Contour-parallel Tool Path Research Articles

Optimization-based non-equidistant toolpath planning for robotic additive manufacturing with non-underfill orientation

Additive manufacturing (AM) technology has achieved universal application in a great number of fields, such as aerospace, medicine, and military industry. As a significant factor causing weak mechanical properties and part flaws, underfill is inevitable in AM based on the conventional equidistant toolpaths with a limited extra cost. To eliminate underfill caused by sharp corners and voids, this paper develops an optimization-based non-equidistant toolpath, i.e., isoperimetric-quotient-optimal toolpath (IQOP). Firstly, an optimization problem minimizing the isoperimetric quotient of toolpaths is designed to generate smooth toolpaths and is convexified. Then, a unilateral rolling circle method is proposed to guarantee the well-defined condition of the optimization-based toolpath planning process. Finally, the application of the depth tree makes the IQOP method adopt slices with complex boundaries and topological structures. The experimental results show that the proposed IQOP achieves an average 88.5% lower underfill rate than the contour-parallel toolpath (CP). IQOP significantly outperforms the dense CP (DCP) on toolpath smoothness and printing efficiency, with better performance on underfill. With obvious advantages on toolpath smoothness and underfill rate, IQOP achieves higher printing quality than CP in the real world. The proposed approach also provides a general framework of non-equidistant toolpath planning for complex slices in AM and computer numerical control (CNC) milling.

Novel Tool Path Generation Method for Pocket Machining Using Sound Field Synthesis Theory

Contour parallel tool paths have been proved to be a preferred machining strategy for their advantage of less tool retractions and less sharp turns. The traditional geometrical algorithm-based tool path generation method often makes it hard to simply and simultaneously solve the problems of self-intersection, no residual, and smoothness at the same time due to their contradictions. To address this issue, a contoured parallel tool path generation method for pocket machining is developed in this study. It is based on sound field synthesis theory inspired by the phenomenon of sound wave propagation. Firstly, the simplified medial axis (SMA) tree of the pocket is extracted and the propagation direction of each SMA segment is calculated on account of the geometric characteristics of the pocket boundary. Secondly, the final tool path is obtained through the synthesis of the sound field. Finally, the novel method is verified on five different pockets to generate a contoured parallel milling tool path. After machining these pockets and measuring the machining time, roughness, and cutting force, the experimental results demonstrate that the tool path obtained by the novel method has advantages in improving machining quality and efficiency.

A framework from point clouds to workpieces

Combining computer-aided design and computer numerical control (CNC) with global technical connections have become interesting topics in the manufacturing industry. A framework was implemented that includes point clouds to workpieces and consists of a mesh generation from geometric data, optimal surface segmentation for CNC, and tool path planning with a certified scallop height. The latest methods were introduced into the mesh generation with implicit geometric regularization and total generalized variation. Once the mesh model was obtained, a fast and robust optimal surface segmentation method is provided by establishing a weighted graph and searching for the minimum spanning tree of the graph for extraordinary points. This method is easy to implement, and the number of segmented patches can be controlled while preserving the sharp features of the workpiece. Finally, a contour parallel tool-path with a confined scallop height is generated on each patch based on B-spline fitting. Experimental results show that the proposed framework is effective and robust.

Generic Tool path planning method of T-spline surface CNC milling

T-spline is a new surface modeling technology, which has superior advantages in complex shape representation over traditional NURBS and mesh surfaces. However, the research in CAM field is insufficient, which hinders the further promotion of this technology. To fit the complex topology of T-spline surface, a general tool path planning method of T-spline surface CNC milling is proposed based on the commonly used zig-zag or contour-parallel toolpath pattern. This method utilizes the 2-dimensional scallop height model to calculate side step, the iso-scallop height method to generate tool paths, the minimum energy method to smooth them, and Hermite curves to join them as one tool path, which can realize the milling of T-spline surface model with arbitrary topology. Finally, a T-spline CAM prototype system is successfully developed to implement simulation analysis and machining experiments to verify the algorithms above. And the results of simulations and experiments prove this method can generate a smooth tool path with uniform iso-scallop height distribution, short length, smooth corner transition, and one retraction, which can achieve better finish machining effect.

A Framework for Adaptive Width Control of Dense Contour-Parallel Toolpaths in Fused Deposition Modeling

3D printing techniques such as Fused Deposition Modeling (FDM) have enabled the fabrication of complex geometry quickly and cheaply. Objects are produced by filling (a portion of) the 2D polygons of consecutive layers with contour-parallel extrusion toolpaths. Uniform width toolpaths consisting of inward offsets from the outline polygons produce over- and underfill regions in the center of the shape, which are especially detrimental to the mechanical performance of thin parts. In order to fill shapes with arbitrary diameter densely the toolpaths require adaptive width. Existing approaches for generating toolpaths with adaptive width result in a large variation in widths, which for some hardware systems is difficult to realize accurately. In this paper we present a framework which supports multiple schemes to generate toolpaths with adaptive width, by employing a function to decide the number of beads and their widths. Furthermore, we propose a novel scheme which reduces extreme bead widths, while limiting the number of altered toolpaths. We statistically validate the effectiveness of our framework and this novel scheme on a data set of representative 3D models, and physically validate it by developing a technique, called back pressure compensation, for off-the-shelf FDM systems to effectively realize adaptive width.

Optimizing the numerical algorithm in Fast Constant Engagement Offsetting Method for generating 2.5D milling tool paths

In the case of 2.5D rough milling operations, machining efficiency can significantly be increased by providing a uniform tool load. This is underpinned by the fact that uniform load has a positive effect on both tool life and machining time. Unfortunately, conventional contour-parallel tool paths are unable to guarantee uniform tool loads. However, nowadays there are some advanced path generation methods which can offer a constant tool load by controlling the cutter engagement angle. Yet, the spread of these non-equidistant offsetting methods is hindered by their dependence on complex calculations. As a solution to this problem, the Fast Constant Engagement Offsetting Method (FACEOM), developed in the scope of our previous study, is seen to be taking a step towards reducing computational needs. In this paper, suggestions for further improvements of FACEOM are presented. Decreasing the number of path points to be calculated is made possible by implementing adaptive step size and spline interpolation. Through simulation tests, it was also analysed which of the numerical methods utilized for solving boundary value problems can be applied to obtain the shortest calculation time during tool path generation. The practical applicability of the algorithm has been proved by cutting experiments. With respect to research results, this paper also describes how a tool path created by the algorithm can be adapted to controllers of CNC machine tools. Solutions presented in this paper can promote a wider application of a modern path generation method that ensures constant tool loads.

Effect of algorithm parameters in development of spiral tool path for machining of 2.5D star-shaped pockets

2.5D pocket milling is extensively used in aerospace, shipyard, automobile, dies and moulds industries. In machining of 2.5D pockets, directional parallel tool-path and contour parallel tool-path are widely used. However, these tool paths significantly limit the machining efficiency. In the present work, an attempt has been made to generate a spiral tool path for machining of 2.5D star-shaped pocket for improving machining efficiency. The successful generation of spiral tool path depends on various algorithm parameters such as mesh size, permissible error and number of degree-steps. The effect of these parameters on spiral tool path generation is discussed and the best values are reported. The spiral tool path is developed using second order elliptic partial differential equation (PDE) and it is free from sharp corners inside the pocket region. Further, the implementation of proposed method is presented on complex non-star-shaped polygon, pocket bounded by free-form curve and pocket with island.

Process planning for adaptive contour parallel toolpath in additive manufacturing with variable bead width

Lightweight structures with slender features and thin walls can be fabricated by the additive manufacturing process. The fabrication process utilizes a contour parallel toolpath, where sets of parallel contours are offset from the boundaries of a geometric structure at predefined intervals to deposit the material layer by layer. Currently, these intervals are set to be constant, which limits its capability to produce near net-shape parts. In recent research, the feasibility to fabricate parts using contour parallel toolpaths with variable bead widths has been explored to increase the production speed, to improve the geometry accuracy, and to manufacture void-free parts. However, existing process planning methods are computationally inefficient and challenging to implement. To resolve these issues, this paper proposes a comprehensive process planning framework for adaptive contour parallel toolpath with variable bead widths. More specifically, this framework includes a toolpath planning algorithm using the level-set method and a process planning algorithm for generating the desired bead geometry using a Gaussian process regression model. To validate the proposed framework, a case study has been demonstrated in the fabrication of benchmark features with a wire and arc additive manufacturing process.

Use of a virtual milling system to generate power-aware tool paths for 2.5-dimensional pocket machining

Traditional (direction-parallel and contour-parallel) and non-traditional (trochoidal) tool paths are generated by specialized geometric algorithms based on the pocket shape and various parameters. However, the tool paths generated with those methods do not usually consider the required machining power. In this work, a method for generating power-aware tool paths is presented, which uses the power consumption estimation for the calculation of the tool path. A virtual milling system was developed to integrate with the tool path generation algorithm in order to obtain tool paths with precise power requirement control. The virtual milling system and the tests used to calibrate it are described within this article, as well as the proposed tool path generation algorithm. Results from machining a test pocket are presented, including the real and the estimated power requirements. Those results were compared with a contour-parallel tool path strategy, which has a shorter machining time but has higher in-process power consumption.

A Voxel Based Automatic Tool Path Planning Approach Using Scanned Data as the Stock

This paper presents a novel automatic tool path planning approach which employs the scanned data as the stock. Using the scan data as the stock allows a new geometrical approach to measure the difference between the stock model and desired final geometry. This approach employs a voxel model as the underlying geometric representation, which enables an efficient parallel tool path planning algorithm based on a general-purpose graphics processing unit (GPGPU) approach. The approach is composed of two stages: first, to identify the excess material to be cut and reduce the required GPU memory, a novel voxelization algorithm for machining was developed to voxelize the scanned data and the model of the part in a single 3D grid. Second, slice-by-slice contour parallel tool paths for roughing and 3D contour parallel tool paths for finishing were generated directly based on the voxel model. The proposed approach has been successfully implemented to machine a part representative of the welded blade tip and a part representative of a cast workpiece with a freeform surface into parts with desired final geometry. The experiment results show that the proposed approach is efficient and feasible in generating tool paths when scanned data was used as the stock.

Image processing-based contour parallel tool path optimization for arbitrary pocket shape

Contour-parallel tool path has been a popular means in milling 2D pocket regions. Traditional contour-parallel path suffers from unsmooth transition and uncut problem around corners, which requires dedicated local geometric treatment. In addition, these two issues contradict with each other; solving one may jeopardize the other. Motivated by these problems, we present a digital image approach to deal with the contour parallel tool path optimization. By initially converting the pocket boundary into a binary image, a greyscale image is constructed via an inward-marching algorithm, such that the greyscale value of every pixel inside the pocket indicates the shortest Euclidean distance (SED) towards the boundary. The contour-parallel tool path can be easily derived from this SED image as the iso-contours. Distinguished from the geometry-based treatments towards unsmooth tool path and uncut regions, the proposed framework takes advantage of various image processing techniques (such as Gaussian blur and unsharp masking) to the SED image. By adaptively optimizing the parameters for image blurring and sharpness, the corresponding iso-SED tool path eliminates the uncut residues while still satisfies a specified level of smoothness. A quantitative analysis together with an image optimization scheme is also presented in this work.

Removal of critical regions by radius-varying trochoidal milling with constant cutting forces

This work presents a novel milling strategy for complex pocket machining by integrating radius-varying trochoidal (RVTR) toolpath with contour parallel (CP) toolpath. Based on a quantitative analysis on the fluctuation of material removal rates (MRR), the proposed strategy is able to precisely identify critical regions from complex pocket geometries, and then by integrating flexible trochoidal radius with adaptive trochoidal step, the proposed approach is able to integrate the RVTR toolpath into CP toolpath under a consistent transition of material removal rate. Moreover, by applying RVTR toolpath, the cutting force can be maintained constantly when machining critical regions. Comparing with the trochoidal milling function available with current mainstream CAM software, experimental investigation has shown that the proposed RVTR-CP toolpath integration strategy offers a better machining condition with minimized fluctuation of cutting forces. Moreover, the total length of toolpath is decreased considerably and hence the machining efficiency is greatly improved.

Contour parallel tool path planning based on conformal parameterisation utilising mapping stretch factors

Parameterisation-based methods for planning tool paths on mesh surfaces have been developing for years. The issue of existing mapping deformation which results in machining error has not been sufficiently addressed. And it still needs particular 3D geometric operations when planning iso-scallop tool path. To handle these, an effective approach to directly generating the iso-scallop paths on the parametric domain utilising anisotropic mapping stretch factors is proposed. The conformal parameterisation algorithm, Angle Based Flattening, is first implemented for the mapping between the spatial mesh and the planar mesh. Then a general method to convert 3D path parameters into 2D, which involves the direction mapping and length mapping based on the mapping deformation analysis, is presented. After that, a non-uniform offsetting method is proposed for retrieving the valid 2D offset paths. Finally, the iso-scallop paths on the parametric domain can be generated using the converted 2D path parameters and the present non-uniform offsetting method, and then the corresponding tool paths are obtained by inverse mapping. Simulation and experimental results are given to validate the feasibility and effectiveness of the proposed methods.

Iso-scallop tool path building algorithm “based on tool performance metric” for generalized cutter and arbitrary milling zones in 3-axis CNC milling of free-form triangular meshed surfaces

In order to generate a tool path, scallop height should be under the given precision requirement and the tool path should be as short as possible. Iso-scallop tool path generation method is traditionally used to generate the effective tool path. This paper discussed a new method to generate a iso-scallop and contour-parallel tool path for a generalized cutter and an arbitrary area on free-form surface in 3-axis CNC milling. The milling surface, originally represented as a triangular mesh, and the generalized cutter surface are converted into voxel-based depth maps. Milling zone map with the same resolution as the part depth map is used to define an arbitrary milling area. First, the points on the boundary of milling area are marked in order to build the tool path that can follow a boundary line. Second, different cutter location positions are tried in order to satisfy two conditions: milling the material from the selected boundary points within the tolerance limit and having the maximum material removal rate on the way from the previous cutter location point. The algorithm is repeated until the whole area to mill is finished. Due to the high demand of computational power, graphics processing unit is employed for the most time consuming operations.

Minimizing time of contour-parallel pocket end-milling by prescribing limit axial and radial depth pairs

The purpose of this study is to minimize pocketing time by exploiting the most beneficial aspects of tool dynamics and the most beneficial chronology of tool passes in contour-parallel toolpath. This is achieved by always prescribing limiting axial and radial depth pairs on a contour-parallel toolpath as a means of minimizing pocketing time within milling machine load specifications. The optimization approach is identified to be of two types: boundary to centre ( b→ c) execution and centre to boundary ( c→ b) execution. Expression for pocketing time is developed for each type taking into account the revelation that the lengths of the bivariately optimized cyclic passes between the first and last cycles constitute an arithmetic series. The presented expressions are demonstrated to be useful in systematic determination of choice of progression (either ( b→ c) execution or ( c→ b) execution) of contour-parallel pocketing operation and choice of limiting axial and radial depth pairs that guarantee minimum machining time. For example, the studied experimentally characterized case gave that ( c→ b) execution is more time saving than ( b→ c) execution by about 3.5565%–15.7690%. This experimentally characterized case also confirms the expectation that continuous tool–workpiece contact of contour-parallel toolpath always ensures a shorter pocketing time than both one-way up- and down-milling toolpaths. It is further seen that it is faster pocketing at high radial depth range of 60%–100% of tool diameter which fortunately is the range of less load restrictions. The benefits of contour-parallel toolpath relative to one-way up- and down-milling toolpaths got more strongly confirmed when the pocket got larger (for example, the time saving of ( c→ b) execution relative to one-way up-milling attained 51.6733% for a larger pocket, while time saving of 41.3385% was recorded for smaller pocket) and when the slope of limiting radial depth versus axial depth got less.

Smooth tool path generation for 5-axis machining of triangular mesh surface with nonzero genus

NC machining of a nonzero genus triangular mesh surface is being more widely confronted than before in the manufacturing field. At present, due to the complexity of geometry computation related to tool path generation, only one path pattern of iso-planar type is adopted in real machining of such surface. To improve significantly 5-axis machining of the nonzero genus mesh surface, it is necessary to develop a more efficient and robust tool path generation method. In this paper, a new method of generating spiral or contour-parallel tool path is proposed, which is inspired by the cylindrical helix or circle which are a set of parallel lines on the rectangular region obtained by unwrapping the cylinder. According to this idea, the effective data structure and algorithm are first designed to transform a nonzero genus surface into a genus-0 surface such that the conformal map method can be used to build the bidirectional mapping between the genus-0 surface and the rectangular region. In this rectangular region, the issues of spiral or contour-parallel tool path generation fall into the category of simple straight path planning. Accordingly, the formula for calculating the parameter increment for the guide line is derived by the difference scheme on the mesh surface and an accuracy improvement method is proposed based on the edge curve interpolation for determining the cutter contact (CC) point. These guarantee that the generated tool path can meet nicely the machining requirement. To improve further the kinematic and dynamic performance of 5-axis machine tool, a method for optimizing tool orientation is also preliminarily investigated. Finally, the experiments are performed to demonstrate the proposed method and show that it can generate nicely the spiral tool path or contour-parallel tool path on the nonzero genus mesh surface and also can guarantee the smooth change of tool orientation.

Optimization of toolpath with circular cycle transition for sharp corners in pocket milling

The toolpath for a sharp corner is important in pocket machining during high-speed machining with numerical control. When milling a sharp corner, a longer tool engagement length generates larger engagement angle and milling force, which reduces the machining precision. To reduce the milling force at a sharp corner and clear up uncut materials, the toolpath at corners must be optimized. In this paper, a new method of generating the toolpath with circular cycle transition at a sharp corner is put forward to provide a new numerically controlled toolpath pattern for pocket milling. On the basis of a common contour-parallel toolpath, minimum radius arcs and circular cycles are added in sequence at a sharp corner. When the cutter comes to the sharp corner, the corner material is removed step by step with a sequence of circular cycles. The validity of the proposed method is confirmed through simulation and tests. The proposed toolpath method is realized by employing the Institute of Manufacturing Technology (IMT) computer-aided manufacturing (CAM) system. The toolpaths at sharp corners generated through commercial software and IMT-CAM software are tested by comparing the corresponding milling forces. It is illustrated in the results that the proposed method reduces the milling force effectively and is of great significance when it is applied in high-speed machining.

Machining of curved geometries with constant engagement tool paths

End milling of the curved surfaces is characterized by significant amount of engagement variation along the tool path which results in deflection-induced surface error on the machined components. Feed rate regulation cannot be used in this case to minimize the surface errors as it necessitates continuous change in feed rate along the tool path which deteriorates surface finish and lowers productivity. Another approach in the form of tool path modification scheme was also proposed in the literature to minimize engagement variation at corners of a pocket. This approach has been effective in reducing fluctuations of engagement at corners and cannot be applied directly in machining of curved geometries where engagement variation is continuous along the entire tool path. Keeping these limitations in focus, a new methodology is proposed in this article to minimize surface error variation that modifies machining strategy for the curved components. The strategy proposes machining of the curved components into two different stages: first, generating the modified semi-finishing geometry using algorithm proposed in this article and then producing the desired geometry using conventional contour parallel tool path. The methodology generates the modified semi-finishing geometry such that the engagement offered is constant when a finishing pass is made. The effectiveness of the proposed methodology has been verified by performing computational studies and machining experiments on typical curved geometries. It is observed that the proposed methodology is quite effective in minimizing the variation in the surface error on the machined components.

Generating HSM-adapted pocketing tool path by region subdivision

High-speed machining (HSM) is an effective manufacturing process to produce parts. In HSM, it is required that the tool path should be smooth and the material removal rate should be constant. However, in geometry, it is nearly impossible to cover an arbitrary pocket with a single form of curves that satisfy both the above two requirements. In this paper, a compromise is made by subdividing the pocket into two kinds of regions: the HSM regions and the low-speed machining (LSM) regions. The HSM regions are selected to be the maximum inscribed circles (MICs) of the pocket. These MICs are calculated in an offset manner. Inside each HSM region, successive concentric circles are filled. The radii of the circles are controlled so that the material removal rate remains constant. The obtained concentric circles are then smoothly linked with pairs of arcs and used as the HSM tool path. For the rest LSM regions, conventional contour parallel tool paths are filled and low cutting speed is applied considering that there might be sharp angles on the pocket boundary. As the HSM regions could take up to 50% of the whole pocket and the cutting speed in HSM regions can be set very high, the average cutting speed for the whole pocket can be enhanced. Several pocket examples are used to verify the feasibility of the proposed HSM tool path generation method.

An efficient contour parallel tool path generation for arbitrary pocket shape without uncut regions

With the aim of improving productivity in pocket machining the uncut regions are one of the most influencing factors on the tool path length and cutting time. We are interested in two kinds of uncut regions; the corner uncuts that appear between the passes and the center uncut. In this paper we have developed a new analytical model of tool path for machining pockets in 2½D with contour parallel offset for an arbitrary shape of pocket boundary whether line-line or line-arc. For the corner uncuts we propose a novel coefficient which ensures the ideal cover area between passes without letting behind any uncut region. For the center uncut we propose an automatically additional reduced looping with a passage segment that depends on the center uncut region geometry. For the purpose of the validation of this new developed method some selected numerical examples are presented. The obtained results show that our new approach minimizes both the tool path length and the machining time compared to other methods. Thus this new method offers an efficient contour parallel offset of tool path generation and an optimized machining.