Empty cannon base is used when repairing dwarf multicannons within the Artisans Workshop. It is obtained after removing the broken cannon cogs from a broken cannon base. Studying it reveals that 10 fixed cogs need to be replaced onto it wither by repairing the broken cogs or by smithing new cogs.
This item cannot be taken outside of the repair area. It can be re-obtained from Isak if you have left the area after obtaining it.