Originally posted by DredSilent:

Its not that you can't upgrade it, its the table level is only 2, and most often you get the upgrade components for the table expansions/addons in the consecutive biomes.

IIRC you can upgrade the forge to level 6 with the materials you get up to the swamp biome, so you can max out the iron armor in the swamp biome, the silver armor in the mountain biome, and the padded armor with the grinding wheel upgrade in the plains biome.

So there are only very few armors that need upgrades from the next biome I think^^