Ultra-precision Raster Milling Research Articles

Numerical and experimental study on the optical performance of micro-pyramid functional surfaces

Micro-pyramid functional surfaces are currently widely used in various advanced scientific, technological, industrial, and engineering fields with their excellent performances of compactness, light weight and higher heat transfer performance. Many recent studies have introduced micro cutting as a method to achieve high-quality micro pyramid surfaces. This paper concerns a micro-pyramid with a height and slope of 60 μm and 30°. Firstly, numerical simulation of the optical effect related to micro-pyramid surfaces was carried out to reveal the mechanism of functional surface imaging and its typical and potential applications. The simulated optical effect shows that the micro-pyramid can form four virtual images on the same side of an object. Next, a novel and simple fabrication method for micro-pyramidal structures was developed by using cross cutting with a 120° V-tip diamond tool by ultra-precision raster milling (UPRM), which can obtain good surface quality, high form accuracy and efficient productivity in contrast to grinding, laser machining and etching. Finally, precision injection molding was employed to replicate a micro-pyramid on the PMMA for optical testing. Although the quality of molded micro pyramid pattern decreased, optical testing showed that clear images were obtained, and the experimental results were consistent with the simulation results.

A Design for a Manufacturing-Constrained Off-Axis Four-Mirror Reflective System

Off-axis reflective optical systems find wide applications in various industries, but the related manufacturing issues have not been well considered in the design process. This paper proposed a design method for cylindrical reflective systems considering manufacturing constraints to facilitate ultra-precision raster milling. An appropriate index to evaluate manufacturing constraints is established. The optimization solution is implemented for the objective function composed of primary aberration coefficients with weights and constraint conditions of the structural configuration by introducing the genetic algorithm. The four-mirror initial structure with a good imaging quality and a special structural configuration is then obtained. The method’s feasibility is validated by designing an off-axis four-mirror afocal system with an entrance pupil diameter of 170 mm, a field of view of 3° × 3° and a compression ratio of five times. All mirrors in the system are designed to be distributed along a cylinder.

Studying the effects of cutting parameters on burr formation and deformation of hierarchical micro-structures in ultra-precision raster milling

Hierarchical micro-structures are composed of a primary micro-structure and secondary micro V-grooves (SMVGs) cut by the UPRM one-step method. The key problem is the existence of burrs and plastic deformation of the top of SMVGs during the fly cutting process. This paper presents a study of the burr and deformation information introduced by one-step cutting operation in ultra-precision raster milling (UPRM). A series of experiments was carried out to investigate the effect of cutting conditions on the quality of hierarchical micro-structures by the one-step cutting method. A single point diamond tool with a 40° tool angle was employed to perform the one-step cutting operation in UPRM. Hierarchical micro-structures were produced on copper material successfully. Large burrs and plastic deformation were formed when cutting with a fixed tool angle. These large burrs and deformation were found to be induced by cutting force and cutting parameters. It was found that large burrs are formed with an increase of the cutting depth, feed rate, and spindle speed; the plastic deformations become serious with an increase of the cutting depth and feed rate, but are reduced with a decrease of spindle speed. An experimental study was established to investigate cutting force as a function of the cutting conditions in relation to micro-burr formation in machining, and explore the problems associated with one-step fly cutting in UPRM. The mechanisms of burrs and plastic deformation of SMVGs are discussed to further improve the machining process.

Surface integrity under diamond tool wear effects in ultra-precision raster milling of a Zn–Al–Cu alloy

In ultra-precision raster milling, the material removal process determines surface integrity. In this study, surface integrity was discussed under diamond tool wear effects in ultra-precision raster milling of a Zn–Al–Cu alloy. The results firstly showed that under high speed cutting in ultra-precision raster milling, quenching took place with phase decomposition (namely twin phase changes) with a deformation thickness of even less than 100 nm. Flank wear enhanced phase changes, promoted surface hardening, degraded surface quality, and increased deformation thickness, but crater wear gave better surface integrity. The intrinsic reason is that flank wear caused more external stress but crater wear was reverse, well supported by finite element simulation in orthogonal cutting. Significantly, it provides a further insight into diamond tool wear effects on surface integrity in ultra-precision raster milling of a Zn–Al–Cu alloy.

A generalized dynamic model for spindle vibration influencing surface topography in different ultra-precision machining processes

Ultra-precision machining (UPM) typically involves ultra-precision diamond turning (UPDT), ultra-precision raster milling (UPRM), and ultra-precision grinding (UPG). In UPM, spindle vibration (SV) is a natural feature majorly influencing surface topography. In this paper, a dynamic model for SV was generalized to study its distinctive effects on surface topography in different UPM processes. The theoretical results were identified well by a series of cutting tests. In UPDT, constant cutting forces induce harmonic SV to produce regular patterns. In UPRM, periodical impulsive cutting forces cause periodical impulsive SV to generate quasi-regular patterns. In UPG, random impulsive cutting forces result in partially random impulsive SV to form partially random patterns. The generalized model can be employed to predict the effects of SV on surface generation in UPM.

Study of materials effects on the single cutting in ultra-precision raster milling

Ultra-precision raster milling (UPRM) rotates a single crystal diamond tool to discontinuously remove materials from the workpiece and can directly produce freeform surfaces with sub-micrometric form accuracy and nanometric surface finishing. This paper presents theoretical and experimental investigations on the workpiece material effect on single cutting profile in UPRM. Experiments are conducted to study the material effect on cutting profile by single cutting four materials including copper, aluminium alloy 6061, brass and aluminium bronze under the same cutting conditions in UPRM. A characterisation method is presented in the paper to study the effect of different workpiece material on the cutting profiles in UPRM. The experimental results show that copper alloy makes the smallest effect on the single cutting profile because of the lowest elastic recovery ratio while the material effect of aluminium bronze is the highest due to the highest elastic recovery ratio and the highest hardness.

Study of materials effects on the single cutting in ultra-precision raster milling

Study of materials effects on the single cutting in ultra-precision raster milling

Effects of cutting speed on phase changes in ultra-precision raster milling of Zn–Al alloy

Ultra-precision raster milling induces external stress to cause phase changes of Zn–Al alloy at a thin surface layer, which further degrade surface integrity of high-precision components. This study focuses on discussing phase changes at the surfaces of the alloy after ultra-precision raster milling under the high cutting speeds of 680 m/min, 1120 m/min, and 1360 m/min. Along with the penetration depth at the machined surfaces, phase changes and surface hardening rapidly declined to vanish with crystal orientation shift back to its standard Bragg angle. As the cutting speeds increased, phase changes, crystal orientation shift, and surface hardening obviously decreased with phase change thicknesses from 323 nm, 241 nm to 87 nm. It is worth noticing that at a high cutting speed phase changes at the machined surface can be greatly reduced to a significant degree.

Modelling and prediction of the effect of cutting strategy on surface generation in ultra-precision raster milling

This paper studies the effect of cutting strategy on surface generation in ultra-precision raster milling (UPRM). By adding the influences of shift length and tool-interference on surface generation, a holistic surface roughness prediction model is built which takes into account the effect of cutting parameters, tool path generation, geometry parameters of diamond tool, the size of the workpiece and machine characteristics. The optimal shift ratio can be achieved by changing factors involved in developing cutting strategy to improve surface quality without decreasing machining efficiency. Conditions for the presence of tool-interference in UPRM are presented. Based on the holistic surface generation model, an integrated system is developed to automatically generate the numerical control program, and predict surface quality and machining efficiency. A series of cutting experiments has been conducted to verify the proposed surface generation model and test the performance of the integrated system. The experimental results agree well with the predicted results from the model and the integrated system.

Characteristics of phase changes induced by ultra-precision raster milling at the surface layer of Zn-Al alloy

Ultra-precision raster milling induces phase changes and crystal orientation changes of Zn-Al alloy at its machined surface up to a thickness of several hundreds of nanometers. In this study, the phase change characteristics of Zn-Al alloy at different penetration depths have been discussed using X-ray diffraction and nano-indentation tests. A phase-change-distribution function to depth has been proposed to calculate phase change thickness based on the X-ray diffraction measurement, which was verified by the nano-indentation tests. It is found that phase changes sharply decrease along the penetration depth and the Bragg angle first decreases and then increases back, less than the original value. Significantly, the proposed method is nondestructive to characterize phase change characteristics and measure phase change thickness.

Deformation-induced phase changes of Zn-Al alloy during ultra-precision raster milling

Metal cutting is a complex material removal process with elastic deformation (ED) and plastic deformation (PD) together taking place in chip formation and surface generation. PD consists of shear PD (SPD) and normal PD (NPD) intrinsically relevant to cutting mechanism, such as pure cutting and ploughing. In ultra-precision machining, PD induces phase changes of Zn-Al alloy once, i.e., phase decomposition. In this study, the distinctive effects of SPD and NPD in surface generation on phase changes of Zn-Al alloy in ultra-precision raster milling (UPRM) with horizontal cutting under up-cutting mode were firstly discussed. At the machined surfaces of Zn-Al alloy, NPD and SPD both results in phase changes, but only NPD changes preferred crystal orientation; and PD under high-speed cutting induces phase changes twice, i.e., four-phase transformation occurs after phase decomposition, namely twin phase changes, which were firstly observed.

A Novel Tool Wear Measurement Method in Ultra-Precision Raster Milling

Tool wear measurement has drawn a significant of attention in the past decades. However, no research has been found on the investigation of tool wear measurement in ultra-precision raster milling (UPRM) process since it is a relative complex cutting process. In the present study, tool wear characteristics were identified by using cutting chip morphologies and a groove cutting. Tool wear investigation using cutting chips is effective because diamond tool wear characteristics can be directly imprinted on the cutting chip surface. Through the inspection of chip surfaces, the profile and location of the tool fracture can be identified. Also, through the groove cutting, the cutting edge retreat due to the tool flank wear can be identified. In this research, a mathematical model was established to calculate the tool retreat. The experimental result shows that the proposed tool wear investigation method is an effective method.

Wetting characteristics of bare micro-patterned cyclic olefin copolymer surfaces fabricated by ultra-precision raster milling

A micro-directional grooved cyclic olefin copolymer surface fabricated by ultra-precision raster milling gives a good sliding performance for a water droplet.

Dynamic Behavior of Ultra Precision Feed Axis at Increased Velocities

The processing speed of machine tools for ultra precision manufacturing are low compared to precision manufacturing to satisfy the precision requirements. To increase the productivity and develop machines for ultra precision high performance cutting (UP-HPC) it is essential to examine and compensate dynamic effects. In this work the positioning behavior of an ultra precision feed axis at different setpoint velocities is analyzed and compared to a high speed feed axis. It is observed that the ultra precision feed axis has a large overshoot of several micrometers if the velocities are increased. Further, the decay time of the ultra precision feed axis exceeds the decay time of the high speed feed axis by more than factor two. These two characteristics increase the production time, because of long tool paths and many reversal points, which are often required to keep the kinematic roughness low, especially in ultra precision raster milling.

A study of cutting factors affecting the generation of functional hierarchical rib array structure in ultra-precision raster milling

Hierarchical ribs can be used to keep balance between heat transfer enhancement and pressure drop reduction. The unique hierarchical structure which features a first-order asymmetric arc rib imposed by second-order microgrooves greatly reduces the pressure drop as well as enhances the heat transfer. However, it is hard for conventional machining techniques to generate such complicated configurations, especially freeform geometries in the micron range. Taking advantage of ultra-precision raster milling (UPRM), a novel one-step machining process, for advanced hierarchical ribs is proposed and demonstrated. In this paper, the principle of the cutting strategy for the second-order microstructures and cutting conditions that affect the basic features of the structures are discussed. In addition, three-level factorial design of cutting speeds, feed rates, and depth of cuts were set and used to machine hierarchical ribs on a copper workpiece. It was found that the cutting strategy was affecting the geometry of second-order microstructures of hierarchical ribs in UPRM. Moreover, the result revealed that at low levels of feed rate and cutting depth, a middle level of cutting speed provides a better quality of second-order microstructures than a middle level of feed rate, high level of cutting depth, and low level and high level of cutting speed. Maintaining a balance between machining efficiency and hierarchical structure generation quality is thoroughly assessed by machining second-order microgrooves on asymmetric arc primary surfaces.

Effect of cutting parameters on heat generation in ultra-precision milling of aluminum alloy 6061

This paper aims to study the cutting-induced heat generation on the machined surface in ultra-precision raster milling (UPRM) by using the time-precipitates-temperature characteristics of aluminum alloy 6061 (al6061). The influences of cutting parameters including depth of cut, spindle speed, and cutting feed rate on heat generation in UPRM are investigated. Isothermal heat treatment is used to build the relationship between the heating time, heating temperature, and average size of the generated precipitates for al6061 alloy. Single cutting tests are conducted to study heat distribution in one revolution during raster milling process, and surface milling is employed to study the influences of cutting parameters on the heat generation and temperature rise on the ultra-precision milled surface. The experimental results show that the increases of spindle speed and feed rate generate more heat during raster milling process while the decrease of depth of cut firstly decreases then increases the temperature rise on the machined al6061. The scratch marks due to the cutting-induced precipitates are dependent on the cutting parameters.

A novel surface quality evaluation method in ultra-precision raster milling using cutting chips

Although online surface quality evaluation is important in ultra-precision machining technologies, very little research has been reported on the topic. In this paper, cutting chips were employed to evaluate machined surface quality online under the occurrence of tool wear in ultra-precision raster milling. Cutting chips were collected and examined by a 3D scanning electron microscope. The inspected cross-sectional shape of ‘ridges’ imprinted by the tool fracture was approximated into two geometric elements so that it could be determined by a mathematical model. The geometric elements for every ridge were assembled into a virtual cutting edge so as to rebuild the machined surface. A mathematical model was established to realize the rebuilt machined surface and calculate the surface roughness under the consideration of tool fractures effects. The theoretical and experimental results show that the ridges imprinted on the machined surface can be predicted by examining a section of the collected cutting chips. It is interesting to note that the quality of the machined surface is reflected in the location and the height of ridges. This demonstrates that the online surface quality evaluation using cutting chips is a novel method as compared to conventional methods, since it can evaluate the machined surface and simulate the machined surface topography without the need to stop the cutting process.

Numerical and experimental analysis of heat transfer in turbulent flow channels with two-dimensional ribs

The repeated ribs surfaces are known for their effective enhancement of heat transfer, which is widely demanded in many scientific and industrial applications. In this paper, further improvements were made by the introduction of a textured asymmetric arc rib structure, on which arrays of secondary micro grooves are superimposed onto a primary asymmetric surface. Numerical simulation has been carried out on the turbulent force convection flow in a single-phase channel with two-dimension (2D) ribbed internal surface in the range of Reynolds number from 20,000 to 60,000. A uniform heat flux is applied on the external surface of channel. In contrast to the conventional optimized symmetric triangular rib, the advanced compound rib could improve the performance of heat transfer while minimizes the pressure drop. These improvements are closely correlated to the promotion of the separated flow which could reattach the channel surface. Since the geometry of the novel compound rib is difficult to fabricate by conventional means, ultra-precision raster milling (UPRM) is used to generate these micro-structures. According to prediction of numerical simulation, the experimental cooling works have been designed and conducted for the considered shaped ribs with the significant dimensionless pitch of 5 and 6. A good agreement was found between the simulation and experimental results.

Development of Cutting Strategy in Ultra-Precision Raster Milling of Freeform Surface

This paper studies the development of cutting strategy in the fabrication of freeform surface in ultra-precision raster milling (UPRM). The tasks of developing cutting strategy in freeform machining involve in the selection of cutting parameters and the planning of tool paths. An integrated system is built in this study to develop the cutting strategy, automatically generate NC program, simulate the tool paths and machining process, as well as make predictions for the machining time and the surface quality of the raster milled freeform surface. Experiment is conducted to verify the developed system and the experimental results show that the system is applicable for the machining of freeform surface in UPRM. This study therefore contributes to avoiding the need to conduct expensive and time consuming trial cutting tests to ensure the product quality in the freeform machining.

An integrated optimization of cutting parameters and tool path generation in ultraprecision raster milling

This paper provides a new methodology for the integrated optimization of cutting parameters and tool path generation (TPG) based on the development of prediction models for surface roughness and machining time in ultraprecision raster milling (UPRM). The proposed methodology simultaneously optimizes the cutting feed rate, the path interval, and the entry distance in the feed direction to achieve the best surface quality in a given machining time. Cutting tests are designed to verify the integrated optimization methodology. The experimental results show that, in the fabrication of plane surface, the changing of entry distance improves surface finish about 40 nm (Ra) and 200 nm (Rt) in vertical cutting and decreases about 8 nm (Ra) and 35 nm (Rt) in horizontal cutting with less than 2 s spending extra machining time. The optimal shift ratio decreases surface roughness about 7 nm (Ra) and 26 nm (Rt) in the fabrication of cylinder surfaces, while the total machining time only increases 2.5 s. This infers that the integrated optimization methodology contributes to improve surface quality without decreasing the machining efficiency in ultraprecision milling process.