The HeadChopper is the newest buildable, featured in Buried. It is basically a steam powered saw blade that swings back and forth. It is unique in that, if placed on the ground, it crawlers the zombies, then kills them. If placed on a wall, it immediatly decapitates the zombies.
All the parts are found in the General store.
The Blade is found in the Barrels opposite the Flag.
The Rachet/Handle part is found on the same shelf as the Trample Steams Motor, just on the other side.
The Base is found in the Back room, in the left corner, opposite the Turbine's Mannequin body.