Diamond Turning Machine Research Articles

Determinant of Dynamics and Interfacial Forces in Ultraprecision Machining of Optical Freeform Surface through Simulation-Based Analysis.

This study delves into the intricacies of ultraprecision machining, particularly in the context of machining optical freeform surfaces using Diamond Turning Machines (DTMs). It underscores the dynamic relationship between toolpath generation, hydrostatic bearing in DTMs, and the machining process. Central to this research is the innovative introduction of Metal Matrix Composites (MMCs) to replace the traditional materials used in designing linear bearings. This strategic substitution aims to dynamically enhance both the accuracy and the quality of the machined optical freeform surfaces. The study employs simulation-based analysis using ADAMS to investigate the interfacial cutting forces at the tooltip and workpiece surface and their impacts on the machining process. Through simulations of STS mode ultraprecision machining, the interfacial cutting forces and their relationship with changes in surface curvatures are examined. The results demonstrate that the use of MMC material leads to a significant reduction in toolpath pressure, highlighting the potential benefits of employing lightweight materials in improving the dynamic performance of the system. Additionally, the analysis of slideway joints reveals the direct influence of interfacial cutting forces on the linear slideways, emphasising the importance of understanding and controlling these forces for achieving higher-precision positioning and motion control. The comparative analysis between steel and MMC materials provides valuable insights into the effects of material properties on the system's dynamic performance. These findings contribute to the existing body of knowledge and suggest a potential shift towards more advanced precision forms, possibly extending to pico-engineering in future systems. Ultimately, this research establishes a new standard in the field, emphasising the importance of system dynamics and interfacial forces in the evolution of precision manufacturing technologies.

In-Situ Measurement and Slow-Tool-Servo Compensation Method of Roundness Error of a Precision Mandrel.

This paper describes a method for measuring and compensating the roundness error of a larger mandrel manufactured by an ultra-precision diamond-turning lathe aimed to obtain a calibration cylinder with a roundness of less than 0.1 μm. The diamond-turning machine has a cross-stacked hydrostatic guideway, produces a cutting depth and feed movement direction, and a dual-spindle system that is firmly connected to the bed. Due to the good repeatability of aerostatic spindles, only in situ rather than online real-time measurements are required. To this end, three high-precision capacitance displacement sensors were utilized to detect the cross-section of the workpiece and the time domain via the three-point error separation technique to separate the roundness error from the rotation motion error. The slow tool servo (STS) cutting technique was employed to compensate for the roundness error, which did not require extra axes, only the excellent dynamic response of the feed axis; hence, the servo control parameters could be suitably adjusted. The experimental results reveal that the low-order harmonic error, often caused by aerostatic spindles, is almost removed completely. For this particular lathe, the experiments indicate that about 60% of the rotational error motion is compensated, and the roundness error is reduced to less than 0.1 μm, which is evaluated by the least-squares circle method.

Real-time Thermal Error Compensation Strategy for Precision Machine tools

Abstract Present manufacturing trend is towards producing precision components with better accuracy. Machine errors like geometrical, thermal and process errors affect the component accuracy. Among these errors, thermal error contributes more than 50-60% of the total machining error. This paper mainly focuses on the development of a real-time thermal error compensation module for precision machine tools and talks about effective modeling of thermal errors, development of thermal error compensation model using feed-forward backpropagation neural network and also simplified model using regression analysis technique, algorithm development for real-time compensation and implementation of module onto the open architecture CNC controller. The developed module has been successfully tested on a Diamond Turning Machine (DTM) by machining the precision component and also verified the effectiveness of the module

Mitigation of damage during surface finishing of sapphire using laser-assisted machining

Machining of hard, crystalline ceramics is time-consuming, expensive and vital in many industries, but getting good results is challenging because of the surface damage it can cause. For sapphire machining damage can be especially detrimental because it can create significant stresses in the surface and sub-surface regions. Worst case, the stresses are relieved by crack propagation, fracture, and chipping at the surface. Conventional machining with a single point diamond-turning machine was compared to the same method with a laser simultaneously focused through a diamond stylus. The laser makes ductile machining more favorable and provides in situ annealing of damage. Micro-Raman spectroscopy was used to quantify residual stresses and optical microscopy was used to identify cracking. A reduction in the residual stresses was achieved when using the laser. This is seen most dramatically at higher laser powers, where fracture, cracking and chipping can be alleviated.

Fabrication of freeform progressive addition lenses using a self-developed long stroke fast tool servo

A long stroke fast tool servo (LFTS) system with a maximum stroke range of ±4 mm based on the voice coil actuator was developed and integrated on a diamond turning machine (DTM) for the fabrication of freeform progressive addition lenses (PALs). The LFTS utilized a three-ring cascade control system for the motion trajectory control of a diamond tool. The numerical control codes of the tool path were transferred from the 3D surface data with freeform features. The complete process chain from tool path generation and diamond machining to power measurement was experimentally demonstrated. The obtained surface form accuracy was 6.7 μm (rms), and the measured power of the PAL agreed well with the theoretical design indicating the desirable processing capability of LFTS for the customized freeform spectacle optics.

In situ measurement and error compensation of optical freeform surfaces based on a two DOF fast tool servo

Freeform optics are widely used in many fields with its excellent optical performances. Fast tool servo (FTS) diamond turning technology are considered to be very promising technique for freeform surface fabrication. However, some elements may influence the machining accuracy, such as the spindle speed drift, periodical change cutting force, and diamond turning machine (DTM) motion errors. In this paper, an in situ measurement method and an error compensation machining technique are proposed to improve FTS technique machining accuracy. The machined surface profile data is obtained by a linear variable differential transformer (LVDT) and the error compensation machine is implemented by a two degree of freedom (DOF) FTS. The experiment results show that the form error is dropped by 20 % and surface roughness of 18.1 %. This technology is easy to take out that contributes to the application of high precision freeform surfaces in aerospace, military, biochemical medical, communication, and many other fields.

Determining issues in optimal turning of micro-structured functional surfaces

Micro-structured surfaces have received ever-increasing requirements in various fields, including optical systems, biological engineering, measurement system, and so on. Fast tool servo (FTS)-assisted ultra-precision diamond turning technique has been extensively regarded to be a very promising technology for the fabrication of the micro-structured functional surfaces with high efficiency and high surface quality. In this paper, the machining flowchart is thoroughly introduced; the key cutting parameters including tool geometry, diamond turning machine (DTM) slide carriage feedrate, and spindle speed are carefully investigated and determined to obtain optimal cutting performances during machining processes. Additionally, a novel toolpath generation method is proposed to minimize the volume of the toolpath data. Comparing with current method, the data volume generated by the new method can be significantly decreased by several times. Finally, the efficiency of the proposed cutting parameter determination method and the new toolpath generation method are verified by fabricating a typical sinusoidal grid surface.

LED 빔조형에 의한 초소형 이미징 장치의 제조 기술

본 연구는 초소형 광학 시스템을 구현할 수 있는 설계 및 제작의 방법론을 제시하고자 한다. 초소형 광학계에서는 조명 및 결상 광학소자의 성능과 소형화가 조광면의 균질도와 결상 된 이미지의 선명도에 중요한 영향을 미치게 된다. 본 연구에서는 얇은 두께로 실효 광도를 배가시키기 위한 초박형의 LED 빔조형 렌즈를 설계 제작하였다. 상기 렌즈는 중앙부의 비구면렌즈와 외각의 전반사 프레넬 가장자리 부로 구성되어 있다. 설계된 LED 빔조형 렌즈(직경 4.7 mm, 두께 0.6 mm)는 다이아몬드 선삭으로 중앙 비구면부의 전반사(TIR) 가장자리가 정밀하게 가공되었으며, LED 의 빔각을 150 도에서 17.5 도로 축소 시켰다. 다른 광학소자들 마이크로 프리즘, 결상광학용 프레넬 렌즈, 광가이드는 다양한 마이크로 나노 크기의 제조공정으로 일체형으로 성형되었다. 시작품으로 제작된 초소형 광학계(6.8 × 2.2 × 2.5 mm)는 마이크로 패턴을 결상의 가능성을 보여주었고, 지문인식용 초소형광학계로서의 성능을 검증하였다.<BR>

Thermal and Structural Deformations During Diamond Turning of Rotationally Symmetric Structured Surfaces

Diamond turning of microstructures on the surface of large cylindrical workpieces has become important with advances made in roll-to-roll manufacturing processes of optical films, drag reduction films, microfluidic devices, and organic electronic components. Micromachined cylindrical workpieces are used as production masters in various printing, embossing, and coating processes. The microstructures machined in this study were 18μm in height and had a pitch of 35μm. These dimensions required control of the location of the single crystal diamond cutting tool that was used for machining to submicrometer levels. The significant error sources identified in the machining process were thermal effects and deflections of the structural loop of the diamond turning machine (DTM) that led to registration errors of the cutting tool between consecutive passes. Environmental temperature variation errors (ETVEs) were measured and modeled as a function of long-term ambient temperature fluctuations. Also studied was the mechanical compliance of the structural loop of the DTM. The height adjustable tool stack and aerostatic spindle were identified as the most compliant components. The cutting forces for radius and V-shaped diamond cutting tools at various depths of cut were measured using the known compliance of the aerostatic bearing to predict workpiece deflections.

Design and testing of a long-range, precision fast tool servo system for diamond turning

Abstract A long-range, precision fast tool servo (FTS) system was developed that is capable of accurately translating the cutting tool on a diamond turning machine (DTM) with maximum accelerations of 260 m s−2 and bandwidths of up to 140 Hz. The maximum displacement range of the cutting tool is 2 mm. The FTS utilizes a flexure mechanism driven by a voice coil actuator, a custom linear current amplifier and a laser interferometer feedback system. This paper describes the design of the electromechanical system, controller configuration and cutting tests to evaluate the system. Initially, low disturbance rejection and poor command following degraded the surface finish of machined test parts. Several techniques to add damping to the dynamic system were investigated to improve the generated surface finishes. Electromotive damping was applied inside the voice coil actuator, and two different viscoelastic damping materials were applied to the flexure mechanism. A control strategy consisting of linear and non-linear feedforward controllers and a proportional, integral and derivative (PID) feedback controller was implemented to accommodate the changed system dynamics. The workpieces were analyzed using form and surface inspection instruments to evaluate the overall system performance. A cylindrical part with five lobes cut across the face had a surface finish value between 20 and 30 nm Ra.

경통고정식 자동무수차점 널 렌즈 테스트법을 이용한 직경 300 mm(f/1.98) 알루미늄 쌍곡면 거울의 형상측정

초정밀 비구면 기공기로 제작한 구경 300 mm이고 f-수가 1.98인 알루미늄 쌍곡면 거울의 형상을 측정하기 위하여 자동무수차점 널(null) 렌즈 광학계를 설계하고 제작하였으며, 이를 이용하여 이 대구경 쌍곡면 거울의 형상을 측정하였다. 이때 널 렌즈간 정렬은 구조적으로 안정하고 부피가 작은 경통고정식을 선택하였고, 널 렌즈 및 경통의 제작 허용오차는 요구되는 측정 정밀도를 만족하도록 공차분석기법을 통하여 계산하고 제작하였다. We design and fabricate an autostigmatic null lens system assembled in a fixed tube mount in order to evaluate the shape of an aluminum hyperbolic mirror with the diameter of 300 mm and the f-number of 1.98, which is fabricated by a high precision aspherical DTM (diamond turning machine). Also, we evaluate the degree of shape of the aspherical mirror by this autostigmatic null lens testing method. The autostigmatic null lens system assembling in a fixed tube mount has several advantages of light weight, good mechanical stability, etc. The permissible fabricating limits of null lenses and a mount are determined by considering various tolerances to assure the measurement reliability.

Development of a compression molding process for three-dimensional tailored free-form glass optics

Because of the limitation of manufacturing capability, free-form glass optics cannot be produced in a large volume using traditional processes such as grinding, lapping, and polishing. Very recently compression molding of glass optics became a viable manufacturing process for the high-volume production of precision glass optical components. An ultraprecision diamond-turning machine retrofitted with a fast tool servo was used to fabricate a free-form optical mold on a nickel-plated surface. A nonuniform rational B-spline trajectory generator was developed to calculate the computer numerical control machine tool path. A specially formulated glass with low transition temperature (Tg) was used, since the nickel alloy mold cannot withstand the high temperatures required for regular optical glasses. We describe the details of this process, from optical surface geometry, mold making, molding experiment, to lens measurement.

Design and fabrication of a freeform phase plate for high-order ocular aberration correction

In recent years it has become possible to measure and in some instances to correct the high-order aberrations of human eyes. We have investigated the correction of wavefront error of human eyes by using phase plates designed to compensate for that error. The wavefront aberrations of the four eyes of two subjects were experimentally determined, and compensating phase plates were machined with an ultraprecision diamond-turning machine equipped with four independent axes. A slow-tool servo freeform trajectory was developed for the machine tool path. The machined phase-correction plates were measured and compared with the original design values to validate the process. The position of the phase-plate relative to the pupil is discussed. The practical utility of this mode of aberration correction was investigated with visual acuity testing. The results are consistent with the potential benefit of aberration correction but also underscore the critical positioning requirements of this mode of aberration correction. This process is described in detail from optical measurements, through machining process design and development, to final results.

IR optics advance - today's single-point diamond-turning machines can produce toroidal, cylindrical, and spiral lenses

IR optics advance - today's single-point diamond-turning machines can produce toroidal, cylindrical, and spiral lenses

레이저 스폿의 칼날주사 방법에 의한 복사계 개구 면적의 정밀측정

검출기의 분광감응도 표준으로부터 출발하여 SI 기본 단위의 하나인 칸델라(candela)눈금의 불확도는 복사계 개구의 면적 측정 불확도에 의해서 제한을 받는다. 칸델라 눈금을 새로 실현하기 위하여 다이아몬드 선삭가공기를 사용하여 개구의 가장자리가 칼날과 같도록 가공하였다. 이 개구를 디지털 되먹임 알고리듬을 채용한 x-y 이송대에 설치한 후, 출력이 안정화된 레이저 광선을 빔 허리가 5 <TEX>$\mu\textrm{m}$</TEX> 이하가 되도록 집속시키고, 이송대를 이동하면서 개구의 가장자리 좌표를 칼날주사 방법으로 검출하였다. 이 좌표들을 지나는 최적의 타원을 최소 제곱법으로 결정하여 개구의 면적으로 구하였다. 측정 불확도를 분석한 결과 제작한 장치의 면적 측정의 상대 측정 표준 불확도(k=1)는 8<TEX>${\times}$</TEX><TEX>$10^{-5}$</TEX>이다. The uncertainty of the detector-based candela scale is limited by the area measurement uncertainty of radiometric apertures. The apertures were fabricated with a diamond-turning machine which trimmed the edge of the apertures as sharply as a knife edge. The positions of the apertures were controlled by a digital feedback algorithm to scan the laser spot with the beam waist less than 5 <TEX>${\mu}{\textrm}{m}$</TEX>. The knife edge scan yielded a set of coordinates on the edges of the aperture. The areas of the apertures were obtained by fitting the coordinates to the ellipses. The relative standard uncertainty of the measurement was estimated to be 8<TEX>${\times}$</TEX>10<TEX>$^{-5}$</TEX>.

Implementation of a fast tool servo with repetitive control for diamond turning

The integration of a fast tool servo (FTS) with piezoelectric actuation on a diamond turning machine (DTM) is presented. The intent of this research is to monitor improvements in surface quality in the areas of surface roughness and waviness observed in the facing of an aluminum work piece. Performance is compared between the utilization of a polynomial controller with position feedback of the tool, a repetitive controller that uses position feedback of the tool in conjunction with position feedback of the spindle face via and eddy current probe, and legacy performance of the DTM with no active control methodology.

Ductile and brittle modes in single-point-diamond-turning of silicon probed by Raman scattering

In recent years, considerable progress has been made on the study of the machinability of fragile materials as crystalline semiconductors because of the demand for faster fabrication processes of complex surface shapes for optoelectronic applications. The most-studied machined semiconductors are silicon and germanium [1– 3]. Some studies indicate that ductility could be related to the high-pressure metallization (brittle-to-ductile) transformation that occurs when these materials are subjected to high hydrostatic pressures [4–7]. Because of this, most of the studies reported previously are devoted mainly to microindented or microcut Si and Ge. Among the experimental techniques used to probe their effects, Raman scattering has been successfully employed, mainly to exploit the presence of residual stresses around the indentations and grooves made by indenters [8]. It is well known that, due to the machining (or to the polishing or lapping) process, semiconductor surfaces can undergo structural damage [9]. Raman scattering is a powerful characterization technique in these cases because the vibrational spectrum of the material is greatly influenced by disorder and residual strains: These lead to changes in phonon frequencies, broadening of Raman peaks and breakdown of Raman selection rules [10]. For bulk crystalline Si (c-Si), the triple degenerate optical phonons display in the first-order Raman spectrum one sharp peak at 521 cm−1. Due to the positive phonon deformation potentials of Si, compressive (tensile) strains lead to positive (or negative) frequency shifts. On the other hand, due to the loss of phonon correlation length and the consequent breakdown of the q = 0 Raman selection rules (q is the phonon wave vector), disorder effects can lead to an asymmetric broadening and shifting of Raman peaks compatible to the dispersion relation of the material [11]. In the silicon case, the frequency and asymmetry point to lower values because the dispersion relationship presents decreasing optical frequencies, increasing phonon wave vectors. At maximum disorder (amorphous material, aSi), the first-order Raman spectrum reflects the phonon density of states: It presents two broad bands centered at about 100 cm−1 (acoustic band) and 470 cm−1 (optical band) [12]. In this letter, an original (macro-) Raman investigation of single-point-diamond-turned silicon samples machined in ductile and brittle modes is presented. To probe the depth profile of disorder effects, the 457.9, 488.0 and 514.5 nm lines of an argon ion laser were used as exciting light. For these lines, the penetration depth of the light is about 140, 270 and 340 nm for c-Si, respectively. For a-Si, the optical absorption coefficient can reach one order of magnitude higher, leading to penetration depths of about tenths of nanometers, depending on the degree of amorphization [13, 14]. All measurements were performed at room temperature, with special care taken taken to avoid overheating the samples. Cutting tests were performed on Si samples on the (1 0 0) surface. The samples were single-pointdiamond-turned using a facing operation on a RankPneumo ASG 2500 diamond-turning machine. Alkalisol 900 coolant cutting fluid was continuously sprayed onto the workpiece during machining to avoid overheating the sample. A 0 (−25) degree rake angle diamond tool with nose radius R= 1.52 mm and a clearance angle of 12◦ was used in all tests. The feed rate used was 12.5 (1.25) μm rev−1 and the nominal depth of cut was 10 (1)μm. These conditions provide brittle (ductile) mode during machining, with an opaque (mirrorlike) surface. The finished surfaces were observed using scanning electron microscopy (SEM). Fig. 1 shows a

A high performance embedded machine tool controller

A Diamond Turning Machine (DTM) can fabricate components with extremely high precision and high quality surface finish characteristics. Existing commercial controllers are limited in their ability to support high performance machining and related advanced control algorithms. Evolving enhancements to the functional capability and control algorithms of a DTM require increased performance and flexibility from the computer system controlling it. To provide this capability the Precision Engineering Center at North Carolina State University has developed and implemented a high performance control system for such a machine tool. The control system is based on a heterogeneous hierarchical multiprocessor computer architecture. This provides the capacity to apply advanced control concepts to the DTM, resulting in improved precision and quality of machined parts. Additionally, the ability to machine non-rotationally symmetric components has been made possible by application of multiprocessor capability.

Application of a fast tool servo for diamond turning of nonrotationally symmetric surfaces

The fabrication of nonrotationally symmetric surfaces by diamond turning requires tool actuation at a bandwidth significantly higher than the rotational frequency of the surfaces. This requirement cannot be met by standard slide drives due to their large mass and consequent low natural frequency. This articles describes the development of a laboratory-scale diamond-turning machine with piezoelectric-driven fast tool servo. The capability of this apparatus will be demonstrated for high-speed features such as sine wave, square wave, and ramp-shaped surfaces. Also described is the implementation of this fast tool servo on a commercial diamond-turning machine. Several nonrotationally symmetric surfaces have been machined, and their images are included.

Finite Element Studies Of Tangent Mounted Conical Optics

This paper presents experimental and analytical results from a study investigating the effect of centrifugal force and gravity on two candidate mirror fixture designs to be used on a diamond-turning ma-chine. The authors illustrate and discuss the use of the finite element method as an aid in the design and fabrication of high precision metallic optical components.