the drills get to the minimum quite fine, drilling one resource every 10 drill animation, so that works.
the problem is how the drill select the square to be mined: it looks like the drill might get stuck on a minimum yielding tile instead of using the best one

(full link as layout is broken: http://i60.tinypic.com/2wd3q04.png )
if you check the screenshot, the drill is digging a square having 0.1 yield, ignoring the nearby tile that still has 420% yield.