I'm attempting to cut some Aluminum sheet 12.5" thick
I will assume you are going to use several passes, maybe 0.25 mm...
My bench mills fastest speed is 2100rpm
If I use a 10mm 3 flute slot drill and cut at 0.5mm per pass , what speed should my table be running at in inches per minute to get best results.
Evil fortune descends on he who mixes his units!
That said, you could start at 400 mm/min and see how that goes. It does depend a lot on your machine.
Is .25mm per pass too little ?
That totally depends on the rigidity and horsepower of your machine. That said, you should be able go up to 0.5 mm/pass without any problems, IF you are using a MILL. If it's a router ... 0.25 might be right.
What is the easiest grade of aluminum to cut?
That's a trick question. imho, the best cutting alloys are Fortal, 7075, 2011 and 6082, in descending order. But they cut OK. I use air-blast and the odd puff of kero/oliveoil mist for them.
The softer bending grades - 5nnn, 6060 etc, are like soggy chewing gum and are a pig. They clog the cutters - which then glow and break.
Cheers
Roger