Online Monitoring and Analysis of Hydroabrasive Cutting by Vibration

Abstract

The paper deals with the investigation of accompanying physical process, vibration, arising from the abrasive waterjet cutting of stainless steel. Samples of the square cross-section with the use of preplanned range of technological factors were cut. During the cutting of target material AISI 309 vibration by piezoelectrical accelerometers PCB IMI 607 A11 was recorded. The accelerometers were oriented perpendicular to the direction of the cut. Scanned data were processed through a virtual instrument created in LabVIEW 6.8. Sampling frequency of the recorded signal was 30 kHz. Investigated was development of the RMS value in particular frequencies. In order to confirm hypothetical assumptions about direct relation between vibration emission and surface quality, further experiment will be done. Current results are possible used for detection of defects during hydroabrasive cutting of materials, in case of focusing tube fracture, orifice damage in the cutting head, in case of fragile material cutting by means of controlled penetration.

1. Introduction

One of the most requirement of technologies is increasing trend of surface quality of materials cut by conventional [1 –3] and nonconventional technologies [1, 4, 5] where belongs abrasive waterjet cutting technology (AWJ) [5]. The surface quality is explicitly expressed in a variety parameters that characterize surface topography created during the AWJ cutting process [4]. Those surface quality parameters are exactly identified by using mechanical or optical methods of surface quality measurement. Measured parameters characterize geometrical and shape accuracy on workpiece surfaces [3 –13]. Level increasing methods of monitored quality parameters for achieving the required quality parameters of cut materials are different [5, 14, 15]. AWJ technical equipment producers are searching for all available paths to modifications and improvement of individual components. The aspects of improvement are both in terms of their design principle, material selection, and manufacturing processes in order to achieve higher operating parameters characterizing the AWJ process, that nowadays restricts the wider use of that technology [14 –17]. Specifically following AWJ parametric factors are, increasing the operating pressure from 600 to 900 MPa, reduce the focusing tube diameter below 100 microns, controlled vibration of focusing tube with minimal amplitude of vibration, reducing of abrasive particles [12]. With the use of technological possibilities determined by above mentioned parameters it is possible to achieve on surfaces of thin metallic materials low numbers values of surface profile parameters [18]. Main parameters are the surface profile parameters R_a and R_z, measured on surface, minimal perpendicular deflection and shape precision of semifinished product [14, 15]. Qualitative characteristics of AWJ technology process it is possible use existing technology facilities with the parameters for common industrial practice by enhancing of AWJ control process using the accompanying physical manifestations-vibration or acoustic emission [19 –26]. These secondary manifestations of AWJ interaction with material are appropriate for monitoring and diagnostic, that allow indirect identification of AWJ erosion process inside the kerf. Manifestation of vibration emissions and its specific component, which is acoustic emission can be measured, analyzed, and composed into a sophisticated form. Based on these obtained data it is possible to monitoring AWJ technological process of material cutting.

2. Related Works and State-of-the-Art Analysis

If we consider the technical level of operation and appropriate AWJ technology development, then the scope of research needed to be moved to the development of an automated control of technological processes that ensure precision cutting [17, 18]. This progress is not possible to achieve without the analytical investigation of the main principles, governing the important functions of physics and mechanics of AWJ disintegration processes in the kerf. The principal difference of the presented approach to the problem of interpretation, in comparison to the current approaches, we see in the importance of the final state of surface and in using AWJ mechanical flexibility to describe mechanical state of system at concrete time. Therefore, we start to study the phenomena ongoing during AWJ cutting by means of mechanical oscillation, vibration, and acoustic emissions. In the field of AWJ cutting tremendous works have been recorded. Great deal has been done by researches under supervision of [15, 21, 22, 27]. In their research reports, we can find a usage of acoustic emission (AE) models of AWJ in the place where they tried to monitor AWJ dissipated energy; this effect has been studied by [27]. Kovacevic used the AE in order to monitor AWJ drilling depth of cut [8, 22, 28 –30]. An identification of different removal mechanisms by AE has been studied by Momber et al. [30], and finally Mohan et al. [29, 31, 32] used AE for monitoring of AWJ depth of cut. Kovacevic et al. [27, 33] published the research work, where the vertical force of AWJ has been measured. The goal of the experimental work was online monitoring of surface roughness. But the great deal of the research object is creates very good base for further research, for using the vibration for surface roughness prediction [11, 34, 35]. The AWJ during the cutting material creates mechanical oscillations and vibrations [19, 20, 23]. Despite the adverse effects of vibration during the cutting of material, vibration in itself bears important information about the technological process. From the perspective of mechanism of cutting, the vibrations also carry information which has importance in monitoring of the immediate state of the technological process. This field of research is a worldwide uncovered and unsolved issue; there are few works which marginally address this issue of using vibration in the technological process of cutting materials by AWJ. Explanatory ability of physical and geometric parameters, mainly geometrical values characterizing the surface topography is an essential analytical tool for mathematical modeling and prediction of relationship AWJ-material. Using information from vibration as a feedback is suitable for modeling the interaction AWJ-material for prediction of material penetration, mechanical oscillations, measurement analysis, and correlation of mutual dependence (Figure 3). Vibration emission is a carrier of information on an instant state of cutting process. Within the frame of technological heritage, there is encoded in the vibration record of cutting material the entire process of cutting including the removal mechanism and output parameters of the required product quality. A number of causes both suggestible and nonsuggestible participate in generation of material vibrations. The same factors participating in generation of material vibrations influence the final surface topography and the examined parameters of surface roughness. The most important group of presumed causes of generation of cutting material vibrations are cutting factors including the traverse speed of cutting head v (mm·min⁻¹). Another group of causes contains properties of cutting material in which the examined vibrations spread. A significant representative of the group of abrasive factors is abrasive mass flow rate m_a (g/min). The group of mixing factors contains factors such as focusing tube diameter d_f (mm) and volume distribution of abrasive in the peripheral part of the tool α which is connected with the abrasive feeding direction. The two remaining groups of causes contain factors disturbingly affecting the recorded signal of vibrations. The given factors significantly act upon the examined parameters and therefore considerable attention is given to them in the following parts of the study.

3. Experimental Setup

Machining test (Figure 1) was conducted on the adjustable precise cutting table from PTV company, designed for application of the AWJ technology. The workpiece was machined by tool generated by pump FLOW 9xD55 with Q = 4.7 L·min⁻¹ with a power p = 60 HP with an Ingersoll Rand cutting head. Experimentally created surfaces were prepared according to experimental condition show at Table 1 and Figure 2. Data collection was carried by NI PXI measurement system (a type of measurement card PXI 4472B, 8-channel simultaneous collection, 24 bit A/D converter, sampling frequency of 102 kHz, the dynamic range of 110 dB) and by the frequency analyzer Microlog GX-S. Data analysis was performed by Lab View Professional Development System, including Sound and Vibration Toolset, and Order Analysis Toolset, and Aptitude Analyst SKF Condition Monitoring (Figure 3).

Table 1:

Experimental conditions.

Factors	Values

Type of intensifier (FLOW 9xD55)	Double acting
Power of intensifier (HP)	60
Pressure in hydraulic circuit (MPa)	20
Intensification ratio	20: 1
Maximal pressure (MPa)	380 MPa
Accumulator volume V (L)	2

Cutting head type	Autoline

Pressure p (MPa)	350 MPa
Water orifice diameter d_o (mm)	0.14
MESH	80
Abrasive mass flow rate m_a (g·min⁻¹)	250, 400
Focusing tube diameter d_f (mm)	0.8
Focusing tube length l_f (mm)	78
Angle of attack ϕ (°)	90
Standoff distance z (mm)	3
Traverse speed v (mm·min⁻¹)	50, 75, 100, 150
Cutting direction (°)	180

Material thickness b (mm)	15

Material AISI 309	Chemical composition (C 0,20%, Mn 2%, Si 1%, Cr 22–24%, Ni 12–15%, P 0,045%, S 0,03%) Mechanical properties (HRB 95, μ = 0,27–0,3, E = 200 GPa, σt = 515 MPa, σK = 205 MPa, A = 40%, Z = 50%)

Figure 1:

Experimental scheme and experimental cutting.

Figure 2:

Experimental design for samples preparation.

Figure 3:

Experimental procedure with proposal of online monitoring and control of abrasive waterjet cutting process (a) causal integrity of equipment-created AWJ tool-workpiece-surface quality, (b) detail sketch of AWJ cutting process, (c) recording of vibration emission via sensors, (d) principle of the noncontact measurement of surface quality, (e) amplitude-frequency spectrum of surface, (f) proposal of functional online circuit for surface quality control and regulation of traverse speed with using vibration and acoustic emission as a feedback.

In total sixteen samples were created in four sets as per Figure 2. The first set labelled by A was produced with setting of constant factors m_a = 400 g·min⁻¹ a d_f = 0.8 mm.

The traverse speed of cutting head was set to values of v = 50, 75, 100, and 150 mm·min⁻¹. The B set was produced with setting of factors m_a = 250 g·min⁻¹ and d_f = 0.8 mm. Variation of traverse speed was maintained unchanged.

4. Result and Discussion

4.1. Time Development

Time records of the cutting of this sample set are illustrated in figures (Figure 4). In the beginning the segment I (Figure 5 (a)) shows the increased amplitudes due to the first impingement of the tool onto material (penetration). This phenomenon is possible to be observed in each time record.

Figure 4:

Signal division into segments as per cutting trajectory. The signal recorded in setting of factors v = 150 mm·min⁻¹, m_a = 250 g·min⁻¹, d_f = 1.4 mm.

Figure 5:

Time development of sample signals obtained during abrasive water jet cutting according to (Figure 2).

The most significant increase of amplitudes in this segment assigned to the material penetration is possible to be observed in the record monitored when the traverse speed of cutting head v = 100 mm·min⁻¹ was used. The lowest deflection was recorded at the speed of v = 150 mm·min⁻¹. In the following segment, the head was moving towards the sensor in the direction of its axis. In this segment A (Figure 5 (a)) the highest amplitudes were recorded at the traverse speed of cutting head of v = 50 mm·min⁻¹ at the beginning of the segment.

In segment III the head was moving perpendicularly onto the sensor. In this segment the values of amplitudes were generally lower in all-time records in comparison with the previous segment. Segment IV, similarly to segment II, is oriented towards the direction of sensor. Contrary to the former segment in this one a slight increase of amplitudes occurred at the traverse speed of cutting head of 50 and 100 mm·min⁻¹. At other speeds substantially higher increase of amplitudes could be observed. The highest value was recorded at speed of v = 150 mm·min⁻¹. Following segment V was directed perpendicularly onto the sensor. The most remarkable change in record occurred again at speed of v = 150 mm·min⁻¹. At this point a significant decline of amplitudes occurred. Segment VI is very similar to segment II. In comparison to it the value of amplitudes was moderately increased in this segment. Segment VII catches the moment of the sample separation from parent material. In the last segment of cutting the sample is being more and more released and even its own frequency changes. The vibration record inherited these changes and included them into entire signal composition. The smoothest time record of signal was recorded at traverse speed of v = 100 mm·min⁻¹. Likewise in case of sample set A also with these set B, higher amplitudes of acceleration of segment I are possible to be observed (Figure 5 (b)). Exception is the record obtained when traverse speed of cutting head of v = 50 mm·min⁻¹ was used in case of which the amplitude slowly moves up to the end of the segment. Also the highest deflections in segment II of samples set B (Figure 5 (b)) were recorded with this setting of traverse speed of cutting head. In segment III the signal amplitudes are of rising character in the records monitored at traverse speed of cutting head of v = 100 and 150 mm·min⁻¹. In segment IV only the record monitored at the traverse speed of cutting head of v = 150 mm·min⁻¹ maintained its rising character. In this record the highest amplitudes of the segment were measured. The record showed the highest deflections also in the following segment V. The most nonhomogeneous development in this segment was recorded at traverse speed of v = 75 mm·min⁻¹. It contains high deflections from the basic data mass in short intervals within the entire segment. The highest deflections in segment VI were recorded at traverse speed of v = 100 mm·min⁻¹. On the whole this area shows higher deflections contrary to previous segments with the exception of the record monitored when traverse speed of v = 150 mm·min⁻¹ was used. In the last segment similar deflections were recorded as in the previous one. The exception is the record monitored at speed of v = 50 mm·min⁻¹ in case of which high amplitudes were recorded shortly after the segment beginning.

4.2. Frequency Analysis

FFT spectra of sample set A are shown in Figure 6 (a). As to the shape the most similar are spectra from the records obtained at speed of v = 75 mm·min⁻¹ and v = 100 mm·min⁻¹. The first noticeable amplitude increase is possible to be observed with frequency of 450 Hz. With this frequency the highest amplitude at traverse speed of cutting head of v = 50 mm·min⁻¹ with value of 0.011 g was recorded. The lowest amplitude of 0.0035 g was recorded at speed of v = 100 mm·min⁻¹. In the following area ranging from 500 Hz to 3500 Hz short accruals of amplitudes up to value of 0.003 were recorded. With frequency of 3500 Hz high increase of amplitudes at traverse speed of cutting head of v = 75 mm·min⁻¹ and v = 100 mm·min⁻¹ could be observed. In the proximity of frequency of 5400 Hz similar increase of amplitudes was observed at traverse speed of v = 100 mm·min⁻¹ and of v = 150 mm·min⁻¹. Amplitudes at FFT spectrum at traverse speed of v = 150 mm·min⁻¹ decline from around 5400 Hz and consequently grow up to frequency of around 10200 Hz. In the surroundings of this value increase of amplitudes was recorded in all FFT records of this sample set. Other frequency in case of which the changes of all records became evident apart from records obtained at speed of v = 50 mm·min⁻¹ is surroundings of frequency of 11800 Hz. On the contrary, with frequency of 14300 Hz higher frequencies became evident at all FFT spectra except for the spectrum belonging to the sample produced by means of using the traverse speed of cutting head of v = 150 mm·min⁻¹. The highest amplitude was recorded right at this spectrum with frequency of 5500 Hz. The amplitude reached the value of 0.13 g.

Figure 6:

Frequency analysis during abrasive waterjet cutting of samples (Figure 2).

FFT spectrum of samples set B is shown in Figure 6 (b). The first significant amplitude increasing at traverse speed v = 50 mm·min⁻¹ was detected at a frequency of 450 Hz. Further increasing of amplitudes occurs at a frequency of 600 Hz, followed by three peaks of frequencies 950 Hz, 1300 Hz, and 2100 Hz. Two peaks with higher amplitudes at frequencies of 2850 Hz and 4500 Hz are followed by bands with lower amplitudes created by three peaks 7250 Hz, in the range between 10 300 Hz and 13 600 Hz. The highest amplitude in the spectrum was recorded at a frequency of 1750 Hz with a value of 0.038 g. Similarly, in the FFT spectrum of processed data detected at traverse speed v = 75 mm·min⁻¹ were found tightly placed higher peaks at low frequencies 400 Hz, 700 Hz, 1000 Hz, 1200 Hz, and significantly high peak at a frequency of 1800 Hz. At higher frequencies amplitudes decreasing till peak occured at 3500 Hz. Furthermore bands of higher amplitudes separated by short lower band were recorded. Bands found from 5500 Hz to 7200 Hz and from 7200 Hz to 11500 Hz are followed by two peaks of frequencies 13500 Hz and 14400 Hz. The highest amplitude of this FFT spectrum was detected at frequency 1800 Hz with value 0,004 g. FFT spectrum of vibration recorded during traverse speed v = 100 mm·min⁻¹ compared with previous spectrum differs by central part. In the front part, like in the previous spectra are tightly placed peaks. They occur at frequencies of 160 Hz, 400 Hz, 730 Hz, and 1800 Hz followed by increased amplitudes ranging from 3400 Hz to 4000 Hz. The next peak is at a frequency of 5200 Hz. The middle part of the spectrum contains increased amplitudes in the form “hill” divided by high amplitudes at frequency 7100 Hz and then by decreasing amplitudes at frequency 7200 Hz. Similar decreasing has been found at frequency 9100 Hz. Furthermore on spectrum were found three peaks with high amplitudes values at frequencies 11100 Hz, 12000 Hz, and 13800 Hz. The highest amplitude of the FFT spectrum was recorded at a frequency of 710 Hz with a value of 0.0043 g. FFT spectrum describing the vibrations recorded at traverse speed of cutting head v = 150 mm·min⁻¹ is different from the previous spectra (Figure 6 (b)).

4.3. RMS Parameter Frequency Analysis

The graphs in Figure 7 show RMS values in dependence on frequency measured with the setting of constant factors of d_f = 0.8 mm, v = 50 mm·min⁻¹ and variable factor m_a = 250, 400 g·min⁻¹.

Figure 7:

RMS value in dependence on frequency, setting of factors: d_f = 0.8 mm, m_a = 250, 400 g·min⁻¹ (a) v = 50 mm·min⁻¹, (b) v = 75 mm·min⁻¹, (c) v = 100 mm·min⁻¹.

The curve belonging to higher abrasive mass flow rate shows significantly higher value with low frequencies. Contrariwise, in the surroundings of 3100 Hz and 4500 Hz the noticeable curve peaks occur belonging to abrasive mass flow rate of m_a = 250 g·min⁻¹. In the surroundings of frequencies of 9500 Hz and 10400 Hz dominant are again curve peaks of mass flow rate yet in comparison to curve peaks belonging to m_a = 250 g·min⁻¹ they are shifted to lower frequencies. A striking shift is possible to be observed in a high frequency zone where curve peak of lower abrasive mass flow rate is shifted by approximately 1000 Hz downwards. The graph (Figure 7 (b)) shows RMS values with the setting of factors d_f = 0.8 mm, v = 75 mm·min⁻¹, and variable factor m_a = 250, 400 g·min⁻¹. Noticeably increased peak on the curve of higher abrasive mass flow rate with low frequencies is obvious even in this case. The curve shows higher values also in the zone of frequencies ranging from 12000 Hz to 14000 Hz. The graph in Figure 7 (c) illustrates RMS values with the same factor setting as in the previous cases. The changed factor is traverse speed which was set to value of v = 100 mm·min⁻¹. In this event were recorded higher RMS values belonging to the lower abrasive mass flow rate up to frequency of 5000 Hz. Significantly higher peaks are at frequencies of 2000 Hz and 4000 Hz. With higher frequencies, the curve of abrasive mass flow rate m_a = 400 g·min⁻¹ is dominant. Between the peaks of both curves a shift is visible. The graph of values obtained with the setting of factors d_f = 0.8 mm, v = 75 mm·min⁻¹ and variable factor m_a = 250,400 g·min⁻¹ is illustrated in Figure 7 (b). Likewise in the previous case, in this one the curve of lower abrasive mass flow rate is dominant in the low frequency part. The phenomenon is visible with gradually and sharply increasing peaks. The curve of mass flow rate m_a = 250 g·min⁻¹ shows significantly higher values with high frequencies and that is with frequencies of 12000 Hz and 14500 Hz.

5. Conclusions and Future Direction of Research

In case of frequency analysis the significant amplitude growth in low frequency spectrum parts was observed. With the change of the abrasive mass flow rate the amplitude shift within the frequency spectrum occurred. Special attention was given to the RMS value with the individual frequencies. The values recorded with abrasive mass flow rate of m_a = 400 g·min⁻¹ were slightly shifted to lower values.

The results are possible to be advantageously used for detection of defects in the course of cutting operation, for instance, in the event of focusing tube breakage or orifice damage in the cutting head or in case of fragile material cutting by means of controlled penetration for special applications [36, 37]. Through the frequency analysis the amplitude shift was observed within the frequency spectrum with the change of abrasive mass flow rate. Using this knowledge, it is possible to detect the changes in abrasive supply or faults in the system of abrasive feed.

The study represents a keystone for further research dealing with the design of abrasive water jet cutting control and with the way of making it more effective. It shall be inevitable to focus on research of the individual technological equipment and its impact upon the final signal that is taken as the whole and is typical for the particular operation at the place of which the experiments were performed. The releasing sample itself influences the final composition of vibration signal as well. In material cutting mechanical changes of material properties occur from the perspective of its dynamics. Its own frequency and shape change which affects the vibration spreading.

Through the prediction of surface quality by vibrations, the practical issues are possible to be solved that most of the engineers face in the process of abrasive water jet cutting for the purpose of maximization of production performance and for setting the parameter values of the process which shall result in the required product quality. Using vibrations for determination of cutting performance shall significantly aid the operator in the field of decision making. Technology control is based chiefly upon the experience of attendance that is rather costly item for the company. Therefore it is necessary to utilize the secondary emission in the online process as an up-to-date method and strategy with the aim to predict the cutting process and to regulate the surface roughness in real time.

Footnotes

Acknowledgments

This work has been supported by Project, VEGA 1/0972/11. For practical and professional comments, acknowledgement is dedicated to Dr. Pavol Hreha, Ing. Pavol Adamčík from Technická diagnostika, Ltd., Prešov. For the possibility to realize the experiments, thanks are due to the executive director Ján Mikita from the company of DRC, Ltd. in Prešov.

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