I need to machine an arbor for a gear-cutting tool. It will look something like

this, but small -- the head diameter will be around 10mm. The cutting tip will protrude beyond the head's diameter by a variable amount, and only on one side, so the stress won't be radially balanced. It will be used to machine soft metals -- like brass and aluminum.

 

I have rod stock of the appropriate size in both aluminum and brass. Which would be a better material to a) tolerate the torsional stress, and b) keep the threads for the set-screws intact for longer?

brass will be better for the screws, but Alu for the Torsion (it work hardens) your bar stock alu, is it a Duralumin alloy? if it you`re laughing! use that!

 

it`s also good for heat transfer too, much better than brass and so will keep the cutting tool cooler.

The brass rod stock I have from an old SmallParts order; it's typed as 'C-360.' The aluminum stock is from a local hardware store, and I haven't the least idea of its metallurgical properties.