An old saw is a tool that is used to make short planks and long planks. The saw uses the same durability mechanic as any other tool, such as the hatchet or pickaxe.
To make short planks, you must have 3 log pile in your inventory. Right click on the saw to create short planks. hatchet cant do it
To make long planks, you have to first find some timber. These can be located around cities, or at sawmills. Right click the saw while near a stack of timber to create long planks.