Experimental study on ultrasonic elliptical vibration cutting of hardened steel using PCD tools

Diamond tools cannot usually be applied for machining hardened steels while applying conventional cutting technique. As an alternative, ultrasonic elliptical vibration cutting (UEVC) technique was successfully applied for obtaining mirror surface on such steels using single crystal diamond (SCD) tools. In order to reduce production cost without compromising mirror surface quality, polycrystalline diamond (PCD) tools may be tested against highly expensive SCD tools. However, study on machining of hardened steel using PCD tools applying the UEVC technique has not yet been reported. The current research presents an experimental study on UEVC of hardened stainless steel (a typical Stavax, hardness 49 HRC) using the PCD tools. Face turning experiments were carried out to investigate the effects of three machining parameters: nominal depth of cut, feed rate, and nominal cutting speed on output performances such as cutting force, tool flank wear, surface roughness, and chip formation. Experimental results show that nominal cutting speed has very strong influence on the output performances, compared to the other two parameters. The surface roughness improves with a decrease in cutting speed. A mirror-like surface of approximately 804 mm² with a roughness value R_a of 11 nm was achieved at a lower cutting speed. Theoretical explanations have been given to support the results drawn from the UEVC experiments. It can be concluded that, while applying the UEVC technique, the inexpensive PCD tools instead of the SCD tools can be effectively applied to obtain optical surface for producing precise molds from the hardened steel.     

Combined ultrasonic vibration and chemical mechanical polishing of copper substrates

This paper introduces an ultrasonic, vibration-assisted, chemical mechanical polishing (UV-CMP) method and an ultrasonic, vibration-assisted, traditional diamond disk (UV-TDD) dressing method. A copper substrate is polished by traditional CMP and UV-CMP. UV-CMP combines the functions of traditional CMP and ultrasonic machining (USM) with small-amplitude, high-frequency tool vibration to improve the fabrication process and machining efficiency. The removal rate of the copper substrate, torque force, and polished surface morphology of CMP and UV-CMP are compared. The polishing pad is also dressed by traditional diamond disk (TDD) and UV-TDD. The pad cut rate, torque force, and pad surface profiles of TDD and UV-TDD are also investigated in experiments. Experimental results reveal that UV-TDD can produce twice the pad cut rate and reduce torque force compared to TDD. Consequently, a dressing time reduction by half is expected, and hence, the diamond life is extended. It is found that the removal rate of the copper substrate polished by UV-CMP is increased by approximately 50-90% relative to that of traditional CMP because in UV-CMP, a passive layer on the copper surface, formed by the chemical action of the slurry, will be removed not only by the mechanical action of CMP but also by ultrasonic action. In addition, the surface roughness improves and the torque force reduces dramatically. This result suggests that the combination processes of CMP/USM and TDD/USM are feasible methods for improving polishing and dressing efficiency.     

Thermal stresses in diamond coatings and their influence on coating wear and failure

Thermal stresses in diamond coatings deposited onto cemented carbide substrates are calculated using the finite element method. The thermo-elastic stress fields for some coating-substrate geometries are presented. The results are compared with experimental data on the tribological behaviour of diamond coatings. Residual stresses can explain many of the observed patterns of coating wear and failure. A model for the abrasive wear of brittle coatings under large compressive biaxial stresses is described. These stresses prevent cracks initiated at the surface to propagate towards the interface and may promote crack paths parallel to the interface, thus causing the formation of a smooth coating surface. Once the smooth appearance is reached it will become extremely hard to initiate and propagate cracks into the coating and consequently the wear rate becomes very low. Thus, large compressive residual stresses increase the already high wear resistance of diamond coatings. When diamond coatings are deposited onto substrate edges, intense concentrations of normal and shear stresses may lead to coating failure by interfacial spalling. These stresses are lowered by increasing the ratio r/h, where r is the edge radius and h is the coating thickness.     

Effect of machining parameters in ultrasonic vibration cutting

The ultrasonic vibration cutting (UVC) method is an efficient cutting technique for difficult-to-machine materials. It is found that the UVC mechanism is influenced by three important parameters: tool vibration frequency, tool vibration amplitude and workpiece cutting speed that determine the cutting force. However, the relation between the cutting force and these three parameters in the UVC is not clearly established. This paper presents firstly the mechanism how these parameters effect the UVC. With theoretical studies, it is established that the tool–workpiece contact ratio (TWCR) plays a key role in the UVC process where the increase in both the tool vibration parameters and the decrease in the cutting speed reduce the TWCR, which in turn reduces both cutting force and tool wear, improves surface quality and prolongs tool life. This paper also experimentally investigates the effect of cutting parameters on cutting performances in the cutting of Inconel 718 by applying both the UVC and the conventional turning (CT) methods. It is observed that the UVC method promises better surface finish and improves tool life in hard cutting at low cutting speed as compared to the CT method. The experiments also show that the TWCR, when investigating the effect of cutting speed, has a significant effect on both the cutting force and the tool wear in the UVC method, which substantiates the theoretical findings.     

Suppression mechanism of tool wear by phosphorous addition in diamond turning of electroless nickel deposits

An appropriate phosphorous addition to electroless nickel deposits remarkably reduces tool wear in diamond turning. To understand the wear suppression mechanism of phosphorous addition, erosion tests simulating tool wear process and ab initio molecular dynamics calculations of interactions between diamond and Ni-P and Ni are carried out. The erosion tests show that carbon diffusion into the workpiece is reduced, and the ab initio calculations suggest that dissociation of carbon atoms on diamond surface due to the interaction with the workpiece is reduced. The results suggest that another possible additive to suppress tool wear can be found by the method proposed.     

Predictive simulation of damping effects in machine tools

A predictive simulation of the different damping effects in machine tools is required to optimize the dynamic behavior and thus increase their performance and working accuracy. Previously, holistic optimization based on damping was not possible due to non-predictive damping models and the lack of adequate modeling approaches. This paper presents a modeling approach, which allows the efficient simulation of the dynamic behavior. By applying this procedure and suitable damping and friction models, the dynamic behavior of a four-axes machining center was simulated with high accuracy – FRAC values above 95% were achieved.     

Effect of the crystallinity of diamond coatings on cemented carbide inserts on their cutting performance in milling

Micro-crystalline diamond (MCD) coatings were deposited on cemented carbide inserts at different temperatures using hot filament chemical vapor deposition technique. For investigating the effect of the developed diamond crystallinity on the fatigue strength and wear behaviour of the prepared MCD coated inserts, inclined impact tests and milling investigations were conducted correspondingly. Raman spectra were recorded for capturing the crystalline phases after the film deposition and their potential changes after the impact and milling experiments induced by the mechanical and thermal loads. Thus, the explanation of the cutting performance of the employed diamond coated inserts with various crystalline phases was enabled.     

Development of monitoring system on the diamond tool wear

Recently, ultra-precision machining using a single crystal diamond tool has been developing very rapidly, especially in the fields of production processes for optical or magnetic parts such as magnetic discs, laser mirrors, polygon mirrors and copier drums. As a result, it has been successfully extended to machine various soft materials, generating mirror-like surfaces to sub-micron geometric accuracy with the ultra-precision CNC machine and the single crystal diamond tool. With the real cutting operation, the geometric accuracy and the surface finish attainable in machined surfaces are mainly determined by both of the sharpness of a cutting tool and stability of the machine vibration. In this study, for monitoring the progress of machining state for assuring the machining accuracy and the surface quality, a new monitoring method of machining states in face-cutting with diamond tool is proposed, using the frequency response of multi-sensors signal, which includes wear state of tool in terms of the energy within the specific frequency band. A magnetic disc is machined on the ultra-precision lathe.     

High surface quality micro machining of monocrystalline diamond by picosecond pulsed laser

In micro machining of monocrystalline diamond by pulsed laser, unique processing characteristics appeared only under a few ten picosecond pulse duration and a certain overlap rate of laser shot. Cracks mostly propagate in parallel direction to top surface of workpiece, although the laser beam axis is perpendicular to the surface. This processed area can keep diamond structure, and its surface roughness is smaller than R_a = 0.2 μm. New laser micro machining method to keep diamond structure and small surface roughness is proposed. This method can contribute to reduce the polishing process in micro machining of diamond.     

Documentation of tool wear progress in the machining of nodular ductile iron with silicon nitride-based ceramic tools

Nowadays, the HPM of cast irons is based on silicon nitride ceramic and CBN cutting tools. This paper characterizes and correlates several outputs of the cutting process of nodular cast iron using uncoated and Al₂O₃/TiN coated Si₃N₄ ceramic tools resulting from wear progress and destruction of tool faces. Investigations include tool wear curves, tribological behaviour of the tool–chip interface and tool wear mechanisms occurring on contact surfaces. The image-based characterization of worn surfaces employs such techniques as SEM, BSE and EDX analysis. The occurrence of various wear mechanisms, such as abrasive, adhesive and chemical wear was revealed.     

超声波对电镀金刚石工具性能的影响

分别在有超声波和无超声波条件下,制备了镍镀层和电镀金刚石工具。采用显微硬度计测试了镍镀层硬度,采用热震法和锉削法测试镀层与基体的结合强度,用显微镜观察镀层金相组织,计算了工具磨削比。研究结果表明:在电镀金刚石工具制备过程中,应用超声波可以细化镀层晶粒,提高镀层硬度,增强镀层与基体的结合强度。上砂前应用超声波可提高电镀金刚石工具磨粒密度和工具的磨削比。     

The material removal mechanism in mechanical lapping of diamond cutting tools

In order to reveal the surface layer removal nature and explain the anisotropy of material removal rate in mechanical lapping single crystal diamond cutting tools, a brittle-ductile transition lapping mechanism is proposed. And then, the dynamic critical depths of cut for brittle-ductile transition in different directions on different planes can be calculated. The lapped surface layer of diamond cutting tool will be removed in plastic mode as long as the embedding depth of diamond grit into the lapped surface is less than the corresponding critical depth of cut. Lapping experiments on the named (110) plane and (100) plane are carried out and the lapped surfaces are measured with atomic force microscope (AFM). The results show that all the lapped surfaces of diamond cutting tools consist of plastic grooves in nanometric scale and the maximal groove depths have prominent anisotropy in different orientations and on different planes, which are consistent with the critical depths of cut well. Therefore, the material removal rate anisotropy of lapped surface layer can be analyzed by comparing the critical depths of cut on different crystallographic planes and in different orientations of the identical plane quantitatively.     

Monitoring of distance between diamond tool edge and workpiece surface in ultraprecision cutting using evanescent light

The interaction between a tool and a workpiece during machining determines the quality of a machined workpiece. This study presents a novel direct monitoring method using evanescent light, which detects the distance between a diamond tool edge and the workpiece surface. In the proposed method, evanescent light is generated around the diamond tool edge, and the intensity of the reflected laser beam corresponds to the distance between the tool edge and the workpiece surface. Experimental results confirmed that the proposed method is capable of monitoring the distance change of a sub-micrometer scale.     

A new approach to determine the wear coefficient for wear prediction of sheet metal forming tools

Accounting for the increase of wear in metal forming tools, it is eminent to have detailed information about the tool lifetime already during the tool design. With the wear simulation tool REDSY—developed at the Institute of Metal Forming and Casting—tool wear can be simulated qualitatively and quantitatively for sheet metal forming processes. The calculations are based on Archard’s wear model, a model using contact mechanics to describe the wear behavior. In this project, a new approach to determine the wear coefficient has been developed using a simple cylindrical cup deep drawing experiment for the wear measurements. Several tool and sheet material combinations were analyzed using a five-stage progressive die tool in a precision automatic punching press in order to achieve a high wear volume in a short period of time. The wear coefficient for the respective material combination could be determined combining the experimental results with simulation. This method is verified by comparing the wear simulation results with actual measurements. This project was funded by Germany’s Bundesministerium für Wirtschaft und Arbeit (BMWA) over Arbeitsgemeinschaft industrieller Forschungsvereinigungen “Otto von Guericke” e.V. (AiF). Project code: AiF14291N.     

High performance grinding of zirconium oxide (ZrO2) using hybrid bond diamond tools

Hybrid bond (metal–ceramic) diamond tools are proposed for grinding zirconium oxide used in medical implants. Compared to conventional grinding tools, material removal rates and tool life time are drastically increased without deterioration in mechanical properties of the workpiece. This is achieved within a selected process window in combination with an elaborate oil cooling system, where material removal is mainly occurring within the ductile cutting mode. Self-sharpening effect of the tool can be observed and the dressability of the tool further improves the grinding performance.     

Wear patterns and wear mechanisms of cutting tools used during the manufacturing of chopped carbon fiber

Chopping carbon fiber is an efficient way to manufacture short carbon fiber reinforced plastic (SCFRP). Drastic wear of cutting tools during this process has been observed because of the highly abrasive nature of carbon fiber. Wear patterns and wear mechanisms need to be investigated to reduce tool wear and prolong tool life. This paper presents the formation and transition of wear patterns during the chopping process. The chopped rate is less than 95% after 21k cuts; thus, tool specimens and data are collected from the initial 21k cuts. Overall, the wear forms a crescent shape along the tool edge at one cutting position. This wear pattern is due to an uneven wear volume caused by the uneven distribution of filaments in the fibers, which forms an oval profile of the chopped carbon fiber. The oval profile results in more crescent shaped wear, which causes a more uneven filaments distribution, leading to a high wear rate. Conical structures at the ends of the chopped carbon fiber and micro scratches on the tool edge are observed. Thus, the abrasive wear mechanism during chopping is verified. The wear pattern on the tool profile is a combination of cutting edge rounding (CER) and rake face wear. Two wear stages divided by 8k cuts are introduced: the CER wear stage (before) and rake face wear stage (after). In the CER wear stage, the wear rate is basically stable at 32 μm/2k cuts and the wear pattern is an increasing CER up to 20 μm. The rake angle is approximately stable at 79°. In the rake face wear stage, the wear rate begins to increase and the wear pattern transitions from the CER wear to massive rake face wear in which the CER decreases and leans towards the flank face and the rake angle rapidly decreases. The CER decreases to 4 μm, and the rake angle decreases to 14° at the end. The wear pattern includes a groove that appears on the tool rake face in this stage, which is similar to crater wear on the rake face of a traditional turning tool because of the low hardness of the tool interior. Moreover, analytical models based on 3-body abrasive wear theory are developed to explain these wear patterns and wear mechanisms.     

Study of machining accuracy in ultrasonic elliptical vibration cutting

The cutting speeds of the tool, the rake angle and clearance angle through the cycles of elliptical vibration cutting for separating type ultrasonic elliptical vibration cutting are defined initially in the present paper. Subsequently, a theoretical model of the thrust cutting force in ultrasonic elliptical vibration cutting is proposed, and the reason of the machining accuracy improvement by applying ultrasonic elliptical vibration is clarified theoretically. Finally, the effect of ultrasonic elliptical vibration cutting on machining accuracy is verified experimentally by utilizing an ultrasonic elliptical vibration cutting system.     

A study of the diamond tool wear suppression mechanism in vibration-assisted machining of steel

Inability of machining steel strongly inhibits the application of diamond machining in manufacturing industry, especially in the fields of ultra-precision and micro machining. In recent years, vibration-assisted machining (VAM) has been proved to be capable of efficiently suppressing the diamond tool wear in cutting steel. Currently, the prevailing speculation claimed by most researchers for such suppression is that the tool–workpiece flash temperature was reduced in VAM, which would slow the chemical reaction between iron on steel and carbon on diamond. However, the correctness of this speculation has not been proved by any experimental or theoretical research. In this paper, in order to understand the true wear suppression mechanism of diamond tools in VAM of steel, a study is conducted by measuring the workpiece temperatures and modeling the cutting energy consumption in both VAM and conventional cutting (CC). Based on the comparison results, it is concluded that the cutting temperature and energy consumption in VAM are not smaller than in CC, and hence the reduced diamond tool wear in VAM should not be caused by the claimed reduced temperature, especially when the material removal rate is very small. Finally, based on the EDS analysis and the comparison of experimental results under different air pressure, two probable reasons are proposed for the significantly reduced diamond tool wear in VAM of steel: (i) increase of gas pressure at the tool–workpiece interface and (ii) generation of an oxide layer on the freshly machined surface.     

A study on the effect of tool nose radius in ultrasonic elliptical vibration cutting of tungsten carbide

Nowadays, ultrasonic elliptical vibration cutting (UEVC) technique is being successfully applied for ultraprecision machining of difficult-to-cut materials. Previous study reported that the tool geometry especially tool nose radius notably influences the performance of 1D ultrasonic vibration cutting (UVC). However, the effect of tool nose radius in the UEVC technique is yet to be studied. This study aims to investigate the effects of tool nose radius on the UEVC performance in terms of cutting force, tool wear and surface finish when machining a hard-to-cut material, sintered tungsten carbide (WC), using PCD tools. The experimental results show that the UEVC technique performs remarkably better in all aspects at a 0.6 mm nose radius compared to a lower (e.g. 0.2 or 0.4 mm) and a higher nose radius (e.g. 0.8 mm). When machining about 412 mm² surface area, an average surface roughness, R_a of 0.010 μm is achieved with a 0.6 mm nose radius. Analyses are conducted to justify the findings in this study.