(A) study on the precision ball-end milling of free-form surfaces

Abstract

The ball-end milling process is widely used in the die/mold industries, and it is very suitable for the machining of free-form surfaces. With the development of CAD/CAM system and CNC machining centers, the accurate evaluation of cutter paths and their control in a machine tool have become possible. But, machining errors, such as under/over cuts, are often found on the machined parts. There are many factors which give rise to these machining errors. Among them, cutter deflection is one of the main causes of these machining errors on the free-form surfaces. In this thesis, a method avoiding these machining errors by rational feedrates has been suggested for the precision ball-end milling. First, a mechanically based model predicting the working accuracy on the free-form surfaces with ball-end mill has been developed. This working accuracy model is based upon the cutting force predictions in the flexible ball-end milling system. The distinct characteristics of ball-end milling process has been accurately modeled, and cutting forces and ball-end mill deflection were computed based on them. The models for the cutting process and ball-end mill deflection are not limited in their scope to consider only rigid systems, but also encompass machining conditions which could be classified as flexible. The models have been verified through the comparisons of measured and predicted data. The forced displacement of cutter is predicted by using stepped cantilever beam theory. The presented working accuracy model has been proved to be able to predict the working accuracies on the free-form surfaces. From the results, it is noted that under/over cuts are found in a restricted region, on the other side, there exist regions which are far from these errors. Hence, a rational feedrate (which effectively keeps the machining error in the allowable range) is required in the machining of free-form surfaces. The working accuracy prediction model is simplified for the evaluation...