Ultraprecision Micromilling Research Articles

Feasibility study on ultraprecision micro-milling of the additively manufactured NiTi alloy for generating microstructure arrays

Nickel-titanium (NiTi) alloy has unique functional properties in medical and microelectronics products. Recently, additive manufacturing has become an attractive way of fabricating NiTi alloy. However, the study of generating microstructure arrays on additively manufactured (AMed) NiTi alloy surfaces remains unclear. Motivated by this, this study proposes the ultraprecision micro-milling (UMM) process to generate microstructure arrays and perform a systematic investigation. First, selective laser melting, as a popular additive manufacturing way, was utilized to fabricate the AMed NiTi alloy. Then, the UMM process was carried out to machine these samples. After the machining experiments, a mirror surface with a surface roughness of 0.014 μm can be acquired, which demonstrates the good machinability of the AMed NiTi alloy. Moreover, the micro-groove array and micro-pillar array with low machining errors were machined on the AMed NiTi alloy surfaces, which verifies the effectiveness of the proposed machining process. The corresponding cutting forces, tool conditions, and chip morphologies were studied to fully understand the machining mechanism of the AMed NiTi alloy. Besides, the surface wettability of microstructure arrays was also quantitatively analyzed. Therefore, this study provides a facile and effective machining process for generating microstructure arrays on the AMed NiTi alloy surfaces.

A novel hybrid technique to fabricate silicon-based micro-implants with near defect-free quality for neuroprosthetics application.

This paper introduces a new hybrid microfabrication technique which combines ultra-precision micro-milling and a ductile sacrificial material deposition process to fabricate a silicon-based implant for neuroprosthetics applications with near defect-free quality at several hundreds of micrometres in thickness. The sacrificial materials can influence the quality of silicon during machining. The cutting mechanism and feasibility of the hybrid technique are studied by molecular dynamics (MD) simulations and experiments. Due to the complexity of modelling PMMA and SU-8 structures in MD environment, only copper was modelled as the simulation is intended to understand the performance of using a ductile sacrificial layer structure in silicon machining. MD analysis shows that the reduced stress intensity and subsurface damage were mainly attributed to workpiece plasticity enhancement, where its mechanism was contributed by better deformability of the ductile sacrificial layer and enhanced thermal softening from the heat generated by the high interfacial stress between the sacrificial layer and silicon substrate. Despite the MD simulation and experiment having different machining scale in terms of cutting parameters, phenomenal behaviours of the cutting performance when observed under the experimental conditions are in good agreement with simulation. Experimental verification shows that near defect-free quality was achieved at large cutting depth of 150μm when silicon is coated either with PMMA or SU-8. An exemplary implant structure was also fabricated to better demonstrate the hybrid technique's capability. In addition, the hybrid technique will be beneficial for low volume high customisation applications as it is a serial process.

Porosity measurements by X-ray computed tomography: Accuracy evaluation using a calibrated object

Accurate identification and measurement of internal voids and porosity is an important step towards improvement of production processes to obtain high quality materials and products. Recently, the importance of knowing the exact size, shape, volume and location of defects has become even higher as tighter requirements and new standards have been introduced in industry. There are several well-established methods for defects evaluation based on various principles (both destructive and non-destructive). However, all conventional methods have various deficiencies and the information about internal voids/porosity that can be extracted is limited. Most of these drawbacks can be overcome by using X-ray computed tomography (CT). Unlike other methods, CT provides full three-dimensional information about shape, size and distribution of internal voids and porosity; however, the accuracy of measurements is still under investigation. Hence, further evaluations on CT porosity measurements must be performed in order to consider X-ray computed tomography a reliable instrument for the assessment and detection of internal defects.A reference object with artificial defects was used in this research work in order to evaluate the accuracy of porosity measurements by CT. The reference object was manufactured by ultra-precision micro-milling. The object contains dismountable components with embedded internal hemispherical features that simulate internal porosity. The artificial porosity was micro-milled on top surfaces of dismountable cylindrical inserts. The hemispherical calottes were thereafter calibrated by traceable coordinate measuring systems and calibrated values were compared to actual values measured by a CT system. The accuracy of CT porosity measurements was then evaluated based on results obtained on various measurands, using different software tools and measuring procedures, comparing real scans to numerical simulations and investigating the influence of CT system parameters settings on measurement results.

An investigation of residual stresses in micro-end-milling considering sequential cuts effect

The distribution of machining-induced residual stresses has significant effects on the fatigue life, corrosion resistance, and precision durability of parts. An analytical model is presented to reveal the evolution regularity of residual stresses in workpiece for micro-end-milling. Considering the characteristics of tool rotation and interrupted cutting, the process of cutting entry and exit of each flute is treated as one cut, and sequential cuts effect is taken into account in the proposed model. The stress state caused by the previous cut is taken as the initial condition for the current cut. In order to improve the prediction efficiency, a new methodology which supposes the tool makes reverse movement is developed to determine the initial cutter position for residual stress calculation. The theoretical model is validated by machining NAK80 steel under different flank wear widths on a 3-axis ultra-precision micro-milling machine. Residual stresses are tested by means of X-ray diffraction. The computed results show that residual stresses are compressive and present a hook-shaped distribution, which is consistent with experimental results. Moreover, the effects of feed rate and radial depth of cut on residual stresses are theoretically investigated. This work can be further applied to optimize cutting conditions to achieve better surface integrity.

Ossicular replacement prostheses from banked bone with ergonomic and functional geometry.

This study shows the fabrication of innovative ossicular replacement prostheses (ORPs) based on banked cortical bone using computer numerically controlled ultraprecision micromilling, as a substantial improvement of "second generation" ORPs. Our aim is to combine optimal middle ear compatibility and surgical manageability in a single device, by releasing off-the-shelf homograft ORPs provided with the appealing features of synthetic ORPs, such as lightness, safety, measurement accuracy, surface decoration, and geometric plasticity. The new total ORP prototype was 13.1 ± 0.1 mg, leading to 81% weight reduction with respect to the previous model. Surface motifs of the head plate were applied to prevent slipping and migration after surgery, as shown by finite element modeling analysis. In addition, bone ORPs were provided with holed head plates to facilitate their surgical positioning while reducing their mass. A comparative measurement of acoustic responses of bone against synthetic partial ORPs in the 250-8000 Hz frequency range demonstrated their superior behavior. This study showed that banked compact bone can be optimally manufactured, eventually enabling the fabrication of light, standardized, and highly performant ORPs. The new bone ORPs may represent the ideal combination of biocompatibility and technology which can ultimately accomplish unmet otosurgical needs. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2495-2506, 2017.

Analytical modeling and experimental validation of micro end-milling cutting forces considering edge radius and material strengthening effects

This paper presents a novel micro end-milling cutting forces prediction methodology including the edge radius, material strengthening, varying sliding friction coefficient and run-out together. A new iterative algorithm is proposed to evaluate the effective rake angle, shear angle and friction angle, which takes into account the effects of edge radius as well as varying sliding friction coefficient. A modified Johnson–Cook constitutive model is introduced to estimate the shear flow stress. This model considers not only the strain-hardening, strain-rate and temperature but also the material strengthening. Furthermore, a generalized algorithm is presented to calculate uncut chip thickness considering run-out. The cutting forces model is calibrated and validated by NAK80 steel, and the relevant micro slot end-milling experiments are carried out on a 3-axis ultra-precision micro-milling machine. The comparison of the predicted and measured cutting forces shows that the proposed model can provide very accurate predicted results. Finally, the effects of material strengthening, edge radius and cutting speed on the cutting forces are investigated by the proposed model and some conclusions are given as follows: (1) the material strengthening behavior has significant effect on micro end-milling process at the micron level. (2) Cutting forces predicted increase with the increase of edge radius. (3) Considering varying sliding friction coefficient can enhance the sensitivity of the predicted cutting forces to cutting speed.

An experimental investigation on micro-milling of polymethyl methacrylate components with nanometric surface roughness

Micro-optical components made from polymethyl methacrylate are increasingly in demand. This article presents an experimental investigation into diamond micro-milling of polymethyl methacrylate components with nanometric surface roughness and its application perspectives. The experimental micro-milling trials with a chemical vapour deposition diamond ball endmill are conducted on a self-developed ultra-precision micro-milling machine (UltraMill) featuring high precision and high dynamic performance. Surface roughness of micro-milled slots using different micro-milling strategies is measured with white light interferometer. Results show that when feed and cutting orientations are perpendicular, smaller surface roughness can be obtained. Micro-milling is carried out on 2 × 2 mm2 areas by applying different micro-milling strategies and process parameters. The results demonstrate that the micro-milling strategy which can generate good surface roughness in slot micro-milling cannot produce expected surface roughness on such a large area (2 × 2 mm2), and machining dynamics plays an important role. By applying two-way joint micro-milling strategy and adjusting process parameters, an optical surface is obtained with roughness of 8.717 nm.

Design and Performance Assessment of a 5-Axis Ultra-Precision Micro-Milling Machine

This paper presents the development and performance assessment of a novel 5-axis ultra-precision micro-milling machine (UltraMill) which enables ultra-precision micromachining of high precision 3D miniature components and micro features. An integrated design approach with motion accuracy, dynamic stiffness and thermal stability prioritized has been proposed and applied to analyze and optimize key machine components and their integration. Direct drives and aerostatic bearings with the squeeze oil-film damper are employed in the micro-milling machine throughout, which offers higher motion accuracy, improved dynamics and loading capacity. Micro-milling trials were performed on OFHC copper using tungsten carbide, CVD diamond and single crystal diamond micro tools. Both micro featured profiles and micro machined surfaces were measured to validate the proposed machine specifications and performance.

Experimental Investigation of the Mold Surface Roughness Effect in Microinjection Molding

Microinjection molding has been drawing more and more attention due to its great advantages such as cost effectiveness and mass production capability. In this work, an experimental study was carried out in order to investigate the effect of the mold surface roughness on the achieved filled length of the molded microfeatures. For this purpose, an aluminum mold insert with microchannels having different surface roughness values was designed and fabricated using ultraprecision diamond machining and micromilling method. The experimental results revealed that increasing surface roughness of the microchannel wall led to a decrease in the filled length of the molded microfeatures. It was also found that with increased melt temperature or injection velocity, the effect of surface roughness was weakened by high-pressure trapped air inside the microchannels during injection process. Finally, the influence mechanism of the mold surface roughness was discussed.

Hybrid optical (freeform) components—functionalization of nonplanar optical surfaces by direct picosecond laser ablation

The performance of optical systems is typically improved by increasing the number of conventionally fabricated optical components (spheres, aspheres, and gratings). This approach is automatically connected to a system enlargement, as well as potentially higher assembly and maintenance costs. Hybrid optical freeform components can help to overcome this trade-off. They merge several optical functions within fewer but more complex optical surfaces, e.g., elements comprising shallow refractive/reflective and high-frequency diffractive structures. However, providing the flexibility and precision essential for their realization is one of the major challenges in the field of optical component fabrication. In this article we present tailored integrated machining techniques suitable for rapid prototyping as well as the fabrication of molding tools for low-cost mass replication of hybrid optical freeform components. To produce the different feature sizes with optical surface quality, we successively combine mechanical machining modes (ultraprecision micromilling and fly cutting) with precisely aligned direct picosecond laser ablation in an integrated fabrication approach. The fabrication accuracy and surface quality achieved by our integrated fabrication approach are demonstrated with profilometric measurements and experimental investigations of the optical performance.

Good Manufacturing Practices—Grade Preformed Ossicular Prostheses from Banked Bone via Computer Numerically Controlled Micromilling

The aim of this study was the fabrication of ossicular replacement prostheses (ORPs) from decellularized banked cortical bone via computer numerically controlled (CNC) ultraprecision micromilling, in order to obtain preformed clinical-grade tissue products, reproducing shape, size, and details perfectly comparable to those of synthetic devices. Banked femoral compact bone was used to fabricate partial and total ORPs via CNC micromilling according to Good Manufacturing Practices procedures. Drawings of ORPs with different shapes and sizes were uploaded to the computer interface, and different surface-finish parameters were tested. The obtained products underwent dimensional, weight, and surface characterizations. A histologic analysis was pursued to compare the bone matrix compactness of the produced ORPs to that of the ear ossicles. Banked-bone ORPs were produced with high dimensional accuracy. Partial ORP weights averaged (+/- SD) 31.2 +/- 0.6 mg, and total ORP weights averaged 69.3 +/- 0.7 mg. The best-finish mode allowed microscale or nanoscale roughness free from machinery textures to be obtained. Finally, the histologic analysis confirmed that the extracellular matrix compactness of the produced ORPs was suitable for ossicular chain replacement. This study assesses the fabrication feasibility of novel banked-bone ORPs of extremely high dimensional accuracy. Such devices are aimed at combining the most favorable aspects of both synthetic (reproducibility, convenience, and biosafety) and biological replacements (total biocompatibility).

Fabrication of microinducer by 5-axis control ultraprecision micromilling

The study deals with the development of an implantable micro axial-flow bloodpump by means of 5-axis control ultraprecision milling technology, which is suitable for fabricating such a complicated shape. 3-D CAD/CAM system for 5-axis control milling is developed in order to fabricate microinducers, while considering the setting error of cutting tool. In the cutting experiment with the diamond ball end mill of 0.2 mm in radius, the axial-flow micro pump of oxygen-free copper is successfully fabricated. The result of performance test shows that the fabricated microinducer works as an axial-flow micropump.

Experimental investigation on micromilling of oxygen-free, high-conductivity copper using tungsten carbide, chemistry vapour deposition, and single-crystal diamond micro tools

Insufficient experimental data from various micro tools limit industrial application of the micromilling process. This paper presents an experimental comparative investigation into micromilling of oxygen-free, high-conductivity copper using tungsten carbide (WC), chemistry vapour deposition (CVD) diamond, and single-crystal diamond micromilling tools at a uniform 0.4 mm diameter. The experiments were carried out on an ultra-precision micromilling machine that features high dynamic accurate performance, so that the dynamic effect of the machine tool itself on the cutting process can be reduced to a minimum. Micromachined surface roughness and burr height were characterized using white light interferometry, a scanning electron microscope (SEM), and a precision surface profiler. The influence of variation of cutting parameters, including cutting speeds, feedrate, and axial depth of cut, on surface roughness and burr formation were analysed. The experimental results show that there exists an optimum feedrate at which best surface roughness can be achieved. Optical quality surface roughness can be achieved with CVD and natural diamond tools by carefully selecting machining conditions, and surface roughness, Ra, of the order of 10 nm can also be obtained when using micromilling using WC tools on the precision micromilling machine.

Development of a PC-based control system for a five-axis ultraprecision micromilling machine ‘Ultra-Mill’ and its performance assessment

In the realm of micro- and nanomachining, high-precision three-dimensional miniaturized components and microstructures from an extensive range of engineering materials are increasingly required in industries such as optics, automotive, medical, aerospace, and defence. Five-axis ultraprecision micromilling machines are essential for manufacturing components and microstructures for these industries. This paper presents the design and development of the control system for a five-axis ultraprecision micromillling machine – Ultra-Mill – and its performance assessment based on two application case studies. The design specifications of Ultra-Mill are based on motion and positioning accuracy, dynamic stiffness, thermal stability, and online monitoring and inspection requirements, which are heavily dependent on the control system for the micromilling machine and associated micromilling processes. The implementation and application perspectives of the control system are therefore presented in relation to the ultraprecision and micro/nano feature capability of the machine and process, including controller dynamics, driver and actuation dynamics, feedback dynamics, online inspection and monitoring, and software dynamics.

A holistic integrated dynamic design and modelling approach applied to the development of ultraprecision micro-milling machines

Ultraprecision machines with small footprints or micro-machines are highly desirable for micro-manufacturing high-precision micro-mechanical components. However, the development of the machines is still at the nascent stage by working on an individual machine basis and hence lacks generic scientific approach and design guidelines. Using computer models to predict the dynamic performance of ultraprecision machine tools can help manufacturers substantially reduce the lead time and cost of developing new machines. Furthermore, the machine dynamic performance depends not only upon the mechanical structure and components but also the control system and electronic drives. This paper proposed a holistic integrated dynamic design and modelling approach, which supports analysis and optimization of the overall machine dynamic performance at the early design stage. Based on the proposed approach the modelling and simulation process on a novel 5-axis bench-top ultraprecision micro-milling machine tool – UltraMill – is presented. The modelling and simulation cover the dynamics of the machine structure, moving components, control system and the machining process, and are used to predict the overall machine performance of two typical configurations. Preliminary machining trials have been carried out and provided the evidence of the approach being helpful to assure the machine performing right at the first setup.

Design of a five-axis ultra-precision micro-milling machine—UltraMill. Part 2: integrated dynamic modelling, design optimisation and analysis

Using computer models to predict the dynamic performance of ultra-precision machine tools can help manufacturers to substantially reduce the lead time and cost of developing new machines. However, the use of electronic drives on such machines is becoming widespread, the machine dynamic performance depending not only on the mechanical structure and components but also on the control system and electronic drives. Bench-top ultra-precision machine tools are highly desirable for the micro-manufacturing of high-accuracy micro-mechanical components. However, the development is still at the nascent stage and hence lacks standardised guidelines. Part 2 of this two-part paper proposes an integrated approach, which permits analysis and optimisation of the entire machine dynamic performance at the early design stage. Based on the proposed approach, the modelling and simulation process of a novel five-axis bench-top ultra-precision micro-milling machine tool—UltraMill—is presented. The modelling and simulation cover the dynamics of the machine structure, the moving components, the control system and the machining process and are used to predict the entire machine performance of two typical configurations.

New tool-workpiece setting up technology for micro-milling

A major issue in micromilling is the setting up with the necessary accuracy the work coordinate system on the machine. The spindle thermal stability during the setting up procedure make this a very difficult task. Thus, it is essential to develop new tool setting up technology to increase the process productivity, reduce machine setting up times and errors, and at the same time improve the precision and quality of machined components. The paper presents a cost-effective and reliable method for setting up the work coordinate system for micro-milling operations employing an on-line tool–workpiece voltage monitoring system. The viability of the proposed method was verified in a series of experiments conducted on an ultra-precision micromilling machine centre. The experimental trials involved the machining of test parts in brass with cutters 100 and 200 µm in diameter. The results of these experiments were analysed and compared with the capabilities of currently available methods and technologies on micro milling machine tools for condition monitoring and setting up the working coordinate systems. Finally, conclusions are made about the effectiveness of the proposed new tool-workpiece setting up technology for micro milling.

Novel biological/biohybrid prostheses for the ossicular chain: fabrication feasibility and preliminary functional characterization

Alternatives for ossicular replacements were fabricated in order to overcome persisting rejections in middle ear prosthetization. Unlike the synthetic prostheses in fashion, we propose biological and biohybrid replacements containing extra cellular matrix (ECM) molecules to improve biointegration. In this study, ECM-containing devices shaped as Partial Ossicular Replacement Prostheses (PORPs) were fabricated reproducing the current synthetic models. Biological PORPs were obtained from human decellularized cortical bone allografts by computer numerically controlled ultraprecision micromilling. Moreover, porous PORP-like scaffolds were produced and cultured with osteoinduced human mesenchymal stromal cells to generate in vitro bone ECM within the scaffold porosity (biohybrid PORPs). The acoustic responses of such devices were investigated and compared to those of commercial prostheses. Results showed that biological PORPs transmit mechanical signals with appropriate frequencies, amplitudes, and with early extinction time. Although signal transmission in biohybrid PORPs showed insufficient amplitude, we believe that tissue engineered constructs represent the new challenge in ossiculoplasty.

New Methods for Tool Failure Detection in Micromilling

A major issue in micromilling is unpredictable tool life and premature tool failure. The specific acoustic footprint and very small removal rates during machining as well as the use of small diameter cutters makes the detection of tool breakage a very difficult task. Thus, it is essential to develop new tool monitoring systems to increase the process productivity, reduce machining costs, and at the same time improve the precision and quality of machined components. This paper analyses three cost-effective and reliable methods for tool breakage detection: an on-line laser system, an on-line tool-workpiece voltage monitoring system, and a programming solution employing an off-line laser system. In order to verify the capability of these three methods, a series of experiments were conducted on an ultra-precision micromilling machine. This involved the machining of test parts in brass, aluminium, steel, and polymethylmethacryate (PMMA) with cutters from 0.5 mm down to 0.1 mm in diameter. The results of these tests are analysed and the technological capabilities of the three studied monitoring approaches are then compared. In addition, the possibility of developing an integrated solution that combines the capabilities of these three monitoring methods is discussed and conclusions made about the feasibility of such an approach.