As pointed out by Nikolay Amiantov, the initial task specification contained a mistake when describing the effect of combining multiple wheels/drills with regard to their duration. In short: it should have been a sum, not a maximum of active times. Please, refer to the corresponding Section 2.2.2 of the updated specification. Also check out the example below illustrating this effect. To see the difference between non-active/active wheels, remove the last Z in the series of moves in the solution file.