Influence of some relevant process parameters on the dimensional accuracy in incremental forming: a numerical and experimental investigation

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Abstract

As known, incremental forming is a flexible and innovative sheet metal forming process which allows complex shape shells forming without the need for any die.

For these reasons, incremental forming is nowadays suggested for rapid prototyping and customised products.

The present paper is focused on material formability in incremental forming and, in particular, on the evaluation and compensation of elastic springback. The latter significantly modifies the imposed shape.

For this purpose, a deeper assessment of the process was developed following three different approaches. First of all, a wide experimental investigation on the influence of some relevant process parameters was developed. At the same time, an explicit FEM analysis of incremental forming process was carried out in order to verify its effectiveness and, as a consequence, its ability to be used as a design tool. Furthermore, the obtained parts were analysed by a reverse engineering technique and the measured geometry was numerically compared with the desired one, with the aim to quantify the geometrical discrepancies.

In this way, an integrated numerical/experimental procedure is proposed in order to limit the shape defects between the obtained geometry and the desired one.

Introduction

Along with the traditional stamping processes, which are nowadays still used in mass productions, several dramatic changes in the last few decades occurred in order to satisfy new relevant and impellent market demands. Among them the production of customised products, the increasing request of processes flexibility and the necessity to reduce the time to market of the products are probably the most important. On the other hand, metal stamping processes are traditionally characterised by relevant equipment capital and tooling costs and, for this reason, the industrial application has to be economically justified with large scale productions. Furthermore, it is not generally possible to fully satisfy the above mentioned demand of flexibility.

These considerations clearly show that current metal stamping processes may maintain a relevant role in modern production routings just if cheaper and more flexible technologies will be developed. In fact, very large research efforts are spent all over the world in order to achieve such objectives. Innovative sheet metal forming processes have been proposed in the last years, opening unexpected scenarios in the industry: water assisted sheet metal forming processes, such as the hydroforming ones, are only a relevant example [1].

Incremental forming may constitute a suitable solution, especially if one or few parts are produced, with a very simple and cheap approach. In fact, in the simplest proposal, the final component shape is determined by the relative movement of a small punch with respect to the blank, rather than by the dies shape. Such processes are usually carried out on CNC machines, where it is possible to assign and control the punch movement according to fixed paths [2].

Nevertheless, incremental forming is not yet a mature process [3], thus an intensive research activity is performed moving toward different directions:

  • knowledge acquisition on process mechanics;

  • very complex shapes forming;

  • incremental forming drawbacks reduction.

The first research line is aimed to clarify some aspects related to the formability enhancing, obtained by using such a process. In fact, due to the extremely localised deformation imposed by a spherical tool, a very suitable stress–strain distribution occurs, extending formability limits well beyond classical formability curves [4].

An important and industrially suitable research line is related to the production of complex parts [2]. Such task is very interesting in some applications, like the reconstruction of components for which the dies are no more available or, in turn, the development of functional prototypes, i.e. for rapid prototyping.

On the other hand, the incremental forming process suffers of several drawbacks probably because it is an innovative and partially unknown technology. First of all, even if the modern machines allow to operate with high feed rates, incremental forming processes are very slow than the traditional stamping processes, resulting in a very low production rate. For this reasons, from an economical point of view, the break even point is reached for small production quantities even if the equipment capital costs, as well as the tooling ones, are very low. Furthermore, although the process mechanics is based on a pure-stretching deformation, some bending zones are not avoidable, close to the clamping fixture. For this reason a sensible springback is generally obtained when an incremental forming process is carried out, compromising, for instance, the geometrical accuracy.

In this paper, the latter phenomenon is accurately taken into account. First of all a complete experimental campaign was designed and executed on AA1050-O sheet specimens at varying of two relevant process parameters, i.e. the tool diameter and the tool pitch.

In the same conditions the process has been numerically simulated by using a modern explicit finite element code.

The deformed geometry of the experimental parts have been acquired by using a laser-based scanning technique which allows to rebuild the surfaces, starting from a cloud of points. In this way, the main discrepancies between the actual part and the desired one have been calculated and compared with the ones predicted by the numerical code.

A very good agreement between the numerical prediction and the actual geometry has been pointed out. Furthermore, the possibility to develop a design strategy in order to minimise the shape errors on the part obtained by incremental forming is foreseen; actually such a strategy involves several further considerations and, as discussed in the coming sections, a preliminary approach is proposed as a conclusion of this work.

Section snippets

The experimental campaign

The research activity was carried out through the two following phases: first of all a set of experiments, characterised by different geometrical conditions, were performed by using a properly designed equipment, able to clamp the sheet metal during the process. Subsequently, after the deformation process, the obtained shape was analysed by using a particular reverse engineering technique (see Section 4).

The experimental campaign was carried out on a 3-axes controlled milling machine and a

Numerical analysis of the problem

Incremental forming is a progressive sheet metal forming operation characterised by large displacement and strains. In order to simulate such operation utilising the finite element analysis, two different approaches could be pursued, namely an implicit model implementing a Lagrangian formulation or an explicit model [6]. The former approach is characterised by a non-linear equations system solved through an iterative procedure; the latter is based on the dynamic equilibrium equation of the

Acquisition of the actual geometry

One typical application of the reverse engineering techniques consists of the comparison between a manufactured product and a virtual one. This task becomes very interesting when it is carried out taking into account the model generated in the design phase and the obtained product; in such a way, in fact, it is possible to evaluate the geometrical errors related to the actual manufacturing process.

A laser-based scanning system as well as a CMM machine are usually utilised to pursue the above

Discussion of the results

As described above, the aim of the paper is the evaluation of the geometrical error that occurs when the one-point incremental forming process has been run. For this reason, the clouds of points, acquired by using the above described technique, have been compared with the expected geometrical model. In the next figures, different sections and view are reported in order to get an immediate representation of the discrepancies between the desired and experimental geometry. The latter can be read

Conclusions

In this paper, the dimensional accuracy of the incremental forming operations was accurately discussed and analysed by using and effective experimental technique, highlighting very relevant discrepancies between the designed surfaces and the obtained ones.

This shape may be satisfactory predicted by using the modern FE codes.

One of the most promising way in order to assess incremental forming also as a net-shape forming process seems to be the design of optimised trajectories that result in more

Acknowledgements

This research was supported by the grant of the Italian Ministry for Education, University, and Research (MIUR).

References (8)

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