Hsi-Yung Feng

Analysis of digitizing errors of a laser scanning system

The digitizing errors of a high-speed 3D laser scanning system are analyzed and characterized in this paper. As the laser scanner is an electro-optical device and based on the principle of optical triangulation, the measurement accuracy is affected by the measured part geometry and its position within the scanning window. Commercial laser scanners are often calibrated in the scanning plane to account for variation of the incident angle of the laser beam. The effects of the scan depth and the projected angle, characterizing the surface normal of the measured part external to the scanning plane, on the measurement accuracy are not considered in the standard calibration process and have been identified by experiments in the present work. Experimental results indicate that the random error of the scanned data is close to the nominal value provided by the manufacturer. The systematic error shows a bilinear relationship with the scan depth and the projected angle and has a maximum value of about 160 μm. The developed empirical model correctly predicts the systematic error with a maximum deviation of only 25 μm.

Machining of an aluminum/SiC composite using diamond inserts

Abstract Experimental results in the machining of a SiC-reinforced aluminum metal–matrix composite (MMC) with diamond inserts are presented in this paper. MMCs are very difficult to machine and diamond tools are considered by far the best choice for the machining of these materials. Two types of commercially available diamond tools were examined: brazed polycrystalline diamond (PCD) tools; and chemical vapor deposition (CVD) diamond coated tools. The present results showed that the initial flank wear on both the PCD and the CVD diamond tools was generated by abrasion due to the very hard SiC particles present in the workpiece material. As machining progressed, thin films of the workpiece material were found to be adhering to the worn areas. Further tool wear in these areas is believed to be caused by a combination of the abrasive wear and the adhesive wear mechanisms. This explains the faster rate of flank wear on the CVD diamond insert than that on the PCD insert observed in the experiments.

The prediction of cutting forces in the ball-end milling process—I. Model formulation and model building procedure

Abstract This paper presents a model for the prediction of cutting forces in the ball-end milling process. The steps used in developing the force model are based on the mechanistic principles of metal cutting. The cutting forces are calculated on the basis of the engaged cut geometry, the underformed chip thickness distribution along the cutting edges, and the empirical relationships that relate the cutting forces to the undeformed chip geometry. A simplified cutter runout model, which characterizes the effect of cutter axis offset and tilt on the undeformed chip geometry, has been formulated. A model building procedure based on experimentally measured average forces and the associated runout data is developed to identify the numerical values of the empirical model parameters for the particular workpiece/cutter combination.

Constant scallop-height tool path generation for three-axis sculptured surface machining

This paper presents a new approach for the determination of efficient tool paths in the machining of sculptured surfaces using 3-axis ball-end milling. The objective is to keep the scallop height constant across the machined surface such that redundant tool paths are minimized. Unlike most previous studies on constant scallop-height machining, the present work determines the tool paths without resorting to the approximated 2D representations of the 3D cutting geometry. Two offset surfaces of the design surface, the scallop surface and the tool center surface, are employed to successively establish scallop curves on the scallop surface and cutter location tool paths for the design surface. The effectiveness of the present approach is demonstrated through the machining of a typical sculptured surface. The results indicate that constant scallop-height machining achieves the specified machining accuracy with fewer and shorter tool paths than the existing tool path generation approaches.

On the normal vector estimation for point cloud data from smooth surfaces

Reliable estimation of the normal vector at a discrete data point in a scanned cloud data set is essential to the correct implementation of modern CAD/CAM technologies when the continuous CAD model representation is not available. A new method based on fitted directional tangent vectors at the data point has been developed to determine its normal vector. A local Voronoi mesh, based on the 3D Voronoi diagram and the proposed mesh growing heuristic rules, is first created to identify the neighboring points that characterize the local geometry. These local Voronoi mesh neighbors are used to fit a group of quadric curves through which the directional tangent vectors are obtained. The normal vector is then determined by minimizing the variance of the dot products between a normal vector candidate and the associated directional tangent vectors. Implementation results from extensive simulated and practical point cloud data sets have demonstrated that the present method is robust and estimates normal vectors with reliable consistency in comparison with the existing plane fitting, quadric surface fitting, triangle-based area weighted average, and triangle-based angle weighted average methods.

Architecture design for distributed process planning

Abstract Todays machining shop floors, characterized by a large variety of products in small batch sizes, require dynamic process planning capabilities that are responsive and adaptive to the rapid changes of production capacity and functionality. To meet the requirement, this research proposes a new methodology for dynamic and distributed process planning. The primary focus of this paper is on the architecture of a new approach using function blocks. The secondary focus is given to the other supporting technologies—machining features and agents. Different from conventional methods, this approach uses a two-layer structure—supervisory planning and operation planning. It is expected that the new architecture can improve the system performance in a dynamic environment.

Iso-planar piecewise linear NC tool path generation from discrete measured data points

Abstract This article presents a method of generating iso-planar piecewise linear NC tool paths for three-axis surface machining using ball-end milling directly from discrete measured data points. Unlike the existing tool path generation methods for discrete points, both the machining error and the machined surface finish are explicitly considered and evaluated in the present work. The primary direction of the generated iso-planar tool paths is derived from the projected boundary of the discrete points. A projected cutter location net (CL-net) is then created, which groups the data points according to the intended machining error and surface finish requirements. The machining error of an individual data point is evaluated within its bounding CL-net cell from the adjacent tool swept surfaces of the ball-end mill. The positions of the CL-net nodes can thus be optimized and established sequentially by minimizing the machining error of each CL-net cell. Since the linear edges of adjacent CL-net cells are in general not perfectly aligned, weighted averages of the associated CL-net nodes are employed as the CL points for machining. As a final step, the redundant segments on the CL paths are trimmed to reduce machining time. The validity of the tool path generation method has been examined by using both simulated and experimentally measured data points.

Integrated tool path and feed rate optimization for the finishing machining of 3D plane surfaces

An integrated approach for the concurrent optimization of tool path and feed rate for the finishing machining of 3D plane surfaces using ball-end milling is presented in this paper. This work is important, as the developed optimization approach is readily applicable to the finishing machining of sculptured surfaces. The concurrently optimized tool path and feed rate correspond to the maximum machining efficiency and satisfy the scallop height and machining error requirements. The cutter feed direction is employed as the optimization variable. For each cutter feed direction, tool path is determined according to the scallop height requirement and feed rate is maximized with the tolerance requirement by using a mechanistic cutting force model for three-dimensional ball-end milling. Optimization results have indicated that the shortest total tool path length, favored by most existing optimization approaches, does not result in maximum efficiency because the corresponding feed rate is often constrained by the specified tolerance. The optimum cutter feed direction is in general not unique but falls within an optimum range in the finishing machining of 3D plane surfaces.

A Flexible Ball-End Milling System Model for Cutting Force and Machining Error Prediction

This paper presents a flexible system model for the prediction of cutting forces and the resulting machining errors in the ball-end milling process. Unlike the previously developed rigid system model, the present model takes into account the instantaneous and regenerative feedback of cutting system deflections to establish the chip geometry in the cutting force calculation algorithm. The deflection-dependent chip geometry is identified by using an iterative procedure to balance the cutting forces and the associated cutting system deflections. A series of steady state 3D cross-feed ball-end milling cuts were performed to validate the capability of the present model in predicting the cutting forces and the resulting machining errors. It is shown that the flexible system model gives significantly better predictions of the cutting forces than the rigid system model. Good agreement between the predicted and measured machining errors is demonstrated for the simple surfaces generated by horizontal cuts.

The prediction of dimensional error for sculptured surface productions using the ball-end milling process. Part 1: Chip geometry analysis and cutting force prediction

Abstract The study of machining errors caused by tool deflection in the balkend milling process involves four issues, namely the chip geometry, the cutting force, the tool deflection and the deflection sensitivity of the surface geometry. In this paper, chip geometry and cutting force are investigated. The study on chip geometry includes the undeformed radial chip thickness, the chip engagement surface and the relationship between feed boundary and feed angle. For cutting force prediction, a rigid force model and a flexible force model are developed. Instantaneous cutting forces of a machining experiment for two 2D sculptured surfaces produced by the ball-end milling process are simulated using these force models and are verified by force measurements. This information is used in Part 2 of this paper, together with a tool deflection model and the deflection sensitivity of the surface geometry, to predict the machining errors of the machined sculptured surfaces.