Generally there is only 2 reason for feeder (extruder) to grind a 'bite' out of the filament :

  1. Feeder tension is to high - Should always be set to half way
  2. There is some restriction in the filament path

Partial or total blockage in the print head is the main reason for grinding as it stops filament from moving forward at the correct rate. 

This requires the nozzle to be cleaned via the 'Hot Pull & Cold Pull Methods' - Often called an Atomic Pull

When doing a Hot/Cold Pull to clean the print head this often need to be done multiple times until you get a ‘clean’ pull which looks like a perfect mould of the inside of the nozzle with not black marks or chunks missing.

Additionally there can occasionally be a restriction in the Bowden tube : 

  • Inspect the Bowden tune for dents or cuts 
  • Remove the Bowden tune from the feeder and the printhead 
  • Then try sliding the PLA through by hand to check for restrictions
  • I have seen ‘pinched’ tubes in the past that can cause issues

In general if everything is clean, free moving and tensioned correctly there should be no excessive grinding on the filament.