yo. drills in your mod mining iron on copper field. wtf?

It's probably because the radius is picking up both ores, is there iron near by? try changing the range of the miner

I stumbled on the same problem, and I can confirm that it happens, as well as it is fixable by what you proposed, lowring the Mining Drill Radius setting (Startup Settings).

So given that, I'll take the opportunity for a feature request. Make the algorithm work by spiraling out from the furnace placement, to prioritise the most close squares containing ore. I'd recon it's just a couple of lines of code, as it has been when I've implemented similar funtionality, although I'm unaware how Factorio modding works.

Thank you for a nice mod btw! : )