Subject: Cog removal.
Aluminium and steel "grow" with oxidation, so items like cogs can develop a "chemical" lock with the hub over a period of time. (This is common with things like wash-basin tap covers).
One way to help free them is with a hammer using gentle (!) tapping. Needless to say, you have to be careful not to damage the item. Work around the perimeter with a series of taps to break the lock. It doesn't always work, but it's worth a try if the proper tools struggle.
On reassembly, make sure an even coating of anti-sieze is applied to both surfaces, thus avoiding future problems.
David Fryer, Brisbane, Australia