High speed pocket milling planning by feature-based machining area partitioning

Pocket milling operations are involved in two and a half-dimensional (2.5D) machining. The machining area of a pocket has to be divided into several sub machining regions (SMRs) to effectively select the machining parameters for ordinary or high speed milling. A SMR of a pocket has its own characteristic geometry, which implicitly provides machining features used for the generation of strategies for high speed machining. This paper presents a methodology to partition a pocket machining area, as well as to identify machining features used for planning of high speed pocket machining. To generate the machining strategy, the attributes of machining features are defined, and evaluated through a machining volume slicing method. SMR-based partitioning rules are developed based on the geometric features of a pocket. The proposed partitioning algorithm is applied to both simple and complex shaped pockets. A case pocket volume is divided into several SMRs, represented by a tree structure containing associated information for pocket milling planning.     

On setup level tool sequence selection for 2.5-D pocket machining

This paper describes algorithms for efficiently machining an entire setup. Previously, the author developed a graph based algorithm to find the optimal tool sequence for machining a single 2.5-axis pocket. This paper extends this algorithm for finding an efficient tool sequence to machine an entire setup. A setup consists of a set of features with precedence constraints, that are machined when the stock is clamped in a particular orientation. The precedence constraints between the features primarily result from nesting of some features within others. Four extensions to the basic graph algorithm are investigated in this research. The first method finds optimal tool sequences on a feature by feature basis. This is a local optimization method that does not consider inter feature tool-path interactions. The second method uses a composite graph for finding an efficient tool sequence for the entire setup. The constrained graph and subgraph approaches have been developed for situations where different features in the setup have distinct critical tools. It is found that the first two methods can produce erroneous results which can lead to machine crashes and incomplete machining. Illustrative examples have been generated for each method.     

Voronoi diagram-based tool path compensations for removing uncut material in 2½D pocket machining

This paper solves the problem of uncut areas, which can arise when 2½D pockets are machined with radial widths of cut greater than half the cutter diameter. Using the Voronoi diagram approach, three types of uncut areas are defined i.e., corner, centre and neck uncut regions. The corner uncut area is further subdivided into five different types, the centre uncut area into four and the neck uncut area into two. Techniques for detecting each type as well as algorithms for generating the tool paths for removing them are developed based on a singularity-free Voronoi diagram approach. These efficient and robust algorithms ensure that no uncut material is left behind even for complex-shaped pockets containing islands. The proposed algorithms even permit the radial width of cut to be increased to its limiting value of tool diameter. Three examples are included to illustrate the procedures for detection and removal of the different types of uncut areas.     

Traversing the machining graph of a pocket

A simple and linear-time algorithm is presented for solving the problem of traversing a machining graph with minimum retractions encountered in zigzag pocket machining and other applications. This algorithm finds a traversal of the machining graph of a general pocket P with N_h holes, such that the number of retractions in the traversal is no greater than OPT+N_h+N_r, where OPT is the (unknown) minimum number of retractions required by any algorithm and N_r is the number of reducible blocks in P (to be defined in the paper). When the step-over distance is small enough relative to the size of P, N_r becomes zero, and our result deviates from OPT by at most the number of holes in P, a significant improvement over the upper bound 5OPT+6N_h achieved [Proceedings of the Seventh ACM-SIAM Symposium on Discrete Algorithms, 1996; Algorithmica 2000 (26) 19]. In particular, if N_h is zero as well, i.e. when P has no holes, the proposed algorithm outputs an optimal solution. A novel computational modeling tool called block transition graph is introduced to formulate the traversal problem in a compact and concise form. Efficient algorithms are then presented for traversing this graph, which in turn gives rise to the major result.     

A corner-looping based tool path for pocket milling

In milling around corners, cutting resistance rises momentarily due to an increase of cutter contact length. NC tool path generation in dealing with sharp corners thus requires special consideration. This paper describes an improved NC tool path pattern for pocket milling. The basic pattern of the improved tool path is a conventional contour-parallel tool path. Bow-like tool path segments are appended to the basic tool path at the corner positions. When reaching a corner, the cutter loops around the appended tool path segments so that corner material is removed progressively in several passes. By using the corner-looping based tool path, cutter contact length can be controlled by adjusting the number of appended tool path loops. The procedures of creating the improved tool path for different corner shapes are explained. The proposed tool path generation was implemented as an add-on user function in a CAD/CAM system. Cutting tests were conducted to demonstrate and verify the significance of the proposed method.     

Applications of genetic algorithms in process planning: tool sequence selection for 2.5-axis pocket machining

Tool sequence selection is an important activity in process-planning for milling and has great bearing on the cost of machining. Currently, it is accomplished manually without consideration of cost factors a priori. Typically, a large tool is selected to quickly generate the rough shape and a smaller clearing tool is used to generate the net-shape. In this paper, we present a new systematic method to select the optimal sequence of tool(s), to machine a 2.5-axis pocket given pocket geometry, a database of cutting tools, cutting parameters, and tool holder geometry. Algorithms have been developed to calculate the geometric constructs such as accessible areas, and pocket decomposition, while considering tool holders. A Genetic Algorithm (GA) formulation is used to find the optimal tool sequence. Two types of selection mechanisms namely “Elitist selection” and “Roulette method” are tested. It is found that the Elitist method converges much faster than the Roulette method. The proposed method is compared to a shortest-path graph formulation that was developed previously by the authors. It is found that the GA formulation generates near optimal solutions while reducing computation by up to 30% as compared to the graph formulation.     

Physics-guided logistic classification for tool life modeling and process parameter optimization in machining

This paper describes a physics-guided logistic classification method for tool life modeling and process parameter optimization in machining. Tool life is modeled using a classification method since the exact tool life cannot be measured in a typical production environment where tool wear can only be directly measured when the tool is replaced. In this study, laboratory tool wear experiments are used to simulate tool wear data normally collected during part production. Two states are defined: tool not worn (class 0) and tool worn (class 1). The non-linear reduction in tool life with cutting speed is modeled by applying a logarithmic transformation to the inputs for the logistic classification model. A method for interpretability of the logistic model coefficients is provided by comparison with the empirical Taylor tool life model. The method is validated using tool wear experiments for milling. Results show that the physics-guided logistic classification method can predict tool life using limited datasets. A method for pre-process optimization of machining parameters using a probabilistic machining cost model is presented. The proposed method offers a robust and practical approach to tool life modeling and process parameter optimization in a production environment.     

Polygon subdivision for pocket machining process planning

This paper presents a new approach to improve tool selection for arbitrary shaped pockets based on an approximate polygon subdivision technique. The pocket is subdivided into smaller sub-polygons and tools are selected separately for each sub-polygon. A set of tools for the entire pocket is obtained based on both machining time and the number of tools used. In addition, the sub-polygons are sequenced to eliminate the requirement of multiple plunging operations. In process planning for pocket machining, selection of tool sizes and minimizing the number of plunging operations can be very important factors. The approach presented in this paper is an improvement over previous work in its use of a polygon subdivision strategy to improve the machining time as well as reducing the number of plunges. The implementation of this technique suggests that using a subdivision approach can reduce machining time when compared to solving for the entire polygonal region.     

Offset tool-path linking for pocket machining

For die-cavity pocketing, contour-parallel offset (CPO) machining is the most popular machining strategy. CPO tool-path generation for pocketing includes geometrical and technological issues: (1) a 2D-curve offsetting algorithm; and (2) optimizing technological objectives, such as tool-path linking. The 2D-curve offsetting solution has been widely studied, because it has so many potential applications. However, though the tool-path linking may seriously affect the machining performance, there have been few reported investigations on optimizing the CPO tool-path linking. This paper presents a CPO tool-path linking procedure optimizing technological objectives, such as dealing with islands (positive and negative) and minimizing tool retractions, drilling holes and slotting. Main features of the proposed algorithm are as follows: (1) a data structure, called a ‘TPE-net’, is devised to provide information on the parent/child relationships among the tool-path-elements; (2) the number of tool retractions is minimized by a ‘tool-path-element linking algorithm’ finding a tour through the TPE-net; and (3) the number of drilling holes is minimized by making use of the concept of the ‘free space’ (negative islands or already machined region).     

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.     

Floor, wall and ceiling approach for ball-end tool pocket machining

This paper presents a floor-wall and ceiling (FWC) approach to determining the tool axes for five-axis pocket machining. The FWC method generates tool axes by simply linking the corresponding points between tool location paths on floor and tool axial paths on ceiling. The geometry of the floor, wall, and ceiling of a pocket are first discussed. The construction and manipulations of the tool location paths and the tool axial paths are then addressed. The relationship between the tool axis variation and the manipulations is analyzed such that the generated tool axes are guaranteed to be smooth, optimized, and collision-free. By using the FWC method, the complexity of determining the tool axes can be significantly reduced. Computer illustrations and example demonstrations are shown in this paper. The results reveal that the FWC method can generate much higher quality tool axes than the traditional methods.     

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.     

Automatic feedrate adjustment for pocket machining

As high-speed machining and unmanned machining become common, the demand for cutting-load regularization increases, so NC machining can be more efficient. To be presented is a simple cutting-load regularization method for pocket machining. As the conventional off-line approaches where cutting-load is predicted and cutting parameters are adjusted before actual cutting, the proposed method requires a cutting force model, which is quite simplified with the function of two independent variables. One is the geometric measure so called 2D chip-load (cutter-engagement angle or effective cutting depth), and the other is the feedrate. Based on the 2D chip-load analysis for the concave line-line segment of the NC tool path, the adjusted feedrate is calculated by using the simplified-cutting force model (SCFM) obtained by the cutting experiment with a tool dynamometer. The concept of the automatic feedrate adjustment (AFA) method to be proposed is very simple, and the implementation requires little effort. Furthermore, the proposed method does not need much calculation time because there are no complex calculations or cutting simulation.     

Automated tool sequence selection for 3-axis machining of free-form pockets

This paper describes an efficient method to find the lowest cost tool sequence for rough machining free-form pockets on a 3-axis milling machine. The free-form pocket is approximated to within a predefined tolerance of the desired surface using series of 2.5-D layers of varying thicknesses that can be efficiently removed with flat-end milling cutters. A graph-based method finds an optimal sequence of tools for rough machining the approximated pocket. The algorithm used here can be tuned to suit any available tool set and preferred cost models. The tool sequence that is obtained is near optimal, and may take into account tool wear, as well as various overhead costs of the machine shop.     

Optimal barrel cutter selection for the CNC machining of blisk

Barrel cutters have been widely used in the flank milling of blisk. In this paper, a new method is proposed to select an optimal barrel cutter in the interest of the high productivity in the CNC machining of blisk. The geometric properties and location of four types of barrel cutters are firstly introduced. Then the optimal size of the tool flute with a greater barrel radius is determined. Meanwhile, the local gouges and kinematic constraints of the CNC machine tool are also taken into account. Furthermore, in order to avoid the global interferences between the tool shaft and the blade in process, the center point of circular arc generatrix is adjusted. Finally, the tool shaft diameter is optimized to improve tool rigidity and avoid the global interferences between the tool shaft and the adjacent blades. The numerical examples prove that a feasible barrel cutter with the higher productivity can be obtained by the proposed method in this paper. Therefore, this method can be directly applied in the CNC machining of blisk or centrifugal impeller.     

Electromagnetism-like algorithms for optimized tool path planning in 5-axis flank machining

Optimization of tool path planning using metaheuristic algorithms such as ant colony systems (ACS) and particle swarm optimization (PSO) provides a feasible approach to reduce geometrical machining errors in 5-axis flank machining of ruled surfaces. The optimal solutions of these algorithms exhibit an unsatisfactory quality in a high-dimensional search space. In this study, various algorithms derived from the electromagnetism-like mechanism (EM) were applied. The test results of representative surfaces showed that all EM-based methods yield more effective optimal solutions than does PSO, despite a longer search time. A new EM-MSS (electromagnetism-like mechanism with move solution screening) algorithm produces the most favorable results by ensuring the continuous improvement of new searches. Incorporating an SPSA (simultaneous perturbation stochastic approximation) technique further improves the search results with effective initial solutions. This work enhances the practical values of tool path planning by providing a satisfactory machining quality.     

Multiple tool path planning for NC machining of convex pockets without islands

This work focuses on machining of pockets with convex angles and containing no islands. The approach is a hybrid contouring – staircasing one. Contouring aims at clearing enough space around the boundary to allow efficient staircasing in the interior. Three tools are foreseen: two for contouring and one for staircasing. The algorithm for contouring is based on creating offsets of the pocket boundary and checking them for self-crossing. Staircasing is based on parametrisation of the tool path with respect to its orientation to the pocket, so as to minimise the tool path length. Machining strategy is complemented by choosing the best combination of diameters of the three tools used. This is simply done by first enumerating all available combinations and then excluding the non-feasible ones, in order to compare the rest according to tool path length (or to machining time when individual feed values are known). The output of the program, which was implemented in Fortran, is the tool path, the CNC part program, which is created automatically, and a numerical comparison of all the tool-angle combinations tried out. A machining simulation based on the CNC program output is conducted on commercial CAM software to demonstrate validity of the result.     

Contour-parallel offset machining without tool-retractions

Contour-parallel offset (CPO) machining uses successive offsets of the boundary curves of the machining region as the tool-path-elements (TPEs). For the efficiency of the CPO machining, it is very important to minimize the number of tool-retractions, which cause additional tool movements and do not contribute to the actual cutting. Presented in the paper is a CPO tool-path linking algorithm, which guarantees ‘zero’ number of tool-retractions. The algorithm employs the concept of a ‘TPE-net’ providing the information on the parent/child relationships among the TPEs. By planning a route through the TPE-net, a CPO tool-path without tool-retractions can be generated.     

A tool path generation method for freeform surface machining by introducing the tensor property of machining strip width

Due to the complexity of geometry, the feed direction with maximal machining strip width usually varies among different regions over a freeform surface or a shell of surfaces. However, in most traditional tool path generation methods, the surface is treated as one machining region thus only local optimisation might be achieved. This paper presents a new region-based tool path generation method. To achieve the full effect of the optimal feed direction, a surface is divided into several sub-surface regions before tool path computation. Different from the scalar field representation of the machining strip width, a rank-two tensor field is derived to evaluate the machining strip width using ball end mill. The continuous tensor field is able to represent the machining strip widths in all feed directions at each cutter contact point, except at the boundaries between sub-regions. Critical points where the tensor field is discontinuous are defined and classified. By applying critical points in the freeform surface as the start for constructing inside boundaries, the surface could be accurately divided to such that each region contain continuous distribution of feed directions with maximal machining strip width. As a result, tool paths are generated in each sub-surface separately to achieve better machining efficiency. The proposed method was tested using two freeform surfaces and the comparison to several leading existing tool path generation methods is also provided.