Nominal Tool Path Research Articles

Process Linearization for Closed-Loop Control of Incremental Sheet Forming

Incremental sheet forming has, despite its great flexibility, not yet been widely adopted by the forming industry, due to its limited geometric accuracy. One of the approaches to overcome this problem is the development of closed-loop control systems. Such control systems are mostly based on linearization with respect to a nominal toolpath. In this work, different approaches to create such linearized process models are reviewed. By implementing the models in a numerical simulation of a model predictive control system for incremental sheet forming, it is investigated how manufacturing accuracy is affected by the chosen linearization. Based on these results, the validity of the most commonly used linearization method is critically discussed. Furthermore, it is shown that the geometric accuracy can be improved by extending the process model with a time-varying component that contains historical information about past control actions.

Automatic profile tracking of deformed surface in mirror milling based on ultrasonic measurement

Mirror milling is an effective and environmentally friendly means to manufacture large thin-walled parts. The remaining wall thickness of such parts is supposed to be strictly controlled to balance strength and weight reduction. However, the clamping deformation will lead to overcut or undercut if the nominal tool path is directly utilized. To address this issue, it would be helpful to scan and reconstruct the deformed unknown workpiece surface and then adjust the nominal tool path. In this paper, an automatic profile tracking approach is proposed to adaptively sample the deformed surface and reconstruct the surface model, based on a multi-ultrasonic-probe measurement system. The adaptive sampling algorithm calculates the position and surface normal of the next sampling point simultaneously based on a curvature sphere, which utilizes the surface information of the local area covered by the multi-ultrasonic-probe system. Moreover, a boundary processing algorithm is proposed to ensure the sampling process is carried out inside the boundary. The feasibility of the proposed automatic measurement method was validated through experiment.

Shape-adaptive CNC milling for complex contours on deformed thin-walled revolution surface parts

Abstract Thin-walled revolution surface parts are extensively used in various fields. Owing to their low rigidity, there is an inevitable deformation caused by the blank forming and clamping processes. This makes high-precision machining of parts with complex contours for industrial applications challenging. To solve this problem, a shape-adaptive milling method for complex contours on deformed parts is proposed. First, the real geometry of a deformed part is acquired by a developed contact-type on-machine measurement (OMM) system. Subsequently, an orthogonal arc-length mapping is mathematically proposed to establish a one-to-one correspondence between the nominal and real surfaces. The method of adjusting the tool path is based on this correspondence is discussed in detail, using which the nominal tool path can be adjusted to adapt automatically to the shape change of the real part. In addition, the methods of refining the tool path, such as eliminating the overcuts and undercuts and reorienting the cutter orientation, are also presented for achieving a good milling quality. Finally, the experiments of milling complex pattern contours on thin-walled bangles, which are typical revolution surface parts with low rigidity, are presented, and the simulation tests on a general revolution surface are also conducted. Experimental results illustrate that the proposed method can produce clear and complete contours on the deformed revolution surfaces.

Minimizing Flute Engagement to Adjust Tool Orientation for Reducing Surface Errors in Five-Axis Ball End Milling Operations

AbstractSurface errors due to force-induced tool and workpiece deflections are one of the major errors in multi-axis machining of parts especially with thin-walled structures. Dominant approaches to reduce these surface errors are re-machining the part, feed scheduling, and tool path modification. These methods are time consuming and computationally costly, and they rely on experimental data which is used in cutting force and deflection predictions. The present paper introduces a pure geometrical approach to reduce surface errors drastically by minimizing the engagement lengths of flutes’ cutting edges when a point on the flute’s cutting edge is in contact with the design surface. The total engagement length of the flutes’ cutting edges when one of them generates a contact point on the workpiece surface is formulated and considered as the minimization objective function of an optimization problem. Tilt and lead angles, which define the tool orientation, are the design variables of the optimization problem subjected to constraints based on the geometrical requirements of the ball end milling process. The optimization problem uses the nominal tool path to generate an optimal tool path with adjusted tool orientations. The presented method is computationally inexpensive and does not need any experimentally calibrated coefficients to predict cutting forces because of the pure geometrical nature of the approach. The method is experimentally validated through five-axis ball end milling experiments in which more than 90% surface error reduction is achieved.

Off-line nominal path generation of 6-DoF robotic manipulator for edge finishing and inspection processes

This paper deals with the development of a Computer-Aided Robotic Machining Process Planning package based on SolidWorks. The main aim of the package is to generate an efficient, collision-free, nominal tool path needed for edge finishing and inspection by utilizing a 6-DoF robotic arm. The overall nominal tool path involves analyzing the geometry of the workpiece, determining and designing an efficient collision-free tool path and finally generating the tool path data for the robot and verifying it. This package significantly simplifies the complexity of planning an efficient path for performing edge finishing and inspection. ABB IRB2000 robot is chosen for executing the generated tool path. Ultimately, such path is to be utilized as the nominal tool path by any control strategy present in the literature for a complete automatic edge finishing process.

Integrated post-processor for 5-axis machine tools with geometric errors compensation

Geometric errors of 5-axis machine tools introduce great deviation in real workpiece manufacture and on-machine measurement like touch-trigger probe measurement. Compensation of those errors by toolpath modification is an effective and distinguished method considering the machine calibration costs and productivity. Development of kinematic transformation model is involved in this paper to clarify the negative influences caused by those errors at first. The deviation of the designed toolpath and the real implemented toolpath in workpiece coordinate system is calculated by this model. An iterative compensation algorithm is then developed through NC code modification. The differential relationship between the NC code and the corresponding real toolpath can be expressed by Jacobi matrix. The optimal linear approximation of the compensated NC code is calculated by utilizing the Newton method. Iteratively applying this approximation progress until the deviation between the nominal and real toolpath satisfies the given tolerance. The variations of the geometric errors at different positions are also taken into account. To this end, the nominal toolpath and the geometric errors of the specific 5-axis machine tool are considered as the input. The new compensated NC code is generated as the output. The methodology can be directly utilized as the post-processor. Experimental results demonstrate the sensibility and effectiveness of the compensation method established in this study.

Feed-rate and trajectory optimization for CNC machine tools

The key task performed by CNCs is the generation of the time-sequence of set-points for driving each physical axis of the machine tool during program execution. This interpolation of axes movement must satisfy a number of constraints on axes dynamics (velocity, acceleration, and jerk), and on process outcome (smooth tool movement and precise tracking of the nominal tool-path at the desired feed-rate). This paper presents an algorithm for CNC kernels that aims at solving the axes interpolation problem by exploiting an Optimal Control Problem formulation. With respect to other solutions proposed in the literature, the approach presented here takes an original approach by assuming a predefined path tracking tolerance-to be added to the constraints listed above-and calculating the whole trajectory (path and feed-rate profile) that satisfies the given constraints. The effectiveness of the proposed solution is benchmarked against the trajectory generated by an industrial, state-of-the-art CNC, proving a significant advantage in efficiency and smoothness of axes velocity profiles.

Adaptation of machining toolpath to distorted geometries: application to remove a constant thickness on rough casting prosthesis

This study proposes a method to adapt the geometry of the toolpath to a specified target. In the case study presented, the geometrical target is to remove a constant thickness on the rough workpiece. This case is normally present in the polishing of the femoral component of knee prostheses. In fact, an operator carries out these operations manually. The aim of this study is to contribute to the automation of prosthesis production, notably, in the preparation of surface polishing. The proposed method can deform and adapt a toolpath to ensure the required geometry of the machined surface. The proposed toolpath deformation method is composed of three steps: alignment, toolpath deformation, and toolpath smoothing. Alignment between the measured surface of the roughcast prostheses and the nominal toolpath is carried out by an Iterative Closest Point (ICP) algorithm. The principle of this algorithm is to find the optimal rigid transformation to readjust the toolpath on the measured surface. Subsequently, the toolpath is deformed to remove the constant thickness of the roughcast prostheses. Next, to increase the machined quality, a smoothing stage is carried out on the obtained toolpath. Experimental tests on industrial prostheses geometry are conducted to validate the effectiveness of this method.

Impact of measurement procedure when error mapping and compensating a small CNC machine using a multilateration laser interferometer

This paper deals with the accuracy of compensation of machine tools using a tracking interferometer using the multilateration method. The measurement strategy and thermal drift compensation of the measurements are studied. It shows that most effects of temperature are accurately compensated by the laser tracking interferometer software. However, thermal drifts of accessories are not taken into account, and are therefore not corrected. To validate the robustness of procedures, the geometrical errors of the same machine tool were measured by five measurement strategies using the same equipment. Each strategy is devised and carried out independently by a different person from several institutions. For each strategy, the geometrical compensations were applied to a set of nominal tool path points. The difference, between the nominal points and the compensated or uncompensated points was calculated. This criterion was used to discuss the procedures employed by the participants.

Simulation-based adaptative toolpath generation in milling processes

Milling toolpath is often designed according to a CAD model which is not always an accurate representation of the real part. In an industrial framework, a significant geometrical variability is sometimes observed in parts that belong to the same batch. Therefore, the conventional approach making use of a unique nominal toolpath for manufacturing series of parts fails to confer both geometrical and dimensional conformity to the machined parts. This study explores a strategy for adapting on-line the toolpath thanks to measurements and data driven simulation. An approximation for the actual geometry is derived from data acquisitions and the path is adjusted accordingly. The POD snapshots technique is employed for building up a reduced basis of approximation by considering geometrical features as well as additional parameters (initial setup) for the part. Thanks to its filtering properties, that basis proves to be robust with respect to measurement noise.

A Method for Measuring the Systematic Errors of CNC-Machine Tools

The method used for measurement and calibration of machine tool errors should be general and efficient. With this method, the machine tool status can be completely identified and its accuracy can be enhanced by software error compensation. The point compensation method can be used as a means for modifying the nominal tool path and on-machine inspection where the machine tool is used as a coordinate measuring machine. The validity of the error calibration method proposed in this' paper was shown using a vertical 3-axis CNC machine with a laser interferometer and a ball bar technique.

Synchronous Adjustment of Milling Tool Path Based on the Relative Deviation

Abstract In machining process, machining accuracy of part mainly depends on the position and orientation of the cutting tool with respect to the workpiece which is influenced by errors of machine tools and cutter-workpiece-fixture system. A systematic modeling method is presented to integrate the two types of error sources into the deviation of the cutting tool relative to the workpiece which determines the accuracy of the machining system. For the purpose of minimizing the machining error, an adjustment strategy of tool path is proposed on the basis of the generation principle of the cutter location source file (CLSF) in modern computer aided manufacturing (CAM) system by means of the prediction deviation, namely, the deviation of the cutting tool relative to the workpiece in computer numerical control (CNC) machining operation. The resulting errors are introduced as adjustment values to adjust the nominal tool path points from cutter location source file from commercial CAM system prior to machining. Finally, this paper demonstrates the effectiveness of the prediction model and the adjustment technique by two study cases.

Tool path accuracy enhancement through geometrical error compensation

Kinematic and geometric errors of CNC machine tools, introduce large deviations in the real path traveled by the cutting tool. Tool path deviation reduces geometrical and dimensional accuracy of the machined features of the component. Tool path modification is an effective strategy to increase accuracy of the machined features. An improved error estimation model based on kinematic transformation concepts has been developed and used to calculate the volumetric overall error. These calculations are applicable for each arbitrary target positions of the machine's work space. Also a NC Program editor software has been developed in order to manage the calculations, modifications and to generate the new compensated NC program. The compensation procedure includes: fragmentation of nominal tool path to small linear elements, translating nominal position of elements to real positions using the Kinematics error model, finding compensated positions using the error compensation algorithm, converting newly generated elements to new tool paths using the packing algorithms and finally editing old NC program using NC code generator algorithm. Experimental tests showed 4–8 times accuracy improvement for linear, and S-pline tool paths deviations.

An error control approach to tool path adjustment conforming to the deformation of thin-walled workpiece

Producing an accurate part mainly depends on the position and orientation of the cutting tool with respect to the workpiece which is mainly influenced by the rigid body motion of the workpiece and the elastic deformation of workpiece–fixture–cutter system. For the purpose of minimizing the machining error, a new modification strategy of the nominal tool path is not that directly compensate the control commands of the machine tools, but that modify the cutter location source file (CLSF) from the computer aided manufacturing (CAM) system by means of the proposed modification model on the basis of the prediction deviation, namely, the deviation of the cutting tool relative to the workpiece in computer numerical control (CNC) machining operation. Therefore, it is not only simpler, but also easier implemented by common manufacturing engineer. The effectiveness of the proposed strategy is verified by a machining example.

모터 동력계를 이용한 공작기계용 NC제어기 시스템의 위치제어 특성 분석을 위한 측정 연구

The gains for NC controller parameter are fixed when the controller is combined with a machine. However, the characteristics of controller could be changed as it has being used by the machine or other environmental conditions. Those result in that the tool positioning accuracy is influenced. The loading torque in servo motor influences on the tool positioning accuracy and it is controlled by the parameter gains. It is required to analyze the torque variation with angular positioning accuracy of the servo motor. This study focus on a measuring method and device for verifying angular positioning accuracy of NC servo motor. We used a high resolution AID converter for acquiring analogue signal of rotary encoder in servo motor. The positional accuracy for a nominal tool path, which is generated by the combination of axial movements (X,Y,Z), is analyzed with the servo motor torque. The current variation signal is acquired at the power line using a hall sensor and converted to the loading torque of servo motor. The method of measurement and analysis proposed in this study will be used for determining the gains of parameter in NC controller. This gain tuning is also necessary when the controller is set up at a machine.

Tool path error prediction of a five-axis machine tool with geometric errors

Predicting the actual tool path of a machine tool prior to machining a part provides useful data in order to ensure or improve the dimensional accuracy of the part. The actual tool path can be estimated by accounting for the effect of the machine tool geometric error parameters. In computer aided design/computer aided manufacture (CAD/CAM) systems, the nominal tool path [or CL (cutter location) data] is directly generated from the curves and surfaces to be machined and the errors of the machine tool are not considered. In order to take these errors into consideration, they must first be identified and then used in the machine tool forward kinematic model. In this paper a method is presented to identify the geometric errors of machine tools and predict their effect on the tool-tip position. Both the link errors (position-independent geometric error parameters) and the motion errors (position-dependent geometric error parameters) are considered. The nominal and predicted tool paths are compared and an assessment is made of the resulting surfaces with respect to the desired part profile tolerance. A methodology is also suggested to integrate this tool within a CAD/CAPP (computer aided process planning)/CAM environment.

An efficient CNC programming approach based on group technology

The key task performed by CNCs is the generation of the time-sequence of set-points for driving each physical axis of the machine tool during program execution. This interpolation of axes movement must satisfy a number of constraints on axes dynamics (velocity, acceleration, and jerk), and on process outcome (smooth tool movement and precise tracking of the nominal tool-path at the desired feed-rate). This paper presents an algorithm for CNC kernels that aims at solving the axes interpolation problem by exploiting an Optimal Control Problem formulation. With respect to other solutions proposed in the literature, the approach presented here takes an original approach by assuming a predefined path tracking tolerance—to be added to the constraints listed above—and calculating the whole trajectory (path and feed-rate profile) that satisfies the given constraints. The effectiveness of the proposed solution is benchmarked against the trajectory generated by an industrial, state-of-the-art CNC, proving a significant advantage in efficiency and smoothness of axes velocity profiles.