Relationship between subsurface damage and surface roughness of optical materials in grinding and lapping processes

Ultrasonic-assisted grinding (UAG) has been extensively employed in manufacturing industries for processing hard and brittle materials. However, its potential has not been sufficiently developed because the material removal mechanism in UAG has not been elucidated. This paper focuses on the material removal mechanism in the UAG of silicon carbide (SiC) ceramics by investigating the material removal behaviors in ultrasonic-assisted scratching (UAS) of SiC ceramics. The UAS test was conducted on an NC (Numerical Control) surface grinder connected to a self-designed ultrasonic unit; a single diamond tool was fixed onto the end-face of the ultrasonic unit to achieve an ultrasonic vibration (UV). Conventional scratching (CS) test was also carried out on the same experimental rig but without UV for comparison. During testing, the material deformation/fracture behavior and the scratching forces were investigated. The experimental results show that (1) there are two scratching modes in the UAS process depending on actual depth of cut: an intermittent mode and a continuous mode; (2) the mean groove depth in UAS is much bigger than that in CS, indicating that the cutting ability of the tool was significantly improved by the assistance of the UV; (3) the critical depth of cut is increased by 56.25% once UV is applied; the stiffness of the experiment setup is improved in the UAS compared to CS; the UV in y- direction strongly contributes to the material removal, whereas the UV in z- direction only results in variation of the cutting trace and hardly contributes to the material removal in the UAS process; (4) in UAS, the scratching forces sinusoidally fluctuate with the same period as that of the UV of the tool when the material removal is in ductile mode, when the material removal is in brittle mode, the forces heavily vary but the period is different from that of the UV of the tool; (5) the cutting efficiency of the tool is improved by the assistance of the UV. The impact and cutting action at the tool tip on the machining surface are the main factors contributing to the material removal. The observed features were rationalized by analyzing kinematic characteristics of the tool and material removal mechanism in UAS. This study confirms that UAG is a highly effective processing method for machining hard and brittle materials.

Material removal behavior in ultrasonic-assisted scratching of SiC ceramics with a single diamond tool

Damage formation and suppression in rotary ultrasonic machining of hard and brittle materials: A critical review

Sapphire, widely used in high-speed integrated-circuit chips, thin-film substrates, and various electronic components, is regarded as one of the most difficult to cut materials owing to its great hardness and low fracture toughness. Ultrasonic vibration-assisted machining (UVAM) is an effective processing method for hard and brittle materials that has been explored in many experimental and theoretical investigations. To expose the material removal mechanism in the UVAM of sapphire, the deformation features of (0001) C-plane sapphire in ultrasonic vibration-assisted scratch (UVAS) tests were compared with those in traditional scratch tests. A self-designed and manufactured spherical diamond indenter was employed. The scratch loads (i.e., the normal forces and tangential forces) were measured with a Kistler dynamometer. The plastic and brittle deformation characteristics were observed by scanning electron microscopy. Raman spectroscopy was employed to reveal the residual stress features of deformation zones in the scratch grooves. The experimental results revealed that the UVAS process can reduce scratch loads, effectively inhibit microcrack propagation, and improve the plastic removal proportion, which was rationalized with elastic and residual stress field calculations. With increasing scratch depth, the distribution of residual stress changes from tensile stress to compressive stress as indicated by the Raman-peak shift.

Ultrasonic vibration-assisted scratch-induced characteristics of C-plane sapphire with a spherical indenter

Tool wear and surface finish in diamond cutting of optical glass

This paper discusses a technique for reducing subsurface damage and/or increasing material removal rate (MRR) in ceramics grinding. An indentation damage model shows that intermittent unloading can produce a lateral crack before the median crack fully develops. It shows that upon reloading, the lateral crack shields against further median crack penetration. Furthermore, intermittent unloading produces shielding even for oblique indentation events and intermediate locations of lateral cracks. Single-grit scratch tests provide experimental validation of the beneficial effects of intermittent unloading. A magnetostrictive actuator modulated the workpiece to create the intermittent unloading. Simulation studies predict that force per grit can be doubled under modulated conditions without an increase in normal damage in the finished workpiece. Our experimental observations show a 62% increase in MRR with minimal associated increase in depth of penetration of the median crack. Alternatively, the experimental results also show that modulations could be used to reduce the depth of median crack penetration by 24% at the same MRR. In addition, we observed that damage depth increases with an increase in the ratio of cutting speed to modulation frequency. Finally, both depth of cut modulation and cutting direction modulation were effective in reducing subsurface damage.

/pdf/using-vibration-assisted-grinding-to-reduce-subsurface-3x3jx3hfs1.pdf

Using vibration-assisted grinding to reduce subsurface damage

A simple stress based defect evolution model is developed to assess the influence of various process parameters on material removal rate (MRR) and induced damage during grinding of brittle materials. Model predictions for normal and lateral damage zones under normal indentations are first compared to fracture models as well as experimental observations on pyrex glass. Process design options for reducing induced damage in the finished part, and increasing MRR are considered next. In particular, the potential of a new design avenue involving intermittent unloading is investigated. Simulation results show that intermittent unloading can potentially facilitate increase in Force/Grit without increasing the associated surface and sub-surface fragmentation in the finished part. Preliminary experimental observations on single grit scratching of pyrex glass also show a similar trend.

Role of Unloading in Machining of Brittle Materials

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Using vibration-assisted grinding to reduce subsurface damage

Role of Unloading in Machining of Brittle Materials