Shape Adaptive Grinding Research Articles

Investigation on Surface Integrity of Nodular Cast Iron QT700-2 in Shape Adaptive Grinding

Nodular cast iron QT700-2 is extensively used in automobile engine crankshaft parts due to its prime mechanical properties. The journal of a crankshaft is a curved surface, and traditional wheel grinding easily causes grinding burn and surface and subsurface damage. Shape adaptive grinding (SAG) is a flexible grinding technology, which has the advantages of low grinding force and temperature, and good grinding quality. It is suitable for machining curved surface parts with complex shapes. Therefore, the SAG surface integrity of nodular cast iron QT700-2 was experimentally investigated. The influence of grinding parameters on grinding force, material removal rate, grinding temperature, and surface integrity was studied, and the machining performance of SAG tools was evaluated. It was concluded that the grain size in SAG is the most important factor affecting the grinding force, material removal rate, and surface roughness; the influence of SAG grinding is very weak, mainly removing the workpiece material. Then, the influence law of SAG technology on the surface integrity of nodular cast iron QT700-2 was summarized, and the optimal grinding parameters were obtained, providing a reference for the curved surface grinding of nodular cast iron QT700-2 in the future.

Hybrid tool combining stiff and elastic grinding

The fabrication process chain for optically smooth surfaces tends to include several time-consuming grinding and polishing steps. To reduce process time, a hybrid tool is proposed in which a stiff grinding tool and a shape adaptive grinding (SAG) tool are fused together, by taking advantage of the elastic nature of SAG tools. The material removal achieved by the hybrid tool is equivalent to discretely using the stiff grinding and SAG tools in sequence. Under similar processing conditions, a smooth surface of ∼0.02 μm Ra can be obtained on BK7 glass with the proposed tool, instead of ∼0.2 μm Ra with the stiff grinding tool.

Predictive model of the surface topography for compliant grinding of brittle materials

During uneven and time-dependent compliant grinding of brittle materials, the surface topography is difficult to predict as ductile and brittle regions are coupled due to compliance occurring in macro/micro tool-workpiece contact. This paper proposed a predictive model for surface topography prediction by considering its ductile-brittle transition in compliant grinding. Shape adaptive grinding and monocrystalline silicon were chosen as an example to validate the proposed model based on progressive grinding tests (spot, line, area). Feed-Spindle Projection Angles are further investigated, revealing that 0° angle can obtain a ground surface with lower area roughness and smaller brittle fragments than 45° and 90°

Theoretical and experimental investigation of material removal rate in shape adaptive grinding of HVOF sprayed WC-Co coating

Shape adaptive grinding (SAG) is conducted on the ground tungsten carbide-cobalt coating using diamond polishing pad. A material removal rate (MRR) model is proposed by considering the indentation of active abrasive particle, cross-sectional area and actual length of the finishing grooves. The effects of tool speed, tool compression, backing pad hardness and abrasive particle size of the polishing pads on the normal force, MRR and surface roughness are investigated. With an increase of the tool compression, the contact pressure increases and the surface peaks get sheared off at an enhanced rate that results in a higher MRR and lower surface roughness. On the other hand, SAG with harder tool (i.e., higher backing pad hardness) produces many deep finishing grooves owing to the aggressive mechanical interaction with the workpiece surface and that increases the surface roughness. Therefore, it is perceived that the harder SAG tool is not suitable as the final finishing step for precision finishing of cermet coating. The effect of tool speed on normal force is found to be insignificant as the contact area or pressure does not vary much with the tool speed. The polishing pad with lower particle size gets partially loaded at a higher tool compression and that impedes the reduction rate of surface roughness. By performing SAG with the polishing pads of reducing particle sizes in sequence, areal surface roughness (Sa) of 7 nm is attained. The residual stress of the finished coating is reduced to - 81 ± 7 MPa (compressive) which is around 29% of the ground surface. No abnormality is observed in the sub-surface of the finished coating.

Predictive topography model for shape adaptive grinding of metal matrix composites

Compliant finishing (e.g. shape adaptive grinding) can routinely achieve nanoscale roughness on diverse metal and ceramic materials. However, when processing multiphase materials consisting of distinct phases with different mechanical properties, non-uniform material removal often occurs and the inherent mechanism remains unclear. Here, a deterministic and stochastic model to predict processed surface topography is derived from the different material removal behaviours across phases. Experimental topographies, profiles and roughness on three selected Si-SiC samples show high consistency with theoretical predictions. Both model and experiments indicate that improved and stable surface finish can be achieved on metal matrix composites with small grain size.

Fabrication of mechanically durable slippery surface on HVOF sprayed WC-Co coating

The limitations of the existing art and the fragility of the developed hydrophobic surfaces limit its applications to the real field. Hence, the fabrication of slippery surface with the required durability has been considered to be an important academic and industrial challenge. The WC-Co coating possesses exceptional mechanical and tribological properties and is considered as one of the most durable surfaces available. The WC-Co coating with nanoscale surface roughness is in high demand in aerospace and automobile industries. In this study, shape adaptive grinding (SAG) is conducted to attain nanoscale surface roughness on hard WC-Co coatings. The static and dynamic wetting behaviour of as-sprayed, ground and nanofinished WC-Co coating are studied by analysing the static contact angle, sliding angle, and contact angle hysteresis (CAH). A model is proposed to predict the apparent contact angle of the coatings. The nanofinished coating exhibits a lower sliding angle and contact angle hysteresis and the water droplets get completely released from the surface in dynamic condition. This is attributed to the lower surface retention force and the presence of trapped air inside the porosities and the nano-well of the coating surface. The durability test of the nanofinished surface is also performed by abrasion test and sand impact test and it is observed that the surface can retain its original topographical features and wetting properties. This report presents a sustainable method for the fabrication of a mechanically durable slippery surface.

Investigation of elastic grinding pins

AbstractIn order to develop a grinding tool for shape adaptive grinding, elastic grinding pins are used as an example to investigate the behaviour of those tools. In the following, we describe the results for test tubes machined with elastic grinding pins. Therefore, the surface profile of the test tubes is analysed for different parameter sets. The varied parameters are the size of the grinding particles and the stiffness of the base material.The interdependencies between the parameters are as expected: larger particles and stiffer material lead to a rougher surface. This is the case for both acrylic glass and steel tubes. However, irregularities in the uniformity of the specimens surface indicate that the manufacturing process has still to be improved at different points.

Theoretical and experimental investigation of material removal mechanism in compliant shape adaptive grinding process

Modeling of grinding process, especially the grinding forces, has been studied extensively. Most previous work concerns the conventional grinding process using a rigid wheel, where the forces are generally based on the preset depth of grinding. However, for compliant grinding such as Shape Adaptive Grinding (SAG) that adopts an elastic tool covered with abrasive pellets, the penetration depth of individual abrasive grains is not simply equal to the preset value. As elastic contact occurs between the tool and workpiece, previous models are not able to estimate the material removal mechanism. Besides, the progressive transitions in grain-workpiece interaction for compliant cutting tools have not been reported yet. To fill these gaps and to offer a fundamental understanding of the compliant grinding process, we propose in this study a new multi-scale model spanning from macroscopic tool-workpiece contact to microscopic grain-workpiece interaction in the unique conditions of using a compliant grinding tool, i.e. SAG tool. Based on the spring-grain model and considering the stochastic nature of grain size and wear flat area, the static penetration and dynamic removal of individual grain can be predicted. Particularly, the removal mechanism is clarified with a new consideration of the transition between rubbing, plowing and cutting stages for each individual grain of the compliant tool. Experiments including scratch tests and removal footprints were carried out, and the high consistency with theoretical predictions validates the proposed model. Finally, considering the progressive abrasive wear in grinding, in-process wear compensation was conducted to enable consistent material removal on aspheric steel molds. The proposed method is not only meaningful to reveal the mechanism of SAG process, but also offers a new foundation for studying other compliant grinding processes in future.

An investigation into the wear mechanism of zirconia-alumina polishing pad under different environments in shape adaptive grinding of WC-Co coating

Shape adaptive grinding (SAG) is a relatively newer finishing process to achieve nanoscale surface roughness of difficult to machine materials. The wear mechanism of zirconia-alumina polishing pads is investigated after performing SAG and chemical assisted SAG (CA-SAG) for the finishing of WC-Co coating. The numbers of active abrasive particles in SAG increases due to the existence of the elastic medium and that leads to a smaller depth of penetration. This phenomenon along with the presence of tough zirconia phase in abrasive particles limits the possibility of the fracture wear. Smooth and elongated grooves along the finishing direction are observed on the wear flats which indicate that the plastic deformation is the predominant wear mechanism rather than micro-fracture. A phenomenological model for the attritious wear of the polishing pads is developed using the experimental findings. The radial pad wear and pad loading are analysed quantitatively using surface texture parameters. The wear mechanism in CA-SAG is immensely affected by the use of chemical reagent in the process. A substantial pad loading is observed owing to the deposition of the passivation layer in the inter-grit spaces. Thus, shelling of the abrasive particles and pad loading are identified as the dominant wear mechanisms.

Ultra-precision finishing of low expansion ceramics by compliant abrasive technologies: A comparative study

Advanced ceramics have many attractive features such as high stability and wear resistance that find broad applications in various fields, e.g. optics, aerospace, etc. However, the accompanying difficult-to-machine property with complex geometry brings great challenges to the commonly used laser machining and rigid wheel based grinding in industry. To achieve optical surface quality with surface roughness below 10 nm Ra, three promising ultra-precision compliant machining technologies using adaptive elastic tools are presented in this paper, including bonnet polishing, compliant pitch polishing and shape adaptive grinding with fine grain size. A comparative study was conducted by machining three different low thermal expansion ceramics while continuously increasing attack angle, spindle speed and tool offset across rectangular regions. Material removal rate (MRR) and surface roughness (Ra) with respect to different process conditions are compared. With sufficient data, the processing ability using above three compliant machining technologies is summarized based on the MRR-Ra plots for different ceramics. In addition, microscopic observation and X-ray diffraction analysis are conducted to characterize differences in material behavior.

Comprehensive study to evaluate the lifespan of flexible polishing pads by 3D surface characterization technique

The topography of a polishing pad plays a significant role to maintain the effectiveness of the finishing process and to preserve the work surface integrity. 3D surface characterization is an efficient tool to reveal the micro-geometry of the pad. A sequential finishing operation of WC-Co coating is performed by shape adaptive grinding (SAG), and chemical assisted SAG (CA-SAG) using zirconia–alumina polishing pads with mean particle sizes of 125 µm, 80 µm and 60 µm. Coherence correlation interferometry (CCI) is employed to investigate the topography of the polishing pads and to assess the microscopic wear and the lifespan of the pads. A new criterion for selecting the optimum sampling interval is proposed to ascertain the peak density precisely. A comprehensive study is undertaken to quantify and analyse the abrasive wear of the polishing pads after both SAG and CA-SAG by employing 13 advanced 3D surface texture parameters. The density and sharpness of the abrasive particles, the coarseness of the polishing pad, the area and volume of the individual motifs and the bearing and fluid retention properties of the pads are characterized. In the case of CA-SAG, the chemical reagent attacks the backing material of the pads and reduces its effective lifespan as compared to SAG.

High efficiency chemical assisted nanofinishing of HVOF sprayed WC-Co coating

Thermally sprayed coatings are widely used to improve surface hardness and wear resistance of components. However, surface roughness of the as-sprayed coatings is higher than the acceptable limit for many applications in aerospace, automobile and printing industries. In the present study, a multi-step finishing strategy is adopted to improve the surface finish of high velocity oxy-fuel spraying (HVOF) sprayed WC-Co coating. Shape adaptive grinding (SAG) is performed using a zirconia-alumina abrasive polishing pad which resulted in differential finishing of WC and Co binder due to their significant hardness difference. To overcome this situation, chemical assisted SAG is performed using Murakami's reagent. A thin passivation layer is formed on the top of coating due to chemical interaction which has hardness many times lesser than the bare coating and facilitates high and uniform material removal rate. X-ray photoelectron spectroscopy measurements confirm the presence of WO3 in the passivation layer. No fracture or WC fragmentation is observed on the surface processed using chemical assisted SAG. Areal surface roughness (Sa) is reduced from 5.04μm to 53nm using the above strategy.

Brittle-ductile transition in shape adaptive grinding (SAG) of SiC aspheric optics

Silicon carbide is a ceramic material with a desirable combination of high thermal and mechanical stability, making it ideal for optical application in aerospace and next generation lithography. It is however notoriously difficult to machine down to super-fine finish when the shape is other than flat or spherical. In this paper, we describe the application of a “semi-elastic” machining method called shape adaptive grinding (SAG), in which an elastic tool is combined with rigid pellets made of nickel or resin, to which super abrasives are bonded. A comprehensive model of the physical interaction between SAG tool and workpiece is proposed, and used to understand the mechanics driving brittle-ductile transition on ceramic materials such as SiC. Machining parameters adequate for optical finishing are then derived from the model and demonstrated on an aspheric silicon carbide workpiece, which was manufactured by reaction bonding and coated with a layer of pure SiC by chemical vapour deposition (CVD). Through SAG processing and final polishing, this aspheric mirror was improved from an initial form error of 40µm down to 112nm Peak-to-Valley, with no residual damage visible on the surface.

Finishing of additively manufactured titanium alloy by shape adaptive grinding (SAG)

In recent years, rapid prototyping of titanium alloy components for medical and aeronautics application has become viable thanks to advances in technologies such as electron beam melting (EBM) and selective laser sintering (SLS). However, for many applications the high surface roughness generated by additive manufacturing techniques demands a post-finishing operation to improve the surface quality prior to usage. In this paper, the novel shape adaptive grinding process has been applied to finishing titanium alloy (Ti6Al4V) additively manufactured by EBM and SLS. It is shown that the micro-structured surface layer resulting from the melting process can be removed, and the surface can then be smoothed down to less than 10 nm Ra (starting from 4–5 μm Ra) using only three different diamond grit sizes. This paper also demonstrates application of the technology to freeform shapes, and documents the dimensional accuracy of finished artifacts.

Process mechanism in shape adaptive grinding (SAG)

Shape adaptive grinding (SAG) is a novel process for freeform machining of difficult materials such as ceramics and hard metals. Despite low stiffness requirement from the machining equipment, due to the “semi-elasticity” of the process (achieved by a combination of elastic tool with rigid pellets), ductile mode grinding can be achieved with high surface finish. In this paper, the SAG process mechanism is investigated by studying how the size and shape of the pellet bonded abrasives evolve over time, measuring grinding forces for various process parameters, and comparing the equivalent chip thickness and specific energy with the observed grinding modes.

1010 Shape Adaptive Grinding (SAG) : a new Process for Finishing of Ceramics and High-performance Alloys

1010 Shape Adaptive Grinding (SAG) : a new Process for Finishing of Ceramics and High-performance Alloys

Shape adaptive grinding of CVD silicon carbide

Because of the direct relationship between removal rate and surface roughness in conventional grinding, ultra-precision finishing of hard coatings produced by chemical vapour deposition (CVD) usually involves several process steps with fixed and loose abrasives. In this paper, an innovative shape adaptive grinding (SAG) tool is introduced that allows finishing of CVD silicon carbide with roughness below 0.4nm Ra and high removal rates up-to 100mm3/min. The SAG tool elastically complies with freeform surfaces, while rigidity at small scales allows grinding to occur. Since material removal is time dependent, this process can improve form error iteratively through feed moderation.