Robotic assisted milling for increased productivity

Ti-6Al-4V具有優異的抗腐蝕性和物理、機械性質，被廣泛運用在生醫領域。本研究探討微銑削寬度0.2 mm、高度2.0 mm，寬高比為10薄壁尺寸誤差量的誤差成因分析與改善策略。本文以有限元素分析探討薄壁工件撓曲變形量成因，模擬結果顯示，隨著薄工件厚度減小，剛性逐漸降低，切削力占變形的比重逐漸上升，而切削熱影響變形的比重過低可以忽略。目前學術研究上，對於薄壁銑削加工適合用順銑法或逆銑法，有著不同的看法。順銑削表面佳易加工，本文先以不同切削速度、進給、切削液搭配進行實驗，得出了在切削速度95 m/min、加工進給3 μm/tooth、切削液MQL下，較原始切削參數下，平均尺寸誤差量小了約49 %；逆銑削表面不佳，表面上有缺陷，本文先探討其產生原因，並且以噴灑MQL、增加0.09 mm預留量、降低切深至0.3 mm等策略，成功切削出表面良好的薄壁件，使其可排除表面因素，進行尺寸誤差量測。相同的參數比較順銑削、逆銑削尺寸誤差量，兩者之間並無太大的差距。最後以本文將以切削力造成刀具撓曲與工件撓曲之間的交互關係為基礎，提出一順銑削與逆銑削的薄壁尺寸誤差模型，並且以實驗驗證之，成功地解釋順銑削、逆銑削薄壁件尺寸誤差的來源。加工策略方面，提出以超音波輔助振動銑削的方式，降低切削力，同時降低薄壁尺寸誤差量，在進給方向進行頻率25,000 Hz、震幅5 μm的一維振動，並且以實驗成功證明可降低側向切削力，在超音波振動輔助順銑削搭配MQL下，側向切削力降低31%，平均尺寸誤差量降低28%。超音波振動輔助逆銑削搭配MQL下，表面橫條紋明顯變少，但是仍然無法全部清除。

Parallel mechanisms with multiple links have been expected to be used in machining because they are higher in rigidity, accuracy, and output power than series mechanisms, such as industrial robots. However, unlike conventional machine tools, which consist of linear and rotary axes, parallel mechanisms have a large number of error factors. In the parallel link mechanism, there is no guide surface that physically guarantees linearity, and all accuracy is determined by the operating performance of the composite axes. This makes it difficult to identify any error factors. Therefore, a kinematics model is devised, and the behavior of the tool tip is checked by inputting the encoder information during the actual operation of a specific axis. Based on the results, we evaluate the machining characteristics of the target machine tool. The target machine tool in this study is a 5-axis machine tool that combines a 3-DOF parallel mechanism consisting of three linear motion axes and a 2-DOF serial mechanism consisting of two rotary axes. In our previous research, we tried to build a forward kinematics model. Although its prediction accuracy was insufficient, it was possible to actually identify the cause of the defect in the quality of the machined surface using the servo position information of the kinematics machine. However, we have not been able to construct an inverse kinematics model that is suitable for calculating the correction position command value to improve the quality of the machined surface. In this study, based on the shape creation theory, we devise and evaluate the kinematics model of a robotic machine tool that has a parallel mechanism. As a result of comparing the kinematics model with the 3D-CAD model in order to evaluate the accuracy of the former, it was confirmed that the proposed method has high simulation accuracy. Then, machining tests were carried out to evaluate the machining accuracy by measuring, based on proposed kinematics model, the machined surfaces in order to identify the mechanism that affects the texture of the machined surface. In addition, we performed a circle interpolation to confirm the effects of reversing the motion of each drive axis on the behavior of the tool tip. As a result, it is considered that the linear motion axis has a large effect on the behavior of the tool tip on the quadrant glitch of each drive axis. It was also found that the effects of the 1st- and 3rd-axes on the behavior of the tool tip are different from those of the 2nd-axis.

The machining process of aluminum alloy usually produces built-up edge and tool sticking problems due to their low hardness and large plastic deformation, which may further affect the machined surface quality and tool life. This paper aims to investigate the influence of different cutting fluids on the machined surface quality and tool life during the milling process of 7050 aluminum alloy. A novel cutting fluid (QC-2803) was considered in the study, which is synthesized by addition of alkyl alcohol amide and chlorinated polyolefin, and the traditional cutting fluid (CCF-10) was used as the control group. The physical and chemical properties of two cutting fluids were characterized. The milling process of 7050 aluminum alloy was carried out under two different cutting fluid conditions. The machined surface morphology, cutting force and tool wear morphology were observed during the process. Results show that the surface tension of the novel cutting fluid is significantly lower than that of the traditional cutting fluid, which makes it easier to produce a lubricating film between the aluminum alloy and tool, and further benefits the machined surface quality and tool life. As a result, the surface roughness and cutting force are reduced by ~20.0% and ~42.9%, respectively, and the tool life is increased by 25.6% in the case of the novel cutting fluid (QC-2803). The results in this paper revealed the important laws of cutting fluid with metal surface quality, cutting performance and tool wear, which helps to control the machined surface quality and tool life by the selection of cutting fluid during metal milling.

Besides the low static stiffness of the machine structure and tool wear, thermal deformations are a major reason for dimensional and geometric work piece errors. As an alternative to cooling, insulation and other design measures, an indirect control compensation offers an inexpensive possibility to reduce thermally caused displacements which occur at the cutting point. The thermo-clastic displacements are calculated from the temperatures of different structure points and are corrected via the NC. In order to reduce the implementation expenditure and to achieve a good transformation capability for other thermal loads, it is necessary to only consider the displacement-relevant temperature measurement points. Finding out the relevant temperatures used to be a difficult and time consuming process. This paper presents a procedure based on a dynamic neural net that automatically determines relevant temperature probes. The quality of neural models and their ambility to generalise depends strongly on the net size, but this parameter is difficult to define in advance. Apart from the calculation of the correction parameters, the developed neural net also modifies its size during the learning phase. After the introduction of this model, compensation results for a milling machine are shown and compared to classical linear equation models.

High-speed machining (HSM) provides a new dimension to machining parts in hard steel because it eliminates intermediate processes and minimizes hand-finishing operations. Lead times and overall manufacturing costs are reduced, but the potential benefits can only be realized when the high-speed processes are implemented correctly. Successful high-speed applications depend on the linkage between contributing factors (machine tool, control system, tooling system, CAM system, CAM operator and machine operator) working effectively. A weakness in one of these areas can jeopardize the geometric form and surface quality in a high-speed operation. Such weaknesses can raise the paradox that as the quality of a machined part is improved through HSM, there is an increasing awareness of smaller defects. This is an acute problem in the final stages of the manufacture of dies, molds, and prototype components, where finished parts are rejected on their overall appearance, even though they satisfy all the standard design quality criteria. Very small, localized surface marks produced during machining operations are perceived as serious blemishes to the eye. One of the most obvious, and frequent surface blemishes are the marks associated with the popular 3D offset machining strategy. These marks are produced every time the 3D offset strategy is used and the parts have to be finished in a manual benching operation. This manual process is indeterminate in terms of time and cost. There is also a great risk that the geometric shapes and tolerances are compromised by the uncontrolled manual operations. This paper reports the results of a series of machining trials to assess the effect of the 3D offset strategy on machined parts, and it considers two modified offset strategies that were designed to alleviate the marking process.

Virtual manufacturing systems can provide useful means for products to be manufactured without the need of physical testing on the shop floor. As a result, the time and cost of part production can be decreased. There are different error sources in machine tools such as tool deflection, geometrical deviations of moving axis and thermal distortions of machine tool structures. Some of these errors can be decreased by controlling the machining process and environmental parameters. However other errors like tool deflection and geometrical errors which have a big portion of the total error, need more attention. This paper presents a virtual machining system in order to enforce dimensional, geometrical and tool deflection errors in three-axis milling operations. The system receives 21 dimensional and geometrical errors of a machine tool and machining codes of a specific part as input. The output of the system is the modified codes which will produce actual machined part in the virtual environment.

While simulation has been successful in tying design and process planning into an iterative loop, machining has traditionally been a downstream terminus of the manufacturing cycle. Simulation of machining has proven difficult due to the highly dynamic and nonlinear nature of the material removal process. Consequently, actual machining must be performed to determine the suitability of designs and process plans. The nature of the programming interface to machine tools has been an obstacle to information flow in both directions. Toward the machine tool, useful design and process information is reduced to primitive tool path motion statements. From the machine tool, no automatic data paths exist, so machinists must translate results into recommendations for design or process changes manually. The advent of STEP (ISO 10303) data for numerical control, or STEP-NC, promises to rectify these problems by providing full product and process data to the machine tool controller at run time and an automatic path back to these upstream activities using technologies such as XML. This paper describes STEP-NC and early experiences that show how these benefits can be achieved.

Energy Consumption Characteristics and Influence on Surface Quality in Milling

When a three-dimensional shape is machined by NC machine tool, motion errors occur around the motion direction changing points of the translational axis. This has a significant influence on a quality of machined surfaces. Therefore, much research on the influence of motion errors around the motion direction changing points of the feed drive system on a machined surface has been conducted to improve the quality of machined surfaces by NC machine tool. Among the motion errors that occur around the motion direction changing points of the translational axis, quadrant glitches with a stick-slip motion have a particularly large influence on the machined surface, and research on compensation methods continues to be reported. However, the transcriptional characteristics of quadrant glitches of the translational axis for machine tools have not been investigated adequately. In this study, the transcriptional characteristics of quadrant glitches of the translational axis on a machined surface were investigated. In addition, the influences of various factors on the transcriptional characteristics of quadrant glitches on a machined surface were investigated using a proposed equation and actual machining tests.

The geometric accuracy of a workpiece represents one of the key parameters defining its quality, and it is affected by the appropriate selection of the machine tool, control system, NC program and cutting conditions. Up-to-date control systems contain advanced compensation functions, which increase the volumetric accuracy of the machine tools. Nevertheless, these functions use correction data measurements within the machine tool’s periodic maintenance plan. This paper introduces a method for ad hoc correction data modification. This modification is based on the difference between the real and nominal workpiece geometries, which are evaluated on a coordinate-measuring machine as a standard process in high-accuracy workpiece production. Correction data are compiled in the form of a three-dimensional structured mesh, where nodes of the mesh contain such correction values that interpolations within the mesh suppress workpiece geometric deviations. The correction mesh calculations are based on the assumption that the nodes are connected by imaginary springs and that they are initially in force equilibrium. Force disbalance is introduced by workpiece geometric deviations evaluated at arbitrary points. Then the new position of force-balanced nodes is calculated. Experimental results on a three-axis machining center have verified the proposed method, where geometric accuracy of the workpiece increased more than 85% without any negative effect on surface quality. The approach presented is efficient for increasing workpiece accuracy without the need for NC program modification.

The dimensional, geometrical, thermal and tool deflection errors which have a big portion of the overall error of machined parts need more attention in precision of components produced by using CNC machine tools. As a result, it is essential to simulate and compensate the errors in the machined components in order to increase accuracy of machined parts. In order to simulate and analyse the real manufactured components in virtual environments, virtual machining systems are proposed. In this paper, application of virtual machining system is investigated in order to simulate and compensate dimensional, geometrical, thermal and tool deflection errors in 5-axis milling operations of free form surfaces. The volumetric error vectors regarding the dimensional, geometrical, thermal and tool deflection errors at each cutting tool location throughout the machining pathways are calculated and compensated utilising the study’s created virtual machining technology. In order to validate the study, a sample workpiece free from surfaces is milled by using the 5-axis CNC machine tool. The machine part is then measured by suing the CMM machine in order to obtain the dimensional, geometrical, thermal and tool deflection errors during milling operations of free form surfaces. Thermal sensors are also installed to the different locations of CNC machine tool in order to measure the thermal error of CNC machine tool during machining operations. Finally, in order to improve accuracy in 5-axis milling operations of free form surfaces, new cutting tool paths regarding the compensated volumetric errors of dimensional, geometrical, thermal, and tool deflection errors are generated. As a result, by utilising the proposed virtual machining system in the research work, precision as well as reliability during 5-axis milling operations of free form surfaces can be enhanced.

Fast flexible multibody dynamic analysis of machine tools using modal state space models

Digital twin based accuracy compensation

Quality Control (QC) is one of the most important phases in the production process. In tradition, the workpiece must be inspected for dimension and shape errors in the QC lab or Coordinate Measuring Machine (CMM) after being machined, which is time-consuming and labor-intensive, especially for large scale parts. Thus the only practical way to do that is to migrate the critical primary inspection operations upstream from the QC lab or CMM to the production floor, and that’s what On-Machine Measurement (OMM) does. OMM technology can inspect the workpiece on its operating station by motion control and the position detection function of CNC machine tool, by which the measuring error caused by the misalignment between measuring datum and manufacturing datum can be avoided accurately. Therefore, it is a feasible way to achieve the real-time control of the manufacturing process and improve the manufacturing accuracy and efficiency. The servo axes of multi-axis CNC machine tool might be redundant for being used in OMM, which will result in that one measurement task could be done by different servo motion schemes. In addition, the map between the machine accuracy and measuring accuracy has not been found out previously. Thus, how to determine metering schemes to measure a workpiece with high accuracy and efficiency is one of the most important problems and also a hotspot for everybody. To solve this problem, a generation method of task-oriented OMM scheme is proposed. Utilizing multi-body system theory and transformation of homogeneous coordinates, a function relationship between the geometrical characteristic of workpiece as independent variables and motions of machine tool as dependent variables is established. All possible metering schemes are obtained by analyzing the solution of functional equation. Then the performance of measurement scheme can be evaluated by comparing the measuring errors of each scheme after building a mapping relationship between the machine tool volumetric errors and measuring errors. In the end, a case study was accomplished, and the correctness and efficiency of the methodology has been verified.

Precision machining is becoming more and more important with the increasing demands on surface quality for various components. This applies, for example, to mirror components in micro-optics or cooling components in microelectronics. Copper is a frequently used material for this purpose, but its mechanical properties make it difficult to machine. In this study, a process strategy for finishing copper surfaces with batch-manufactured micro-grinding tools in an electrochemically assisted grinding process is demonstrated. The tool heads are manufactured from a polyimide-abrasive-suspension and silicon as a carrier substrate using microsystems technology. The matching shafts are milled from aluminium. The tools are then used on pure copper and oxidised copper surfaces. By using finer abrasives grains (1.6–2.4 µm instead of 4–6 µm) than previously, similar surface roughness values could be achieved (Ra = 0.09 ± 0.02 µm, Rz = 1.94 ± 0.73 µm) with the same grinding process. An optimised grinding process that combines the use of rough and fine tools, on the other hand, achieves significantly better surface finishes in just four grinding iterations (Ra = 0.02 ± 0.01 µm, Rz = 0.83 ± 0.21 µm). In order to achieve a further increase in surface quality, this optimised grinding process is combined with the anodic oxidation of the copper workpieces. The surface modification is done to increase the machinability of the surface by creating an oxide layer. This is confirmed by the results of scratch tests carried out, which showed less force acting on the tool during machining with the oxide layer than with a pure copper surface. To realise this within the machine tool, an electrochemical cell is shown that can be integrated into the machine so that the oxidation can be carried out immediately before the grinding process. The copper layers produced inside the electrochemical cell in the machine tool show similar characteristics to the samples produced outside. Processing the oxidised samples with the optimised grinding process led to a further reduction of about 17% in the Rz values (Ra = 0.03 ± 0.01 µm, Rz = 0.69 ± 0.20 µm). The combination of the shown grinding process and the integration of anodic oxidation within the machine tool for the surface modification of copper workpieces seems to be promising to achieve high surface finishes.