An example of selection of the cutting conditions in broaching of heat-resistant alloys based on cutting forces, surface roughness and tool wear

Despite the fact that broaching has been used for long a time as a machining process for manufacturing highly accurate, complex profiles, little work has been published on the selection of cutting conditions to maximise tool life while achieving the required surface quality and level of cutting forces. This is even more important when notorious difficult-to-cut materials, such as Ni and Ti alloys, used in the aero-engine/power generation industry where high geometrical accuracy, along with restricted surface quality are required. The paper describes a multi-step methodology to select the cutting conditions for the broaching of Ni and Ti alloys used in the manufacture of aero-engines and power generators. Based on the application of the Taguchi technique, the cutting conditions (cutting speed, rise per tooth, rake angle, coolant type) were selected in order to obtain fine surface quality along with reasonably low levels of main cutting force (Fz) and perpendicular cutting force (Fy). This was followed by a reduced number of tool life tests which were carried out in order to select the final cutting conditions. Scanning electron microscopy (SEM) and chemical composition analysis on the flank and rake faces of the tools were employed to characterise the worn tools. It was found that when broaching forces, surface roughness and tool life are considered as process output measures, and taking into consideration generic process constraints on process productivity, machine tool stability, and tool stiffness, only a "pseudo-optimal" solution for the cutting conditions could be specified.