Spell Ruling: Heat Metal Ruling

Heat metal is a great 2nd level spell but I'm going to say that it does not work against a creature made of metal even if that creature is manufactured. If cast it can heat up a metal creature allowing it to do an additional 1d8 damage if it hits or 2d8 if it grapples another character.

Heat Metal Spell:

 

Choose a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor, that you can see within range. You cause the object to glow red-hot. Any creature in physical contact with the object takes 2d8 fire damage when you cast the spell. Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again.

If a creature is holding or wearing the object and takes the damage from it, the creature must succeed on a Constitution saving throw or drop the object if it can. If it doesn't drop the object, it has disadvantage on attack rolls and ability checks until the start of your next turn.

  As ruled during a game, a Modron is a creature (even though he is a construct made mostly of metal.) Therefore, the spell does not work since the target of the spell must be an object.
The question came up "What about animated armor since the spell specifically mentions metal armor?"
I did some research and there are a variety of rulings out there and the general consensus is "it's up to the GM."
My ruling for the Modron stands for any metal creature (construct) or otherwise including Iron Golems and Animated Armor, swords etc.

My reasons:

1. The target of the spell is an object, not a creature.
2. In turn, the heated object does damage to a creature that is in contact with the object, not the object itself.

The object itself does not take damage, it simply heats up. A small "object" like a shield, sword or even armor would typically have about 10hp if 5e allowed you to target that object directly. If heat metal damaged the object itself, the item would not possibly last 10 rounds!