Increase of the reliability of the machines of cut by water and abrasive through the monitoring

The technology of cut by stream of water and abrasive is a process of mechanised flexible, cheap and fast that it can mechanise any type of material, even the ones of drop maquinabilidad as they can be the Inconel and the Titanium. However, had to especially to the fault of integration of systems of monitoring and control of the process, the reliability that offers this type of industrial machines is drop in comparison with other technologies.

During the operations of mechanised that use the technology of cut by water and abrasive can exist multiple deviations of parameters with regard to his values of consigns that they cause that the machines work but without achieving good pieces. Ésto In addition to producing a high waste of time and of energetic resources supposes a high economic risk to cut pieces of high value added in which the cost of the prime matter is high, and therefore, ensure a high reliability of the process is crucial.

The technology of cut by water and abrasive (AWJ) bases in a stream of water to high pressesure to the cual add him particles of abrasive that they are directed by a filter focalizadora. In the cabezal forms then a stream composed by water presurizada, air and abrasive to speeds around 300 and 1000m/s that it is able to delete the material wished by means of the erosion of the same. That is to say, the technology of cut by water and abrasive (abrasive water jet) consists in doing incidir the stream on a material the sufficient time so that the stream cross the same [1].

Is a process of mechanised flexible, cheap and fast that it can mechanise any type of material. However, the process of cut by AWJ presents some practical limitations that condition his productivity, the evolution in the machines of water has centred in the increase of this, increasing for this the hydraulics pressesure. It is true that to greater pressesure, obtains a greater power of cut, but this increase in the power of cut carries associated other collateral damages, as for example, an increase of the wear of the components of the system of high pressesure or a greater wear of the tool of cutting. This evolution in the systems of high pressesure has left in evidence other problems concernientes to the technology, like the low incorporation of sensors to arrive to a complete automation of these commercial machines.

Exist multiple failures or deviations of the process with regard to his values of consigns that they cause that the quality of the pieces obtained was not the required. These failures can have to to the wear of the filter, to the jam of abrasive in the filter or in the pipe of abrasive, to the break of the filter or to the escape of water in the pipe of water to high pressesure. These limitations hamper the production neglected of the machines of AWJ, by what is necessary to develop a system for the detection of failures associated to these causes.

In the bibliography can find several technicians for the monitoring of the possible failures that can occur in the machines of cut by water. Annoni Et al [2] studied the possibility to control simultaneously the discharge of abrasive and the transversal speed, for what developed an able device to control continuously the discharge of abrasive. Louis and Meier [4] relate the diameter of the filter with the flow of air succionado by the filter through the tube of feeding of abrasive, as well as the losses of pressesure in the tube of feeding of abrasive. If the tube focalizador changes his form of significant way during the manufacture of a piece is impossible to carry out a precise cut [5], of here the need to monitor the wear of these pieces to ensure the quality of cutting. The detection of wears and breaks is also possible with sensors of acoustic broadcasts and with the measurement of the level of empty in the camera of mixes [7].With the aim to control the distance of trabajof, Jurisevic et al. [8] they realizar a study relating this distance of work with the audible level that generates in the process, . The results reached showed that it is possible to filter this signal and relate with the distance of work but always under conditions of work controlled, but showed difficulties to be implemented in conditions of real work where external factors generate levels of noise that disturb the original signal of the process. Axinte And Kong [9] measure the remanent strengths of the stream after the cut using for this a table Kistler, the aim that pursue is to relate this strength with the estriacionesde the pieces cut.

Has remained patent the importance to treat to control the maximum number of parameters that affect to this process, but everything Is important to control the maximum of possible parameters to be able to detect in addition to the possible failures owed to the supply of abrasive and to the wear of the pieces, any another problem (as it can be a failure in the pressesure supplied by the bomb) that it can influence in the quality of the pieces cut.

The failures by jams of abrasive are very common and can give place to a jam of the filter or to a jam in the pipe of abrasive. In the cases in which it obstructs the pipe of abrasive the machine is still working, to the not having of abrasive in the stream loses the capacity of cut of hard materials or of high thickness, by what although the machine is still working, only achieves mark the sheets and not to cut them, wasting so much time like prime matter and power.

Poses a system for the monitoring of the instantaneous conditions of the process, resulting an able system to detect of effective form the situations of jam in the pipe of abrasive and of the filter.

To develop this system previously carried out several essays to arrive to the final system. Can use different methodologies for the detection of a jam in the supply of abrasive. One of the options is by means of the detection of the sound (LAT) isolating the different sources of noise. Jurisevic And Junkar [3] have achieved a system based in this principle that can integrate in the machine. However in the essays carried out in Tecnalia R&I the measure of the noise does not result to be a very robust method, since they exist multiple sources that cause levels of high noise (although in his elder splits instantaneous) that at all have to see with the consumption or no of abrasive: movements in empty of the machine, changes of steering, perforaciones, entrance or exit of the stream of the sheet to cut and others.

Another of the options studied consists in the measurement of the electrical resistance between entrance and exit as well as the temperature in the pipe of supply of abrasive, since taking into account that the water is conductive and heats can detect the presence of water in the pipe of abrasive. In the interior of the tube the water causes a brusque change of temperature of at least 30°C. The temperature registered in normal operation is not constant, have measured slow swings between 28°C and 10°C, in function mainly of the rebounds and salpicaduras of water on the system of detection. The flow of air (or air and abrasive) by the interior of the tube cools the sensor of temperature and if the salpicaduras are low, the resultant temperature is lower to the environingingment. If they occur salpicaduras, the temperature can reach the almost 30°C. In any case, the variations been due to these reasons are slow. When it occurs a jam of filter, the rise of temperature is sudden, almost instantaneous and of temperature very upper to the 30°C.

Finally, the most reliable option bases in the monitoring of the depressesion generated in the tube of abrasive because of the effect Venturi that causes the stream of water in the camera of mixes. This depressesion depends on the speed of the stream and of the opposition generated in the tube. It deduces that it will exist a value of depressesion for normal conditions of cut, that if the tube of abrasive atasca in a point between the point of measure and the camera of mixes will not exist depressesion and that if atascara in a point between the point of measure and the taking of abrasive the depressesion will be maximum. However, this depressesion in usual conditions is very small, therefore opts for entering a narrowing in the pipe of abrasive to generate a sufficient depressesion to be differentiated, but that to his time allow to circulate the discharges of abrasive originals. They have carried out essays with different estrangulamientos commercial of the mark FESTO, arriving to the conclusion that so that the measurement of the fall of depressesion was reliable is recommended the use of an estrangulamiento with a different inner geometry to the used in the estrangulamientos commercial. Therefore it has designed an estrangulamiento suitable for being able to obtain an optimum signal to the hour to detect any possible failure without that his insertion in the machine suppose any variation in the process.

In the Figure 2 shows a chart of the depressesion generated in the tube of abrasive when it forces a jam in the same with the estrangulamiento designed in Tecnalia. Can see that the fall of pressesure is sufficiently stable and differentiable, what allows that this was a reliable system for the detection of jams in the tube of abrasive.

Has developed a system for his installation in machine, that thanks to the utilisation of the estrangulamiento designed and to a sensor of pressesure installed in the pipe of abrasive, is able to detain the machine when it produces a jam in the same (Figure 3). The system will work so that when it exist a demand of abrasive and was not arriving to the cabezal of cut, this detect it and stop the machine. When it do not exist demand and there is not neither consumption, the machine will continue working usually to the equal that when it exist demand and there is consumption.

3.1 Measurement Wear out Filter and Sapphire

So much the filter like the sapphire suffer wear in the cabezal of cut due to the fact that through them happens the water to high pressesure constantly, if the particles of abrasive arrived until the sapphire can even produce the break of the same so that the stream does not arrive to form properly (Figure 4).

The monitoring of the wear of these pieces can carry out of distinct ways. The same system of control for the supply of abrasive consistent in the measurement of the depressesion created in the tube of abrasive serves also to detect a break or wear of filter and sapphire. In the proofs realizar has detected that the value of the depressesion varies in function of if these pieces find in good condition or no. In the Figure 5 can appreciate the signal collected of depressesion in the tube of abrasive using abrasive in different state (new, worn out and broken), sees that according to the health of the worsens the depressesion generated is greater.

The detection of the noise is another of the options to the hour to detect a wear of the pieces of the cabezal of cutting. It situates a microphone beside the cabezal of cutting and measures the noise generated during the process relating the variations in the signals collected with wears or breaks of the distinct pieces. In this case, because of the noise that comports the process is complicated to detect signals of clear noise that can indicate a wear with reliability. In the Figure 6 can see as in the signal of noise, although it appreciates that it exists a change, is not easily identifiable since it is a small variation and besides treats of a little stable signal.

The detection of wears and breaks is also possible with sensors of acoustic broadcasts and with the measurement of the level of empty in the camera of mixes [7]. It situates a sensor in the subject filter by means of a metallic piece and another on the material in which it is working (can see this configuration in the Figure 9). With this method will be able to detect if the sapphire is in good condition and if it produces wear in the filter.

Have carried out essays with a sensor of acoustic broadcasts installed in the filter of the cabezal of cut changing the parameters of the stream (pressesure and discharge of abrasive) with the end to know if the variations in the stream are detectables by means of this type of sensors. As it can see in the Figure 7 observes that by means of the obtaining of this signal can arrive to detect small variations in the stream, as it is the case of a variation of abrasive of so alone 50g/min.

This fact opens the doors to a wider experimentation with the end to establish the ranks in which it will find this signal as they go wearing out sapphire and filter.

3.2 Power of the stream

The process of learning AWJ consists in accelerating the abrasive particles with a stream of water to high speed. Once the stream of water to high pressesure goes through the orifice situated in the high of the camera of mixes turns into a stream to high speed in which they inject the particles of abrasive, these accelerate in his trascurso by the filter generating a stream with a big power of cutting that bumps against the material eroding it. According to the size of the sapphire and the filter, the strength that will have the stream will be distinct, by what controlling the strength of impact of the stream on the material will be able to detect possible wears in orifice and filter. It is possible to relate also the marks that exist in the material cut with possible variations in the power of the stream, what allows to know the quality of the cut in function of the signals of the strengths of impact.

One of the possible ways to measure the strength of the stream is using a table dinamométrica to measure the excess power of the stream after cutting the material and relate it with the qualities of cutting (Figure 9). It measures the impact of the stream in the table to the exit of the cut, in which they remain small estriaciones whose size and frequency will depend on the power of the stream in this moment. What worse was the quality of the lower cut will be the strength that will exert the stream on the table.

In the corresponding essays in addition to the obtaining of the strengths with the table Kistler, plant also sensors of acoustic broadcasts to detect if it exists some relation between the estriaciones that produce in the piece and the signals collected of strength and acoustic broadcasts.

Arrives to the conclusion that the strengths measured with the table Kistler do not save a lot of relation with the estriaciones measured on the piece. However, in the study of the signals of acoustic broadcasts can appreciate that realizar an analysis in frequency the values obtained correspond with those measured on the piece (Figure 10).

Therefore, deduces that by means of the analysis of the signals of acoustic broadcasts can arrive to know the estriaciones that will obtain in the final piece.

3.3 Pressesure and escapes

The fundamental base of the process is the water to high pressesure, a variation in the pressesure can be signal of desgate of some component of the bomb or of an escape in the pipes of high pressesure. It knows thanks to the experimentation realizar that monitor the pressesure can help to know the health of the sapphire, have obtained data with different sizes of sapphire that indicate that the pressesure that arrives to the cabezal is different according to the diameter of the sapphire. In the Figure 11 can see the difference that exists in the signal of pressesure when it changes the size of the sapphire.

Having of two sensors of pressesure can arrive to detect the escapes or the wears of the components monitoring at all times the difference of pressesure between a sensor and another.

With the measurement of the pressesure in the cabezal of cutting has seen that the pressesure that arrives to the same is not the same that the one who provides the bomb and indicates the machine, but they always exist some losses of pressesure owed to the number of elbows, length of the pipe, etc. Controlling the value of the pressesure in cabezal controls that the pressesure of cut is the suitable and knows for the treatment of the signals and the back calculations the real value of the pressesure that is using during the essays [10].

What pretends with the introduction of a second sensor to the exit of the bomb is to know really if the value of the pressesure in the cabezal is because of the own losses of the distribution of the machine or because the water does not go out of the bomb with the value of pressesure comandado from the control.

In this article show the main problems that produce during the mechanised by means of stream of water and abrasive in addition to some of the technicians of monitoring that can be used. It has showed that they are many the parameters that take part and have effect in the cut with this technology. Each one of these parameters of the process has a distinct effect in the pieces cut. The control and monitoring of the distinct parameters can help to do of this process, that in these moments is little monitored, a more reliable process and precise so that it was easy so much predict a wear in the filter or the sapphire like detecting an error in the piece by fault of pressesure or of abrasive, being able to like this stop the machine and avoiding of this big form quantities of material and time wasted.

Although the technicians showed are reliable to level of laboratory, the main problem finds in how incorporate all these sensors inside a so aggressive environingingment like the water, the dust and the abrasive.

The authors want to appreciate the support and the implication during this project to the company MTorres, in addition to appreciating the support received from the Basque Government through programs of support to projects of industrial investigation and experimental development of strategic character.

References

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[3]. B.Jurisevic And M.Junkar, Adaptative Control Constraint (ACC) of Abrasive Water Jet Cutting, presented at the International Conference on Water Jetting Machining WJM, Cracow, 2001, pp.71-76.

[4]. H.Louis and G.Meier, Methods of Process Control for Abrasive Water Jets, presented at the 6th American Water Jet Conference, Houston, Texas, 1991.

[5]. J.J Rozario Jegaraj And N.Reamersh Badu, To soft it approach for controlling the quality of cut with abrasive waterjet cutting system experiencing orifice and focuing tube wear, Journal of Materials Processing Technology, vol. 185, number.1-3, pp.217-227, apr.2007

[6]. T.Liu, Omax Corporation USES, Evaluation of nozzle health states for AWJ machining, 19th International Conference on Water Jetting.

[7]. M. Hashish, D.Or. Monserud, and P.D. Bondurat, To new abrasive toilet jet nozzle for automated and intelligent machining, in 7th American Water Jet Conference, Seattle, Washington, 1993.

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[9]. D.To.Axinte And M.C.Kong, An integrated monitoring method to supervise waterjet machining, CIRP Annals – Manufacturing Technology, vol.58, number.1, pp.303-306,2009.

[10]. To.Lebar, B. Jurisevic And M. Junkar, Monitoring of the AWJ cutting in the submerged conditions, in 2003 WJTA American Waterjet Conference, August 17-19, 2003, Houston, Texas.