FRF Estimation through Sweep Milling Force Excitation (SMFE)

AbstractInaccurate machine dynamics characterization is thought to be one of the main sources of errors in current cutting stability models. For this reason, traditional dynamic characterization procedures have been called into question. A new method for frequency response function (FRF) estimation using the real milling force as the input excitation is proposed. It consists of exciting the structure through several cutting tests at increasing or decreasing spindle speed while measuring its response. This sweep milling force excitation (SMFE) procedure allows obtaining the FRF under real cutting conditions. The results obtained have improved stability prediction with respect to conventional impact tests

Optimum selection of variable pitch for chatter suppression in face milling operations

Cutting capacity can be seriously limited in heavy duty face milling processes due to self-excited structural vibrations. Special geometry tools and, specifically, variable pitch milling tools have been extensively used in aeronautic applications with the purpose of removing these detrimental chatter vibrations, where high frequency chatter related to slender tools or thin walls limits productivity. However, the application of this technique in heavy duty face milling operations has not been thoroughly explored. In this paper, a method for the definition of the optimum angles between inserts is presented, based on the optimum pitch angle and the stabilizability diagrams. These diagrams are obtained through the brute force (BF) iterative method, which basically consists of an iterative maximization of the stability by using the semidiscretization method. From the observed results, hints for the selection of the optimum pitch pattern and the optimum values of the angles between inserts are presented. A practical application is implemented and the cutting performance when using an optimized variable pitch tool is assessed. It is concluded that with an optimum selection of the pitch, the material removal rate can be improved up to three times. Finally, the existence of two more different stability lobe families related to the saddle-node and flip type stability losses is demonstrated

Damping of in-process measuring system through variable stiffness tunable vibration absorber

International audienceIn external cylindrical grinding machines, measuring systems are occasionally mounted on a gantry type frame. The modes of this structure are potentially harmful within the operation range of the grinding wheel in a standard machine configuration, since the rotation of the wheel can enter into resonance, thus avoiding a correct determination of the on-line measurement. The resoncance problem can be succesfully dealt with the use of a variable stiffness vibration absorber which autonomously adapts its stiffness to tune according to the mode to be damped, increasing the dynamic stiffness along the whole operation range of the wheel. In this work, a variable stiffness self-tunable vibration absorber prototype has been built and a new tuning function has been derived in order to minimize the response at the frequency coincident with the rotating speed of the wheel. Finally, validation tests have been performed in a scale supporting structure and the vibration reduction improvement comparison with respect to a standard fixed tuning strategy has been evaluated

Tool path pattern and feed direction selection in robotic milling for increased chatter-free material removal rate

Robotic milling becomes increasingly relevant to large-scale part manufacturing industries thanks to its cost-effective and portable manufacturing concept compared to large-scale CNC machine tools. Integration of milling processes with industrial robots is proposed to be well aligned with the aims and objective of the recent fourth industrial revolution. However, the industrial robots introduce position-dependent and asymmetrical dynamic flexibility, which may reflect to the tool tip dynamics under several conditions. Under such circumstances, the stability limits become dependent on the machining location and the feed direction. In this respect, selection of machining tool path patterns is crucial for increased chatter-free material removal rates (MRR). This paper proposes an approach to evaluate and select tool path patterns, offered by the existing CAM packages, for increased chatter-free MRR. The machining area is divided into number of machining locations. The optimal feed direction is decided based on the absolute stability at each region considering the asymmetrical and position-dependent tool tip dynamics. Then, the alternative tool path patterns are evaluated and the corresponding optimum feed direction is decided for increased chatter-free material removal. The application of the proposed approach is demonstrated through simulations and representative experiments

Imaging Outcomes in Clinical Trials of Treatments for Glaucoma

Currently, all therapies for glaucoma have been licensed based on their ability to lower intraocular pressure (IOP). However, the main outcome of interest to people with glaucoma is vision-related (VR) quality of life (QoL). Instruments measuring VR QoL are unlikely to be sensitive enough to function as the primary outcome for clinical trials, 1 but they remain important as secondary outcomes to capture side-effects of treatment. Although lowering IOP has been shown to slow visual field (VF) loss, 2 IOP is a far-removed surrogate for VR QoL in glaucoma. Furthermore, IOP obviously would be an inappropriate outcome for a trial of a neuroprotective treatment with no effect on IOP. In contrast, the association of VR QoL measures with VF loss and other measures of vision has been established. 3 Measurements of visual function are recognized by regulatory authorities as the appropriate primary outcome measure for clinical trials in glaucoma, 4 and the major clinical trials that have evaluated vision function as the primary outcome have used progressive VF loss as the main outcome measure

OCT Signal Enhancement with Deep Learning

PURPOSE: To establish whether deep learning methods are able to improve the signal-to-noise ratio of time-domain (TD) optical coherence tomography (OCT) images to approach that of spectral-domain (SD) OCT. DESIGN: Method agreement study and progression-detection in a randomized, double-masked, placebo-controlled, multi-centre trial for open-angle glaucoma (OAG) [UK Glaucoma Treatment Study (UKGTS)]. PARTICIPANTS: Cohort for training and validation: 77 stable OAG participants with TDOCT and SDOCT imaging at up to 11 visits within 3 months. Cohort for testing: 284 newly-diagnosed OAG patients with TDOCT from a cohort of 516 recruited at 10 UK centres between 2007 and 2010. METHODS: An ensemble of generative adversarial networks (GANs) was trained on TDOCT and SDOCT image pairs from the training dataset and applied to TDOCT images from the testing dataset. TDOCT were converted to synthesized SDOCT images and segmented via Bayesian fusion on the output of the GANs. MAIN OUTCOME MEASURES: 1) Bland-Altman analysis to assess agreement between TDOCT and synthesized SDOCT average retinal nerve fibre layer thickness (RNFLT) measurements and the SDOCT RNFLT. 2) Analysis of the distribution of the rates of RNFLT change in TDOCT and synthesized SDOCT in the two treatments arms of the UKGTS was compared. A Cox model for predictors of time-to-incident VF progression was computed with the TDOCT and the synthesized SDOCT. RESULTS: The 95% limits of agreement between TDOCT and SDOCT were [26.64, -22.95], between synthesized SDOCT and SDOCT were [8.11, -6.73], and between SDOCT and SDOCT were [4.16, -4.04]. The mean difference in the rate of RNFL change between UKGTS treatment and placebo arms with TDOCT was 0.24 (p=0.11) and with synthesized SDOCT was 0.43 (p=0.0017). The hazard ratio for RNFLT slope in Cox regression modeling for time to incident VF progression was 1.09 (95% CI 1.02 to 1.21) (p=0.035) for TDOCT and 1.24 (95% CI 1.08 to 1.39) (p=0.011) for synthesized SDOCT. CONCLUSIONS: Image enhancement significantly improved the agreement of TDOCT RNFLT measurements with SDOCT RNFLT measurements. The difference, and its significance, in rates of RNFLT change in the UKGTS treatment arms was enhanced and RNFLT change became a stronger predictor of VF progression

Implicit subspace iteration as an efficient method to compute milling stability lobe diagrams

A new method for calculation of chip removal machining stability diagrams is proposed. The method can be considered as an application of the Floquet theorem by repeated time integrations and represents an alternative to the previously presented time domain stability methods (semi-discretisation, time finite elements and so on), without the requirement to build the transition matrix. In this way, the computation effort is very much reduced, especially when the required number of segments is large. As a result, the computing time depends on the number of segments with an exponent 1.5, instead of 2.8 that is the exponent for the optimised semi-discretisation. This results in that the presented method is the most efficient among the previous ones. As a further advantage, the memory requirements for the method are much lower, allowing computing very-high-order stability lobes. As a drawback, for the computation of high-order lobes, the method is not as efficient as could be expected, due to the slow convergence of the eigensystem resolution when many eigenvalues of similar magnitude exist. Even in that case, the method is five times faster than the optimised semi-discretisation, but a more efficient eigenvalue resolution method is sought for

Chatter suppression in a high speed magnetic spindle by adding damping

Magnetic bearings are used in several applications, but they didn’t succeed in the machine tool industry, due basically to their low damping, with its associated low chatter free cutting capacity. This work shows the development of a high speed milling spindle supported on magnetic bearings with high chatter resistance. That is achieved by using control strategies which add damping to the bearings. Excitation by means of the magnetic bearings is used to identify its dynamic behaviour and to optimize the damping parameters. Finally the results of the industrial validation are presented; showing an increase of six times in the cutting capacity

Chatter avoidance method for milling process based on sinusoidal spindle speed variation method: simulation and experimental results

International audienceThis paper investigates the effectiveness of Sinusoidal Spindle Speed Variation (SSSV) technique as a chatter suppression method in milling process. On this purpose, a twofold study was carried out: on the one hand, a simulation analysis, and on the other, experimental machining. First, a time domain model of the cutting process was modified to include simulation capabilities concerning spindle speed variation. The results obtained by SSSV techniques mainly depend on the relationship between the chatter frequency and the tooth passing frequency. The SSSV-based strategy for chatter avoidance was embedded in a Computer Numerical Control (CNC) to run experimental tests. They were carried out to show the SSSV technique performance, focused on both high and low spindle speeds. To summarise, this technique lifts the asymptotic stability limits mainly at low spindle speeds