Quote Originally Posted by guy_ View Post
I won't go more than 1mm cut depth for a slot. As I'm not cutting for cash, I don't mind high cycle times and although my machine is capable of cutting deep pockets in Al, I don't want to risk any more carbide cutters.
There's part of your problem straight away, Carbide likes to be pushed hard and 1mm Doc isn't enough with a 6mm tool. Could easily double this and more without breaking the tool.
Next is the intermittent cut caused by the holes you drilled, Carbide doesn't like shocks and intermittent cutting, I would stop doing this.

I find Mist cooling with plenty of air is better than flood for slots.