Tool Path Compensation Research Articles

Precision improvement of robotic bioprinting via vision-based tool path compensation

Robotic 3D bioprinting is a rapidly advancing technology with applications in organ fabrication, tissue restoration, and pharmaceutical testing. While the stepwise generation of organs characterizes bioprinting, challenges such as non-linear material behavior, layer shifting, and trajectory tracking are common in freeform reversible embedding of suspended hydrogels (FRESH) bioprinting, leading to imperfections in complex organ construction. To overcome these limitations, we propose a computer vision-based strategy to identify discrepancies between printed filaments and the reference robot path. Employing error compensation techniques, we generate an adjusted reference path, enhancing robotic 3D bioprinting by adapting the robot path based on vision system data. Experimental assessments confirm the reliability and agility of our vision-based robotic 3D bioprinting approach, showcasing precision in fabricating human blood vessel segments through case studies. Significantly, it minimizes the printing layer width disparity to just 0.15 mm compared to the 0.6 mm in traditional methods, and it decreases the average error for curved filaments to 7.0 mm2 from the previous 12.7 mm2 in conventional printing. While these results underscore the significant potential of our innovation in creating precise biomimetic constructs, further investigation is necessary to tackle challenges such as accurately distinguishing closely stacked layers using a vision system, especially under varying lighting conditions. These limitations, coupled with issues of computational complexity and scalability in larger-scale bioprinting, emphasize the importance of enhancing the reliability of the vision-based approach across various conditions. Nonetheless, our innovation demonstrates substantial promise in creating precise biomimetic constructs and paves the way for future advancements in vision-guided robotic bioprinting, including the integration of multi-material printing techniques to enhance versatility.

Fabrication of Polymethyl Methacrylate (PMMA) Hydrophilic Surfaces Using Combined Offset-Tool-Servo Flycutting and Hot Embossing Methods.

Polymethyl methacrylate (PMMA) is a material with good surface wettability and has unique and widespread applications in industrial fields. However, fabricating this material in an environmentally friendly way while maintaining its mechanical robustness remains a challenging task. One effective way is through the rational design of microstructure surfaces. The current study fabricated a pyramid microstructure array on a mold surface using offset-tool-servo flycutting, which was then combined with hot embossing to replicate an inverted pyramid microstructure array on a PMMA surface. Firstly, a toolpath compensation algorithm was developed to linearize the arc toolpath and reduce the cost of ultra-precision lathe. Then, the algorithm was further developed to achieve automatic linear toolpath intersection, aiming to ensure the machining accuracy and improve machining efficiency. An experiment testing the linear toolpath intersecting at 90° was conducted, fabricating a pyramid microstructure array with nanoscale roughness on the mold surface. This surface was then employed for replicating an inverted pyramid microstructure array on the PMMA surface using hot embossing. Furthermore, the accuracy of replication was evaluated, and the experimental results demonstrated excellent replication fidelity, exceeding 98%. The microstructural surface of the PMMA exhibited a change in surface wettability. The wettability test showed a water-droplet contact angle reduction from 84.8° ± 0.1° to 56.2° ± 0.1°, demonstrating a good hydrophilic effect. This study introduces a novel, environmentally friendly and high-precision method to fabricate a functional PMMA surface with an inverted pyramid microstructure array. The results of this study also provide strong technical support and theoretical guidance for micro-nanostructure functional surface machining and replicating.

Posture optimization in robotic drilling using a deformation energy model

Robots have various advantages such as multiple degrees of freedom (DOFs), flexibility, and cost efficiency. Although robots have mostly been utilized for painting, welding, pick-and-place, and assembly tasks, robot machining has been actively introduced in many fields to maximize productivity. The main obstacle to robot machining is the poor stiffness of robots, which is lower than that of conventional machine tools. Low stiffness induces compliance errors that deteriorate machining quality. One effort to improve stiffness is tool path compensation, in which compliance errors are predicted and compensated for by utilizing a stiffness model. Another approach to solving the stiffness problem is posture optimization, in which a performance index is defined based on the stiffness model. Such a performance index evaluates stiffness according to the configuration of a robot. Using this index, one can optimize the posture of a robot to maximize stiffness during machining. In previous studies, optimization was performed for the redundant DOF in the spindle axis direction at a given position, assuming a fixed workpiece and vertical feed direction. However, workpiece placement also has to be optimized in the robot workspace to obtain the optimal machining posture. This study introduced a novel approach to posture optimization. A deformation energy model that can evaluate stiffness from an energy perspective was proposed. With the proposed method, the posture of the robot and the workpiece position can be optimized simultaneously. Global optimization was performed within the entire workspace, and simulations were conducted to verify the optimization results. Additionally, local optimization considering the working environment was performed to deal with the practical problems in real scenarios. The results of optimization were verified experimentally.

A Review on Part Geometric Precision Improvement Strategies in Double-Sided Incremental Forming

Low geometric accuracy is one of the main limitations in double-sided incremental forming (DSIF) with a rough surface finish, long forming time, and excessive sheet thinning. The lost contact between the support tool and the sheet is considered the main reason for the geometric error. Researchers presented different solutions for geometric accuracy improvement, such as toolpath compensation, adaptation, material redistribution, and heat-assisted processes. Toolpath compensations strategies improve geometric precision without adding extra tooling to the setup. It relies on formulas, simulation, and algorithm-based studies to enhance the part accuracy. Toolpath adaptation improves the part accuracy by adding additional equipment such as pneumatically or spring-loaded support tools or changing the conventional toolpath sequence such as accumulative-DSIF (ADSIF) and its variants. It also includes forming multi-region parts with various arrangements. Toolpath adaptation mostly requires experimental trial-and-error experiments to adjust parameters to obtain the desired shape with precision. Material redistribution strategies are effective for high-wall-angle parts. It is the less studied area in the geometric precision context in the DSIF. The heat-assisted process mainly concentrates on hard-to-form material. It can align itself to any toolpath compensation or adaptation strategy. This work aims to provide DSIF variants and studies, which focus on improving geometric accuracy using various methodologies. It includes a brief survey of tool force requirements for different strategies, sheet thickness variation in DSIF, and support tool role on deformation and fracture mechanism. Finally, a brief discussion and future work are suggested based on the insights from several articles.

An automatic compensation method for improving forming precision of multi-layer multi-bead component

Wire and arc additive manufacturing (WAAM) is a promising technology for manufacturing large-sized metal components. However, the material shortage region (MSR) at the edge of each slicing layer can influence the forming precision and surface flatness of components. To solve these problems, this paper proposes a shape follow-up edge cycle compensation (SECC) method and model for predicting the weld width and weld height to improve the efficiency of the WAAM process. First, the prediction model was used to determine the weld width and weld height for various welding parameters. The predicted width was then used to obtain the optimal overlap distance, and the filling path of each layer was generated. The same weld height was used for slicing of the 3D model and the tool path compensation cycle was generated. Second, the influence of the MSR on the morphology of multi-layer multi-bead (MLMB) components was analyzed. The MSR results in a height difference between the edge height and the middle height of every deposited layer, and the height difference increases as more layers are added and the height of the component increases. Furthermore, the influence of the MSR gradually extends from the edge to the middle, such that the upper surface presents a parabolic shape. Finally, a mathematical model was established to determine the height difference based on the area of the MSR. When the height difference reaches the weld height, an edge compensation weld is added to eliminate the height difference. Our experimental results show that the proposed forming control strategy improves forming precision and surface flatness. The method is highly feasible and can be applied to a wide range of WAAM applications.

Inverse dynamics toolpath compensation for CNC machines based on model predictive control

The use of model predictive control (MPC) as a form of inverse dynamics compensation for multi–axis CNC machines, to subdue the inaccuracies incurred by axis inertia and damping, is investigated by both simulation studies and experimental performance analysis using a 3–axis milling machine governed by an open–architecture software controller. The results indicate that MPC is a viable tool for inverse dynamics compensation with a controller sampling frequency f = 1024 Hz running on a 500-MHz processor, with only modest prediction horizons offering excellent performance in terms of feedrate accuracy and contour error suppression. Unlike inverse dynamics schemes based upon linear time–invariant dynamic models, the MPC scheme provides the flexibility to compensate for nonlinear physical effects such as backlash in the machine axes and hard constraints on axis accelerations imposed by motor torque constraint.

Simulation of compensated tool path through virtual robot machining model

Nowadays, industrial robots could be a successful alternative to machine tools for milling of large parts with complex geometry. As it is known, poor accuracy which is most influenced by the stiffness of robot structure is recognized as a limiting factor for successful use of robots in milling tasks. Since there are different sources of error in robots, virtual manufacturing systems provide a useful means for products to be manufactured without the need of physical testing on the shop floor. This paper presents the developed virtual robot machining model, as a part of digital twin, for the simulation of a modified tool path generated by the compensation algorithm for the errors induced by cutting forces due to robot compliance. This part of digital twin includes the robot kinematic model, the Cartesian space robot compliance model, the model of cutting forces and the developed program based on off-line compensation algorithm. Development of such virtual robot machining model is important for the validation of modified tool path before the machining on a real robot. The developed virtual robot machining model is verified via several experiments, where the simulated surfaces are compared with the real machined surfaces generated by tool movement through a modified, i.e., corrected, trajectory on an available industrial robot programmed in G-code.

Cutting force validation and volumetric errors compensation of thin workpieces with sensory tool holder

The purpose of this study is to improve the machining precision and processing efficiency in the milling process of thin workpieces. A static analysis is conducted in commercial software ANSYS with the predicted cutting force exerted on the feature points. In this study, the cutting forces are measured by a sensory tool holder and the data coordinate transformation between the tool and workpiece has been established. Modified tool paths for compensation volumetric errors were built according to the deformation data as a CL (cutter location) file. Moreover, three program interfaces are developed based on MATLAB for calculating the cutting constants, predicting cutting forces, and generating the compensated tool path, respectively. A thin workpiece machining example was made for demonstration and verification, including a tool path simulation in NX CAM (computer-aided manufacturing) software and practical machining on a CNC milling machine. The result shows that the presented method successfully improved machining precision and processing efficiency.

Shape Accuracy Improvement Obtained by μ-SPIF by Tool Path Compensation

In this paper, the focus will be on the micro single-point incremental forming technique (μ-SPIF) consisting of forming thin sheets (less than 1mm) using a hemispherical tool. The formability limits for incremental forming are higher than for conventional plastic deformation processes. The second advantage of this technique is that it does not require specific tools (dieless process). Although the μ-SPIF process increases flexibility and reduces installation costs, it is constrained by many problems related to large decreases in thickness and significant shape defect. The purpose of this communication is to propose a technique for compensating, after the fact, for shape defects related to forming and springback. The proposed technique consists in defining parametric tool paths using B-Splines curves. The trajectory thus applied to the CAD geometry is generated. The forming of a part is then carried out using this initial trajectory. After digitizing the obtained shape, a comparison is made between the obtained geometry and the specified geometry. A compensation algorithm is then applied to generate an optimized trajectory that takes into account forming and springback defects. After applying this new path, the part is then made and the interest of this approach is demonstrated by comparing the specified, obtained and optimized geometries.

Planar tool radius compensation for CNC systems based on NURBS interpolation

This paper introduces the realization of a tool radius compensation algorithm for NURBS trajectory. First, a single-segment NURBS trajectory tool radius compensation algorithm is developed. Different from the straight line and arc trajectory, the self-intersection phenomenon is prone to happen when calculating a single NURBS tool center trajectory, and the self-intersection will cause the overcut of workpiece. To avoid this situation, the algorithm introduced in this paper can detect whether the NURBS tool center track has caused overcut, and deal with the self-processing. Second, the tool radius compensation algorithm with multi-segment NURBS trajectory is implemented. The focus of this part is the tool radius compensation of the trajectory transfer, and the trajectory transfer is divided into two types: the extension type and the shortened type. For the shortened type transfer, cross-processing is needed to avoid the overcut of workpiece at the transfer. When calculating the tool radius compensation of the shortened type, we not only need to find the intersection of the tool center trajectory of two adjacent NURBS curves, but also need to select the intersection we need when a number of intersections exist. For the extension type transfer, in order to ensure the continuity of the tool center trajectory, we need to extend the tool center trajectory or add arc-segment at the transfer. The proposed algorithm can automatically decide where to extend the tool center trajectory or add arc-segment to achieve the best efficiency. Finally, the algorithm can output the calculated NURBS tool center trajectory in the form of linear segment interpolation G code or NURBS interpolation G code according to the processing needs. Simulations on VERICUT and experiments on three-axis CNC machine tool shows the effectiveness and validation of the tool path compensation algorithm.

Form error compensation in the slow tool servo machining of freeform optics

Advancements in diamond turning technology with tool servo configurations enables the generation of precise freeform surfaces. However, the profile accuracy is mainly limited due to non-availability of an efficient tool path compensation techniques and precise alignment methods. The aim of this study is focused on developing a tool path compensation routine for slow tool servo machining of freeform optics. A seven-order polynomial freeform surface, designed for hyperspectral imaging is selected for experimentation. Alignment strategy by utilizing the available fiducials is presented to ensure the precise re-mounting of surface during machining and metrology. The contact type profilometer is used to measure the fabricated surface by taking 25 numbers of two-dimensional raster scans at an interval of 0.5 mm. The scans are then stitched to get the 3D surface measurement. The residual form error map is used to compensate the tool path. Significant reduction in form error, i.e., from peak to valley (PV) of 9.27 to 0.75 μm with surface finish (Ra) of 11.82 nm, is achieved by performing four machining iterations of compensation. The simulation studies are also presented to investigate the effects of various misalignments on manufacturing accuracies. The developed compensation process is effective for fast convergence of form error and to manufacture the precise freeform optics for various imaging and non-imaging applications.

Control of Process Settings for Large-Scale Additive Manufacturing With Sustainable Natural Composites

We present a system for 3D printing large-scale objects using natural biocomposite materials, which comprises a precision extruder mounted on an industrial six-axis robot. This paper highlights work on controlling process settings to print filaments of desired dimensions while constraining the operating point to a region of maximum tensile strength and minimum shrinkage. Response surface models relating the process settings to the geometric and physical properties of extruded filaments are obtained through face-centered central composite designed experiments. Unlike traditional applications of this technique that identify a fixed operating point, the models are used to uncover dimensions of filaments obtainable within the operating boundaries of our system. Process-setting predictions are then made through multi-objective optimization of the models. An interesting outcome of this study is the ability to produce filaments of different shrinkage and tensile strength properties by solely changing process settings. As a follow-up, we identify optimal lateral overlap and interlayer spacing parameters to define toolpaths to print structures. If unoptimized, the material’s anisotropic shrinkage and nonlinear compression characteristics cause severe delamination, cross-sectional tapering, and warpage. Finally, we show the linear scalability of the shrinkage model in 3D space, which allows for suitable toolpath compensation to improve the dimensional accuracy of printed artifacts. We believe this first-ever study on the parametrization of the large-scale additive manufacture technique with biocomposites will serve as reference for future sustainable developments in manufacturing.

Reduction of geometric deviation by multi-pass incremental forming combined with tool path compensation for non-axisymmetric aluminum alloy component with stepped feature

Incremental sheet forming (ISF) is a new and rapid forming process which produces parts without using specific die. One of the key challenges is geometric accuracy especially for complex sheet metal parts. In the presented work, a typical non-axisymmetric part of AA5052 with stepped feature was studied through orthogonal experiments with 4 parameters in 4 levels to determine the optimal process parameters for better geometric accuracy. Based on the optimum process parameters, the strategy on further reducing geometric deviation was investigated, and the compensation was made on each tool path during multi-pass incremental forming. The intermediate configurations were designed to avoid the interaction of different features and improve the smoothness of material flow. Based on the proposed methodology, the geometric accuracy of non-axisymmetric part with stepped feature has been obviously improved, particularly at the stepped feature.

Using the Segmented Iterative Learning Control Method to Generate Volumetric Error-Compensated Part Programs for Three-Axis CNC Milling Machine Tools

This study proposes using the iterative learning control method to adjust the volumetric error-compensated tool path, where the working volume motion accuracy of three-axis computerized numerical control (CNC) milling machine tools is increased by segmented modification of the part program. As the three-axis CNC milling machine tools generally have volumetric error of working volume, this study refers to the measured and established table of volumetric errors and uses the method of the modifying part program for volumetric error compensation of machine tools. This study proposes using part-program single-block positioning segmented for volumetric error compensation, as the generated compensated part program with multiple compensated blocks can effectively compensate the volumetric error of working volume in the tool moving process. In terms of the compensated tool path computing method, this study uses the iterative learning control (ILC) method and refers to compensated tool path and volumetric errors along the compensated tool path for iterative computation. Finally, a part program with multiple blocks is modified by the converged optimal compensated tool path, in order that the modified part program has higher-precision volumetric error compensation effect. The simulation result shows that the rate of improvement of error of the volumetric error compensation method proposed in this study is 70%. The result of cutting tests shows that the average rate of improvement of the straightness error of the test workpiece is 60%, while the average rate of improvement of height error is 80%. Therefore, the results of simulation and cutting tests can prove the feasibility of using the ILC method for segmented modification of the volumetric error-compensated part programs proposed in this study.

Cryogenic machining of PDMS fluidic channel using shrinkage compensation and surface roughness control

Polydimethylsiloxane (PDMS) is widely used in the fabrication of fluidic chips. Recently, a direct mechanical micromilling process using cryogenic cooling was considered to cut PDMS in order to reduce development time and prototyping costs. In this paper, the characteristics of cryogenic machining of PDMS were experimentally studied. The machined surface was analyzed with a variety of machining parameters, such as spindle speed and feed per tooth, and their effects on cutting temperature were examined. To improve geometric accuracy, a tool path compensation method that takes into account the shrinkage of PDMS at cryogenic temperatures was developed. Based on the preliminary results, the fabrication of a PDMS fluidic chip demonstrated that fluidic flow can be controlled by differences in the flow friction.

Error compensation model considering tool wear in milling difficult to cut materials

In this paper, error compensation considering tool wear is studied to improve machining precision of difficult-to-cut material in milling process. First, an error model considering tool wear in milling process is established based on the relationship between the position angle and the radius of the section circle of ball-end milling cutter, and the established error model is solved and verified. Then, based on the verified error model, a new error compensation method based on a scale division idea is proposed to determine the required compensation value and generate the compensated tool path by tracing the initial tool path and tool wear deviations. Based on the above error compensation method, the detailed compensated processes for a plane, cylindrical surface and sculptured surface are analysed and an off-line error compensation software is developed by using Visual C++ programming tool. Finally, experimental tests performed with stainless steel 1Cr18Ni9Ti and cemented carbide cutters, are used to validate the reliability of the proposed error compensation method in milling operations. Experimental results show that the proposed error compensation method can reduce machining error and improve machining surface quality.

5-Axis adaptive flank milling of flexible thin-walled parts based on the on-machine measurement

Deformation of the part and cutter caused by cutting forces immediately affects the dimensional accuracy of manufactured parts. This paper presents an integrated machining deviation compensation strategy based on on-machine measurement (OMM) inspection system. Previous research attempts on this topic deal with deformation compensation in machining of geometries in 3-axis machine tools only. This paper is the first time that concerned with 5-axis flank milling of flexible thin-walled parts. To capture the machined surface precision dimensions, OMM with a touch-trigger probe installed on machine׳s spindle is utilized. Probe path is planned to obtain the coordinate of the sampling points on machined surface. The machined surface can then be reconstructed. Meanwhile, the cutter׳s envelope surface is calculated based on nominal cutter location source file (CLSF). Subsequently, the machining error caused by part and cutter deflection is calibrated by comparing the deviation between the machined surface and the envelope surface. An iteration toolpath compensation algorithm is designed to decrease machining errors and avoid unwanted interference by modifying the toolpath. Experiment of machining the impeller blade is carried out to validate the methodology developed in this paper. The results demonstrate the effectiveness of the proposed method in machining error compensation.

Closed door machining error compensation of complex surfaces using the cutting compliance coefficient and on-machine measurement for a milling process

Closed door machining is a strategy for producing a part within tolerance using on-machine measurement and automatic process adjustment as opposed to manual gauging interventions. This paper presents an integrated methodology for compensating errors detected using on-machine probing. In a multi-cut process, intermittent probing which is achieved through replacing the cutting tool with a touch probe, after each cut, can detect machining errors caused by deflection and the tool offset error. A cutting compliance coefficient model is used to estimate corrections to the tool path at the finishing cut based on a finite number of measured errors at discrete locations for previous cuts. The model also anticipates compliance changes and the effect of the compensated depth of cut. The complex surface to be machined is represented by a B-spline model. The compensated tool path is obtained from B-spline deformation techniques applied to the initial tool path according to the discrete corrections. Milling tests are carried out with and without compensation demonstrating a reduction of machining error from +140 to ±20 µm.

Tool path design for enhancement of accuracy in single-point incremental forming

One of the variants and simplest form of incremental sheet metal forming process is single-point incremental forming in which a spherical ended tool is used to form features on one side of the initial plane of the sheet. Dimensional deviation is observed in the region of component opening (due to sheet bending) and at the wall as well as base regions. This article presents a methodology to minimize the dimensional deviation in the wall and base regions of single-point incremental forming formed components. The authors observed that the sheet deflection due to axial force and tool deflection due to radial force are the two main factors leading to the geometric deviation in wall as well as base regions. An analytical model has been developed for estimating these two deflections. These deflection values are incorporated in the tool path generation, and the components formed using the compensated tool path resulted in acceptable dimensional accuracy in the wall region. Force predictions carried out by force equilibrium method are used to calculate the deflection compensations as its predictions are in reasonably good agreement with the force predictions of finite element analysis and experimental measurements. Significant improvement in accuracy is achieved by using the deflection compensated tool paths.

V-grooving using a strip EDM

This study suggested use of strip electrical discharge machining (strip EDM) for fabricating V-grooves. Traditional methods, such as cutting or grinding, make it difficult to deal with hard-to-cut metals and burr formation on machined portions is a concern. Although EDM does not cause these problems, this machining method has the disadvantage of electrode wear. The wear causes shape error and low productivity. To overcome these problems, an experiment using strip EDM was conducted to produce V-grooves. A thin conductive strip used as an electrode was bent around an electrode guide and its corner was shaped into a right-angle triangle. This shaped electrode was used to fabricate a V-groove. Since the suggested process was not concerned with electrode wear, new electrodes for finish cuts and tool path compensation were not required during the machining process. It was also possible to complete high precision machining because no burr or deformation was caused by the cutting force when compared with traditional machining methods. In the practical machining of this research, the patterns of V-grooves, prisms and pyramids were fabricated on a stainless steel workpiece.