Abstract: A cable core advances through an extruder head and then past an ultrasonic test set. In the extruder head, an annular plastic jacket is formed about the core. The test set generates four signals which correspond in magnitude to the jacket thickness in orthogonally spaced quadrants of the jacket. The signals are then applied to apparatus for centering the core with respect to the extruder head. Additional apparatus generates control signals in response to the signals from the test set. The control signals are applied to a mechanism for adjusting the jacket thickness in diametrically opposite quadrants with respect to the thickness in the quadrants adjacent thereto to control the uniformity of the jacket thickness. The control signals are also applied to a mechanism for controlling the overall jacket thickness. The signals give priority to the function of controlling the uniformity of the jacket thickness before the overall jacket thickness is reduced to a predetermined thickness range.