a support base body along which a through opening is obtained for the passage of the bar,

oblong centring elements, which are connected rotatably to said support base body and arranged circumferentially thereto to maintain the bar axially aligned on the spindle of the machine tool,

an elastic element arranged for urging elastically said oblong centring elements to a radial collecting and contraction position in which they are in mutual proximity, wherein

each of said oblong centring elements is provided with friction reducing members including revolving members configured for facilitating the corresponding sliding of said bar with respect to the oblong centring element,

said revolving members including at least one revolving bearing the rotation axis of which is tiltable, in an adjustable manner, with respect to an axial plane of said support base body, each revolving bearing being rotatably supported by a respective shank protrusion that projects from the respective oblong centring element, wherein

said shank protrusion is connected to the respective oblong centring element in an adjustable manner by suitable fixing elements that, once loosened, enable the orientation of the respective axis to be varied, to select the position and tilt of the respective revolving bearing in a desired manner in function of specific needs and on the basis of the geometry of said bars, tubes or similar to guide to the machine tool.