As Rory says, you need to shave the surface when cutting with aluminium.
I have successfully milled 1.2 mm thick aluminium sheet using small diameter two flute end mills with no air or liquid cooling. The grade of aluminium is important. Too soft/pure (e.g. 1050A) and it melts the aluminium.
I even managed to mill a 20 mm thick aluminium block with no cooling (once) using the Stepcraft.
Take a look at my posts. I have several YouTube videos illustrating this process and I mention appropriate feeds and speeds which work repeatably.
Doug, what sort of cut did you do with the 20mm aluminium? I have spent the last 9 days!!!! trying to machine some relatively simple parts for a friend. my feeds are slow as, solid carbide 4 flute, cutting oil, cooling spray, 0.2mm pass, 4mm bit, HF spindle, etc etc. I have loads of experience, however the v2 840 has rattled its guide bearing loose 5 times now, worn the Y axis lead nuts, loosened the collet...…. no missed steps, no stress on any of the steppers, 37degs C in 24deg weather...….. I`m of the thought this machine is not fit for purpose.... id love for someone to prove me wrong without loads of mods? When the X access or Y axis tensions bolts loosen they cutters grabs the material and because there is zero rigidity it gets exacerbated violently with zero high draw protection. this isn't helped of course by the performance upgrade kit that has SS pins in the ally coupler that simply bend the ally and fall out...….