Tuesday, September 14, 2010

Tech Tip Tuesday

Well, yesterday's post was indeed apropos.

As part of the kayak build, it is necessary to bend some wood for coamings, hatch covers and rims. I've spoken about bending wood before - and canoe ribs specifically - but I'll recap quickly here. You can cut pieces of wood so that they are curved after cutting - often done with crooks or knees so that the grain direction follows the intended curve to yield a strong part. You can laminate thin layers of wood around a curved form like we do with our stems, or you can heat the wood by boiling it or steaming it to soften it for bending.

In my case, I'm bending long strips about an inch wide at their widest and about 8 feet long. The stock cannot tolerate the tight bend that I'm contemplating for any of the parts. I tried one method that I've seen used successfully - taking a damp T-shirt and steam iron to heat the strips (because they are s thin) and bending them around the forms. This didn't work too badly, but my scrap rate was a bit high for comfort. (The rims are going to be cherry!) I decided that I'd better bite the bullet and just steam them.

Alas - no good way to create steam. Time to make one.

I like the brute force behind a propane fired steam generator (think large kettle on big propane burner with 100 gallon propane tank and regulator), but don't want the expense or safety issues associated with a set-up like they have at WoodenBoat. I decided that an electric steam generator would work best for me. I'd seen some that I liked. Here's my first cut:

First, before thinking of undertaking this type of project, be sure to know and understand what you're doing - it can be dangerous if you are not careful. Live steam, electricity, flammables and other hot parts can hurt your badly or even kill you if not treated with respect. Details are important - like choosing the correct wire gage for the cable, proper grounding, avoiding electric shock and not creating a sealed area where dangerous amounts of pressure can build up. Think before doing.

I've shown part of the steam generator before, but let me describe it a bit. What we have is a frame to support the apparatus and on the left is a bucket which gets filled with water. (both for feeding water and counter-balance - note that the feed on the pipe is higher than the bucket's bottom) on the bottom of the bucket is nipple which lets the water feed into the high-temperature black heater hose, through a ball valve, and into the "T" on the pipe assembly to the right. Located in the pipe at the bottom is a 1500 watt electric hot water heater element. I was very careful to ground the pipe with the cable and to cover the connections with a silicone cover to isolate them from water and to prevent electrocuting the operator. I also located the element at the bottom to avoid boiling it dry - I've seen similar steam generators with elements in the "T" and fed from the bottom. The water is boiled in the tube and expands as steam exiting at the top brass fitting and into another piece of heater hose which is held in the steam box with a rag.

Because of the length, this presented some issues. I picked up some ABS pipe to use as both a soak tank (wet wood steams better...) and another piece for the steam box.

It's all R&D.

Lessons learned? Yup. Bigger diameter pipe would have been better. First, it would have created a larger surface area making more steam. Second, the expanding steam wouldn't push the hot water up the relatively narrow column; I'd started with some shorter pipe, but the expanding steam pushed hot water up and out of the top of the device - like Old Faithful! That's one of the reasons that this is such a tall device - this one is about 3-1/2" tall. I'd also make a more robust custom cap for the bottom to protect the electrical elements.

"If they don't find you handsome, they should at least find you handy."

- Red Green

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