drilling aluminum, imperfect circle [TANKS]

[Amir Tahvildaran <adt22-at-drexel.edu>]

I'm drilling through a 1/8" walled 1" aluminum square tube using a S&D 
5/8" black oxide bit.  I've found that the holes aren't perfect circles 
in some cases.  Anyone have tips?

I'm using a drill press at 560 rpm (which is correct for 11/16 holes in 
alum) Should I go up one step?

I'm using "household oil" while drilling

I'm not predrilling a smaller hole which is the next thing I'll change 
(3/8" is the next smallest metal bit I have). 

The only other thing I've thought could help would be to clamp on and 
drill through a piece of wood first to help stabilize the bit for when 
it gets to the metal.

-Amir