Possible Clues:

• Fires there when it gets rough
• Makes rasping sound, hard on ear
• Makes harsh sound by rubbing
• Shreds cheese
• Where there's firing, roughly speaking
• Makes harsh noise by rubbing - on nerves?
• You get no thanks in them if smoothness is lacking
• Makes harsh rubbing noise - on nerves?
• The fire does not go smoothly into them
• Hardly soothes