Operational space control has a number of desirable characteristics but is sensitive to model accuracy. For heavy machines the dynamics are difficult to model due to their friction and dynamic coupling, thus making full compensation imprecise. This work presents an approach in which a simplified model gives partial compensation via an open-loop feedforward input, pre-calculated in forward simulation. In this way, effects that are difficult to compensate for can be partially corrected without causing instability. Since the reference trajectory is known a priori, dynamic model parameters are tuned in its neighbourhood, reducing the burden of global modelling. The feasibility and performance of this approach is shown experimentally via improved free motion tracking of an excavator arm. This framework further supports efforts for direct impedance control between bucket tip and soil.