What effect does dickens create with the imagery of the people at the grindstone?

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.