The problem with cutting near pure aluminum is that it's so soft. You need an acute rake angle on cutting tools to get them to penetrate properly, and a standard die set will usually be set up for steel. That usually means they cut oversize first pass. Reno's answer of a cutting 'gunk' plus cycling the die down is the solution. Aluminum also tends to 'gall' or weld itself to cutting edges, so it's advisable to check and clean any galls off regularly.
