A machining assembly for processing a semi-finished product, said machining assembly comprising: at least one shaft having a longitudinal axis, 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; 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 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 (103, 106), 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 (10, 30) 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, 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; 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 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 (103, 106), 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 (10, 30) 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.