Problems steam bending

[cs]

If you remember correctly I was having some problems bending a keel strip for my canoe. So I built a steam box, and still having problems.

Situation:
5/16" thick doug fir bending on a 16" diameter stem form. I esitmated steaming time (1" per hour) to be 20 minutes. I steamed anywhere from 20 minutes to 45 minutes and all I managed to do was make kindling. As looking at the image below the pieces were splitting out between the third and fourth holes and the fifth and sixth hole in the stem form (as counted down from the top).

http://a6.cpimg.com/image/64/13/14952036-71a8-028001E0-.jpg

My next step is to pre-soak the pieces and than try to steam them. I may even rip them a little thinner.

Any suggestions? BTW the photo below shows the steam box I built.

http://a6.cpimg.com/image/78/14/14952056-d143-028001E0-.jpg

Rest of photos (http://members6.clubphoto.com/chad414100/654655/guest.phtml)

Chad

[Dave Fleming]

Trick for problem steam bent pieces.
A steel strap wrapped over the curve and with a clip end to grab over the...end. Then clamp in form. The steel strap compresses the wood fiber where on the outside of the curve they are spreading and at the same time trying to compress on the inside.
Do not remove for several days, ya folla?

[reddog]

Chad:
What Dave said about the bending strap is sage advice.You say douglas fir for the keel?Is it air or kiln dried?If KD that may be the problem.Did you consider using ash or another wood which takes steaming better?Just asking.
If your fir is real dry then a good soaking,(3 or 5 days),may limber it up some.
Good luck;
Earl

[WRB]

Do you have the wood raised up on a rack inside the box? Steam has to get all around the wood. Also, make sure you have a really good head of steam up before you put the wood in. Stuff the rags loosly in the end so that you get a good flow through of steam.

[imported_Conrad]

In my experience, steaming fir, especially if kiln dried, does little more than get it wet. I would seriously suggest switching to ash for the curved sections. After much frustration with steaming oak that was kiln dried, I'm convinced even soaking it doesn't return it to the state of being required for proper steam bending. It's as though kiln drying forever cooks the "soul" out of it- or some such! tongue.gif

[Jim Budde]

Just a thought. If you have a container that will accept your strips, try boiling for an hour or so. I prefer ash, but have bent fir before by boiling first. Grab the wood from the boiling water and immediately use the strap method mentioned above.

[Mrleft8]

First, I think your steam box is WAY too big. It should be just big enough for the stock you're putting in it. The way you have it set up you're steaming the inside of the box, not the stock.
Second, Be sure your stock is straight grained with NO run out (even long diagonal grain will split).
Third, try soaking the stock in HOT water with fabric softener in it for a few hours before you steam it. The fabric softener is basically wax that allows the wood fibers to slide along each other instead of separating.

[Frank Wentzel]

Check out Lee Valley's steam bending info at :

http://www.leevalley.com/wood/page.asp?page=31161&category=1,45866,45867&abspage=1&ccurrency=2&SID=

(Cut and paste into your address bar)

They have all you wanted to know and more about steam bending.

/// Frank ///

[TomRobb]

You need green wood with no grain runout.
KD Doug Fir wouldn't be my first choice.
I imagine it breaks where it does because the radius tightens there.
In despairation you could always laminate.

[Bill Perkins]

Thanks for the link Frank that lookes like good stuff.

[Bruce Hooke]

If you are determined to use Douglas Fir I would laminate the stem. Otherwise I would switch to ash or white oak, both of which steam bend quite readily. Softwoods are notoriously unresponsive to steaming.

Given how thin the strip is relative to the radius of the bend I think you would do just fine with a kiln-dried piece of stock that has been soaked in water for a few weeks, as long as it has nice straight grain. Even without soaking it might work if you just up the steaming time a little. However, you don't want to oversteam because that can also cause problems. Finally, make sure that you are generating plenty of steam throughout the steaming process. For a similar size steambox I use two 4 quart cooking pots at a good rolling boil.

One mistake I think people make with soaking is thinking that they can soak somthing for a day or two and have it make much difference. Remember, it takes a long time to get water out of wood, so it also takes a long time to get water back into wood. Fortunately, it is somewhat quicker getting water into wood because the difference in moisture level between the water tank and the dry wood is much greater than the difference between air and fairly dry wood, which means the process of getting water into the wood is faster than getting it out -- otherwise soaking would take months to work. Still, it does, I think, take more than a day or two to effectively soak a piece of wood. I have bent kiln-dried, but soaked white oak of a similar thickness to a similar radius without much trouble.

[Thaddeus J. Van Gilder]

First my steam box is about 12 feet long and about9 by 12 or so. It works just fine for small stuff. But I have never been able to steam doug fir. It just doesn't work. It's too brittle.

[Dave Fleming]

Can't speak with any authority about that slender a piece of Doug Fir but we sure as hell steamed a lot of planks of the stuff over the years.
Ayup, on occassion there would be a split out. We would steam the plank, clamp in place and let cool overnight. If anything was going to happen it would be that first night cooling down so in the morning if all was well as it normally was, the boring for fasteners and such was done.
IIRC, the most problems were where there was a severe twist (near the stern) and a bend to the plank. That was when you crossed your fingers and hoped it would not split.

Agreed, that KD stock is not really the way to go green or air dried is. Steam should be WET steam and I don't know what the size of the steam box has to do with anything except length of time to build up steam and fuel used.
Yards usually had one size steam box, Extra Extra Large.

[ 12-16-2002, 02:00 PM: Message edited by: Dave Fleming ]

[Carl Simmons]

I used Douglas Fir for my Stripper Canoe and used three laminated sections. I didn't have any problems with cracking, in fact after they cooled and dried (day or two later) I took them off the mould and they held their shape nicely. You might want to check out Moores book "Canoe Craft" I believe about making the stems.

Carl.

[cs]

Thanks for all the help. Been kinda busy and ain't had time to reply.

Yes I do have the wood supported so that it gets a good flow of steam all around it. My next step was to try pre-soaking, but today I found a supplier for some Ash. I think I will pre-soak it and try it.

I'm thinking about doing the whole canoe in ash. It only cost $1.82 a board foot. Do you think it would be better than doug fir?

Chad

[Todd Bradshaw]

Too heavy and I'm not sure you would get as good a bond with your glass, in terms of preventing delaminations on impact.

[T.KAMILA]

A couple of points to remember when steam bending wood. The first is do you have a good supply of steam? Start timing when you approach maximum temperature. The second is the surface of the wood to be bent should be smooth with the outside corners eased to remove any stress concentrations caused by saw marks intersecting at the corners. The third suggestion is to allow the wood time to stretch and compress as you bring it around the form. Wrapping it around the form too quickly can cause failure. Also as suggested, buy some cold roll steel strap at Home Depot and clamp it on top as it is soft and will bend around the form as you tighten the clamps. It contains the fibers that want to split out and spreads the pressure of the clamp.

Good luck
Tom

[cs]

I will probably re-try bending tommorow night. Just got through ripping up some ash strips and got them soaking in water. BTW I found another use for the cheap canoe. ;)

Chad

[imported_Steven Bauer]

Those compression straps are really important. I had many failures with white oak and ash during my early tobbaggan making attempts. Once I had the compression straps - not a single failure. The ones I used were Lexan - polycarbonate. (Like plexiglass but won't break :D Worked like a charm.
Steven

http://www.imagestation.com/picture/sraid39/p0bda0c0b8ea82af2db8abdcdbddb7efa/fd07026c.jpg

Quartersawn ash on walnut frames. I got all the planks out of one 12' long, 2 1/4" thick plank. Fastened with ss screws.

[Mad McGee]

Well, I think it has all been said...but...

I steamed ash strips for my cedarstrip canoe and they bent beautifully. It was like handling (for about 20 seconds) wet noodles.

Now your bend is more acute then mine but I think ash is the way to go.

ARM

[Mrleft8]

The size of the box does matter ;) . If the steam chamber is too big, the stock is not getting as much moisture as it needs in relation to the heat it is being subjected to, and you get a drying effect. Ideally you want as much moisture as possible, with the heat added to help it absorb into the wood structure. As we all know if you pour very cold water on very cold wood, most of it just rolls off before it absorbs. Once you heat both water and wood, the absorbtion rate is signifigantly higher. I'm no damned scientist, but empirical studies have shown me that if your steam box is too big, the steam doesn't get into the damned wood.

[Mrleft8]

Of course if you stuff other things like scraps of wood into a bigger box, that should also alleviate the problem, as there is less air space....

[T.KAMILA]

If I remember correctly, it’s more about the heating of the wood that allows it to become plastic. The moisture helps the heat transfer to the wood and keeps it from drying out from the heat. It doesn’t take long for the wood to cool down and become ridged. In that short time moisture content has not changed.

Tom

[Mrleft8]

If that last post were true.... Wouldn't firewood melt before it burned? Moisture is the more important issue I think. If you could force water into the wood w/o heat it would become more flexible than if you forced heat into the wood w/o moisture. The real point however is the correct amount of both.

[ion barnes]

My reference material, "Fine Woodworking on Bending Wood says that Doug Fir is not as suitable to bend as others but can be done with the understanding that you may incur a larger % of failure to get your finished product. Its a good book wherth about 10US from Tauton books and Video It has plans for a lapstrake Sea Urchin as a bonus. Good reading about all kinds of bent wood projects and generally how to do it.

As an example, the limiting Radii of curvature (in inches) for 1inch stock, is

Ash unsupported - 13.0 in
strap support - 4.5 in

Doug Fir unsupported - 33.0 in
strap supported - 18.0 in

A couple of other tips are lots of wet steam and I mean lots. An electric kettle wont do the job. Its not pressure that reqired, its the water moisture to transfer the heat. oh yah thats been said. Now if you over do it the wood will show puckers on the inside of the bend that indicates that the wood has collapsed. Not good. Just as grain breaking out because of too cold or no backup strap. Long time ago WB had an article about bending wood. Anyone got a issue # ?

[cs]

After all the discussion and all the time building a steam box. After snapping many pieces of doug fir trying to make the bend (even with steam). After more talk and buying ash. After almost burning up my saw ripping the ash, and than more discussion after that. After pre-soaking the wood for not 1 day nor 2 days, but 3 days.

Well to cap this off after all of this work and still no keel piece bent for my canoe I started to bend it last night. The wood was cold and wet and so I brought some pieces in the shop and started up the steam box. I loaded the box with 3 times the amount of strips I needed and waited for it to build up a good head of steam.

Well you know while I waited I thought that I would try to bend a piece of ash on the forms just to see how well it would bend. Keep in mind that this was an un-steamed and cold piece of wood. Well guess what. It bent on to the form like a wet noodle.

http://a5.cpimg.com/image/0D/84/15050765-b241-028001E0-.jpg

If I had steamed this piece I could have probably tied it into a knot.

Thanks for the help.

Chad

[Mrleft8]

HOORAY!!!!
Doncha just LOVE it when something works the way you want it to? smile.gif

[Bruce Hooke]

Originally posted by Mrleft8:
If that last post were true.... Wouldn't firewood melt before it burned? Moisture is the more important issue I think. If you could force water into the wood w/o heat it would become more flexible than if you forced heat into the wood w/o moisture. The real point however is the correct amount of both.Heat can, and in some cases is used on it's own to make wood more flexible. I believe this is how guitar sides are usually (or traditionally) made. It's just easier to make wood flexible with steam because the moisture transfers the heat better without drying out the wood, and because with just heat you are more likely to scorch the wood or set it on fire. That much I feel quite confident about.

Now I will speculate a bit...I don't think that melting is necessarily the next step beyond something becoming more flexible. On the other hand, maybe wood would melt if you could keep it from burning! Say, I wonder what would happen if you heated a piece of wood up to, say, 800 degrees F in an environment devoid of any oxygen so that it couldn't burn? Now that would be an interesting experiment. It would, however, be a bit hard to do! :D

[cs]

Bruce I think you are right about the guitars. Saw a special on the history channel where they were at the Martin factory. To bend the sides of the guitar they had these irons that were about 3" or 4" in diameter and they would roll the sides back and forth while applying pressure. He said the trick was not to stay in one place too long so you wouldn't scorch the wood.

Chad

[Mrleft8]

Don't you suppose that there is SOME moisture in those guitar sides that is being heated up by those irons? If you look at a guitar side as it is being bent you can see the wood turning color as it is heated. That is the moisture moving to the outside of the piece of wood. It's not MUCH moisture I agree. BUT there IS moisture involved in the process... (Yeah.... I'm gonna defend this until I get flogged to death.... :D )

[imported_Steven Bauer]

How about those Asian boatbuilders that suspend the board over a fire with a weight at the other end. I think that some moisture is helpful but it's really the heat that allows the wood to bend.
Steven

[T.KAMILA]

I think it is the lignin that softens when heated. The lignin is what holds all the fibers together to form wood. So I guess in a sense the wood melts or part of it any ways.

Masonite is manufactured without glue. It is the lignin from the wood that holds everything together.

Tom

[ion barnes]

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