Metal Cutting

Not knowing specifically what the part is, the first guess is too many risers, making it difficult to remove them. Remove then with a hand saw to start until there is enough clearance to use a metal cutting band saw. Do not attempt to cut them flush, remove final part of riser on a belt sander.

several questions, is the casting round? what kind of quantity do you need to make? If it is high volume and round it might be worth the expense of a lathe. Typically cast aluminum has silicon in it and will be abraisive to your tooling. But, more info is needed to help.

Scooter

I get the feeling this is a one-off, and there does seem to be too many risers for the size of the casting. It has been a few years since college and the casting professors rants though. I was never a big fan of turning castings, cast faults can make them unstable when turning.

Maybe a cold saw would be in order here.

Hydraulic bolt cutter with the jaws adapted to do a close crop in aluminium!

Water jet.

pneumatic chisel then follow with belt or disc sander...