Are you using a two flute mill or a four flute mill?
Are you using coolant/lubricant or cutting dry?
Generally with 6061 a two flute cutter with a high helix is best because there is more room for the chips and the helix helps evacuate them from the cut.
Also 6160 needs some coolant/lubricant. You do not have to flood it on when using small cutters on small machines but at least spray a bit on every few seconds.