Free-form Surface Machining Research Articles

A simplified mathematical model development for the design of free-form cathode surface in electrochemical machining

ABSTRACTHigh-performance machining of free-form surfaces is highly critical in automotive, aerospace, and die–mold manufacturing industries. Therefore, electrochemical machining (ECM) process has been used in such cases in that sense. The most important challenges of using ECM process are the lack of accuracy and difficulty in designing proper machining tool (cathode) surfaces. In this article, a simplified mathematical model is presented to obtain a cathode surface for ECM of free-form surfaces which have high curvatures. In this theoretical approach, the finite-element method (FEM) is used to solve the 3-D Laplace equation and to determine the potential distribution between the anode (workpiece) and cathode (tool) surfaces. A compact and simple program was developed to obtain a proper cathode surface that only requires some nodal coordinates on the anode surface and boundary conditions. In this work, a trial cathode surface is constructed for a given gap distance. For the determined ECM parameters, cathode shape that satisfies the boundary conditions is obtained for the 45th layer. The results are compared with the literature and ANSYS Workbench for verification. The developed theoretical approach benefits simpler and faster FEM solutions, accurate cathode surface, and consequently correct form of machined surface.

Optimal Workpiece Setup for Time-Efficient and Energy-Saving Five-Axis Machining of Freeform Surfaces

Energy consumption in five-axis machining of freeform surfaces can be considerably large for large-size parts. This paper presents a study on how to setup the workpiece in order to minimize the energy consumption without modifying the toolpath itself. For an arbitrary freeform workpiece, the way how it is setup on the working table highly affects the machine's kinematic behavior, which dominates the overall processing time and energy consumption. Taking into account the speed and acceleration limit of each axis of the machine, we first establish the energy consumption model as a function of the workpiece setup. However, this original model involves certain critical physically pertinent coefficients (such as the moment of inertial of a rotary table) which are usually unavailable in practice. Instead, by exploring insightful geometric characteristics of the five-axis machine, an alternative energy consumption model is established which is independent of those hard-to-obtain coefficients. A simple algorithm is then designed to optimize this model. Both computer simulations and physical cutting experiments demonstrate that, when compared with an arbitrary setup, the optimized workpiece setup is able to achieve a significant saving (as much as 50%) in both energy consumption and total machining time, both using a same tool path.

Efficiency-optimal iso-planar tool path generation for five-axis finishing machining of freeform surfaces

Traditional works in five-axis iso-planar tool path generation mainly aim at finding the best direction of the parallel drive planes so that the total length of the tool path is minimized. However, an iso-planar tool path optimized this way is often found to be inferior in improving the machining efficiency, as it ignores the kinematic constraints of the specific machine tool. In this paper, we present an efficiency-optimal iso-planar tool path generation algorithm for multi-axis machining of freeform surfaces that takes the kinematic capacities of the machine tool in full consideration. Rather than trying to maximizing the cutting strip width alone, the proposed algorithm aims at globally maximizing the material removal rate which considers both the cutting strip width and the machine’s kinematic capacities and hence is a more comprehensive reflection of machining efficiency. In addition, we further enhance the machining efficiency by automatically adjusting the tool orientation along the tool path. Both computer simulation and physical machining experimental results show that, when compared with the traditional iso-planar tool path generation algorithms, the proposed method is often able to substantially improve the machining efficiency.

Artificial immune algorithm implementation for optimized multi-axis sculptured surface CNC machining

This paper presents the results obtained by the implementation of an artificial immune algorithm to optimize standard multi-axis tool-paths applied to machine free-form surfaces. The investigation for its applicability was based on a full factorial experimental design addressing the two additional axes for tool inclination as independent variables whilst a multi-objective response was formulated by taking into consideration surface deviation and tool path time; objectives assessed directly from computer-aided manufacturing environment A standard sculptured part was developed by scratch considering its benchmark specifications and a cutting-edge surface machining tool-path was applied to study the effects of the pattern formulated when dynamically inclining a toroidal end-mill and guiding it towards the feed direction under fixed lead and tilt inclination angles. The results obtained form the series of the experiments were used for the fitness function creation the algorithm was about to sequentially evaluate. It was found that the artificial immune algorithm employed has the ability of attaining optimal values for inclination angles facilitating thus the complexity of such manufacturing process and ensuring full potentials in multi-axis machining modelling operations for producing enhanced CNC manufacturing programs. Results suggested that the proposed algorithm implementation may reduce the mean experimental objective value to 51.5%

A new toolpath generation method with feed sensitive zones inspection based on inverse evaluation mechanism

Toolpath generation is the crucial task of free-form surface machining, and the quality of toolpath affects the final machining precision and efficiency. In conventional method, machining efficiency is approximately evaluated by the length of toolpaths. However, due to the curvature of toolpath distributing irregularly, the axis drive has to accelerate or decelerate to avoid exceeding its limits. For this reason, interpolation situation would affect toolpath quality as well, which is not taken into consideration in the traditional toolpaths generation mechanism. This paper proposes a toolpath generation method to plan toolpath with time-optimal, which has bidirectional information flow between CAM system and CNC system. This architecture is called inverse evaluation mechanism. By this mechanism, an offline simulation interpolation module is integrated into the CNC system; with this module, the feed sensitive zones are inspected by CNC system and the surface is divided into normal zones and low-velocity zones. The low-velocity zones are transmitted to CAM system as the feedback information. This paper gives the whole architecture of inverse evaluation mechanism and the details of how it works. The proposed method is verified with simulation examples; the result shows that this method can automatically find the low-velocity zones and modify the toolpaths.

Chip volume and cutting force calculations in 5-axis CNC machining of free-form surfaces using flat-end mills

This paper presents two new methods and associated algorithms for numerically modeling chip geometry and calculating cutting force for parts with free-form surface during 5-axis CNC machining using a flat-end mill. Extending the computational geometry-based Alpha shape method that can only predict cutting chip geometry, a parallel slice local volume modeling approach has been added to predict cutting forces as well. To demonstrate the validity and capability of these new methods, simulation of the cutting and chip forming process of 5-axis CNC machining on a free-form surface has been carried out. Physical validation experiment in controlled conditions has been carried out on a 3-axis micro CNC machine with the two cutter rotation angles set to be zero. The predicted and measured cutting forces are in reasonably good agreement both in trend and magnitude. The presented chip volume and cutting force method can be used to perform cutting force estimation for generating optimal tool path and orientation during 5-axis milling. The method requires longer computational time than traditional analytical methods, but it supports the ultimate goal of chip volume modeling and calculation—accurate dynamics cutting force prediction.

CAx-technologies for hybrid fast tool/slow slide servo diamond turning of freeform surface

Most of commercial CAM software solutions (SolidCAM, Unigraphics and etc.) are commonly employed for generating tool paths on machining of freeform surfaces. However, these CAM software solutions are presently unable to support fast tool/slow slide servo diamond turning of freeform surface due to difference in coordinating systems. In contrast to traditional machining processes, fast tool/slow slide servo diamond turning is controlled by both linear and angular positions of the workpiece and is known as polar or cylindrical coordinate system. Hence, a special CAM post-processor is required to generate the spiral tool trajectory in fast tool/slow slide servo diamond turning. However, these special CAM software solutions are very expensive. Manufacturers have been searching for solutions to sustain their competitive advantage in mass producing products at the shortest time to market and at a most economical cost. Hence, these drive the needs for an alternative and economical option of generating accurate tool trajectory for fast tool/slow slide servo diamond turning of freeform surfaces. This article proposes an attractive solution with an integration of Visual Basic application programming interface into SolidWorks to generate spiral tool trajectory for diamond turning of freeform surface. In this proposed methodology, the spiral tool trajectory can be extracted directly from computer-aided design models of freeform surface without utilizing any expensive CAM software. In addition, the critical cutting parameters and tool geometrical angles were also optimized with the developed methodologies for the accurate machining of freeform surfaces in the hybrid fast tool/slow slide servo process.

Deterministic polishing of freeform optical surface by iterative intersection tool path in an off-axial three-mirror anastigmat imaging system

Deterministic polishing as the final step of freeform surface machining can acquire preferable form accuracy. In this article, a deterministic polishing model based on iterative intersection tool path is presented to meet the requirement of high-form accuracy in freeform surface. In the polishing process, on-machine measurement and point set registration method are adopted for installation error extraction and form error calculation. The iterative polishing can be finished without discharging of workpiece by on-machine measurement strategy, which will reduce the processing time and improve the machining efficiency. In addition, a boundary extension method is employed to diminish the surface edge collapse generated by edge effect. Finally, the polishing experiment of freeform optical surface in an off-axial three-mirror anastigmat imaging system is conducted to verify the effectiveness of the proposed model.

Optimization of free-form surface machining using parallel planes strategy and torus milling cutter

Machining by parallel planes is a widely used strategy for end-milling of free-form surfaces on 3-axis numerically controlled machines. In industry, this type of machining is generally performed with a hemispherical tool. However, numerous studies have shown the benefits of torus-end mills over ball-end or flat-end mills. More than anything, the machining direction has much influence on productivity while using a torus-end mill. In this context, the choice of the machining direction is of paramount importance when using a torus-end mill in the machining of free-form surfaces. This paper presents an optimization of part machining direction allowing the machining time to be minimized while respecting the maximum imposed scallop height. This optimization methodology is then applied to an industrial part and measurements are performed on this part. The study highlights the interest of optimizing the machining direction and the benefits that can be drawn with respect to machining using a non-optimized direction.

Manufacturing of a maxillofacial prosthesis from an axial tomography using simulation technologies with a virtual machine tool and four-axis machining

<p>The surfaces of a personalized maxillofacial prosthesis were manufactured in a relatively short time with moderate cost. The topography of the surface was generated with a Computer-Aided Design system from a Computerized Axial Tomography of a maxillofacial area. The design of the machining manufacturing process, its simulation and verification were facilitated by the use of a virtual machine tool equivalent to the real machine tool available. Finally, the manufacturing process was successfully achieved by using a conventional 3-axis vertical machining center equipped with a fourth external rotational axis. Using a 3-axis machine tool with an additional axis is less expensive than using a 5-axis machine. There is abundant literature about machining of free-form surfaces using a 5-axis machine tool, but there are few precedents for the manufacturing of this kind of surface using a 4-axis machine.</p>

Towards efficient 5-axis flank CNC machining of free-form surfaces via fitting envelopes of surfaces of revolution

We introduce a new method that approximates free-form surfaces by envelopes of one-parameter motions of surfaces of revolution. In the context of 5-axis computer numerically controlled (CNC) machining, we propose a flank machining methodology which is a preferable scallop-free scenario when the milling tool and the machined free-form surface meet tangentially along a smooth curve. We seek both an optimal shape of the milling tool as well as its optimal path in 3D space and propose an optimization based framework where these entities are the unknowns. We propose two initialization strategies where the first one requires a user’s intervention only by setting the initial position of the milling tool while the second one enables to prescribe a preferable tool-path. We present several examples showing that the proposed method recovers exact envelopes, including semi-envelopes and incomplete data, and for general free-form objects it detects envelope sub-patches.

Surface Quality Prediction in Case of Steep Free Form Surface Milling

The machining of free form surfaces is a current and important issue in die and mould industry. Beside the complex geometry, an accurate and productive machining and good surface quality are needed. The finishing milling carried out by a ball-end or toroid milling cutter defines the surface quality, which is characterized by the surface roughness and the tool path trace. The surface quality is defined by the properties of the milling cutter, the type of surface and its position, as well as the cutting parameters. This article focuses on the z-level milling of steep surfaces by 2.5D milling strategy. The importance of the different elements of the tool path is presented, the effect of cutting parameters is investigated, and a formula to predict the surface roughness is suggested.

Spherical Approximation of Free-Form Surface Closed to a Sphere in Semi-Finishing

The free-form surface closed to a sphere of brittle material has been used widely, but it is difficult for machining and the efficiency of processing is insufficient. In order to get a product, several processes are needed, such as rough machining, semi-finishing and finishing. Axisymmetric curved surface can take place of the free-form surface in roughing or semi-finishing for wiping off the mass allowance efficiently. Therefore, a spherical approximation algorithm of free-form surface closed to sphere is presented in which free-form surface optical lens will be replaced by a spherical surface in semi-finishing and get the approximate sphere of the free-form surface. It can be certified in the test that this method is simple and reliable. The efficiency and precision in machining is excellent and the distribution of allowance for finishing is uniform in the whole surface, which has great practical significance in machining of optical free-form surface of brittle materials.

Modelling Topology of Freeform Surfaces with Ball-end Milling

Machining of freeform surfaces, viz. dies, moulds, blades, etc. is one the most difficult processes nowadays. Ball-end milling having either the constant tool pitch for flat and oblique surfaces or the variable tool pitch for freeform surfaces is required to improve the surface quality before the final machining. Among other parameters of the surface quality is the surface topology which is described in the paper. Various strategies of the pre-final working passes in ball-end milling of three different types of the machined freeform surfaces (oblique, convex, concave) of three strategies – “along the trace”, “across the trace” and “at an angle to the trace” of the rough machining passes are investigated, six different models representing the combination of the freeform surfaces and the trace shape of the topology of the surface before machining are offered. The calculated dependencies allow forecasting of cutting forces, and, hence, the predictability in obtaining the surface accuracy.

NX CAM post processing errors: Machine data file generator vs. post builder

Replacing or upgrading software packages requires a detailed analysis and testing. This is especially important for software modules that are part of the production process. Machining of free-form surfaces always requires using of a five-axis milling machine. Tool path post-processing for these machines always attracts special attention. The most important part of the postprocessor is implemented IKS (Inverse Kinematic Solver). Implemented IKS in the postprocessor must accurately describe the kinematics of milling machine in the production plant. Military Technical Institute performed parallel testing of two postprocessing modules of CAD/CAM system Siemens PLM NX: and old GPM (Graphics Postprocessor Module and a new module MOM (Manufacturing Output Manager). Complete analysis was sent to the NX support center and application engineers confirmed that the old and new postprocessors do not behave identically. The paper describes in detail the analysis and explains the solution of the presented problem.

Barrel cutter design and toolpath planning for high-efficiency machining of freeform surface

Improvement of machining efficiency is always one of the most interesting problems in CNC machining. Traditionally, the ball-end cutter is widely used in sculptured surface machining for the highly flexible controllability. But the process efficiency is low especially for freeform surface machining, which generally needs multiple tool-paths. To improve the machining efficiency of multi-axis flank milling of freeform surface, a novel barrel cutter design method is proposed in this paper. There are two principle parameters determining the revolution surface of the barrel cutter: the radius of the generatrix curve and the maximum rotating radius. The main work in this paper is to calculate the two parameters for strip-width-maximization machining without local over-gouging. Firstly, the curvature properties are introduced by the geometry model of the barrel cutter. Then the contact condition between the barrel cutter and the freeform surface is analyzed, relationship between cutter parameters and surface curvature is derived. To avoid the over-gouging, based on the curvature constraint, the designed surface is fitted into the cutter surface by surface approximation theory, so that the cutter surface can approach to the designed surface as close as possible. After that, toolpath planning method for milling of freeform surface with the barrel cutter is presented. Finally, machining implementations are presented to verify the effectiveness of the proposed method.

Simultaneous optimization of the feed direction and tool orientation in five-axis flat-end milling

The two additional rotational motions of five-axis machining make the determination of the optimal feed direction and tool orientation a challenging task. A new model to find the optimal feed direction and tool orientation maximising the machining width and avoiding local gouging at a cutter contact (CC) point with a flat-end cutter considering the tool path smoothness requirement is developed in this paper. The machining error is characterised by a signed distance function defined from a point on the bottom tool circle of the cutter to the design surface. With the help of the differential evolution approach, the optimisation model can be resolved to determine the optimal tool orientation and feed direction at a given CC point, and generate the smooth tool paths following the optimal feed direction. Simulation examples demonstrate the developed techniques can improve the tool orientation and feed direction at a CC point to increase the machining width, improving the efficiency of freeform surface machining.

L-Shaped Machining of Anisotropic Woods with a Fine Wire Cutting Tool

A fret-saw blade is commonly used in micromachining or curve machining of various woods. However, there is a curvature limit for machining of free-form surfaces because a fret-saw blade has a thickness of several hundred microns and a width of several millimeters. Additionally, cutting with a fret-saw blade produces much wood meal as chips. If a fine wire cutting tool is used, more flexible machining, such as machining of high curvature free-form surfaces, is possible and the quantity of chip production drastically decreases. The main purpose of this study is to clarify the fundamental machinability of anisotropic materials cut with a fine wire tool. In this report, we describe the machinability of various woods that are naturally anisotropic materials using a fine wire cutting tool that has electrodeposited diamond grains on its surface. In addition, this report discusses the performance of a trial manufactured hand tool employing the same wire cutting tool. The main conclusions obtained in this study are as follows. Acceptable machining of anisotropic woods is possible using a fine wire cutting tool, and the kerf width produced with this wire tool is narrower than that produced with a fret-saw blade. Additionally, the wood species and the cutting direction with respect to the wood grain have a significant influence on the machinability of various woods. Moreover, a relatively smooth cross section is provided when wood is cut by the hand tool using the fine wire tool.

A Novel Method of Efficient Machining Error Compensation Based on NURBS Surface Control Points Reconstruction

A novel method to improve the efficiency of error compensation in free-form surface machining based on the Non-Uniform Rational B-Splines (NURBS) surface control points reconstruction is proposed in this article. With the presented method, a relatively small number of inspection points are needed to be measured for error compensation. The machined surface is obtained by reconstructing the control points of the designed surface based on the on-machine measurement data. The machining error of the surface is obtained by calculating the difference between the machined surface and the designed one. Then a compensate surface is achieved using the mirror symmetry model and surface modification method to compensate the machining error. Experimental validation for the milling of a NURBS surface shows that the machining accuracy of the surface is improved by 62.57% through use of the proposed method.

A multi-perspective dynamic feature concept in adaptive NC machining of complex freeform surfaces

This paper presents a new concept of feature for freeform surface machining that defines the changes in feature status during real manufacturing situations which have not been sufficiently addressed by current international standards and previous research in feature technology. These changes are multi-perspective, including (i) changes in depth-of-cut: the geometry of a feature in the depth-of-cut direction changes during different machining operations such as roughing, semi-finishing and finishing; (ii) changes across the surface: a surface may be divided into different machining regions (effectively sub-features) for the selection of appropriate manufacturing methods for each region such as different cutting tools, parameters, set-ups or machine tools; and (iii) changes in resources or manufacturing capabilities may require the re-planning of depth-of-cuts, division of machining regions and manufacturing operations (machines, tools, set-ups and parameters). Adding the above dynamic information to the part information models in current CAD systems (which only represent the final state of parts) would significantly improve the accuracy, efficiency and timeliness of manufacturing planning and optimisation, especially for the integrated NC machining planning for complex freeform surfaces. A case study in an aircraft manufacturing company will be included in this paper.