Multiresolution analysis as an approach for tool path planning in NC machining

Wavelets permit multiresolution analysis of curves and surfaces. A complex curve can be decomposed using wavelet theory into lower resolution curves. The low-resolution (coarse) curves are similar to rough-cuts and high-resolution (fine) curves to finish cuts in NC machining. In this paper, we investigate the applicability of multiresolution analysis using B-spline wavelets to NC machining of contoured 2D objects. High-resolution curves are used close to the object boundary similar to conventional offsetting while lower resolution curves are used farther away from the object boundary. Experimental results indicate that wavelet-based tool path planning improves machining efficiency. Tool path length is reduced, sharp corners are smoothed out thereby reducing uncut areas and larger tools can be selected for rough-cuts.     

A machining process planning activity model for systems integration

A key issue of integrating process planning systems with design systems and production planning systems is how to overcome barriers in data exchange and sharing amongst software systems. A machining process planning activity model was developed to address some of the barriers. This model represents functional components and data requirements in process planning systems. The purpose of the model is to create the context in which data requirements and data flow for numerically controlled machining process planning are defined. Furthermore, the model was developed as a unification of many previously developed process planning activity models.     

A feature-based method for NC machining time estimation

Machining time estimation plays an important role in manufacturing process planning and scheduling. Existing NC machining time estimation methods are all based on material removal rates, NC programs, and machine characteristics. However, the machining condition which is related to the geometry-process information is also an important impact factor of the NC machining time estimation. As existing methods cannot satisfy the requirement of timeliness, accuracy and efficiency, this paper presents a feature-based method for NC machining time estimation. Experiment results show that the proposed approach is feasible and practical. It is particularly useful in real time manufacturing process planning and scheduling systems.     

Estimation of NC machining time using NC block distribution for sculptured surface machining

The estimation of NC machining time is of importance because it provides manufacturing engineers with information to accurately predict the productivity of an NC machine, as well as its production schedule. NC programs contain various machining information, such as tool positions, feed and speed rates, and other machine instructions. Nominal NC machining time can easily be obtained based on the NC program data. Actual machining time, however, cannot simply be found due to the dynamic characteristics of a NC machine controller, such as acceleration and deceleration effect. Hence, this study presents an NC machine time estimation model for machining sculptured surfaces, considering such dynamic characteristics of the machine. The proposed estimation model uses several factors, such as the distribution of NC blocks, angle between the blocks, federates, acceleration and deceleration constants, classifying tool feed rate patterns into four types based on the acceleration and deceleration profile, NC block length, and minimum feed rate. However, there exists an error for the actual machining time due to the lack of the measurement equipment or tools to gauge an exact minimum feed rate. Thus, this paper proposes a machining time estimation model using NC block distributions, lowering down the error caused by the inaccurate minimum feed rate. The proposed machining time estimator performs at around 10% of mean error.     

A process matching approach for flexible workflow process reuse

Matching between two workflow processes is the key step of workflow process reuse. This paper presents an inexact matching approach for flexible workflow process reuse. A multi-valued process specialization relationship is defined based on the definition of activity specialization and the characteristics of workflow process. The matching degree between two workflow processes is determined by the matching degrees of their corresponding sub-processes or activities. The matching degree between two activities is determined by the activity-distance between them in the activity-ontology repository. A set of process specialization rules enables a new process matching to be derived from the existing matchings. Users are provided with an SQL-like command to retrieve the required processes in an inexact query condition from the workflow-process-ontology repository.     

Efficient feature-based process planning for sculptured pocket machining

Feature is recently known as the core concept necessary to realize a fully integrated CAD/CAM system. The information contents in a feature can be easily conveyed from one application to another in the manufacturing domain. However the feature generated in one application may not be suitable for another without being modified with more information. The objective of the paper is to present the methodology of decomposing bulky features of the sculptured shape of pocket to be removed into compact features to be efficiently machined. It is possible to reactively and efficiently machine the sculptured shape of pocket by segmenting horizontally and vertically a bulky feature and by applying variable cutting condition to each feature.     

Multiresolution offsetting and loose convex hull clipping for 2.5D NC machining

A complex curve can be decomposed using wavelet-based multiresolution analysis into lower resolution curves. In this paper B-spline wavelet theory is applied to NC machining of contoured 2D objects. High-resolution curves are used close to the object boundary, while lower resolution curves are used farther away from the object boundary. This reduces tool path length and sharp corners are smoothed out thereby reducing uncut areas. The wavelet curves are clipped against a loose convex hull of the 2D object contour to further improve the efficiency of the generated tool paths. Simulation results indicate that the wavelet based tool paths and clipping improves machining efficiency.     

A flexible quantitative method for NC machining verification using a space-division based solid model

A geometric-modeling method called Graftree and a system design of an NC (numerical control) machining verifier based on Graftree are proposed. Graftree is constructed by combining Oct-tree and constructive solid geometry (CSG) so as to simulate machining processes precisely in three-dimensional space. Using Graftree, the Boolean operation produces no risk of yielding topological conflicts, which often cause the simulation to stop abnormally. Based on the properties of Graftree, an NC machining verifier is designed to consist of three individual subsystems: data-input operations, geometric simulation, and interactive verification, i.e., visuluatzation of machining scenes and evaluation of the machined product using models of the measuring instruments. This design lets the users watch selectively only the machining steps they want to check precisely, and verify the machining according to dimensional tolerance.     

Accurate tool position for five-axis ruled surface machining by swept envelope approach

This paper presents a swept envelope approach to determining tool position for five-axis ruled surface machining. The initial tool position is traditionally located to contact with two directrices of a ruled surface. The swept profile of the tool is then determined based on the tool motion. By comparing the swept profile with the ruled surface, the tool position is corrected to avoid machining errors. The cutter's swept envelope is further constructed by integrating the intermediate swept profiles, and applied to NC simulation and verification. This paper presents the explicit solution for the swept profile of a taper-end cutter in five-axis ruled surface machining. The relation of the ruled surface geometry, the tool motion and the machining errors is developed. Therefore, the error sources can be detected early and prevented during tool path planning. The explicit swept envelope indicates that the machined surface is not a ruled surface in five-axis ruled surface machining. Manufacturing industries should take extra care in high precision ruled surface machining. Computer illustrations and example demonstrations are shown in this paper. The results reveal that the developed method can accurately position tool location and reduce machining errors for five-axis ruled surface machining.     

Rough and finish tool-path generation for NC machining of freeform surfaces based on a multiresolution method

Progressive fitting and multiresolution tool path generating techniques are proposed in this paper, by which multi-level (LOD) models fitting for different subsets of sampled points are obtained, and then multiresolution rough-cut and finish-cut tool paths are generated based on the LOD models. The advantages of the proposed method are: (1) the user need not care for data reduction in CAD modeling; (2) final result is obtained by interpolating two lower-level reconstructed surfces, and each lower multiresolution CAD representation can be used to generate rough-cut tool paths; (3) different manufacturing requirements utilize different level models to generate tool paths; (4) selective refinement can be applied by interpolating selceted areas at different levels of details. The key avantage of the prograssive fitting algorithm is that it can use different level surfaces to generate adaptive rough-cut and finishi-cut tool path curves directly. Therefore, based on the proposed techniques, tool path length is reduced. Sharp concers are smothed out and large tools can be selected for rough machining. The efficiency of this algorithm has been demonstrated, and it results in a 20% reduction in machining time.     

The sweep plane algorithm for global collision detection with workpiece geometry update for five-axis NC machining

A new algorithm based on the sweep plane approach for global collision detection for five-axis NC machining is presented. This algorithm takes into account not only collisions between the tool and workpiece, but also collisions between the other parts of the CNC machine, especially the change of the workpiece geometry is included in the detection process. The workpiece and machine bodies are firstly approximated by an octree of bounding spheres. Collision detection is conducted between these spheres. If there is any interference between these bounding spheres, their subspheres are further tested. The subdivision process is recursively performed until the resolution reaches the desired precision level. If there is no interference between the spheres, there is no need to subdivide any more. When the interference is detected between the spheres in the last octree level, the slices within these colliding spheres are further checked by using the sweep plane algorithm to determine whether the enclosed objects really collide with each other. In the sweep plane algorithm, most of the slices of the moving bodies stay parallel and their collisions are detected by checking the interference between these parallel slices using 2D polygon clipping. Whereas, if the slices are not parallel to the reference slicing direction (due to the rotary axes), the interference detection is conducted by examining overlaps of the projections of these slices on the three perpendicular planes XY,YZ, and ZX. The accuracy of the algorithm can be adjusted by changing the distance between the sweep planes. The algorithm can be applied to any five-axis CNC machines.     

A machining potential field approach to tool path generation for multi-axis sculptured surface machining

This paper presents a machining potential field (MPF) method to generate tool paths for multi-axis sculptured surface machining. A machining potential field is constructed by considering both the part geometry and the cutter geometry to represent the machining-oriented information on the part surface for machining planning. The largest feasible machining strip width and the optimal cutting direction at a surface point can be found on the constructed machining potential field. The tool paths can be generated by following the optimal cutting direction. Compared to the traditional iso-parametric and iso-planar path generation methods, the generated MPF multi-axis tool paths can achieve better surface finish with shorter machining time. Feasible cutter sizes and cutter orientations can also be determined by using the MPF method. The developed techniques can be used to automate the multi-axis tool path generation and to improve the machining efficiency of sculptured surface machining.     

ISO 14649-based nonlinear process planning implementation for complex machining

Increasing attention is being paid to complete machining, i.e., machining of the whole part in a single machine tool, in the metal working industry. For this purpose, complex machine tools equipped with machining components, such as multiple spindles and turrets have been developed by leading machine tool builders. The efficiency of complex machine tools is largely dependent on how the machining components are utilized. The main thrust of this paper is twofold: (1) Proposition of a nonlinear process planning based on the STEP-NC (STEP-compliant data interface for numerical controls) paradigm whose data model is formalized as ISO 14649, and (2) Development of an optimal solution algorithm for process planning for complex machining. The developed algorithm is based on the branch-and-bound approach and heuristics derived from engineering insights. The developed process planning method and optimization algorithm were implemented and tested via the TurnSTEP system developed by our research team. Through the experiments, we are convinced that the new process planning and algorithm can be used as a fundamental means for implementing the third type of STEP-NC [Suh S. TurnSTEP: Tools to create CNC turning programs. In: White paper presented on STEP Implementers’ Forum ISO TC184/SC4 Meeting. 2004], i.e., an Intelligent and Autonomous STEP-NC system for the CAD-CAM-CNC chain supporting e-Manufacturing.     

A novel flexible activity refinement approach for improving workflow process flexibility

Flexible activity refinement plays an important role in improving process flexibility and addressing uncertainties of business processes. However, it is still a challenge to refine flexible activities, and the existing researches on flexible activity refinement such as the refinement principles and methods, and their combination with factors such as constraints and contexts is still lacking. Aiming at this, a novel dynamic refinement approach for flexible activities is proposed, which combines both vertical decomposition and horizontal extension refinements, with the impact of constraints and contexts considered. In particular, we summarize five typical refinement categories, and present a set of activity refinement rules based on them. Furthermore, the decomposition refinement, including the activity decomposition principles, the related rules for trigger event delivering and execution condition transferring is discussed in detail. The extension refinement, which realizes the horizontal refinement, can be integrated with other kinds of refinement and uses constraints to specify activity selection, activity temporal relationships, etc. Then, a tree-like activity refinement graph (ARG) is proposed to represent the refinement process, based on which the refinement cost and refinement degree can be computed to benefit the finding of the potential optimal refinement path. As a further implementation of the proposed refinement approach, a general refinement algorithm is described. Finally, a case study of urolithiasis therapy process and its application are given, and the results indicate the effectiveness of our proposals.     

A model for integrating process planning and production planning and control in machining processes

The goal of process planning is to propose the routing of a previously designed part and results in a sequence of operations and their parameters. It concerns and requires detailed information about the process. The goal of production planning, on the other hand, is to schedule, sequence and launch the orders introduced on the routing sheet into the job-shop according to the enterprise's strategic goal and the actual conditions of the production plant. The goals, information and decisions taken in process planning and production planning and control are often very different and, because of that, it is very difficult to integrate them.

The objective of this work is to develop a model that can be applied in the future to the development of an integrated process planning and scheduling tool using an integrated definition (IDEF) methodology to design an activity model, which integrates process and production planning in metal removal processes. An activity model will be used to develop a system that allows the user to plan the process and the production at the same time in collaborative engineering work. To design the activity model, a wide range of parts were evaluated and processed in an actual job-shop factory. Several activities were developed in detail to be tested in real cases, and an example of one of them is introduced in this article.     

A novel energy demand modelling approach for CNC machining based on function blocks

Energy efficiency remains one of the major issues in the machining domain. Today's machining systems are confronted with a number of new challenges, such as turbulent product demand and variations in production resources. Rapid and flexible energy modelling in a distributed and collaborative machining environment emerges as a new research area. Energy demand models in such an environment need to be practical, accurate, effective, scalable and reusable. Energy analysis and optimisation cannot be carried out once for all at the beginning. Instead, it is an on-going process. In this paper, the function block technique, i.e. IEC 61499, is used for the development of energy demand models as it brings advantages such as modularity, encapsulation, extensibility and reusability. A brief review on energy modelling and research on function blocks are given in the first part. A novel energy demand modelling approach based on function blocks is then proposed and elaborated. Three types of function blocks have been developed, i.e. machine tool dependent function blocks, state transition function blocks, and service interface function blocks. The first type, as the fundamental building blocks, is divided into two sub-types, machine component function block and machining state function block. Two case studies, based on a small 3-axis milling machine and an industrial production line respectively, are presented to demonstrate the possible applications using the function block-based model. Comprehensive discussions are given thereafter, including a pilot application of a distributed process planning system and a unique energy evaluation scheme. A confidence level associated energy rating system is proposed as the first step to turn energy consumption figures into useful indicators. The energy demand model based on function blocks developed here enhances the energy modelling and their practical implementations.     

支持实体数控铣削仿真验证的Cuboid-Array模型的研究

研究了Z-Buffer,Dexel和Ray-representation建模思想和方法,提出Cuboid-array的实体近似建模方法．采用基于设计模型的实体数控仿真验证技术,以Cuboid-array表示干涉元素集合体,提出基于B-rep和Cuboid-array的双体建模的近似实体数控仿真验证技术．给出了基于Cuboid-array的数控仿真验证算法,研制了基于双体建模和双体显示的数控仿真验证实验系统,实现了面向过程和结果的实体数控仿真的集成、面向实体数控仿真和实体数控验证的集成,并给出了实验用例．     

New capability indices for measuring the performance of a multidimensional machining process

Engineering tolerance plays an important role in the process capability analysis for determining whether a manufacturing process is capable of making good quality products. In contrast with the engineering tolerance region in a multivariate manufacturing process, the multidimensional machining process or the nano-cutting process has a special engineering tolerance called the positional tolerance. Positional tolerance is a special type of geometric dimensioning and tolerancing which describes the tolerance region between the actual location of machining results and the target location. In the past few years, several capability indices have been developed for measuring the performance of a multidimensional machining process under the assumption that the variances of machining results on different directions are equal. However, this assumption may not be true in most practical situations. In this paper, we propose three novel capability indices for measuring the performance of a multidimensional machining process under the assumption that the variances of machining results on different directions may not be equal. The statistical properties of the point estimators and their confidence intervals for the new capability indices are derived. Both the simulation results and numerical examples show that the new capability indices outperform the predecessors.