the sleeve 13 small tubes or channels 19 connect these various connecting holes to the coupling 8 moreover in the intermediary plate 1a and at the location corresponding to each of the clamping devices 3 a cylindrical hole 20 is machined to allow for the free axial passage of the head 9 this being accomplished to provide a great deal of tolerance on one side of the hole 20 the same side for each hole which in the case of the fig5 showing the example selected is the left side a short t channel 21 is machined which opens into the hole 20 and allows for the hookup of the head 9 the two horizontal prepositioning stops 4 and the vertical positioning stop 5 are arranged such that the first two on the fixed framework of the machine are disposed opposite the high position of the slide 2 the latter being disposed on the same slide accordingly when the intermediary plate 1a abuts the truing up edge 7 all the clamping heads 9 are hooked into their respective channels 21 as represented in fig4 and 5 to completely change the unit 1 and replace it by a new unit the first step is to move the slide 2 of the machine to its high position place the unit 1 in a sling to support it later by a piece of handling equipment of some kind and to then connect the hydraulic system by means of coupler 8 so as to communicate the necessary oil pressure for simultaneous unclamping of all the heads 9 the unit 1 then rests against its positioning stop 5 instructions are then given to advance.