Bending bamboo strips?

[tharlow]

Help!
I tried an experiment that did not work.
I've seen those lamps with tripod-like legs formed from bending strips of bamboo at the base of a culm. I used a bandsaw to cut a green 4" diameter Ph. vivax into 8 strips about 10 or 12 inches into the section of culm.
I proceeded to heat the strips with a propane torch, while I tried to bend the strips at the point I was heating.
All I accomplished was breaking off burned strips of bamboo. What did I do wrong and how do I do it right?

TIA, Tom

[Mark Meckes]

Hi Tom,
It sounds like you may have held the propane torch in one place too long, and/ too close to the bamboo, if it got badly burnt.
Also, if the bamboo was dry, rather then green or slightly dry, it is harder to bend, and dry bamboo will also get burnt faster.
A steady back and forth motion, a few inches away from the bamboo is needed.
The process I think you are doing, that is, bending bamboo outwards, or against the curve, is harder to do, and may require shaving away some of the inside of the culm to make it more flexible.
Here's a suggestion: After you have made the 4 cuts to make 8 the strips from the 4" dia. bamboo, slip another piece of 4" bamboo inside the culm which will begin to splay the pieces outward.
Now gradually heat the strips with the propane torch, gently stroking each piece with the flame until all pieces have been equally heated.
As they heat up you should be able to push the bamboo that's inside the work, downward, which will push the strips outward.
There's a limit how far you can splay these pieces outward before something splits or breaks. Trial and error can be your guide.
One option is to splay them out to a certain point, then dunk the piece in cold water, or let it cool naturally, with the jig still inside the piece. It will then keep its' shape.
Now that there's room to get a knife in between the strips, carefully shave some of the inside of the strips away to make the strips more flexible.
Then repeat the heating process till you have the desired spread.
If you haven't done much of this before, I'd suggest that you experiment on some separate strips to explore the process, and define the limitations.
This is definitely a skill that is refined with practice.
I hope it becomes easier done then said.
Mark Meckes

[tharlow]

Thanks Mark
I knew you would have the answers. The water dunk would never have come to me. I suspected I was trying to do it too fast. Thanks for the detailed guidelines!

--Tom

[tharlow]

Success!
Using a splint, water, and longer cuts were the key.
The third time was the charm.
The first time was my original attempt already posted.
The second attempt resulted in no charing but I did crack the legs. This last attempt resulted in only one leg with minimal
cracking. I filled in the crack with polyurethane woodglue. Hopefully this floor lamp will stay intact as it completely dries. Am I looking at a year for this green culm to dry out?

One comment on the splint I used. I ended up using a half culm of B. vulgaris to slide down the cuts. The rounded side did nicely, pushing the slat out without the need to slide it low.
Again, thanks Mark! I can't believe I got it to work. I have a ways to go towards making polished pieces, but this was a huge step.
Thanks!

--Tom

[bambooda]

Hello Tom,
Another technique that you might consider using is to thin out the inner meat of the culm in the area of the bend.
If you are working with 4"diam. stock you might be able to reach in with a rotary die grinder and a 1"diam carbide burr and grind out the inner circumference in the area that will be
heated and bent. The fibers are much denser towards the outer circumference and will still maintain much of the strength even with the inner meat removed.
If this is B. vulgaris that you are using, the wall thickness is
pretty heavy and I congratulate you if you were able to get some splits to bend without cracking.
Note that vulgaris is at the very top of the yummy list for powder post beetles.

Regards, Cal

[tharlow]

Thanks Cal,
I do have a Black and Decker imitation Dremmel. I had tried shaving inside the culm with a utility knife, but a grinder/burr sounds much more effective.
I was not able to maneuver well with the utility blade.
I'm sure I wasn't clear with the terminology. The vulgaris was
used like a wedge to force the strips of Ph. vivax outward.
It seems to me that vivax is thin enough to be bent but it is a bit brittle.
Which of the clumpers are best suited for heat bending?
I have a bit of vivax, but my supply on runners will be harder to refill than most clumpers.
I know vulgaris is low on the totem pole of quality, but in
my apprenticing it's readily available.

Thanks again, Tom

[bambooda]

Hello Tom,
Here in Hawaii I haven't come accross much mature vivax and have not worked with it at all so I'm not sure of it's working characteristics.
It's rather thin walled is it not?
The species that I am familiar with in regards to
heat bending is the Phylostachys and I believe they take a bend pretty well compared to a lot of other bamboos.
Most of my experience is with P. bambusoides, aurea, and pubescens. I consider them to be medium walled with
medium length internodes and the fiber structure very resilient.
With something in the range of 4" diam. the wall thickness would be roughly 1/2" and I would grind off around 1/4" of the inside meat before attempting a bend over 15 degrees. The age of the culm makes a difference also.
The basket (market quality) makers use culms around two years old because the fibers are more pliable and bend without breaking.
For my sculpture work I prefer culms 5-7 years old as the lignin and cellulose in the parenchyma has reached optimum strength, but they are more difficult to bend.
When the culm gets too old, it may look perfectly sound, but it is very brittle.
A dremel is a far cry from a die grinder by the way.
I use Makitas. They are about 12" long - 1/4" collet - 25,000 rpm - draws 2 amps. I use them a lot.

Regards, Cal