Luthier CNC

[Voicecoil]

The best taps I've found for through holes in ali are the blue ring spiral point types like this:

They're made specially for aluninium, the interrupted thread higher up seems to help with binding. Typically I've found you can tap a hole in 12mm plate in 3...4 seconds with one of these in a cordless drill with some cutting fluid. For blind holes as Mekanik said you need to use a spiral flute tap (to get the chips out), again I've found the blue ring types are marginally the best.

[mekanik]

This might be of interest
http://www.europatool.co.uk/threading/application-taps
Regards
Mike

[m_c]

In a nut shell for taps-
Spiral Point - Only good for through holes, as they push the swarf forward through the hole.
Spiral Flute - good for blind holes, as the swarf comes up around the shank, but are more prone to binding if the swarf doesn't clear.

My preference is always spiral point, as they're the stronger option.
If you're only going through relatively thin section, I'd avoid the interrupted flute ones Voicecoil posted a pic off, as a bit inadvertent sideways pressure with only two or three support flutes engaged can quickly jam the tap and snap the tap/rip the threads out the hole.
As always, lubrication is key. For tapping aluminium, a drop of good quality cutting oil is essential to stop things sticking.

There are also threadforming / roll forming taps which can give a stronger thread in aluminium, but drill hole size is very critical, so I would advise against them unless you can control the hole size well. Undersize and the tap will quickly jam, oversize and you won't have much of a thread.