Under the control of a microprocessor first and second motors alternately drive first and second ends of a shear bar for adjusting the position of the shear bar relative to cutter head. The shear bar is adjusted so that it is substantially parallel to the cutter head even though the cutter head and shear bar may not be parallel at the time the adjustment is initiated. A knock sensor is provided for sensing impacts between the cutter head and shear bar, the knock sensor output signal being converted to a digital value for controlling the adjusting sequence. A knocker is provided for inducing vibrations in the shear bar to thereby test the operability of the knock sensor and its output circuitry. The sensitivity of the adjustment system to the output of the knock sensor is set automatically before an adjustment sequence begins to account for variations in the knock sensor, its output circuitry, and "noise" vibrations induced in the shear bar by normal machine operation.