Waveguides in lithium niobate fabricated by focused ultrashort laser pulses

We report on investigations of the bulk microstructuring of lithium niobate crystals with intense femtosecond laser pulses. In different crystal cuts, optical waveguides were produced whose properties depend strongly on the processing parameters. To explore the origin of the refractive index changes, we subjected the crystals to different conditions (like temperature, illumination, etc.) while monitoring the waveguide output. This way several mechanisms for the change in refractive index could be singled out. These include the photorefractive effect, inhomogeneous ion concentrations and stress in the crystalline lattice. As an application, we demonstrate frequency doubling of 1064 nm laser radiation in a microstructured phase-matched waveguide.     

Femtosecond laser machining of electrospun membranes

We demonstrate that a femtosecond laser can be used to machine arbitrary patterns and pattern arrays into free-standing electrospun polycaprolactone (PCL) membranes. We also examine the influence of various laser irradiation settings on the final microstructure of electrospun membranes. A beam fluence of 0.6 J/cm² is used to ablate holes in 100 μm thick PCL membranes. The machined holes have an average diameter of 436 μm and a center-to-center spacing of 1000 μm. Based on these results, the femtosecond ablation of electrospun membranes shows great potential for fabricating a variety of functional tissue scaffolds. This technique will advance scaffold design by providing the ability to rapidly tailor surface morphology, while minimizing and controlling the deformation of the electrospun fibers.     

黑腔芯轴平台微细电火花加工技术

采用电火花成型加工技术,在黑腔芯轴侧表面加工平台。采用白光干涉仪对平台表面轮廓及粗糙度测量,结果表明:平面部分表面粗糙度小于0.5μm,最大峰谷高度小于15μm。通过奥林巴斯测力显微镜对平台尺寸测量,结果表明:平台的轴向尺寸加工精度可控制在±10μm,同一电极加工的平台尺寸一致性可控制在±2μm。分析了电极损耗对零件形状精度的影响规律以及平台表面粗糙度的影响因素,并通过负极性加工去除电极损耗对平台尺寸精度的影响。     

Rotary ultrasonic machining of CFRP: A mechanistic predictive model for cutting force

Cutting force is one of the most important output variables in rotary ultrasonic machining (RUM) of carbon fiber reinforced plastic (CFRP) composites. Many experimental investigations on cutting force in RUM of CFRP have been reported. However, in the literature, there are no cutting force models for RUM of CFRP. This paper develops a mechanistic predictive model for cutting force in RUM of CFRP. The material removal mechanism of CFRP in RUM has been analyzed first. The model is based on the assumption that brittle fracture is the dominant mode of material removal. CFRP micromechanical analysis has been conducted to represent CFRP as an equivalent homogeneous material to obtain the mechanical properties of CFRP from its components. Based on this model, relationships between input variables (including ultrasonic vibration amplitude, tool rotation speed, feedrate, abrasive size, and abrasive concentration) and cutting force can be predicted. The relationships between input variables and important intermediate variables (indentation depth, effective contact time, and maximum impact force of single abrasive grain) have been investigated to explain predicted trends of cutting force. Experiments are conducted to verify the model, and experimental results agree well with predicted trends from this model.     

Preliminary study on rotary ultrasonic machining of CFRP/Ti stacks

Reported drilling methods for CFRP/Ti stacks include twist drilling, end milling, core grinding, and their derived methods. The literature does not have any report on drilling of CFRP/Ti stacks using rotary ultrasonic machining (RUM). This paper, for the first time, reports a study on drilling of CFRP/Ti stacks using RUM. It also compares results on drilling of CFRP/Ti stacks using RUM with reported results on drilling of CFRP/Ti stacks using other methods. When drilling CFRP/Ti stacks using RUM, cutting force, torque, and CFRP surface roughness were lower, hole size variation was smaller, CFRP groove depth was smaller, tool life was longer, and there was no obvious Ti exit burr and CFRP entrance delamination. Ti surface roughness when drilling of CFRP/Ti stacks using RUM was about the same as those when using other methods.     

Discussions on on-machine measurement of aspheric lens-mold surface

At present, regarding to the machining of aspheric lens mold, there are two major methods to carry out the on-machine measurement (OMM) — contacting method (CM) and non-contacting method (NCM). Here such two methods are reviewed in detail. CM is mainly based on the contacting probe which is scratching aspheric surface of lens mold to achieve profile data. To be efficient, an idea with 45° tilt of probe is proposed for OMM of lens mold by Suzuki. But generally speaking, the contacting OMM is not so efficient and can only deal with axisymmetric aspheric lens mold. On the contrary, NCM mostly uses laser to achieve aspheric profile without any contact. On ultra-precision lathe, laser scanning system or laser interferometer is mounted on the frame of lathe and transfers measurement data to machining system efficiently. However, most NCMs need stable environment and low working noise except instantaneous phase-shifting shearing interferometry (IPSSI). Therefore, a new idea about IPSSI is proposed in this paper to realize OMM of lens molds. Unfortunately, it's also difficult to test the high numerical aperture aspheric or free-form lens molds. By comparison, the newly-developed fringe reflection (FR) method is becoming the promising method because it features the high efficiency and high accuracy. However, this method has not been used for OMM system yet. Much research should be conducted for FR OMM technology.     

Vibration reduction in ultrasonic machine to external and tuned excitation forces

The dynamic response of mechanical and civil structures subject to high-amplitude vibration is often dangerous and undesirable. Sometimes controlled vibration is desirable as in ultrasonic machining (USM). Ultrasonic machining (USM) is the removal of material by the abrading action of grit-loaded liquid slurry circulating between the workpiece and a tool vibrating perpendicular to the workface at a frequency above the audible range. A high-frequency power source activates a stack of magnetostrictive material, which produces a low-amplitude vibration of the toolholder. This motion is transmitted under light pressure to the slurry, which abrades the workpiece into a conjugate image of the tool form. This can be achieved via passive and active control methods. In this paper, multi-tool techniques are used in the ultrasonic machining via reducing the vibration in the tool holder and providing reasonable amplitudes for the tools represented by the absorbers. The coupling of the tool holder and absorbers simulating ultrasonic cutting process are investigated. This leads to a multi-degree-of-freedom system subject to external and tuned excitation forces. Multiple scale perturbation method is applied to obtain the solution up to the second order approximation. Different resonance cases are reported and studied numerically. The stability of the system is investigated applying both phase-plane and frequency response techniques. The effects of the different parameters of the absorbers on the system behavior are studied numerically. Comparison with the available published work is reported.     

Figure Measurement of Concave Mirror by Fiber-Grating Hartmann Test

A new type of Hartmann test was developed using an optical fiber grating instead of the traditional Hartmann screen with multiple holes. A fiber-grating is made of short optical fibers arranged in single layers with no gaps between them. On illumination with laser light, the fiber-grating generates fan-like diverging multiple rays of almost equivalent intensities. Two gratings are overlapped at right angles to make multiple beams diverging in two directions. Rays reflected from test mirror surfaces converge to make a point focus provinding the sulface is free from aberrations. A CCD camera detects a spot pattern of beams near but out of focus, and a personal computer analyzes the spot patterns and calculates the figure error from a ideal surface. A few concave mirrors were analyzed by this method and the results compared with those obtained with a Fizeau interferometer. The data using the two systems were consistent with each other to within 1/10A.     

High Precision Diamond Machining of Hybrid Sol-Gel Coatings

Flexible and economic production of complex reflective optical elements is achieved by high-precision machining of aluminum and copper with diamond tools. There is also an increasing demand for complex refractive optical elements like micro lens arrays, Fresnel lenses or prismatic surfaces on silicon wafers or metallic surfaces. For the production of these optical elements, hybrid sol-gel coatings based on methacryloxypropyltrimethoxysilane (MATMS) and zirconium (IV) tetra n-propoxide (ZTP) were deposited on aluminum substrates by spin-coating. The influence of the rotational velocity and the chemical sol composition on the coating thickness was determined. The hardness and elastic modulus of these coatings was measured as a function of the chemical composition. The machining characteristics of these coatings were investigated by high precision turning and fly cutting with diamond tools of different geometry. The resulting surface finish obtained was determined as a function of the machining parameters.     

Effects of cutting depth on the surface microstructure of a Zn-Al alloy during ultra-precision machining

The microstructural changes and phase decomposition at the surface of an ultra-precision machined Zn-Al based alloy were studied using optical microscopy, back-scattered electron microscopy, electron back-scattered diffraction and X-ray diffraction techniques. It was found that with increasing DoC the phase decomposition was accelerated and further increasing DoC might cause a microstructural recovery on the surface of the UPM specimen. UPM induced phase decomposition was discussed in relation to the structural evolution of the chips.