segments 93 the segments may be extended against the force of compression springs 94 by means of actuator members 95 this being effected by means of a rod 96 which is vertically displaceable in the center of the support column 34 and which carries a cone 97 at its lower end the presser ring 98 which is illustrated merely partially in fig8 and 9 is illustrated in its totality on a reduced scale in fig1 as may be seen therefrom the presser ring 98 is vertically displaceable by a certain amount at the upper cross head by means of pins 99 the upper limit position of its vertical displacement being its abutment against the upper cross head left half of fig1 whereas its lower limit position of the displacement movement is formed by abutment rings 100 of the pins 99 with the tool fig8 left half open for the insertion of the workpieces to be welded together the presser ring 98 is located in its lower displacement position see also fig1 right half in this position the lower edge of the presser ring 98 is located below the upper edges of the centering pins 92 as well as the segments 93 when then a coverplate 61 is placed from below over the plunger 90 against the presser ring 98 their correct centering is effected automatically by the centering pins 92 when then the presser plate 27 together with the.