Single Point Diamond Fly-cutting Research Articles

Formation Mechanism of Dkdp Surface in Single Point Diamond Fly-Cutting Process and the Resulting Degradation of Laser-Induced Damage Performance

Formation Mechanism of Dkdp Surface in Single Point Diamond Fly-Cutting Process and the Resulting Degradation of Laser-Induced Damage Performance

Interaction Mechanism of Thermal and Mechanical Field in KDP Fly-Cutting Process.

As an important nonlinear optical material, potassium dihydrogen phosphate (KDP) crystal is used in high-power laser beams as the core element of inertial confinement fusion. It is the most general method of single point diamond fly-cutting (SPDF) to produce high precision and crack-free KDP surfaces. Nevertheless, the cutting mechanism of such material remains unclear, and therefore needs further analysis. Firstly, the stress field, cutting force and cutting temperature under different working conditions are calculated by a KDP crystal cutting simulation model. Then, the rules and the cause of change and interaction mechanisms of force and temperature are analyzed by comparing the measurement experiments with simulations. Furthermore, the causes of chip formation and micro-cracks on the machined surface are analyzed based on thermo-mechanical coupling and chip morphology. The conclusion can be deduced: Although the temperature has not reached the phase transition temperature during the finishing process, under high cutting speeds and large unformed chip thickness, such as semi-finishing and roughing, the temperature can reach up to 180 °C or higher, and KDP crystals are very likely to phase transition—chip morphology also verifies this phenomenon.

CGHMM based fault identification for single point diamond flycutting machines

The ultra-precision single point diamond flycutting method is commonly used to produce fine optical surfaces. But the motion errors of machine tool are very weak, it is hard to achieve the error detection for single point diamond flycutting machines. To solve this problem, different measuring data such as vibrations and temperatures of different components were collected synchronously, and then we established the fault identification method based on hidden Markov model (HMM) to analyse the acquired measuring signals. The method includes signal analysis of raw data, fault recognition based on continuous Gaussian mixture density hidden Markov model (CGHMM) and information fusion. It combines the advantages of HMM and information fusion, to identify fault information accurately. Results show that the identification rates of the temperature signals to classification status by GRMS can reach 92% and vibration signals are little related to the surface quality of the workpieces classified by GRMS.

Study on thermal field in fly-cutting process of DKDP crystal

As an important nonlinear optical material, deuterated potassium dihydrogen phosphate (DKDP) crystal is used in laser beams as the core element of inertial confinement fusion. The most general method to produce smooth and crack-free DKDP surface is single point diamond fly-cutting (SPDF). However, the phase-changed temperature of DKDP is only about 120 °C which could be exceed in fly-cutting process. To analyze temperature field in cutting process, the formula based on the heat source method was introduced to calculate the temperature values on tool rake face under different cutting speed in DKDP crystal cutting process; then, thermo-mechanical FE analysis under the same process parameters was carried out to simulate the distribution of temperature field. The calculation and simulation results are basically in agreement, and the conclusion can be deduced: as the cutting speed increases, the temperature in the cutting zone increases, the highest temperature under common cutting parameter is up to 140 °C, which is validated by the dynamic infrared camera; however, the cutting temperature growth rate decreases with the same trend. The experiment results indicate that the chip morphology appears small spherical structure at high cutting speed, which illustrates the great possibility that the temperature exceed the phase transition temperature.

Investigation on the influence of aerostatic pressure upon surface generation in flycutting

Single-point diamond flycutting is an important technology for cutting flat KH2PO4 (potassium dihydrogen phosphate) crystals of large size. However, there always exist some undesirable waviness errors on the machined surface, which can directly reduce the optical performance of the potassium dihydrogen phosphate crystals. This article presents a kind of low-frequency waviness errors with wavelength about 26 mm along the feeding direction in single-point diamond flycutting, which has not been described yet. In order to find the main source of the mentioned waviness errors, the relationship between the displacement of the cutting tool and the aerostatic pressure was quantitatively studied for the first time. And then, surface simulation considering the aerostatic pressure fluctuations was carried out based on the relationship. Besides, a novel method that can achieve online submicron feeding along axial direction in single-point diamond flycutting without complex structure was proposed considering the spindle motion errors, the spindle dynamic characteristics and the aerostatic pressure. The experimental results validate that the mentioned waviness errors are mainly generated by the bolt stretched phenomenon and deformation of the big disk flycutting head due to the aerostatic pressure fluctuations. And the proposed method can achieve a cutting depth of about 120 nm when the aerostatic pressure increases from 0.52 to 0.56 MPa, which can reduce the cutting force and is beneficial for the performance of single-point diamond flycutting.

Investigation on the generation of the waviness errors along feed-direction on flycutting surfaces

Ultra-precision single-point diamond flycutting (SPDF) is an irreplaceable technology for cutting KH2PO4 (KDP) crystals with large size. However, some undesirable waviness errors along feed-direction exist on the machined surface, which could greatly deteriorate the optical performance of KDP crystals. The spindle dynamic characteristic of SPDF plays an important role in the formation of such waviness errors. This paper presents an on-line measurement method with five-capacitance displacement sensors in nanometer range for monitoring the motion errors of the aerostatic spindle. An error transform theory which is based on kinematic error propagation was introduced to obtain the three-dimensional trajectory of the cutting tool. Then, the spindle motion errors and the cutting tool trajectory were analyzed in frequency domain to investigate the main source of the waviness errors. Simultaneously, the three-dimensional topography of work-piece was simulated by combining the motion trajectory and the geometrical morphology of the cutting tool. The results showed that the waviness errors along feed-direction were caused by the spindle rotation fluctuation with a frequency about 10.8 Hz, and the topography simulations showed good agreement with the machining results. Besides, the experimental results indicated that the error can be diminished by changing the spindle control system to the digital control method. This study established the contact between the measured spindle motion errors and the machined surfaces, which contributes to deeper understanding of the dynamic characteristics of the spindle in SPDF.

Wafer-level hermetic thermo-compression bonding using electroplated gold sealing frame planarized by fly-cutting

In this paper, a novel wafer-level hermetic packaging technology for heterogeneous device integration is presented. Hermetic sealing is achieved by low-temperature thermo-compression bonding using electroplated Au micro-sealing frame planarized by single-point diamond fly-cutting. The proposed technology has significant advantages compared to other established processes in terms of integration of micro-structured wafer, vacuum encapsulation and electrical interconnection, which can be achieved at the same time. Furthermore, the technology is also achievable for a bonding frame width as narrow as 30 μm, giving it an advantage from a geometry perspective, and bonding temperatures as low as 300 °C, making it advantageous for temperature-sensitive devices. Outgassing in vacuum sealed cavities is studied and a cavity pressure below 500 Pa is achieved by introducing annealing steps prior to bonding. The pressure of the sealed cavity is measured by zero-balance method utilizing diaphragm-structured bonding test devices. The leak rate into the packages is determined by long-term sealed cavity pressure measurement for 1500 h to be less than Pa m3s−1. In addition, the bonding shear strength is also evaluated to be higher than 100 MPa.

Investigation on the generation of the medium-frequency waviness error in flycutting based on 3D surface topography

Undesirable medium-frequency waviness error along the cutting direction was often left on the machined surface of KDP crystals adopting ultraprecision single-point diamond flycutting (SPDF), reducing the optical performance of KDP crystals and restricting their capability in the inertial confinement fusion (ICF). In order to reduce such medium-frequency waviness error, an indirect method for investigating the generation of the error was proposed in this paper. The tool-tip vibration was indirectly extracted by combining the intermittent cutting trajectory of the flycutting process and the medium-frequency waviness error, which was extracted from the 3D surface topography using 2D wavelet transform method. The main frequency of the extracted tool-tip vibration was calculated and compared with the modals of the machine tool spindle system, which was obtained by FEM analysis and modal test. Finally, an improved spindle system was proposed based on the FEM analysis, and contrast tests between origin and improved machine tool were carried out. Results show that the tool-tip vibration can be effectively extracted by the proposed method, which deduces that the vibration that produced the medium-frequency waviness error is mainly generated by the sixth-order modal of the spindle system. In addition, the quality of the machined surface has been greatly improved with the improved spindle system.

An experimental and theoretical investigation on the brittle ductile transition and cutting force anisotropy in cutting KDP crystal

As a typical brittle material, Potassium Dihydrogen Phosphate (KDP) crystal exhibits anisotropic mechanical property during processing. The most general method to produce smooth and crack-free KDP surface is single point diamond fly-cutting (SPDF). In processing KDP by SPDF, cutting direction has great influence on the cutting force and the quality of the machined surface. Thus, selecting an optimal cutting direction is of great significance in decreasing the cutting force and improving the surface quality. In this paper, influence of cutting direction on the brittle ductile transition (BDT) depth and cutting force in machining KDP crystal has been investigated. Cutting experiments are carried out on the (001), Doubler and Tripler plane of KDP crystal to find out the change law of cutting force and BDT depth related to cutting direction. Theoretical models for calculating the cutting force and conditions for achieving crack-free surface in cutting by circular edge cutter have also been established. The predicted results coincide well with the experiment results, which have proved the validity of the proposed models. The experimental results in this study can provide guidelines for optimizing the processing parameters in fly-cutting of KDP crystal, and the theoretical models can be extended to study the cutting mechanism of other brittle materials.

Research on single-point diamond fly-grooving of brittle materials

The purpose of this paper is to investigate the machining mechanisms that accompany the single-point diamond fly-cutting operation in grooving of brittle materials. Single-point diamond fly-cutting is widely used in precision machining of free-form optics, semiconductor devices, and micro-electromechanical system (MEMS) components among many others. The undeformed chip zone was analyzed and its relation to the critical brittle/ductile transition depth of cut was discussed. Then, a mechanics-based model was proposed to describe the material stress condition under the diamond tool. The machining parameters were incorporated into the model to understand fly-cutting behavior. It was shown that the fly-cutting technique is highly suitable for the ductile removal of brittle materials by generating large compressive pressures in the chip formation zone. This condition can be further enhanced by a small feedrate and a large negative rake angle of the diamond tool used. The theoretical results were substantiated and verified by fly-grooving experiments performed on mono-crystalline silicon.

Research on compensation of suction deformation error of potassium dihydrogen phosphate crystal

Potassium dihydrogen phosphate (KDP) crystals used in high power laser systems require high figure accuracy. Deformation caused by vacuum suction is one of the most significant factors affecting the final surface figure accuracy. It is difficult to compensate the error caused by the suction deformation when using a single point diamond flycutting method. This paper presents a spiral turning method to machine KDP crystals on a common ultraprecision lathe, and the transfer rule and compensation theory of the suction deformation error are studied. The in situ measurement of deformation error is accomplished with a displacement test apparatus, which can acquire submicron precision, and then compensates for it by utilizing the three-axis servo control technology. Meanwhile, cutting parameters are strictly controlled to achieve full-aperture ductile material removal. The compensation experiments for both reflected and transmitted wavefront error are carried out on a Φ270 mm KDP crystal and the transmission wavefront error of 0.591λ (peak-to-valley, λ = 632.8 nm) and 25 nm/cm (gradient root-mean-square) are obtained after one compensation with full-aperture surface roughness of about 2 nm RMS.