I've been asked about this a few times so makes sense to just post a walkthrough

Tinkering should be high-if tinkering is not high enough you will not get a golem that can smelt ore.
Using the tinkering menu go to customs and build an Iron Clockwork assembly. You will need the following materials in your bag:
1 Clock Part
1 Spring
25 Iron Ingots
5 Blackrock

Once your Iron Clockwork assembly is made double click it.
The following items must be in your pack:
10 Power Crystals (from Exodus dungeon)
7500 Iron Ingots
3750 Bronze ingots
50 Gears

Congrats..you've just made an Iron Mining Golem!

I have one of these and a suit to make them, they work and will mine with whatever your mining skill is