Quick and Dirty Steam Box

For a current project I need to steam bend some pieces of Bolivian Rosewood, and the problem is that I have neither steam bent anything nor have a steam box handy. For about $25 and an hour of work, here is what I came up with.

For parts, I used
     -42" of 4" diameter, schedule 40 PVC pipe for the body of the hot box,
     -1 4" end cap
     -1 pair of a threaded cap and coupling that it screws into
     -1 4" female-female coupling (not pictured, to join the screw coupling and pipe)
     -12" of 2" pipe
     -2" 45 degree elbow
     -$1.50 Kettle from the thrift store (half off... What a deal!)

I would have gone with a shorter length of pipe had the hardware store been able to accommodate the length I needed, but they could not so I'll save the extra for something else. The reason I went with 42" is because I plan on using the tube for etching patternwelded longswords (given the infrequency I obviously need a steam box).

Despite what seems to be common in other PVC designs, I will not be drilling holes in the length of the pipe for dowel supports. This limits the pipe to the single purpose as a steam box and is not strictly necessary. While I am cutting my boards to width just barely smaller than the inner diameter, they will be off the bottom of the pipe, although I could also have made some removable supports to slide in underneath to keep the bottom face open to steam circulation.

The ends both need holes drilled in them to release steam and prevent it from rupturing violently. I don't have a functioning power drill, so I turned to the brace.

The first hole is sized to fit my thermocouple probe so I can track temperature throughout the process. A regular meat thermometer will work fine provided it can accurately measure around 230 (F).

Probe test fitted. No need to make the hole overly tight.

Next up I drilled three holes of arbitrary size (9/16") on the other side of the end cap. These will be the primary steam vents. Never having done this before, it was a guess as to size and number, but this combination worked well.

Originally, I intended the elbow to fit inside the kettle mouth itself, but that clearly did not work out. To remedy this, I cut a piece of pipe to join the two.

This little bit is enough to keep it snug inside the kettle, but not too deep as to inhibit the steam flow by dipping below the surface of the water.

Here we come to the most difficult part (for me working with hand tools). I had to drill an angled hole in the threaded cap to join the elbow to the steam chamber. Due to the complex surface, it was difficult to position the bits in such a way that they effectively drilled. This took a majority of the time, but would have been inconsequential with a drill press or power drill.

Even using the largest bit I have, the hole was slightly undersized for the 2" PVC to fit into. So, deburring and widening a hair with a file opened it up to the perfect size.

What a fit!

And that's the finished box! (or pipe)

Naturally, I tested it on some scrap wood before committing to the real thing. For the actual steaming, I will be building a support to hold the kettle end of the pipe due to the softening of the smaller 2" pipe during use.

A rule of thumb for steam bending wood:
For every inch of thickness, steam for an hour.

For my purposes, I will be bending 1/8" wood, which comes out to 7.5 minutes, but with the 1/4" piece I tested, I may go longer.

If I were to use wood of this width, I would use supports to keep it higher in the tube. However, it is only a test so I did not care to go through the wasted effort.

The test was of mixed results. Taking the wood out of the steam box, it felt about the same in terms of moisture content and rigidity as it did going in. I was not able to bend it in the tighter inner corners of the frame, but it also did not break. Perhaps I need to wet it first? I'm not quite sure...