The Roland SRM-20 is NOT made to machine aluminum.
The Roland MDX-540 is made to machine aluminum but is quite a bit more expensive.
I have the MDX-40A which also is NOT made to machine aluminum. Many jewelers, dental labs and artists use it to make wax models. They use the wax models to cast molds and then cast metals into the molds. The results are superb.
I think you could get excellent results using the SRM-20 with a wax model - cast mold - cast metal process.