a tool - holder for a milling machine is composed of a first rotating portion with a coupler attachable to a spindle drive shaft of the machine , a second rotating portion turning as one with the first portion and equipped with a chuck to hold a tool , and a mechanism designed to detect undulations on a surface being milled . the second rotating portion is capable of axial movement relative to the first rotating portion , so that when the tool - holder is traversed along a prescribed trajectory referable to a nominal surface , the undulations on the actual surface of the panel and the extent of their deviation from the nominal surface can be sensed and quantified through the agency of detection mechanism , and a corresponding axial movement induced in the second portion of the tool - holder to compensate the variation and maintain a constant milling depth , thereby improving the quality of finish - milling operations on technological materials such as panels of carbon fiber composite .