Large Plate is a part for advanced tools such as hammers, cleavers, and battle axes. Crafting a large plate requires 8 parts of the material. This part is crafted normally at part builder but won't do the player any good until they get the tool forge, which has the advanced tools needing this part.
You can also use an Obsidian Large Plate as a modifier to add Reinforced to your tool. If you are looking to make your tool unbreakable, you need 80 obsidian (64+16)
Large Plate Cast.
Large Plate Stencil