Engineering method for determining the possibility of internal buttress thread milling

Thread milling on CNC machines is widely used in practice as a highly productive method of machining. In the case of using three-coordinate CNC machines, when the axes of the thread being cut and the milling cutter are parallel, this feature does not prevent obtaining metric threads. However, when cutting threads with a buttress profile, in which the angle of inclination of one of the lateral sides is just a few degrees, such machining often becomes impossible. The reason for this is the phenomenon of enveloped path of the original tool surface generation, in this case – the surface of revolution, the nominal surface of the part, here the screw surface. The consequence of such enveloping is possible undercutting of the thread or the formation of unmachined areas on it. There are many methods for profiling such cutters, but all of them are quite complex, associated with transcendental and trigonometric equations, which requires the use of appropriate computer programs. In this regard, engineers of manufacturing enterprises need a simple practical method that gives the opportunity to evaluate the possibility of such processing at the first stages of design work and, if necessary, select the initial parameters of the required tool. Based on the analytical approach, a simple inequality has been obtained that can be implemented using typical office programs and can serve as a basis for the specified choice. In the case of cutting threads with a large pitch, when the phenomenon of enveloping significantly affects the resulting thread profile of the part, the method enables profiling and then sharpening the milling cutter to minimize errors in the resulting surface of the buttress thread.