Cog in the Wheel is a short errand Ian sets you in the middle of The 
Minecrawler Nest quest, down in the Old Mine. Before he will help with the plan to break into the nest, he wants you to fetch a cog from the tunnels below. It does not take long, but Minecrawlers are guarding the spot.
You pick this up by exhausting Ian's dialogue in the middle section of the Old Mine, partway through The Minecrawler Nest quest. He asks for a cog before he will commit to the plan.
Go to the cave where the Minecrawlers were, the same ones you kill for 
Gor Na Bar's mandibles, and head down to the bottom corner. Pass the guard on your right. At the back you will find two sets of ladders leading down.
Climb down the first ladder normally. For the second, jump down rather than climb, because Minecrawlers are waiting at the bottom and will attack you while you are stuck on the rungs. Kill them, then look to the right and pick up the cog.
Climb back up both ladders and return to Ian. Hand him the cog to complete Cog in the Wheel. He is pleased, though he still needs a little more convincing before The Minecrawler Nest plan can move ahead.