I'm a bit of a heathen, but for soft copper, I use the .046" thick cutting wheel that fits on an angle grinder. You can get wheels that are for alum and other non-ferrous metals at welding shops, but even the steel wheel does OK, bit of temporary build up on the wheel. Goes through soft copper like a hot knife through butter, no distortion. Then I hit the corners with a sanding block.

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1986 300SDL, 362K
1984 300D, 138K