Good advice above. Tip must be perfectly sharp and square to the blade. As soon as you loose a corner it will deflect blade away from dull corner. If tip is not square to the blade it will plow sideways. If there is any welding of aluminum on the tip it will have the same effect so must avoid heat at all costs. Using lots of lube to keep tip and alu cool might help. Parafin, heating oil, or kerosene might work better than WD40. Alas some types of aluminum are so gummy that parting is a nightmare even when done right. Makes the option of hacksaw and then facing the part look pretty good.