Balancing in two planes is the ideal representation of several individual unbalances in a long toolholder. In this case, and in addition to the existing centre-of-mass error, the main axis of interia and the axis of rotation no longer run parallel. This condition is called 'dynamic' unbalance. The resulting unbalances thus generate a wobbling movement of the toolholder. This is the case with long tools and all tools operated at very high operating speeds of rotation (over 20,000rpm). The following rule of thumb can be used to determined the need for dynamic unbalance correction: L > 2 x D.

Independent of the above guidelines, all drilling and boring tools having a single cutting edge should be balanced dynamically.