Like this one from Saw V:
What primitive shape/s would you start out with? How would you transform the primitive shape/s to give it a nice, smooth, sharp blade on one side without distorting the entire object in a weird way? I tried starting out with a cylinder and then subtracting the top half using a duplicate cylinder and a difference modifier, but I ended up distorting the entire object when I tried to pull the "blade" edges together. I think I need to add lattices to smoothly "sharpen" the edge of the blade.