I needed to make something to hold a pen that has a
spring-loaded up-and-down motion. I decided to modify
one of the air-operated cylinders that the hand already
fit. I took it apart, removed the "O" rings, and replaced the
spring with a lighter one.

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The parts that make up an air-operated cylinder.

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This is how the pen needs to attach

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The cylinder extended.

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The cylinder retracted.

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I had to grind this down to fit inside the pen barrel.

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Modified and original cyclinders.

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The final hand/pen holder.

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And the final hand/pen holder attached to the robot arm.