Wear mechanisms of diamond coated dental burs

Abstract

Wear mechanisms of diamond burs consist of diamond wear-out, diamond pull-out, clogging by debris and degradation of the diamond binder material. These have been reported in the scientific literature and several discrepancies were found by the authors, which in itself, justifies an independent study. Diamond coated dental burs before and after use on human teeth were, therefore, compared in order to identify the predominant cause of wear. Fifteen new diamond coated burs were characterised using scanning electron microscopy before and after use on human teeth. The study focused on the condition of the same diamond particles before and after use. Clear evidence of diamond particle wear was detected rather than evidence of diamond pullout, clogging by debris or degradation of the diamond binder material.     

ABRASIVE MACHINING OF GLASS-INFILTRATED ALUMINA WITH DIAMOND BURS

The abrasive machining characteristics of a glass-infiltrated alumina used for fabrication of all-ceramic dental crowns were investigated using a high-speed dental handpiece and diamond burs with different grit sizes. The material removal rate, surface roughness, and extent of edge chipping were measured as a function of grit size. The removal rate decreased substantially with decreasing bur grit size from supercoarse (180 μm) to fine (40 μm) and ultrafine (10 μm). The removal rate with the supercoarse burs was approximately twice that achieved with the fine burs and four times the removal rate with the ultrafine burs. Both surface roughness and edge chipping damage were sensitive to diamond grit size. Chipping damage was severe and the surface roughness substantial with the supercoarse burs, while negligible edge chipping and smooth surfaces were obtained with the ultrafine burs. The removal rate also decreased with continued machining for all grit sizes. The observed reduction in removal rate was found to be primarily due to wear of the diamond grit and accumulation of debris on the bur (i.e., bur loading). After prolonged use, a significant loss of diamond grit was observed that led to a substantial loss of cutting efficiency. It is concluded that, with respect to material removal rate and surface integrity, diamond machining is a feasible machining process for glass-infiltrated alumina in the final infiltrated state. However, caution should be exercised in the use of diamond grit larger than 40 μm. Such burs may result in excessively rough surfaces, chipped edges, and strength limiting surface and subsurface microcracks.     

The quantitative effect of diamond grit size on the subsurface damage induced in dental adjustment of porcelain surfaces

Diamond burs with different grit sizes are often applied to adjust ceramic prostheses in restorative dentistry. However, the quantitative influence of diamond grit size on subsurface damage in adjusting ceramic prostheses is unknown. The aim of this study was to investigate and visualize the quantitative effect of diamond bur grit size on subsurface damage in dental adjusting of a feldspar prosthetic porcelain. Diamond burs with coarse (106-125 rm), medium (53-60 microm), and fine (10-20 microm) grit sizes were selected. Dental adjusting-induced subsurface damage was quantitatively investigated with the aid of finite element analysis (FEA) and scanning electron microscopy (SEM). Significant differences in subsurface damage depth were found among the coarse, medium, and fine diamond burs (ANOVA, p < 0.05). Coarse diamond burs induced approximately 6-8 times deeper subsurface damage than fine burs. Diamond grit size is confirmed to be a controlling factor in determining the degree of subsurface damage. Subsurface damage depths also significantly increased with removal rate (ANOVA, p< 0.05). The correlation of the SEM-measured subsurface damage depths and the diamond grit sizes supports the FEA predictions. From a practical standpoint, dental porcelains should be adjusted using smaller diamond grit sizes with lower removal rates to minimize subsurface damage.     

Prediction of cutting force for ball end mill in sculptured surface based on analytic model of CWE and ICCE

Abstract

The force prediction is the precondition of improving equipment utilization ratio and optimizing process for CNC machining. Cutter-workpiece engagement (CWE) and in-cut cutting edge (ICCE) are the keys. In this article, a new analytic method of CWE and ICCE is proposed for ball end milling of sculptured surface and the prediction model of milling force is established. The sculptured surface is discretized into a series of infinitesimal inclined planes corresponding to cutter location points. The geometry relationships of cutter axis, feed direction and inclined plane are defined parametrically. The boundary curves and the boundary inflection points of the CWE are obtained by intersecting spatial standard curved surfaces with rotation transformation of coordinate system. The effective intersection points of the CWE and the cutter edge curve in X_c-Y_ctwo-dimensional plane are the upper and lower boundary points of ICCE. Based on the instantaneous chip thickness considering arbitrary feed direction, the force prediction model for ball end mill of three-axis surface milling is established. Simulation and experiment show that CWE and ICCE calculated by analytic method are well consistent with those of solid method. The predicted cutting forces match well with the measurements both in magnitude and variation trend.     

Theoretical model for cutting force in rotary ultrasonic milling of dental zirconia ceramics

Rotary ultrasonic machining or ultrasonic vibration assisted grinding has superior performance in machining hard and brittle materials, such as dental zirconia ceramics. However, there are few reports about cutting force modeling of rotary ultrasonic milling (RUM) for dental ceramics, especially for cutting force model in feed direction. In this study, the theoretical model of cutting force both in axial direction and feed direction is proposed under the assumption that brittle fracture is the primary mechanism of material removal in RUM of dental ceramics. The effective cutting time and material removal volume have been analyzed to develop the cutting force model. Besides, the number of active abrasive particles has been calculated for the first time during the modeling. The effect of overlapping and intersection of fracture zone in peripheral direction on material removal volume has also been considered via the parameters K ₁ and K ₂. In addition, the relationships between the cutting force and input variables are revealed through the theoretical model. Finally, pilot experiments of RUM on dental zirconia ceramics are conducted to verify the theoretical model. The experimental results are consistent well with the model predictions. Therefore, the theoretical model can be applied to evaluate the cutting force in RUM of dental ceramics.     

Micromorphology analysis and bond strength of two adhesives to Er,Cr:YSGG laser‐prepared vs. Bur‐prepared fluorosed enamel

Preservation of enamel during composite veneer restorations of fluorosed teeth could be achieved by conservative preparation with Erbium lasers. This study evaluated the effect of fluorosed enamel preparation with Er,Cr:YSGG vs. conventional diamond bur on the micromorphology and bond strength of a self‐etch and an etch‐and‐rinse adhesives. Er,Cr:YSGG laser or diamond bur preparation was performed on the flattened midbuccal surfaces of 70 extracted human premolars with moderate fluorosis (according to Thylstrup and Fejerskov index, TFI = 4–6). Adper Single Bond (SB) with acid etching for 20 or 40 s and Clearfil SE Bond (SEB) alone or with additional etching was applied in four laser groups. The same adhesive procedures were used in three bur groups except for 40 s of etching along with SB. After restoration, microshear bond strength was measured (MPa). Data were analyzed using ANOVA and Tamhane tests (α = 0.05). Six additional specimens were differently prepared and conditioned for scanning electron microscopy evaluation. The highest and lowest bond strengths were obtained for bur‐prepared/SB (39.5) and laser‐prepared/SEB (16.9), respectively, with a significant difference (P = 0.001). The different adhesive procedures used associated to two adhesives exhibited insignificantly lower bonding in laser‐prepared groups compared to bur‐prepared ones (P > 0.05), with the exception of additional etching/SEB, which bonded significantly higher to bur‐prepared (36.4) than to laser‐prepared enamel (18.7, P = 0.04). Morphological analyses revealed a delicate etch pattern with exposed enamel prisms on laser‐prepared fluorosed enamel after acid etching and less microretentive pattern after self‐etching primer. The etch‐and‐rinse adhesive was preferred in the laser‐prepared fluorosed enamel in terms of bonding performance. Microsc. Res. Tech. 77:779–784, 2014. © 2014 Wiley Periodicals, Inc.     

In-process estimation of radial immersion ratio in face milling using cutting force

In order to prevent tool breakage in milling, maximum total cutting force is regulated at a specific constant level, or threshold, through feed rate control. Since the threshold is a function of the immersion ratio, an estimation of the immersion ratio is necessary to flexibly determine the threshold. In this paper, a method of in-process estimation of the radial immersion ratio in face milling is presented. When an insert finishes sweeping, a sudden drop in cutting forces occurs. These force drops are equal to the cutting forces that act upon a single insert at the swept cutting angle and they can be acquired from cutting force signals in the feed and cross-feed directions. Average cutting forces per tooth period can also be calculated from the cutting force signals in two directions. The ratio of cutting forces acting upon a single insert at the swept angle of cut and the ratio of average cutting forces per tooth period are functions of the swept angle of cut and the ratio of radial to tangential cutting force. Using these parameters, the radial immersion ratio is estimated. Various experiments are performed to verify the proposed method. The results show that the radial immersion ratio can be estimated by this method regardless of other cutting conditions.Nomenclature F_T, F_R tangential and radial forces - F_X, F_Y cutting forces in feed direction and cross feed direction - dF_X, dF_Y cutting force differences before and after the immersion angle in X and Y direction - K_s specific cutting pressure - a depth of cut - r ratio between tangential force and radial force - s_t feed per tooth - instantaneous angle of cut - _s swept angle of cut - _T tooth spacing angle - w radial width of cut - R cutter radius - z number of inserts     

A Novel Atomic Force Microscope with High Stability and Scan Speed

Abstract

A novel atomic force microscope (AFM) has been developed. Unlike conventional AFM systems, its cantilever and tip were set up in the X direction with respect to the sample. It can practically eliminate the crawling effect of the probe itself. Meanwhile, using a beam splitter, we devised a unique path optical beam deflection method for the measurement of the cantilever's displacement with its minimized structure and an optimal light‐amplifying ratio; the position of the sensitive detector (PSD) is just in front of the visual sight of operator. This makes observation and operation easier. Furthermore, the PSD attached to the translation stage can be adjusted and, thus, adapts the setpoint of imaging force to different samples. In this way, our new AFM provides high stability and scan speed. The highest scan rate is about 40 lines/s or 10s for a 400×400‐pixel, 3 µm×3 µm image.     

Effect of MQL flow rate on machinability of AISI 4140 steel

Abstract

Many studies were performed about the influence of minimum quantity lubrication (MQL) technique on cutting performance in the literature, but there is no paper examining the effect of different MQL flow rates and cutting parameters on machinability of AISI 4140 material as a whole. In this study, the effects of different MQL flow rates and cutting parameters on surface roughness, main cutting force and cutting tool flank wear (VB), with great importance among the machinability criteria, and forming as a result of the machining of AISI 4140, were revealed. At the end of the experiments, it was determined that rise of flow rate affected main cutting forces positively to a certain extent; yet, it exhibited no significant effect on surface roughness, but reduced VB. Also, it was observed that both main cutting force and surface roughness increased with the increase of feed, while generally decreased with the increase of cutting speed. It was seen that flank wear was positively affected by the increase in flow rate; and this decreased with the increase in flow rate. R² values obtained as 99.8% and 99.9% for main cutting forces and surface roughness values modeled statistically with the help of quadratic equations, respectively.     

Effect of in‐office bleaching agents on the surface roughness and morphology of different dental composites: An AFM study

The aim of this study was to evaluate, using atomic force microscopy, the effect of two different bleaching agents on the modification of dental composites materials. This modification will be judged by analyzing the variation of surface roughness and surface morphology of two different composites: one containing nanoparticles and other consisting of microhybrid resin. The bleaching was performed by using two different concentrations of hydrogen peroxide: HP Blue (20% hydrogen peroxide) and Whiteness HP Maxx (35% hydrogen peroxide). Disks of Esthet X and Filtek? Z350 composites were used. Atomic force microscopy was used for analyses of the same place of the sample before and after treatment. A total of 12 analyses were performed per group (n = 12). The samples were analyzed qualitatively by evaluating morphological changes in the images and quantitatively by using roughness parameters (Ra). Data were analyzed statistically using Kruskal–Wallis, Mann–Whitney, and Friedman tests (P < 0.05). Changes were observed both qualitatively and quantitatively only in the groups where Esthet X resin was used. The use of hydrogen peroxide bleaching agents caused changes only in the surface of microhybrid composites, with no changes being observed in the composite containing nanoparticles. Despite being even significant, these alterations are clinically slight and can be eliminated by polishing them. Microsc. Res. Tech. 76:481–485, 2013. © 2013 Wiley Periodicals, Inc.     

Experimental simulation of phosphites additives tribochemical reactions by gas phase lubrication

Abstract

Tribochemical reactions of phosphites additives on steel surface have been simulated by gas phase lubrication. Trimethylphosphite (TMPi), P(OCH₃)₃, has been used as model molecule for phosphites additives. It has been introduced under gas phase up to 5 hPa in a new tribometer dedicated to gas phase lubrication. Friction tests have been carried out at ambient temperature and 100°C. Chemical analyses by X-ray photoelectron spectroscopy and by Auger electron spectroscopy have been conducted inside and outside of the track. Two kinds of analysis have been carried out: ex situ and in situ surface analyses after tribological test. Indeed, a new environmentally controlled tribometer allows friction test then accurate analyses without air exposure of the formed tribofilm. Tribotests conducted under TMPi gas phase show a reduction of friction coefficient until 0˙2 instead of 1˙4 under high vacuum. Jointly, formation of tribofilm has been confirmed by optical microscopy and ex situ chemical analysis. Comparison between analyses performed inside and outside of the wear scar indicates that the friction induces the formation of phosphide compound that could reduce friction. Moreover analyses show the formation of methoxy group (CH₃O) and carbonate originally from the decomposition of TMPi under friction into H₂ and CO. In situ analyses clearly show the importance to investigate an uncontaminated tribofilm in order to obtain a better characterisation of it and then a better comprehension of the tribochemical mechanisms.     

ABRASIVE MACHINING OF GLASS-CERAMICS WITH A DENTAL HANDPIECE

Dental restorations are commonly prepared from machinable glass-ceramics using modern dental CAD/CAM systems. Unfortunately, little is understood about the influence of machining parameters on material removal rates and any damage which could be introduced into the restoration during the abrasive machining processes employed with these systems. These effects are investigated for three experimental machinable glass-ceramics with varying microstructure and one closely related commercial material. Abrasive machining is performed with dental burs containing coarse and fine diamond particles. The results show that the microstructure of the glass-ceramic, the size of diamond grit in the burs, and the load applied to the burs during machining have significant effects on the machining behavior. By increasing the size of the mica platelets within the glass-ceramics or by increasing the load on the burs, material removal rate increases. However, chipping damage at groove edges increases as either the load is increased or as the size of the mica platelets is decreased. The use of coarse burs does not necessarily result in high material removal rates but increases the extent of chipping damage. Surface roughness is found to be relatively independent of the microstructure or applied load but is strongly dependent upon coarseness of the diamond particles in the burs.     

Investigation of the Er: YAG laser and diamond bur cavity preparation on the marginal microleakage of Class V cavities restored with different flowable composites

The purpose of this study was to evaluate the efficiency of the Er:YAG laser and diamond bur cavity preparation on the marginal microleakage of Class V cavities. Group 1: bur preparation (bp) + Vertise Flow (VF); Group 2: laser preparation (lp) + VF; Group 3: bp + Adper Easy One (AEO) + Filtek Ultimate Flowable Composite (FUFC); Group 4: lp + AEO + FUFC; Group 5: bp + Clearfil S3 Bond (CSB) + Clearfil Majesty Flow (CMF); Group 6: lp + CSB + CMF. Data were analyzed by Kruskal–Wallis and Mann–Whitney U tests (p < .05). More microleakage was observed in cervical regions compared to occlusal regions in all groups (p < .05). No significant difference was observed among all groups in terms of occlusal and cervical surfaces, respectively (p > .05). The use of the Er:YAG laser for cavity preparation with different adhesive systems and flowable composites did not influence microleakage.     

THE ELECTRICAL DISCHARGE MACHINING (EDM) HANDBOOK

Abstract

Recycled plastics are increasingly being used to manufacture planks used in large-volume applications, including decks, garden, and cloakroom chairs. These products, although manufactured near-to-net shape, often require drilling for assembly purposes. There are very limited data on the machining of plastic material. Manufacturers often rely on data and models established for metals. The machining of plastics, although limited to assembly purposes, or to the removal of excess materials, requires an understanding of the behavior of these materials during the machining in order to obtain better quality parts. It is even more important for recycled plastics, which are inhomogeneous, contain pores, and most often, are made with more than one type of product. This work analyzes the machining of recycled plastics in order to establish and compare their machining models with those traditionally used for metals, and to better understand the behavior of the plastics during machining. The workpiece is drilled at different process conditions and at different temperatures. The process performance indicators such as cutting forces, chip formation, and chip form are analyzed. The originality of this work resides in its study of chip formation and the effects of the preset workpiece temperature on the drilling mechanisms. It is found that there is a range of critical temperatures of transition for plastics similar to the Charpy impact ductile-brittle temperature separating the domain of low cutting force and long and spiral chip from that of high cutting force corresponding to the accordion-type of chip. A parameter describing this phenomenon is defined. It is also found that for low- to moderate-speed operations, the cutting speed has very little effect on the cutting forces, which depend mainly on the feed rate and the workpiece temperature. The relationship between the drilling forces and the feed rate established for metals remain valid, but the exponent of the feed rate for the thrust force is lower. The thrust force and the tangential force are proportional to the feed rate exponent 0.4 compared to 0.8 for metals when drilling workpiece at room temperature or below.     

Three-dimensional chatter stability prediction of milling based on the linear and exponential cutting force model

Stability lobes are widely used to avoid chatter which restricts the machining quality and productivity. A lot of work has been done to predict the stability lobes fast and accurately. However, most of them are based on the linear force model, and the chatter stability limit is formulated as independent on the feed rate, which is inconsistent with the machining practice. By referencing with the zero-order solution, this paper investigates the chatter stability prediction based on the exponential force model. Focusing on the cutters with a lead angle (i.e., inserted face mill, the ball-end mill, and bull-nose end mill) where chatter is likely to be brought up in Z direction, the stability model is extended to three-dimensional. Taylor equation is utilized to linearize the exponential expressions when computing the directional coefficients in order to solve the stability limit analytically as the linear force model. Simulation results show that the exponential force model agrees with the measurements as well as the linear force model in the cutting force prediction, and it is able to demonstrate the feed rate effect on the stability limit. The stability limit is found to be increased as the feed rate increases, which is evidenced by the time domain simulation. Cutting tests are performed in the end to verify the stability model. The proposed model could be reduced to either X/Y dimensional or linear force model-based stability model by further simplifications.     

Integration of cutting force control and chatter suppression control into automatic cutting feed adjustment system design

Abstract

This study designed an automatic cutting feed adjustment system for computer numerical control (CNC) turning machine tools, which integrate the operational characteristics of cutting force control and chatter suppression control to shorten the machining time and maintain the quality of workpieces. The setting of appropriate machining conditions (such as cutting feed, spindle speed and depth of cut) to consider both machining quality and efficiency often causes difficulties for machine tool operators. Therefore, this study uses cutting force control to design an automatic cutting feed adjustment method for cutting tools, and then, the chatter suppression control design is used to modify the cutting force command to suppress cutting chatter. The experimental results of the CNC turning machine tool show that the use of the cutting force control to adjust the cutting feed can shorten the machining time; however, the cutting chatter results in larger surface waviness on the workpiece surface. When the cutting force command is properly modified by actuating the chatter suppression control, the workpiece shows better surface roughness with prolonged machining time. Therefore, the cutting tests demonstrate that the proposed system is feasible for satisfying the machining requirements of the manufacturing processes of mechanical parts for high speed and high accuracy.     

Experimental Investigations of Machinability in the Turning of Compacted Graphite Iron using Minimum Quantity Lubrication

Unique mechanical properties of the compacted graphite iron (CGI) attracted attention of manufacturers and suppliers mainly in automotive industry in last decades. However due to the low machinability of the CGI material, more efficient machining strategies need to be implemented. Improvement in the cost-effective and environmentally sensitive processing of compacted graphite iron (CGI) is one of the major concerns of the manufacturing world because of the allure of CGI's mechanical properties. This study assesses the efficiency of minimum quantity lubrication (MQL) in CGI turning when compared to the dry-cutting condition. The turning tests were conducted across a wide range of cutting parameters: three different cutting speeds (100, 200, 300 m/min) and three different feed rates (0.1, 0.2, 0.3 mm/rev), all at a constant depth of cut (1 mm). The MQL efficiency is evaluated through cutting force and surface roughness measurements, optical and SEM analyses of chip formation and tool-wear analysis. The results showed that MQL usage provided a reduction in the resultant cutting forces by 2–5%, a reduction in surface roughness by 25%. The SEM analysis also revealed much clearer and smoother cutting edges on tool surfaces used in the MQL tests.     

Effect of MQL and nanofluid on the machinability aspects of hardened alloy steel

Abstract

The current research comprises of various machinability aspects of 4340 hardened alloy steel which are scrutinized with in context of improvements in main cutting force, tool flank wear, crater wear, surface roughness, micro-hardness, machined surface morphology, chip morphology, chip reduction coefficient and apparent coefficient of friction under three different cutting fluid applications i.e. compressed air, water soluble coolant and nanofluid (using eco-friendly radiator coolant as the base fluid and Al₂O₃ as the nanoparticle) using uncoated cermet cutting inserts and a comparative assessment was performed to select which fluid performed better in terms of various machining attributes among three cutting fluids. The minimum quantity lubrication technique (MQL) was used in which a smaller volume of coolant sprinkled at high pressure. This method is found as the most effective alternative to minimize health risks and machining costs, which is quite high in other setups. The test specimen was machined at three different cutting speeds i.e. 100, 120 and 140?m/min along with two machining parameters i.e. feed and depth of cut were kept constant respectively at 0.2?mm/rev and 0.4?mm. Outcomes made a conclusion that Al₂O₃ enriched ecofriendly nanocoolant outperformed both compressed air and water soluble coolant in terms of every machinability aspects.     

Effects of docosahexaenoic acid or arachidonic acid supplementation on the behavior of cardiomyocytes derived from human pluripotent stem cells

Background: Human heart changes its energetic substrates from lactate and glucose to fatty acids during the neonatal period. Noticing the lack of fatty acids in media for the culture of cardiomyocytes derived from human pluripotent stem cells (hiPS-CM), researchers have supplemented mixtures of fatty acids to hiPS-CM and reported the enhancement in the maturation of hiPS-CM. In our previous studies, we separately supplemented two polyunsaturated fatty acids (PUFAs), docosahexaenoic acid (DHA) or arachidonic acid (AA), to rat fetal cardiomyocytes and found that the supplementations upregulated the expressions of mRNAs for cardiomyocyte differentiation, fatty acid metabolism, and cellular adhesion. The enhancement in cellular contractility was attributed to the improvement in intercellular connection rather than a direct enhancement of the contractile force. Methods: This study reports the successive results of the effects of DHA or AA supplementation on hiPS-CM. In addition to the contractile force and mRNA measurements used in the previous study, we further investigated the effect of different cellular aggregations on the contractile force output by means of finite element analysis, measured glucose and fatty acids metabolites, and assessed cTNT and MLC2v expressions through immunofluorecsence evaluation. Results: It showed that the sole supplementation of albumin-conjugated DHA or AA can be taken up by hiPS-CM without other uptake-enhancing factors, and the supplementations may activate the CD36_­ERRγ metabolic pathway. DHA or AA supplementation increased the cellular contractile ratio on collagen gels and AA supplementation stimulated hiPS-CM aggregation to form cellular clusters. The enhancement effect on the hiPS-CM contractile force was modest since the increase in contractile force was not significant. AA supplementation was more effective than DHA supplementation because it significantly upregulated mRNA expressions of P300 and CD36. However, finite element analysis showed that the formation of clusters on a collagen gel attenuated the contractile force exerted by the gel on its surroundings. Conclusion: DHA and AA, as having been supplemented in infant formulas, have no direct and significant enhancement effect on the performance of the hiPS-CM when they were supplemented individually, although they were able to enter the cellular metabolic system. The AA supplementation showed some auxiliary effect on the maturation of hiPS-CM, which is worthy of further investigation under the consideration of membrane composition alteration and remodeling of membrane molecules.     

Stability analysis for SiC grinding based upon harmonic wavelet and Lipschitz exponent

Abstract

In order to effectively identify the stability degree of grinding and obtain better surface integrity of machined workpiece in grinding application, in this paper, a quantitative assessment approach based on harmonic wavelet and Lipschitz exponent is proposed, using silicon carbide as an example. To begin with, the Lipschitz exponent is defined and calculated by harmonic wavelet modulus maxima (HWMM) and linear regression algorithm, respectively. Then the Kurtosis based on Lipschitz exponent (KLE) is defined as a quantitative indicator. In grinding experiment, the acceleration signals and grinding force signals of the X/Y directions are selected, and experimental parameters including wheel speed, workpiece speed and grinding depth are employed to investigate the grinding stability in grinding stage. Experimental results show that the grinding stability is relatively worse at low wheel speed, low-high workpiece speed and large grinding depth in terms of KLE index, which may provide a new guidance to explore optimized grinding parameters and thus obtain good surface integrity in engineering applications.