Abstract: The mandrel has a cylindrical body provided at the periphery with a plurality of jaws which are capable of radial displacement under the action of control means. Each jaw is supported by two radial columns which are capable of translational motion and each placed at one end of the jaw. Both support columns are driven by means of two first crank-arms rigidly fixed to two toothed wheels which are disposed in meshing engagement with each other, one toothed wheel being coupled with a second crank-arm. An actuating rod is mounted in the axis of the mandrel body and capable of translational motion along the axis, the rod being intended to actuate all the second crank-arms which correspond to the plurality of jaws.