Aluminum wants speed - 350-400 feet/min, so for a 12mm drill that works out to ~2800 RPM. Obviously this is higher than you would want to go for most work but crank up the RPM to help out if possible.
Have you checked your tailstock alignment? It could be binding in the hole if it is misaligned.
Make sure your drills are properly sharpened - they may have a very aggressive grind causing them to dig in. Look into 4 facet drill sharpening. Can be done off hand on the grinder with a little practice and gives much better performance.
Also, I tend to step drill such that the drill's web is slightly larger than the hole left in the previous operation. If the drill is cutting only on the outer edges it will get very grabby and digg in badly.
I have never had an issue drilling to 1/2" on my 7x12 even in steel - the only metal I don't like to drill is brass because it does grab and dig in so badly (use a slightly dulled bit for brass)
Aaron |