In chapter 2 of book 3, Dickens introduces the image of the grindstone: “a roughly mounted thing which appeared to have hurriedly been brought there from some neighbouring smithy, or other workshop.” Turning a grindstone is extremely hard work, hence our expression “back to the grindstone.” This stone is turned by two “ruffians,” as many people come to sharpen their already bloody and blunted weapons to prepare to go back and fight. The imagery is horrific, serving to reflect the horrors of the revolution and its effect upon the citizens, who come across as bloody, violent, and relentless. As Jarvis Lorry looks out at the end of the chapter, Dickens compares the turning earth (as the sun sets, leaving a red hue) to the “lesser” grindstone. Dickens is making a comment about humankind in general here, not just those involved in the French Revolution.
The grindstone is a ghoulish image. I would say the main effect is that at that point we really do not feel sympathy for the revolutionaries. We sympathize for Lucie and Charles, and we are worried about them. The grindstone evokes an image of visceral, barely controlled, overzealous violence. It is the joy in violence as portrayed that is most disturbing and off-putting.