Hello,
1500 seems a bit fast for that diameter... I'd slow it down to half that and work my way up... 2 tricks I use for tee slots: 1. Don't cut full depth on the clearance slot. Leave about .25mm so the bottom edge of the bit is engaged in the material. This helps to reduce chatter - stablizes the cut. 2. Use a blow gun to evacuate the chips. Might be a mess with your flood coolant (portable shields with magnetic bases can help with this). The fewer chips that get recut the longer the edges will stay sharp and the better the finish. I saw one of your other posts regarding the correct flood coolant for material so you seem to be savy on that point. However, I like to use A-9 on aluminum. Smells like parifin and contaminates the bit to the point that the ally
normally won't stick to it. May not be available in the UK but I'm sure like products are. Won't go through a flood coolant pump tho. So you'd have your hands full manually cranking the table, holding the blow gun and trying to swab the oil on... maybe mount the blow gun near the spindle, huh?
I hope you find this helpful,
Doug