David,
The speed charts you get with endmills and drills are good to study. Both vary with different materials. SFM is the "surface feet per minute" and I like to remember the base calculation so I am not depending on a special calculator, chart, etc. From Engineering School, I learned that if I can dervie an equation, I can bring it back ten years from now. If I am running a 1/2" diameter endmill at 1500 rpm, what is my SFM? Well, the circumference of the endmill is Pi (3.14) * the diameter, so that is 1.57" At 1500 revs/minute, the outer edge of the endmill is travelling 1.57*1500 inches per minute (2355 inches per minute) Then you have to divide by 12 inches per foot to get feet per minute, so it's 2355/12 = 196 SFM.
Let's run it backwards to check it. Let's say you have a 1/2" endmill and you want to run it at 196 SFM. 196 SFM *12 inches/foot = 2355 inches per minute. The circumference of the 1/2" endmill is still 1.57", so if we divide 2355 inches per minute by 1.57" per revolution, we get 1500 revolutions per minute again. The constast 3.82 comes from dividing 12 by Pi (3.14)

Roughly, with a TiALN coated carbide endmill, you can cut mild steel at 300-400 SFM with full engagement. Some guys run much higher with smaller widths of cut. Aluminum has no real speed limit except what might make the material gall and melt to the cutter. Coolant with oil becomes really important there. If you go to the cutting tool sites, you find speed and feed charts. www.sgs.com, www.garrtool.com, www.hanita.com