Ductile Mode Grinding Research Articles

Critical cutting thickness model considering subsurface damage of zirconia grinding and friction–wear performance evaluation applied in simulated oral environment

Ductile mode removal is always a popular topic in zirconia ceramic cutting and grinding. The completely undamaged ductile mode grinding is extremely inefficient and difficult to control, which limits the clinical application of dental zirconia ceramics. Hence, considering the effect of subsurface damage, this paper redefines the ductile mode grinding removal of ceramic materials and establishes a critical cutting thickness model for zirconia ceramic grinding. Results show that the better the lubrication condition, the smaller the subsurface damage degree and the larger the removal range of ductile zone. To verify the model, grinding experiments of zirconia ceramic under different lubrication conditions are conducted. The results show that grinding under nanofluid minimum quantity lubrication obtains the lowest friction coefficient, which is reduced by 58.54 % and 18 % compared with dry grinding and minimum quantity lubrication (MQL), respectively. Under nanofluid minimum quantity lubrication (NMQL) condition, the critical cutting thickness is 2.27 µm, which is 160.92 % larger than that under dry grinding. The error between the theoretical model and the experimental results is 10.67 %, which verifies the correctness of the theoretical model. Furthermore, using the workpiece material obtained from grinding experiments, friction–wear experiments are conducted on dental zirconia ceramics under different loads in a simulated oral environment. The results show that zirconia ceramics obtained by NMQL grinding achieve better anti-friction and anti-wear performance than that of MQL. When the maximum cutting thickness of zirconia ceramics processed by NMQL grinding is 2.27 µm, the wear rate of the workpiece is 3.06 × 10−5 mm3/N·m, and at 2.6418 µm, the wear rate of the workpiece is 2.01 × 10−3 mm3/N·m. This paper aims to provide theoretical guidance and technical support for the ductile mode removal of hard, brittle materials.

Beneficial stress of a coating on ductile-mode cutting of single-crystal brittle material

Innovative techniques have been proposed to overcome the challenge of the strong tendency of brittle materials to crack during machining. One of them is the use of an epoxy coating to serve as a crack formation restraint. However, this only serves to achieve ductile-mode grinding along the uncut shoulders. Therefore, this study evaluates the effect of a layer of epoxy resin on the machined surface perpendicular to the micro-cutting direction of a brittle material, single-crystal calcium fluoride. An increase in the ductile–brittle transition was observed in micro-cutting experiments on the (111) plane of calcium fluoride with the use of a 4-μm thick epoxy resin coating, ranging from 167–347 nm to 213–476 nm for the 0° rake angled tool. A similar increase was also observed with the +5° rake angle, ranging from 91–233 nm to 187–310 nm. An energy-based ductile–brittle transition predictive model is introduced, which characterises the anisotropic behaviour of the single crystal and incorporates the additional stress induced during cutting with a coating as a fraction of the coating hardness. The analytical model accurately predicts a consistent range of improvement from 69–325 nm to 257–485 nm for the 0° rake angle and from 62–285 nm to 215–460 nm for the +5° rake angle. The ductile-mode cutting energy increases to preserve its dominance over brittle-mode cutting and delays the onset of brittle-mode activation. The validity of the model extends the understanding of a surface coating as a restraining technology to include the beneficial stress acting in the deformation zone during cutting.

Thrust Force Directional Vibration-Assisted Cutting Based on a Removal Process of Ductile-Mode Grinding

Our research introduced a 40kHz thrust force directional vibration-assisted ductile-mode grinding of hard and brittle materials. And, the vibration-assisted grinding was adopted in order to avoid brittle fracture on the ground surface. But, the cause of this result could not be proved, because it was difficult to investigate removal process by many and various abrasive grains. Therefore, this study describes an evaluation of ductile-brittle transition with thrust force directional vibration-assisted cutting. Because the movement of cutting tool edge is similar to that of a grain on grinding wheel with the vibration. Then, cutting tool is oscillated by Fast Tool Servo actuator, and the frequency 1.00 kHz and amplitude 1.33 µm. In this cutting process, some intermittent cutting marks occur in the sub-micron depth. The rake angle of diamond tool is -35 and -65 degrees. Workpiece is synthetic silica glass. As an experimental result, the vibration-assisted cutting of the silica glass has a larger ductile-mode surface at removal depth of cut 0.09-0.15μm. And, it guessed that this increase of ductile-mode surface is affected by plastic flow of the workpiece under the tool, and ease of cut into workpiece by movement of cutting tool.

Mechanisms of ductile mode machining for AlON ceramics

This paper aims to reveal the mechanisms of ductile mode machining for AlON ceramics. The removal characteristics during machining were studied through ultra-precision grinding experiments. The machined surface consists of fractured and smooth areas, which were generated by brittle and ductile removal, respectively, of the individual AlON grains. The material removal mode has a determining effect on the surface/subsurface quality. The proportion of the fractured areas on the ground surface decreased gradually with a decrease in the depth of cut. The crystal indices of the grains most prone to brittle removal on the workpiece surface were determined using micro-area X-ray diffraction (μXRD) analysis performed using a beam with 50 μm diameter. The results showed that the ductile removal of the {111} planes is critical for the ductile mode machining of AlON. Nanoindentation tests based on electron back-scattered diffraction (EBSD) indicated that AlON shows strong anisotropy in its mechanical properties and machinability. The (111) plane has the highest hardness and lowest fracture toughness, at 22.91 GPa and 1.8 MPa m1/2, respectively. The material removal mechanism during the grinding of AlON was discussed in detail. The minimum and maximum dc(hkl) values must be known for classifying whether the removal mode of the workpiece is brittle or ductile. A damage-free surface could be obtained during ductile mode grinding by controlling hmax to be less than dc(111). The subsurface deformation mechanism during ductile mode grinding was analysed. An amorphous layer was observed close to the ground surface. Further, dense dislocations with no particular orientation were present beneath this amorphous layer. As the crystal structure became clearer with an increase in the depth, the plastic deformation shifted to stacking faults parallel to the {111} planes.

Electrical Discharge Machining and Grinding of SiC Single Crystal Using a Disc-type Hybrid Tool of Polycrystalline Diamond

Silicon carbide (SiC) single crystal was machined using a disc-type hybrid tool of polycrystalline diamond (PCD). The tool can be used to perform electrical discharge machining (EDM) as a roughing process and grinding as a finishing process. The width of the discharge gap and the thickness of the resolidified layer caused by EDM were measured, the results of which were then used to optimize the subsequent grinding conditions. As a result, ductile mode grinding was realized by removing the EDM-induced resolidified layer completely. In addition, a low surface roughness was obtained after grinding by utilizing the electrical discharges in EDM to dress the PCD tool surface. Finally, high surface integrity and productivity were successfully sustained by optimizing the EDMgrinding cycles.

Ductile-brittle transition behavior in the ultrasonic vibration-assisted internal grinding of silicon carbide ceramics

Silicon carbide (SiC) ceramics play a key role in various engineering applications due to their desirable properties, however, they are typical hard-brittle materials and famous for their poor machinability. Ultrasonic-assisted grinding, a processing method hybridizing the conventional grinding and ultrasonic vibration (UV) machining, is employed as the one applicable machining method for hard-brittle materials. This study focuses on internal grinding of SiC ceramics with the assistance of the UV, and its ductile-brittle transition behavior in the grinding process were investigated experimentally. Following the processing principal of internal grinding and UV machining (i.e., UV-assisted internal grinding, UVAIG), the UVAIG experiment rig were constructed. Conventional internal grinding (CIG) tests, i.e., internal grinding without UV, were also performed on the constructed rig for comparison. In addition, the grinding force and the grinding chips variation behavior with the UV amplitude were also investigated to explore the ductile-brittle transition mechanism in UVAIG. The experiment results evidence that (1) ductile mode grinding is easily achieved in UVAIG, and the critical depth of the cut is deeper in UVAIG than that in CIG, i.e., 0.072 μm in CIG and 0.093 μm in UVAIG; (2) increasing of the critical depth of cut in UVAIG is likely owing to the lower grinding force in UVAIG than that in CIG; and (3) the ultrasonic vibration acted on the axis of the grinding wheel helps in the removal of material on the work-surface, but decreases the grinding energy in the ductile-brittle transition.

Ductile mode grinding of reaction-bonded silicon carbide mirrors.

The demand for reaction-bonded silicon carbide (RB-SiC) mirrors has escalated recently with the rapid development of space optical remote sensors used in astronomy or Earth observation. However, RB-SiC is difficult to machine due to its high hardness. This study intends to perform ductile mode grinding to RB-SiC, which produces superior surface integrity and fewer subsurface damages, thus minimizing the workload of subsequent lapping and polishing. For this purpose, a modified theoretical model for grain depth of cut of grinding wheels is presented, which correlates various processing parameters and the material characteristics (i.e., elastic module) of a wheel's bonding matrix and workpiece. Ductile mode grinding can be achieved as the grain depth of cut of wheels decreases to be less than the critical cut depth of workpieces. The theoretical model gives a roadmap to optimize the grinding parameters for ductile mode grinding of RB-SiC and other ultra-hard brittle materials. Its feasibility was validated by experiments. With the optimized grinding parameters for RB-SiC, the ductile mode grinding produced highly specular surfaces (with roughness of ∼2.2-2.8 nm Ra), which means the material removal mechanism of RB-SiC is dominated by plastic deformation rather than brittle fracture. Contrast experiments were also conducted on fused silica, using the same grinding parameters; this produced only very rough surfaces, which further validated the feasibility of the proposed model.

Analysis of critical process parameters of ductile mode grinding of brittle materials

Abstract The process of ductile mode grinding has been analyzed experimentally. Besides the traditional approach, controlling the depth of a cut on feed-controlled ultra-precision machines (UPMs), two alternative approaches have been tested. By applying fluid jet polishing (FJP) using a grinding slurry, a stable ductile mode grinding could be set up. Subsequently, loose abrasive grinding processes on load-controlled traditional spindle machines have been tested to be suited for ductile mode grinding identifying the dependence of crack initiation on the type of slurry fluid being applied. In that way, substantially improved levels of surface roughness could be achieved, although local brittle cracking within the generated clear apertures still remained. Furthermore, critical process parameters were identified determining the process window of feed-controlled ductile grinding applied on state-of-the-art UPM machineries. These have been analyzed experimentally, and it was found that the critical depth of a cut significantly depends on the set of critical process parameters being applied. Finally, the critical ductile grinding process parameters could be identified determining the generated level of surface roughness achieving 0.83 nm Ra on tungsten carbide.

Surface roughness modeling for grinding of Silicon Carbide ceramics considering co-existence of brittleness and ductility

Ground surface roughness is always hard to be predicted due to the random distribution of abrasive grits and various materials properties, especially for brittle materials. In grinding of brittle materials, because of the brittle and hard nature, the material will be removed in both ductile and brittle modes, which make the modeling of surface roughness difficult. Therefore, this paper is intended to propose a new surface roughness model for brittle materials by considering the co-existing of machining-induced ductility and brittleness. A Rayleigh distribution function will be used to model the randomness of abrasive grits by assuming that the chip thickness for a single grit conforms to this distribution. In the meanwhile, the critical chip thickness model, considering both the materials properties and process parameters, will be adopted to divide the ductile and brittle grinding for a single grit. Thus, the probability for ductile mode grinding can be obtained through integral calculation of the Rayleigh distribution function. The grinding damage model from the indentation fracture mechanics is used to model the brittle surface roughness. The surface roughness and chip thickness model was calibrated and validated through a series of experiments on grinding of Silicon Carbide. The results show that the model predictions have a good match with the experimental results and the ductile surface roughness is much smaller than the brittle mode. Therefore, it can be concluded that the surface roughness modeling of brittle materials could benefit from considering the co-existence of brittleness and ductility governed by the machining kinematics, abrasive topology and process parameters, as well as the grinding induced surface damages. Moreover, the ductility-dominant grinding, under a higher wheel speed or lower chip thickness, can help to improve the surface roughness with a lower level of damage.

Maximum undeformed equivalent chip thickness for ductile-brittle transition of zirconia ceramics under different lubrication conditions

Abstract This study investigates the critical maximum undeformed equivalent chip thickness for ductile-brittle transition (DBhmax-e) of zirconia ceramics under different lubrication conditions. A DBhmax-e model is developed through geometry and kinematics analyses of ductile-mode grinding. Result shows that DBhmax-e decreases with increasing friction coefficient (μ). An experimental investigation is then conducted to validate the model and determine the effect of dry lubrication, minimum quantity lubrication (MQL), and nanoparticle jet minimum quantity lubrication (NJMQL) conditions on DBhmax-e. According to different formation mechanisms of debris, the grinding behavior of zirconia ceramics is categorized into elastic sliding friction, plastic removal, powder removal, and brittle removal. Grinding forces per unit undeformed chip thickness (Fn/h and Ft/h) are obtained. The lubrication condition affects the normal force and ultimately influences the resultant force on workpiece. In comparison with dry grinding (DBhmax-e = 0.8 μm), MQL and NJMQL grinding processes increase DBhmax-e by 0.99 and 1.79 μm respectively; this finding is similar to model result. The theoretical model is then assessed by different volume fractions of nanofluids under NJMQL condition with an average percentage error of less than 8.6%.

Controlled material removal mode and depth of micro cracks in precision grinding of fused silica – A theoretical model and experimental verification

Abstract A critical function for crack propagation for the single grit scratching of fused silica is developed based on the fracture mechanics. The effects of original crack density on the surface, strain rate and grinding coolant are considered in the function. A theoretical model for controlled material removal mode and depth of micro cracks precision grinding is presented based on the critical function for crack propagation. It can be predicted by the model that the material removal mode in the grinding of fused silica with original cracks damage will change from a ductile mode to a semi-brittle mode, a full-brittle mode and a semi-brittle mode in sequence with the increasing single grit scratching depth. It was found that the micro crack damage depth of fused silica does not increase with the single grit scratching depth after a full brittle mode grinding and it is always smaller than that after a semi brittle mode grinding even with a smaller single grit scratching depth. These interesting results are explained by the fracture mechanics. The ductile mode grinding is a recognized desirable process of fabricating fused silica while the full-brittle grinding is also a feasible process for its shallow subsurface damage, high efficiency, low grinding force and energy consumption. Therefore, the depth of micro cracks after grinding can be controlled by choosing suitable grinding parameters. Grinding experiments are conducted on fused silica. The undeformed chip thickness of randomly distributed effective grits is simulated based on 3D reconstruction of wheel topography to reveal the relationship between the grinding parameters and the single grit scratching depth. Ground surface roughness, sub-surface damage (SSD) depth and grinding force are measured and discussed. It is shown that the model predictions correlate well with the experimental trend of grinding modes.

Basic Study on Ductile-Mode Grinding of Optical Glass Lenses with Rubber Bonded Diamond Wheels

Large aperture lenses with high surface quality are demanded for professional imaging products such as single-lens reflex cameras and astronomical telescopes. Large aperture optical lenses are shaped by ultra-precision grinding and finished by prolonged polishing. However, the prolonged polishing process leads to deterioration of the form accuracy. In order to reduce the amount of polishing, ductile-mode ultra-precision grinding is demanded. In this study, a rubber bonded wheel, which has a low elastic modulus, is used for grinding of spherical glass BK7, and influence of the hardness of the rubber bonded wheel and abrasive chip thickness on brittle fracture and surface roughness are experimentally investigated.

Application potential of coarse-grained diamond grinding wheels for precision grinding of optical materials

Fine-grained resin bonded diamond tools are often used for ultra-precision machining of brittle materials to achieve optical surfaces. A well-known drawback is the high tool wear. Therefore, grinding processes need to be developed exhibiting less wear and higher profitability. Consequently, the presented work focuses on conditioning a mono-layered, coarse-grained diamond grinding wheel with a spherical profile and an average grain size of 301 µm by combining a thermo-chemical and a mechanical-abrasive dressing technique. This processing leads to a run-out error of the grinding wheel in a low-micrometer range. Additionally, the thermo-chemical dressing leads to flattened grains, which supports the generation of hydrostatic pressure in the cutting zone and enables ductile-mode grinding of hard and brittle materials. After dressing, the application characteristics of coarse-grained diamond grinding wheels were examined by grinding optical glasses, fused silica and glass–ceramics in two different kinematics, plunge-cut surface grinding and cross grinding. For plunge-cut surface grinding, a critical depth of cut and surface roughness were determined and for cross-grinding experiments the subsurface damage was analyzed additionally. Finally, the identified parameters for ductile-machining with coarse-grained diamond grinding wheels were used for grinding a surface of 2000 mm2 in glass–ceramics.

A critical energy model for brittle–ductile transition in grinding considering wheel speed and chip thickness effects

The ability to predict the critical depth for ductile-mode grinding of brittle materials is important to grinding process optimization and quality control. The traditional models for predicting the critical depth are mainly concerned with the material properties without considering the operation parameters. This article presents a new critical energy model for brittle–ductile transition by considering the strain rate effect brought by the grinding wheel speed and chip thickness. The experiments will be conducted through a high-speed diamond grinder on reaction-sintered silicon carbide materials under different grinding speed and chip thickness. Through detailed analysis of the strain rate effect on the dynamic fracture toughness, a new fracture toughness model will be established based on the Johnson–Holmquist material model (JH-2) and calibrated through experiments based on the indentation fracture mechanics. Then, the new critical model for brittle–ductile transition will be established by introducing the dynamic facture toughness model considering the wheel speed and chip thickness. According to scanning electron microscope observations, the results show that ductile-mode grinding can be obtained through a combination of higher grinding speed and smaller chip thickness. Moreover, the critical value for ductile grinding of brittle materials can be improved through the elevation of the grinding speed or reduction in the chip thickness.

Crack-free ductile mode grinding of fused silica under controllable dry grinding conditions

A crack-free ductile mode grinding of fused silica was realized by a controllable dry grinding process in this research, which is attributed to the improvement of fused silica's ductile machinability induced by the high grinding temperature. The plastic deformation of fused silica consists of shear flow and densification. Plastic deformation mechanisms and cracking behaviors related to densification were investigated firstly by high temperature nanoindentation experiments to reveal the ductile–brittle transition mechanisms. Fused silica exhibits less densification and more shear flow at high temperature than room temperature. The critical ductile–brittle transition load of fused silica is higher at high temperature than room temperature. These results may lead to the improvement of the fused silica's ductile machinability at high temperature. Dry grinding experiments were conducted to investigate the effect of grinding depth. A mathematical model is established to predict the maximum temperature in workpiece. A novel infrared radiation (IR) transmission on-line measurement method was presented to acquire the workpiece temperature in the contact zone directly. The predicted results coincide well with the experiment results. Contrary to the conventional experience, a large grinding depth is beneficial for the surface quality and integrity in the dry grinding of fused silica due to the increased grinding temperature; however, the excessive grinding depth results in grinding wheel burn. The ductile grinding depth of the fused silica increases from sub-micrometers to 5μm by dry grinding which makes the grinding process more controllable and effective.

Theoretical study on brittle–ductile transition behavior in elliptical ultrasonic assisted grinding of hard brittle materials

The elliptical ultrasonic assisted vibration can lead to a higher comprehensive performance with better machined quality and lower grinding force during brittle materials machining. The undeformed chip thickness in grinding can be defined as the distance between two consecutive grinding surfaces formed by the adjacent abrasive grain trajectories. The ductile–brittle transition point of chip formation in brittle materials machining is believed to be represented by the threshold value of undeformed chip thickness (critical undeformed chip thickness). In this paper, an energy based method is proposed to predict the critical undeformed chip thickness in elliptical ultrasonic assisted grinding (EUAG). The effects of vibration parameters on grinding force and specific grinding energy are analyzed in detail. Results show that the axial vibration amplitude leads to a slight reduction in grinding force, whereas the vertical vibration amplitude results in a significant reduction of grinding force. The increase of wheel speed and ultrasonic vibration frequency can reduce the grinding force as well. Nevertheless, the grinding force ratio remains relatively steady fluctuating from 1.37 to 1.56. Additionally, the critical undeformed chip thickness increases with the increasing grinding speed, axial vibration amplitude and ultrasonic vibration frequency, while firstly increases and then decreases with the increasing vertical vibration amplitude. Especially, a ductile-mode grinding in micron-level can be achieved when the axial vibration amplitude is above 3μm and a totally brittle fracture occurs when the axial vibration does not exist. Thus, a reasonable selection of vibration parameters in EUAG process is required. The above theoretical evaluations coincide well with the variable experimental results.

Process optimization in optical fabrication

Predictable and stable fabrication processes are essential for reliable cost and quality management in optical fabrication technology. This paper reports on strategies to generate and control optimum sets of process parameters for, e.g., subaperture polishing of small optics (featuring clear apertures smaller than 2 mm). Emphasis is placed on distinguishing between machine and process optimization, demonstrating that it is possible to set up the ductile mode grinding process by means other than controlling critical depth of cut. Finally, a recently developed in situ testing technique is applied to monitor surface quality on-machine while abrasively working the surface under test enabling an online optimization of polishing processes eventually minimizing polishing time and fabrication cost.

An Overview on Nano Metric Surface Finishing Techniques

This paper deals with free form optics, ELID, partial ductile mode grinding, ductile regima grinding, four domains of material removal, selection of tool materials, ultra precision grinding, aspherical surface generation grinding wheel. Advantages, disadvantages, applications, operation of the above methods were also discussed in brief manner for understanding the concepts. This paper would be useful for the readers to know the basic concepts in the nano metric surface finishing techniques.

Machining Technology of Ultrasonic Assisted Grinding for a Silicon Carbide Wafer Carrier

The machining technology of ultrasonic assisted ductile mode grinding for the silicon carbide (SiC) wafer carrier. The machining tool is designed and analyzed by ANASYS 14.0 and the machining technology is studied for the 6-inch SiC wafer carrier. The ultrasonic tool holder with the resonance frequency 26 kHz is designed and fabricated. The advantageous machining parameters are proposed that spindle rotation rate is 3000 rpm, feed speed is 200 mm/min, and ultrasonic output power is 250 W. The measurement results show that the average roughness (Ra) of the SiC is 0.18 μm, the MRR is over 2.4 mm3/min, the tool tips are only minor abrasion, and the cutting temperature are reduced. Obvious, the machining technology has the advantage of high surface quality, high machining efficiency and long tool life.

High-efficiency ductile-mode grinding of optical glass lens by controlling tool feed rate and workpiece velocity

High-efficiency ductile-mode grinding of optical glass lens by controlling tool feed rate and workpiece velocity