You can use the PET Bottle Cutter to cut sprips from the PET bottles with selected width. The width is defined by spacers which are put under the blade. The PET strip can be then used in machine which converts it to filament for 3D printers.
I'm using this bottle cutter for some time and it works very well. You will gain the skill after a few bottles.
The metal sheet from the bottle can protects the plastic body because the edges of the bottle and strip are quite sharp and can scratch the plastic.
Bill of material:
Assembly:
You can follow the photos.
Here you can see how I cut a bottle using this cuter: