/diy/ - Do It Yourself

Is there any reason I can't use a double fluted 6mm cutter for milling slots out of aluminium box section? I keep asking the company's machinist and he just shakes his head like there's something I'm not understanding but then never actually explains it. I'm not actually trained or qualified to do any of this btw the machinist just comes over once a month and stands around explaining whatever the fuck he feels like for a few hours and then fucks off and I do the actual hours because my hourly is like a tenth of his