Dave,
Aluminum bronzes have the ability to workharden as you machine them. It is not as great as some of the 200 series stainless steel alloys, but it is there and, like many precipitation hardening materials, you need to take a fairly aggressive cut to avoid this. There is a good chance that carbide cutters will help, but you also need a rigid set-up which is often beyond the capabilities of lightweight machines.
Good luck!