A machining assembly for processing a semi-finished product, said machining assembly comprising: at least one shaft having a longitudinal axis (X1), a first end or tool-holder end for supporting a machining tool , and a second end , opposite to the first end along said longitudinal axis (X1); a support head in which is formed at least one longitudinal housing for housing said at least one shaft ; said at least one shaft being movable from a first non-operating position to a second operating position, and vice versa, along said longitudinal axis (X1) partially inside said longitudinal housing ; a chamber of said longitudinal housing being designed to be powered pneumatically by means of a gas, in particular compressed gas, preferably compressed air, for acting on said second end of said at least one shaft for moving it from said non-operating position to said operating position, or vice versa; said at least one shaft having a first through channel passing between said two ends , in order to put in fluid communication said chamber with a first hole formed in said tool-holder end , said machining assembly being configured so that when said chamber is fed pneumatically, said at least one shaft passes from said first non-operating position to said second operating position, or vice versa, and said fluid flows out via said hole of said tool-holder end ; characterized in that said at least one shaft further comprising, at the relative second end , a channel for connecting fluid between said first through channel and said chamber in such a way as to allow the passage of said gas, when the chamber if fed pneumatically, from the chamber to the first through channel of said at least one shaft . at least one shaft having a longitudinal axis (X1), a first end or tool-holder end for supporting a machining tool , and a second end , opposite to the first end along said longitudinal axis (X1); a support head in which is formed at least one longitudinal housing for housing said at least one shaft ; said at least one shaft being movable from a first non-operating position to a second operating position, and vice versa, along said longitudinal axis (X1) partially inside said longitudinal housing ; a chamber of said longitudinal housing being designed to be powered pneumatically by means of a gas, in particular compressed gas, preferably compressed air, for acting on said second end of said at least one shaft for moving it from said non-operating position to said operating position, or vice versa; said at least one shaft having a first through channel passing between said two ends , in order to put in fluid communication said chamber with a first hole formed in said tool-holder end , said machining assembly being configured so that when said chamber is fed pneumatically, said at least one shaft passes from said first non-operating position to said second operating position, or vice versa, and said fluid flows out via said hole of said tool-holder end ; characterized in that said at least one shaft further comprising, at the relative second end , a channel for connecting fluid between said first through channel and said chamber in such a way as to allow the passage of said gas, when the chamber if fed pneumatically, from the chamber to the first through channel of said at least one shaft . <EOS>