Alan,
What composition is the material in question? There seem to be so many core material types, from resin bonded fibreglass to PTFE, PE, PVC, a variety of foams of different densities, etc…
I did actually try make some panels from a PVC cored Ali substrate – was not a great success. The PVC was difficult to machine – did not work with high RPMs ( 1200rpm..) and would go gummy and tear the Ali surface. This was quite a few years ago, when I was still in SA, and then managed to find a single flute reverse spiral 4mm carbide mill which worked reasonably – around 800rpm if I remember well and quite aggressive feed rates. The reverse spiral help push everything down, had to use compressed air on the cut line to get swarf out and keep the cutter cool. The Ali still had a rough edge – was not really acceptable to me. I tried some of the HDPE Foam core as well – I never managed with that at all.
Joe