45: Living Hinges

 

When I first heard about living hinges (not the plastic ones), it was the same time I learned what a useless machine was. Useless machines are entertaining, but it was for some reason shocking to me that I could laser cut a particular pattern into certain materials to make them flexible.  In reality, it’s a trick I’ve seen dozens of times, from the wooden snakes my father used to leave around the house as tricks and kerf-cut wood, but applying it to lasers made it shiny and new to me!

Early kerf cut tests were weak and hardly deep enough to flex.
Early kerf cut tests were weak and hardly deep enough to flex.

Before I set out to recreate the same type of living hinge in the video above, I wanted to try out traditional kerf cutting on the laser, hoping it would produce a more precise result than the hand sawed versions I’ve seen.  The cut tests—labeled with their power, speed and pulses-per-inch settings—were all cut with 1/16″ between each cut on a long inch-wide strip of 1/8″ bamboo plywood. Only one of my initial settings was strong enough to produce a kerf cut deep enough to allow flexibility in the wood, and even then it was nothing to cheer over.

A few attempts at kerf cutting, one of which mostly went straight through.
A few attempts at kerf cutting, one of which mostly went straight through.

I was hoping for something far more flexible.  After all of the testing, including one example with cut lines spaced only 1/32″ apart, I learned why living hinges are the best option for flexible wood.  The kerf cut has to be so deep that you run the risk of cutting straight through, and at the thickness I was testing at the wood’s grain was creating an uneven cut that did actually cut straight through a few times.  Worst of all, even a little flexing in the opposite direction of the expected bend would snap the wood straight in half, as seen in the 1/32″ test pictured.

My first actual living hinge was awfully flexible.
My first actual living hinge was awfully flexible.

The living hinges fared much, much better. I continued to space the cuts 1/16″ apart, opting to use a portion of the cut pattern in the video example. The result was amazing! The super-flexible hinge was fun to twist and turn, and I was surprised to see how it handled my abuse. I wonder if cutting with the grain as opposed to against the grain would have produced such sturdy results. I could hear the wood cracking a bit here and there, but it easily exceeded my expectations. It was downright floppy.

Flexible, sure, but also shown here is my failed attempt to depaper a living hinge.
Flexible, sure, but avoid masking paper. It’s a huge pain to remove from living hinges.

I took that success and at Jennifer’s keen suggestion turned it into a bracelet. A simple stroke engraving might have weakened the piece, though, as I accidentally snapped a piece while stress testing at a much lighter level than I’d given the previous piece.  I am willing to bet that spacing the cuts out farther, perhaps to 1/8″, would solve that immediately.

As flexible as they are, it's still bamboo and can be broken.
As flexible as they are, it’s still bamboo and can be broken.

I’m not done exploring these yet, so you might see another living hinge or two before the year is out!

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