instead? But what would you do with an entire palletable bow? Unless you had a full length bow press. I might try out a heating pad on piece of wood shellaced or wrapped with plastic sometime. Has anybody tried a heating blanket?
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Steve,
Where is the conversation about the micro-wave? Thats what I use....