Friday, February 20, 2009

The Wonders of Steam

We had two days of warming weather so it was time to make hay whilst the sun shines 'cause the Nat'l Weather Service sez it wasn't to last. As I mentioned, the colder the environment the quicker your workpiece looses plasticity; and since I'm using a propane camp stove as one of the heat sources the doors and windows need to be open for ventilation.

News Flash: McKennawerks goes high-tech!

Thanks to the technical expertise of Generica, My Lady Wife, I'm beginning blogging this from the laptop down in the bowels of the McKennawerks. Gen, with the able assistance of our computer guru friend Gregor, were finally able to get the wireless router to recognize the laptops, so we can compute while sitting in front of the warm woodstove, instead of at the main computer which lives in the coldest corner of the house. Apparently wireless routers don't like dial-up, and it took some doing to convince it that, yes, while we are still stuck in the middle ages courtesy of Verizon, we really do want to use more than one computer.

I've also added a hit counter just for the heck of it. I looked at adding a guestbook as well, but after checking out some on other blogs it appears that the comments are mostly banal, off-topic, spam, or obscene. I'll just stick with google's comment widget. And of course you can email me at McKenna(one word)werksatgmaildotcom. I don't normally bring anything of a sensitive electronic nature down here due to the ever present dust and potential for (fragile and expensive) things to be accidentally knocked to the concrete floor, but we're in a holding pattern because we've started to steam.

So it's time to talk about some of the specifics of steam bending wood. It requires one hour per inch thickness of wood, plus (in this case) bringing 3 gallons of water up to a rolling boil. Fine Woodworking has a good book on many different bending techniques. If you're interested in some in-depth knowledge about how the pros do it you should check it out. I've not steam bent anything in ages and ages, I much prefer cold lamination with glue for bending to steaming. The steaming part of steam bending is easy and straight forward: where the problems arise is in the bending part.

To bend wood with steam we need:
--patience of the saints
--safety equipment
--a source of steam
--wood to be bent
--a form to bend the wood over

Patience. While wood, especially green wood, is flexible to an extent, it doesn't like to bend and stay bent. Depending on many factors, especially thickness and grain of the workpiece and amount of curve, you can expect a failure rate of 50% or more. This should elicit no more than a muted, "Darn." If breaking wood is going to send you into a cat-throwing, obscenity yelling, jumping-up-and-down rage, stick with lamination.

Time. You need an hour per inch thickness of wood, and it doesn't start until a good head of steam is built up. Then you have to get the wood instantaneously into the form while maintaining a serene calm.

Safety. For safety for this project I wore high leather boots in case of a spill: I didn't want boiling water splashing my ankles and soaking into sneakers. There was a fire extinguisher close to hand, and I wore good leather gloves throughout the process in case I needed to grab something hot.

As in metal casting, though, if something starts to "go" it's usually a lot smarter to back off and let it and clean up the mess, than to risk a burn by grabbing at something. Eye protection, of course, is essential.

Steam. For the source of the steam I'm using an old turkey fryer that I actually bought with steam bending in mind. Several years ago we held an early spring outdoor event where we made a big pot of lentil soup (and roasted a pig, yummm). Since then the cooker has been sitting at the back of the shop awaiting the day I finally decided to steam bend.

For this bend I started by heating about 3 gallons of water with a hot plate, but it took a long time to get it up to a low boil, and couldn't really get it up to the rolling boil we need. So I switched to a propane camp stove. This was an improvement, but if I weren't doing a one-off project, I think I'd bite the bullet and fill up the propane tank for the gas grill and use the turkey fryer stand.

In the lid of the fryer I drilled a hole for ¾" copper pipe and just stuck it in place with friction. I still want to be able to use the fryer as a stock pot, so I didn't silver solder it in place. There was an existing hole in the lid for inserting a thermometer, which I loosely plugged with a bit of sanded down dowel. It's very important that you DON'T heat in a closed system, this plug served as a safety valve.

For the steam box I used an old piece of 4" thin wall pvc drain pipe. I cut a plywood plug for one end and drilled out a hole for the inlet pipe near the bottom edge of the plug. As water condenses from the steam in the steambox it will run back into the boiler. Again, avoiding a closed system, I left the irregularities in the edge of the plug from cutting it out with the sabre saw in place. The pipe clamp you can see around the end was due to concern that the heat might soften the plastic to where the friction fitted plug would slip out. It's just helping the pipe retain it's shape--it is not clamping the plug in place.

The piece of pipe I had lying around was too short for the 9' long poles so I extended it with flexible aluminum clothes dryer hose.

I had planned to use the dryer hose for the entire pole if it proved too curvy to fit in the pvc pipe, but after releasing them from the temporary clamping they sprang back fairly straight. If you look carefully at the plug (which is a 4" round cut from a 2x6) you'll note two holes. They are there for allowing stem to escape and the thermometer. The center hole was later drilled wider to accept a piece of 1" copper pipe used as a union for the automobile heater hose.

A test run showed the long box was too long for the amount of steam generated with the small camp stove. The thermometer only registered about 160 degrees F. at the far end.

So I supplemented the system by putting a camp coffee pot on the hot plate at the opposite end of the steam box. I stuffed an automobile heater hose scrap (left over from a wagon tire) into the spout and jammed a few small bits of cloth into the gap. As Tennessee Tuxedo used to say, "If you make do with what you've got, then you won't need what you have not."

When the pot began to boil the temperature quickly rose to 212 deg. F and more steam began to leak from the untightened lids and fittings, showing an increase in steam pressure within the box. I'm using the meat thermometer which came with the turkey fryer--waste not, want not. (I'm just full of aphorisms today)

Wood. The wood I'm using is (probably) ash. Oak, ash, and hickory are widely considered the best woods to use steam bending with. Don't bother with pine-it'll split. The thicker the piece, the longer it needs in the steambox (one hour per inch) and the more likely it is to split. The tighter or more extreme the curve the more likely it is to split. Also try to use pieces with long straight grain, it's more likely to crack where the grain 'runs out' to the edge of the wood. It's easier to stream green wood. Kiln dried wood does not steam well. If you're stuck with MegaLoweMart lumber, I'd strongly suggest considering glued up laminations.

Form. The form for bending was made from scrap lying around the shop. There is a good distance between points where the blocks push against the piece to be bent. If we were making furniture or boats where precision is a prerequisite we'd make the bending form almost continuous along the bend, but such precision isn't needed here, and would not have been bothered with by our hypothetical medieval craftsman.

To use it we need lots of clamps. Lots and lots of clamps. If you don't have them there are work arounds to clamps that I covered in an earlier post. The other thing is to make sure you do a dry run. Have all the clamps laid out to hand. Have a hammer ready to hand to help tap along the piece if necessary to overcome friction against the mold. It's really better to do one piece at a time, but since one piece was so close to the desired bends, to save clamping and drilling holes in the bending form to take clamp ends I went with doing both pieces at once.

To do this I placed small pieces of 1/4" dowel between the pieces as spacers,and used cable ties to hold them together in rough alignment. Once the boilers had built up a good head of steam they were ensconced in the steam box. Then we wait.

After a tad over an hour (the pieces are a tad over an inch thick) I pulled them from the steam box and stuffed the front ends under a cross piece and forced the back under another cross piece at that end. Then frantically began clamping. The starboard piece went in fine, but the port was a little stiff. Unfortunately, after about 20 minutes in the form there was a sudden crack! and the port piece split at the compound curve where the harness hitch would be.

This is not a total disaster, as I have plenty of wood to make another pole from, and I can cut pieces from the split pole to use in other parts of the cart.

Post mortem analysis shows that I left the split piece a little wider than the other intending to trim it down to size after bending--I should have made it the same width as the other. Secondly, when I was clamping it there was more resistance than the other so I should have aborted the clamping, let the wood cool for 24 hrs and started over. Impatience is often my besetting sin. Also, I probably shouldn't have done both at the same time. There's a lot of truth in the old saw, "if you don't have time to do it right, will you have time to do it over?"

As I said above-expect splits. I'm disappointed it didn't go perfectly, but it's not going to impact the deadline.


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