The Ore Miner is a unit capable of picking up AC Ore and converting it to anticreeper storage for the Apex Ship.
The Ore Miner is capable of picking up AC Ore within a 14 tile radius from its center. The Ore Miner will pick up 1 tile of AC Ore every 3 frames (10 per second) and each tile converts to 1 packet containing 10 anticreeper.
The Ore Miner gives access to anticreeper ammo to supply Maker ships. The Ore Miner requires no ammo to function, so it is effectively free after the 20 energy build cost plus a potential cost of getting access to a supply of AC Ore.
The question of when you build an Ore Miner is more a question of your ease of access to AC Ore and how useful anticreeper production would be. Rushing anticreeper can be a good idea on some missions and a terrible idea on others, you will need to use your own judgement when to build your Ore Miner(s).