An armature for an electric motor has a lamination stack having slots in which magnet wires are wound. An armature shaft extends coaxially through the lamination stack and a commutator is disposed on the armature shaft to which ends of the magnet wires are electrically coupled. Plastic is molded around the magnet wires and the armature shaft to increase the stiffness of the armature shaft, which increases the critical frequency of the armature shaft. In an aspect, an electric motor has such an armature. In an aspect, a power tool has an electric motor having such an armature.