Some washers designed to fit either side of the blade for a better fit, but still thin enough not to cause problems.
Download the main butterfly knife parts here:
https://www.printables.com/model/590041-butterfly-knife-trainer-toy
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Print 4 of these, and I suggest using ironing to make sure both sides are smooth. I also used a touch of white lithium grease on mine and it turned out perfectly.
The author remixed this model.
A washer to fix the blade's spacing.
Print 4 of these.