I do a lot of aluminum cutting, making parts for experimental airplanes.
I always use a piece of MDF under my aluminum sheet. I cut parts from 1 X 1 inches up to 12 X 15 inches from aluminum of .016, .025, .032, .060, and .125. In all cases I never cut more than .035 in depth. I use only 2 flute carbide bits, usually between .09375 and .125 in diameter.
What I find most critical is the RPM of the tool and the linear cutting speed. If the tool RPM is too high and or the cutting speed is too fast you develop a lot of heat at the site of the cut. This causes the aluminum to "bubble" upward at the site of the cut, due to the heating and swelling of the aluminum around the cut. I have a simple test to test if the cutting speeds are right. I simply run my finger along the cut, just BEHIND the cutting tool. If the metal is just warm to the touch, and there is no swelling of the metal, all is going well. I typically use cutting speeds of around 2000 to 2400 RPM and linear cutting speeds of no higher than 10 inches per minute.
I have tried 4 flute cutters and they just are not suitable to cutting aluminum sheet. Actually the best cuts and lowest temps when cutting come from single flute carbide cutters. They have long life and do a very clean cool cut.
I forgot to mention, I hold the material in place with 3 or 4 "Clamp Down" devices around the perimeter of the sheet being cut. I use ones made by Incra, they really work very well
Hope this help some,
George