Plunging into aluminum at close to 20 ipm is pretty fast, I try to plunge at half the speed I cut at, and for a non flood coolant machine you are working it, must be a pretty rigid machine. But, you now have half the thrust you had with a .1" pitch leadscrew, the ballscrew being .2" pitch, this is a large difference I'd say, and the source of your problem.
Dave.