You can buy mining machines on the gold factory and the mine. The first one will grant you faster gold obtaining, and the second one, fast iron bar obtaining.
Since you can buy 10 iron bars with 100 gold, and taking into account the progression, what is the best strategy to maximize the iron/gold obtaining faster?
If you increase your gold production with the iron by buying mining machines, then if you need iron, dig the tunnel all of the way through until you hit the converter. Use any gold you get to purchase large quantities of iron at once and repeat the process. Works every time!
Simply buy iron miners. I personally have 1345 iron miners, each one costing 12070 iron (buying 100 is better than buying one at a time, too) and get 250 iron per second. At gold, I have 840 miners and am making 84 gold per second (+1 gold for every 10 miners) and one miner costs 9148. That's a difference of 4000 iron and I have roughly 3x higher iron production than gold. Iron is definitely the way to go.
Buy some iron bars and use the iron mining, since mining helps you to get more irons for free rather than buying in the shop
