Cutting Force Model And Experimental Study On Core Drilling By Ultrasonic-Vibration-Assisted Grinding

Ultrasonic-vibration-assisted grinding (UVAG) is a hybrid machining process which combines diamond grinding and ultrasonic machining. With impact mode produced by ultrasonic vibration, grinding mode produced by tool rotation, erosion mode produced by both ultrasonic vibration and tool rotation, and the fatigue of workpiece surface layer produced by high-frequency ultrasonic vibration, the core drill with metal bonded diamond abrasives removes material from workpiece with lower cutting force. Therefore, UVAG gets much higher material removal rate than conventional ultrasonic machining, and attractive applications on hard-to-machining materials, especially the hole drilling on hard-to-machining materials. However, the hole drilling in UVAG of ductile and brittle materials under the condition of constant feed rate has not been systematically studied. In China, due to lack of equipment, not much experimental research has been done on hole drilling by constant-feedrate UVAG, especially no research on fundamental mechanisms under the drilling condition of constant feed rate has been reported in author's knowledge, hindering the application of constant-feedrate UVAG. In this dissertation, two predictive cutting force models have been developed for constant-feedrate UVAG of ductile materials and brittle materials, respectively. The effects of input variables on cutting force and exit edge-chipping have been studied. The results in this dissertation could provide theoretical guidance for choosing reasonable process variables and designing diamond drilling tool and UVAG equipment.The main research contents and conclusions are as follows:1. A physics-based predictive cutting force model is developed for ductile materials with these assumptions and simplifications:workpiece material is rigid-plastic; diamond grains are rigid spheres of the same size; diamond grains located on the tool end surface have the same extrusion, and all of them take part in cutting during each ultrasonic vibration cycle; the volume of material removed by a diamond grain in one vibration cycle is approximately equal to the intersection volume between the diamond grain and the workpiece. With this developed model, the effect trends of input variables on cutting force are studied. It shows that in constant-feedrate UVAG, as diamond grain number, diamond grain size and feedrate decrease, cutting force decreases; as ultrasonic vibration amplitude and spindle speed increase, cutting force decreases; ultrasonic vibration frequency has no significant effects on cutting force. The model results are verified through experiments.2. A mechanistic predictive cutting force model is developed for brittle materials under these assumptions and simplifications:the workpiece material is an ideally brittle material; diamond grains were rigid spheres of same size; diamond grains located on the tool end surface had the same height of extrusion, and all of them took part in cutting during each ultrasonic vibration cycle. The maximum indentation depth is calculated with theory of fracture mechanics. It shows that as diamond grain number, diamond grain size, ultrasonic vibration amplitude, and spindle speed increase, cutting force decreases; as feedrate decreases, cutting force decreases; ultrasonic vibration frequency has no significant effects on cutting force. The model results are also verified through experiments.3. Based on these models, a full-factorial design of experiments is utilized to study the main effects and interaction effects of input variables on cutting force systematically. There are no significant interaction effects among these input variables in constant-feedrate UVAG of ductile materials. However, there are significant interaction effects among input variables in constant-feedrate UVAG of brittle materials. Therefore, in constant-feedrate UVAG of brittle materials, it is important to determine the input variables which have interaction effects.4. Three cutting tool designs are proposed to improve the exit edge-chipping since it is common in UVAG of brittle materials. Finite element analysis is utilized to study the effects of tool design and process variables on exit edge-chipping for brittle materials in constant-feedrate UVAG and the simulation results are verified by experiments. It shows that, with increase of feedrate, the edge-chipping thickness and size increase; with increase of spindle speed and ultrasonic vibration amplitude, edge-chipping thickness and size decrease; with increase of tool angle, the edge-chipping thickness increases; with increases of wall thickness of tool, the edge-chipping thickness varies. Among the three cutting tools, the outer tool gets the lowest exit edge-chipping thickness, followed by inner tool and normal tool.