You can use the PET Bottle Cutter to cut sprips from the PET bottles with selected width. The PET strip can be then used in machine which converts it to filament for 3D printers. This cutter is using sharpened bearings, they works like scissors. The width of the strip can be set by the knob.

I'm using this bottle cutter for some time and it works very well. You will gain the skill after a few bottles.

Here you can see how I cut a bottle using this cuter:

Bill of material:

• M8 screw rod with length about 40-45cm
• 2x M8x60mm screw
• 1x M8x30mm screw
• 7x M8 washer
• 6x M8 nut
• 2x M8 nylon lock nut
• 4x M3.5x20 woodscrew

Assembly:

• Print the cutter body 
• Print the knob with M8 nuts inserted inside during print
• Sharpen the bearings using a grinder
• Cut slot into the M8x30mm screw (see photos)
• Assembly the cutter accorning to the photos
• The bearing must be touching but also rotating freely
• Print the top protector
• Cut the M8 screw rod to length
• Add M8 nut and top protector to avoid an injury

Check also my PET Strip Spooler and Spool Holder and my other tools for making filament from PET botles.