Antonio Scippa

A heuristic approach to meet geometric tolerance in High Pressure Die Casting

Abstract In High Pressure Die Casting (HPDC), geometrical distortions usually happen during the cooling phase, due to the reduced cooling time and the high thermal gradient inside the product itself. This phenomenon affects most the thin walled products. The usual die design practice considers only the linear shrinking of the product during the cooling as a consequence of the difficult to take in account also the geometrical deformations. In this essay a simple finite element design strategy that allows the designer to improve the die shape is presented. The proposed approach uses an automatic iterative optimization technique based on a heuristic algorithm, which could be easily applied to most of the Finite Element (FE) commercial software: the basic concept of the method is simply to move the nodes defining the die surface in the opposite direction to the error due to the cooling phenomena. An automotive component has been selected as a case study: the aim was to improve the planarity tolerance of a planar surface of the casted product. Results show the efficiency of the proposed method that, despite its simplicity, is able to provide an optimal solution with a small number of iterations.

On the effect of testing uncertainties in the homologation tests of motorcycle helmets according to ECE 22.05

Industrial products designed for a specific task need to prove their capability to fulfil their design goal through experimental homologation procedures. Homologation standards define the necessary steps for these experimental tests in a very detailed manner but still are not entirely capable of accounting for any imprecisions in the definition of the testing procedure and variability in its carrying out. This paper addresses the issue of testing uncertainties that are related to the homologation standard ECE 22.05 for motorcycle helmets. Finite Element simulations are performed in the framework of an uncertainty analysis based on fuzzy-valued testing parameters, which are an adequate and very practicable method to represent the homologation uncertainties. Through the uncertainty analysis, large uncertainties are found for the relevant homologation quantities. They are entirely in agreement with the homologation standards. Moreover, the most important testing parameters are identified and recommendations for improving the homologation standards are formulated.

Numerical investigation of chatter suppression in milling using active fixtures in open-loop control

Among the chatter suppression techniques in milling, active fixtures seem to be the most industrially oriented, mainly because these devices could be directly retrofittable to a variety of machine tools. The actual performances strongly depend on fixture design and the control logic employed. The usual approach in the literature, derived from general active vibration control applications, is based on the employment of adaptive closed-loop controls aimed at mitigating the amplitude of chatter frequencies with targeted counteracting vibrations. Whilst this approach has proven its effectiveness, a general application would demand a wide actuation bandwidth that is practically impeded by inertial forces and actuator-related issues. This paper presents the study of the performance of alternative open-loop actuation strategies in suppressing chatter phenomena, aiming at limiting the required actuation bandwidth. A dedicated time-domain simulation model, integrating fixture dynamics and the features of piezoelectric actuators, is developed and experimentally validated in order to be used as a testing environment to assess the effectiveness of the proposed actuation strategies. An extensive numerical investigation is then carried out to highlight the most influential factors in assessing the capability of suppressing chatter vibrations. The results clearly demonstrated that the regenerative effect could be effectively disrupted by actuation frequencies close to half the tooth-pass frequency, as long as adequate displacement is provided by the actuators. This could sensibly increase the critical axial depth of cut and hence improve the achievable material removal rate, as discussed in the paper.

Effects of cutting conditions on forces and force coefficients in plunge milling operations

The modeling of milling forces is a crucial issue to understand milling processes. In the literature, many force models and experiments to identify force coefficients are found. The objective of this article is to develop a new approach, based on the traditional average force method, able to measure and compute the cutting coefficients for end mills used in plunging operations. This model has been used to evaluate the effect of the radial engagement on the cutting coefficients themselves, proposing a new strategy to update these values for different cutting parameters. This dependency of the cutting coefficient is particularly important for the determination of the stability lobe diagrams, used to predict the chatter conditions. In this article, the method to assess the cutting coefficients, the results of the experimental tests, and the effect of condition-dependent cutting coefficients on process stability are presented.

Investigation and Correction of Actual Microphone Response for Chatter Detection in Milling Operations

Integrating sensors in machine tools for monitoring purpose entails dealing with different issues, not only related to accessibility and safety but also to measureable bandwidth and linearity of the sensors. Those factors could be related to the sensor itself but also to sensor–machine interaction that could drastically affect sensor performances and reliability. This paper presents a dedicated experimental investigation of the actual response of microphone transducer inside the machine-tool chamber, highlighting the effects of the machine-tool chamber in altering response linearity. The identified response is then processed with specifically developed equalization filters to correct the measured response and rescale the amplitude of frequency contributions, as required by most chatter detection techniques. The main aspect of both the experimental identification procedure and the development of an effective correction approach are presented and discussed. Finally, the technique is tested in processing signals acquired in experimental chatter tests to estimate the achievable improvements.

Two-points-based receptance coupling method for tool-tip dynamics prediction

ABSTRACT Tool-tip frequency response function (FRF) is essential to predict chatter vibration in milling. This key input can be acquired by experimental tests, but a new test has to be performed for every tool clamped on the machine. To avoid such time-consuming procedures, receptance coupling methods have been developed, allowing coupling of the experimental dynamic response of the machine to the numerical model of the tool. Such techniques require joint rotation response, which is hard to experimentally identify. Inversion of receptance coupling technique is usually performed on additional experimental measurements to overcome this issue. This procedure amplifies measurement uncertainties, reducing accuracy of the coupling approach. In this article, a novel receptance coupling technique is presented. Machine and toolkit are connected through two distinct points, eliminating the experimental phase and computation of rotational degrees of freedom (DOFs). Only translation responses are required, acquired by a single test setup. Proposed technique was experimentally validated on different case studies.

Analytical – FE simulation of a multi-jet electrospinning process to predict material flow

Abstract Electrospinning is a technology used for the production of nanometric fibers starting from a solution of material spun by a needle in an electrostatic field. The jet starts from a needle and its diameter is reduced thanks to the instability of the process that stretch the fiber till nanometric dimension. The productivity of a single needle is very low so multiple needles facing the same collector is the simplest and most used apparatus to achieve an adequate productivity. However jets so produced repel each other making their path diverge from the axis of the needles; this effect can be corrected introducing a system of electrostatic lenses. As soon as the diameter of the filament is commonly tens of nanometers the FE simulation of the process in the work area is nearly impossible due to the very large number of elements required. This paper presents an hybrid approach that couple together an analytical analysis with an FE approach in order to reduce the computational time. The developed model is able to predict the divergence of electrospun multi-jets with and without the corrective effect of an electrostatic lens. The developed approach has been validated thanks to laboratory experimental tests that has proven its accuracy. A simulated test campaign using the Design of Experiment approach has been performed to create a mathematical model to predict the deflection of the filaments with different process parameters.

Case Study 1.3: Auto-adaptive Vibrations and Instabilities Suppression in General Milling Operations

In general rough-milling operations, unstable tool vibrations due to the interaction between process forces and tool flexibility could arise. The onset of these unstable vibrations, usually referred to as chatter, poses limitations in terms of the achievable material removal rates, hence directly impacting on the productivity. Moreover, chatter vibrations generally lead to an increase in tool wear, imposing premature tool changes and careful monitoring of the process, potentially impeding unmanned operations. Within the INTEFIX project, an active fixture prototype was developed to detect and mitigate the level of chatter vibrations in general rough-milling operations with the purpose of improving the achievable material removal rates. This contribution covers the main aspects of the global development of this prototype, from the mechanical design to the adaptive control logic used in order to drastically reduce the inputs and expertise required for its operability.

Development of an artificial vision system for the automatic evaluation of the cutting angles of worn tools

This article presents a new method to evaluate the geometry of dull cutting tools in order to verify the necessity of tool re-sharpening and to decrease the tool grinding machine setup time, based on a laser scanning approach. The developed method consists of the definition of a system architecture and the programming of all the algorithms needed to analyze the data and provide, as output, the cutting angles of the worn tool. These angles are usually difficult to be measured and are needed to set up the grinding machine. The main challenges that have been dealt with in this application are related to the treatment of data acquired by the system’s cameras, which must be specific for the milling tools, usually characterized by the presence of undercuts and sharp edges. Starting from the architecture of the system, an industrial product has been designed, with the support of a grinding machine manufacturer. The basic idea has been to develop a low-cost system that could be integrated on a tool sharpening machine and interfaced with its numeric control. The article reports the developed algorithms and an example of application.