The distribution and characteristics of surface cracking (i.e. sub-surface damage or SSD) formed during standard grinding processes has been investigated on fused silica glass. The SSD distributions of the ground surfaces were determined by: (1) creating a shallow (18-108 {micro}m) wedge/taper on the surface by magneto-rheological finishing; (2) exposing the SSD by HF acid etching; and (3) performing image analysis of the observed cracks from optical micrographs taken along the surface taper. The observed surface cracks are characterized as near-surface lateral and deeper trailing indent type fractures (i.e., chatter marks). The SSD depth distributions are typically described by a single exponential distribution followed by an asymptotic cutoff in depth (c{sub max}). The length of the trailing indent is strongly correlated with a given process. Using established fracture indentation relationships, it is shown that only a small fraction of the abrasive particles are being mechanically loaded and causing fracture, and it is likely the larger particles in the abrasive particle size distribution that bear the higher loads. The SSD depth was observed to increase with load and with a small amount of larger contaminant particles. Using a simple brittle fracture model for grinding, the SSD depth distribution has been related tomore » the SSD length distribution to gain insight into ''effective'' size distribution of particles participating in the fracture. Both the average crack length and the surface roughness were found to scale linearly with the maximum SSD depth (c{sub max}). These relationships can serve as useful rules-of-thumb for nondestructively estimating SSD depth and to identify the process that caused the SSD. In certain applications such as high intensity lasers, SSD on the glass optics can serve as a reservoir for minute amounts of impurities that absorb the high intensity laser light and lead to subsequent laser-induced surface damage. Hence a more scientific understanding of SSD formation can provide a means to establish recipes to fabricate SSD-free, laser damage resistant optical surfaces.« less

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Sub-surface mechanical damage distributions during grinding of fused silica

YAG single crystals are the primary host materials for solid-state lasers at multi-kW scale and must be processed using ultra-precision grinding to achieve a satisfactory dimensional precision and surface integrity. However, the deformation mechanism of YAG crystals is not well understood, which has thus hindered the development of high efficiency grinding technology for the crystals. In this work, precision grinding of YAG single crystals was investigated. Ductile-like surfaces that are free of cracks and brittle-ductile surfaces that consist of fractured spots and ductile striations were found after grinding. The deformation mechanisms associated with the two types of surfaces were explored with the aid of transmission electron microscopy (TEM). The results indicated that the deformation involved in the formation of the ductile-like surface was mainly caused by the slippage of (0 0 1) crystal planes, along with the formation of dislocations and stacking faults and the distortion of atomic planes. The brittle-ductile surfaces were generated by the plastic deformation due to the formation of nanocrystals and nanovoids, combined with brittle fracture caused by the crack propagation initiated at intersections of slip lines. A theoretical model was developed to predict the grinding force in the ductile-like grinding process, which has taken the combined effect of strain rate, random distribution of abrasive radii and elastic-to-plastic transition depth into account for the first time. The key model parameters were obtained using a genetic algorithm trained using the experimental force data. The modelled force agrees well with the measured. This model enabled an in-depth understanding of the deformation mechanism of a crystal solid involved in ultraprecision grinding and the effect of strain rate on its material removal.

Deformation mechanism and force modelling of the grinding of YAG single crystals

Traditional fluid jet polishing (FJP) is limited by its low material removal rate and its applicability to medium-large size surfaces. This paper presents a novel multi-jet polishing (MJP) process and tools based on FJP which can implement high-efficiency polishing on large-scale surfaces or lens array surfaces. The MJP makes use of a purposely designed nozzle which possesses many regularly distributed holes, whose number can be a few to several hundred. Moreover, each hole can spray out a high-energy fluid jet leading to a dramatic increase of material removal. Its feasibility is firstly analyzed through a Computational Fluid Dynamics (CFD) simulation. Hence, its surface generation mechanisms in the integrated polishing mode and discrete polishing mode are studied. After that, a series of polishing experiments on different materials are conducted to validate its polishing performance as compared to single jet polishing (SJP). The experimental results show that the MJP tool can realize a much higher material removal rate, together with compatible surface roughness to SJP. Hence, the MJP tool has the potential to implement high-efficiency polishing on medium-large size surfaces and lens array surfaces.

A novel multi-jet polishing process and tool for high-efficiency polishing

Compliant grinding and polishing refers to a class of fine material processing methods relying on one or more system element being compliant with the workpiece surface in a controllable and reversible manner, thus being distinct from conventional rigid wheel grinding and polishing. The resulting surface adaptability greatly facilitates the ultra-precision machining of freeform surfaces, with a level of accuracy and productivity difficult to achieve with conventional rigid processing systems. Aiming for a comprehensive review of signal advances on this topic in the last decades, this paper introduces a multi-level compliance categorization system to sort between distinctive processes. For each category, various perspectives are explored that pertain to principles, methodologies, and physical properties such as material removal and surface integrity. Finally, relative merits are analyzed, together with a discussion of the outlook and future trends.

Compliant grinding and polishing: A review

Novel rotating-vibrating magnetic abrasive polishing method for double-layered internal surface finishing

Theoretical modelling and analysis of the material removal characteristics in fluid jet polishing

Multi-scale modeling and simulation of material removal characteristics in computer-controlled bonnet polishing

Fluid Jet Polishing (FJP) has the advantages of localized force and cooling of the debris, and of a stable and controllable material removal function with very little tool wear. Due to the complex machining mechanism, it is still difficult to model the material removal characteristics and simulate the surface generation for FJP. In the present study, an attempt has been made to better understand the material removal and surface generation mechanisms in the FJP process. Hence, a theoretical model is built for predicting and characterizing the material removal characteristics and surface generation in FJP based on computational fluid dynamic modelling. A series of spot and pattern polishing tests, as well as simulation experiments by the theoretical model, were conducted. The results show that the theoretical model satisfactorily predicts the surface generation under different polishing conditions and enables a better understanding of the polishing process in FJP.

Modelling and characterization of surface generation in fluid jet polishing

A theoretical and experimental investigation of material removal characteristics and surface generation in bonnet polishing

High-precision inner surfaces are difficult to be machined to a sufficiently high surface quality. This paper presents the development of a novel multi-jet polishing process for precision polishing of inner surfaces through adopting a rod-shaped nozzle designed with a linear array of orifices at its side face. The material removal characteristic on inner cylindrical surface was modelled based on computational fluid dynamic method. Four groups of material removal experiments were conducted to validate the proposed material removal model and investigate its material removal characteristics. Moreover, the surface generation model was also developed and validated based on the material removal model. A series of polishing experiments were conducted on 304 stainless steel cylindrical inner surfaces. The results show that the proposed multi-jet polishing process with the newly designed nozzle is able to achieve high efficiency and precision inner surface finishing on the inner surface.

Zhong-Chen Cao

Papers

Theoretical modelling and analysis of the material removal characteristics in fluid jet polishing

Multi-scale modeling and simulation of material removal characteristics in computer-controlled bonnet polishing

Modelling and characterization of surface generation in fluid jet polishing

A theoretical and experimental investigation of material removal characteristics and surface generation in bonnet polishing

Development of a multi-jet polishing process for inner surface finishing