It is really bad idea to cut copper with this discs. I`m not sure that they can cut even aluminium piece of such thickness.
If you don`t have big tools you can try to use small reciprocating saw. Better way is milling, if you can`t have milling machine you can try drill press like Old-Lumens here: Tear down and mod videos for the Small Sun ZY-T08 .
Those who have outgrown dremel and proxxon with 20000+ rpm and less than 1nm torque can try this tools that are made in Russia:
6000-12000 rpm, up to 20nm torque:
