Effects of bearing clearance on the chatter stability of milling process

In the present study, the influences of the bearing clearance, which is a common fault for machines, to the chatter stability of milling process are examined by using numerical simulation method. The results reveal that the presence of bearing clearance could make the milling process easier to enter the status of chatter instability and can shift the chatter frequency. In addition, the spectra analysis to vibration signals obtained under the instable milling processes show that the presence of bearing clearance could introduce more frequency components to the vibration responses but, however, under both the stable and instable milling processes, the generated frequency components will not violate the ideal spectra structures of the vibration responses of the milling process, which are usually characterized by the tooth passing frequency and its associated higher harmonics for the stable milling process and by the complex coupling of the tooth passing frequency and the chatter frequency for the instable milling process. This implies that, even under the case with bearing clearance fault, the stability of the milling process can still be determined by viewing the frequency spectra of the vibration responses. Moreover, the phenomena of the chatter frequency shift and the generation of more components provide potential ways to detect the bearing clearance in machines. (C) 2010 Elsevier Ltd. All rights reserved

A dynamic model of cylindrical plunge grinding process for chatter phenomena investigation

In the paper, chatter vibrations in the cylindrical plunge grinding process are investigated. An improved model of the grinding process was developed which is able to simulate self-excited vibrations due to a regenerative effect on the workpiece and the grinding wheel surface. The model includes a finite-element model of the workpiece, two degrees of freedom model of the grinding wheel headstock and a model of wheel-workpiece geometrical interferences. The model allows to studying the influence of different factors, i.e. workpiece and machine parameters as well as grinding conditions on the stability limit and a chatter vibration growth rate. At the end, simulation results are shown and compared with exemplified real grinding results

An analytical study of wheel regeneration in surface grinding

Machining vibrations have been a topic of active research since the beginning of the 20th century, because of their harmful potential to deteriorate the surface finish of the workpiece, reduce the lifetime of the machine tool and limit the overall productivity of manufacturing operations. Grinding, often being a finishing operation responsible for the final quality of the workpiece, can be especially affected by unwanted process vibrations. Machining vibrations can arise from a number of sources, but they are usually classified as forced and self-excited vibrations. Forced vibrations, such as those resulting from runout or unbalance, are generally easier to avoid. However, self-excited vibrations, also known as chatter, originate in the machining process itself and are typically more difficult to predict and suppress. When it comes to grinding operations, chatter can occur as a result of uneven surface regeneration on both the wheel and the workpiece. In this paper we perform an analytical study of grinding chatter, to explore the intricate nature of wheel regeneration (i.e. chatter arising from uneven wear on the wheel) in surface grinding. Whilst there has been a great deal of previous research into grinding chatter, the present study explores a contrarian approach whereby a circumferential variation of the specific grinding energy occurs during the onset of instability. The specific energy is a fundamental quantity in grinding that relates the necessary grinding power to the prescribed material removal rate. It is also related to wheel wear and thus to grinding forces, since a worn wheel requires more grinding power and produces greater grinding forces to sustain the same material removal rate than a sharp one. Therefore, by characterising the specific energy variation around the circumference of the grinding wheel as a function of wheel vibration, we derive a stability model and discuss its practical potential

An alternative wheel regenerative mechanism in surface grinding : distributed grit dullness captured by specific energy waves

Regenerative chatter is a serious problem in machining. It is an unstable relative vibration between the workpiece and the cutting tool that adversely affects virtually all chip formation processes. This paper addresses regenerative chatter in grinding, which is one of the most widely used abrasive processes today. As a result of significant tool wear in grinding, surface regeneration (which is a prerequisite for regenerative chatter) can occur not only on the workpiece but also on the grinding wheel. This article is concerned with the regenerative mechanism by which wheel-related instability develops. In the present study, the role of distributed grit dullness alone is explored. A new chatter model is formulated and validated by both numerical simulations and experimental data. The new theory accurately predicts the existence of stable regimes in grinding, for the first time. This is in contrast to the published literature where the consensus has been that grinding cannot be stable with respect to wheel regeneration. Consequently, the present contribution enables a novel opportunity to increase the productivity of industrial grinding operations

Quantitative impacts of regenerative vibration and abrasive wheel eccentricity on surface grinding dynamic performance

In grinding, regenerative vibration and forced vibration due to grinding wheel eccentric rotation are main excited-vibration sources that interact with grinding material removal mechanism. In the paper, instantaneous undeformed chip thickness in down-grinding cutting phase may consist of two components, i.e. linear kinetic thickness and nonlinear dynamic thickness. Considering abrasive grit-workpiece interaction in the grinding contact zone, the grinding vibration system is presented by a new set of differential equations of two degrees of freedom (DOF) with a close-loop feedback control system models. Conventional grinding control parameters, including wheel spindle speed, work-speed in feed direction and radial cutting depth, are often regarded as linear constants in many existing simplified models. When considering time delay, they can be transferred to nonlinear variables, so the capability of prediction and the accuracy of solution of the grit-workpiece dynamic performance are improved. Based on quantitative comparison of force and vibration magnitudes, the influence of the eccentric rotation of abrasive wheel and the negative rake angle of working grit cutting edges on grinding performance are demonstrated in the paper. © 2018 Springer-Verlag London Ltd., part of Springer Natur

Oxygen reduction reaction features in neutral media on glassy carbon electrode functionalized by chemically prepared gold nanoparticles

Gold nanoparticles (AuNPs) were prepared by chemical route using 4 different protocols by varying reducer, stabilizing agent and solvent mixture. The obtained AuNPs were characterized by transmission electronic microscopy (TEM), UV-Visible and zeta potential measurements. From these latter surface charge densities were calculated to evidence the effect of the solvent mixture on AuNPs stability. The AuNPs were then deposited onto glassy carbon (GC) electrodes by drop-casting and the resulting deposits were characterized by cyclic voltammetry (CV) in H2SO4 and field emission gun scanning electron microscopy (FEG-SEM). The electrochemical kinetic parameters of the 4 different modified electrodes towards oxygen reduction reaction (ORR) in neutral NaCl-NaHCO3 media (0.15 M / 0.028 M, pH 7.4) were evaluated by rotating disk electrode voltammetry and subsequent Koutecky-Levich treatment. Contrary to what we previously obtained with electrodeposited AuNPs [Gotti et al., Electrochim. Acta 2014], the highest cathodic transfer coefficients were not obtained on the smallest particles, highlighting the influence of the stabilizing ligand together with the deposits morphology on the ORR kinetics