The effect of cutting head vibrations on the surfaces generated by waterjet cutting

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Abstract

After a first period in which the research has been focused on the optimisation of the process parameters, the attention is now focused on aspects that were usually neglected. However, they are very important in order to understand the physics of the waterjet/abrasive waterjet cutting process and to improve the cutting quality.

Particularly, it has been demonstrated that, in the pure waterjet cutting (and in the abrasive waterjet cutting too), there are irregularities, called striations, along the generated surface. The striation formation depends mainly on the jet instability caused by vibrations during the cutting process. Vibration signals have been measured whilst varying the cutting conditions. A model has been studied which estimates the mean spacing and the frequency of the striations, as a function of the period and the amplitude of the jet vibration. This model has been completely validated through measurements of plasticine surfaces generated by waterjet cutting.

Introduction

The waterjet and abrasive waterjet technology is the object of a concentrated research. This research area is oriented to understand the material removal mechanisms and to optimise the process parameters (fluid dynamical and technological) of various applications.

The optimisation of the process parameters is especially difficult because of their instability during the process. Many authors have inquired into the physical–mechanical aspects of the interaction between the jet and the mechanical properties of the material and they developed models and different interpretations. However, the existing models, mostly, try to describe the cutting mechanism only for AWJ technology [1].

In AWJ cutting, the surface is characterised of two different zones: the upper one shows a random roughness profile while the lower one is characterised of a pronounced undulation (striations). The striations are formed when the ratio between the available energy of the jet and required energy of the destruction becomes comparatively small. Thus, the striations can be reduced by the reduction of the traverse rate. This, however, damages the productivity and, what is the most important, does not eliminate the striations.

Research works have highlighted that the striations are caused by internal and external mechanisms of the AWJ cutting process [2], [3], [4], [5].

The traditional interpretation of the mechanism of striation formation in AWJ cutting [6] claims that striations appear beyond a certain depth and they are seen especially in cutting of very thick materials. This result is in contrast with what has been found for WJ technology. In fact, along the whole waterjet cutting surfaces, there are irregularities (striations) with dimensions comparable to the jet diameter. Therefore, the AWJ models do not explain the mechanism of striation formation in WJ cutting.

Section snippets

State of the art

Hashish [7] found the responsible causes of the striation generation in AWJ cutting and he subdivides them into:

  • process causes: the striation are the result of the removal material mechanism;

  • causes involved in the process parameters control: the instability of the process parameters (pressure, abrasive flow rate, feed rate, etc.) is responsible for the striation formation;

  • causes involved in the support equipment: the vibrations of the piece and/or the nozzle during the cutting process cause the

Experimental procedure

The vibrations can be subdivided in external, or exogenous, and in intrinsic, or endogenous. The vibrations on the workpiece caused by the jet, which interacts in air with the workpiece, the equipment and the water in the catcher, are exogenous. The inherent vibrations of the jet caused by the cutting head vibrations, by the movement system, by the intensifier system and by the nozzle, are endogenous.

Every source of vibration has been investigated and their magnitude and effects on surface

Exogenous vibrations

From the mean measures at different depth of the two tests (standard equipment and splash-back damping equipment), reported in Table 1, the following observations come out:

  • as the depth of measure increases, every parameter shows a growing trend, either the mean (Fig. 5a) or the dispersion (Fig. 5b);

  • the surface quality using the splash-back damping equipment (case B) is better than that obtained using the standard equipment (case A). At the same depth of measure, in fact, the case B indicates a

The geometric model

The cutting of very soft materials with the lowest resistance to the jet penetration, like rubber, has shown the striation morphology on the separation surface even from the top edge of the material. This result, in addition to the vibration measurements carried out in several positions of the cutting head, has shown that the striations are caused by multiple vibration phenomenon generated during the cutting process. These are in accordance with the latest trends of research. Therefore, after

Model validation: experimental procedure

In order to validate the proposed model, the experimental procedure is made up of two stages:

  • theoretical estimation of the striation spatial period (X=u/fv) and the striation height (Rw), at fixed operative conditions;

  • comparison of the theoretical values (obtained at the end of the first stage) with the measured values on the cut surfaces.

The model validation is realised by two distinguished phases:

  • measurement of the frequency and the amplitude of the jet oscillation (through the indirect

Conclusions

The WJ cutting quality depends strongly on the exogenous and endogenous vibrations.

A considerable reduction of these vibrations allows to obtain remarkable improvements on the surface quality, especially as regards the striation morphology. In particular, dampened splash-back has effects on the roughness surface: the roughness synthetic indices indicate a considerable improvement.

A decrease on the exogenous vibrations is achievable using simple devices: anchorage of the workpiece-grates to

Acknowledgements

The authors are grateful to UE and MIUR (Italian Ministry of Education, University and Research) for the financial support and Tecnocut for the utilisation of their cutting system.

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