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An instrument for measuring abrasive water jet diameter
ARTICLE IN PRESS International Journal of Machine Tools & Manufacture 49 (2009) 843–849
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An instrument for measuring abrasive water jet diameter H. Orbanic´ , M. Junkar, I. Bajsic´, A. Lebar University of Ljubljana, Faculty of Mechanical Engineering, Askerceva 6, SI-1000 Ljubljana, Slovenia
a r t i c l e in fo
abstract
Article history: Received 10 March 2009 Received in revised form 21 May 2009 Accepted 27 May 2009 Available online 6 June 2009
In order to improve the accuracy of abrasive water jet (AWJ) machining the precise value of the jet diameter has to be known. Because of an aggressive environment caused by high velocity abrasive grains, the diameter is not easily measured. That is why a measuring device consisting of a load cell and a wear resistant probe was developed. The device measures the force of the jet while it passes over the edge of the probe. If the feed rate of the jet is constant and the time needed for jet to pass is known, the diameter can be determined. Because of probe wear issue several preliminary tests were made with water jet only in order to determine the measuring uncertainty and accuracy of the device. In the end the measurement of the AWJ was performed for two different focusing nozzles of different diameters. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Abrasive water jet Jet diameter Jet force Measurement uncertainty Measurement accuracy
1. Introduction Cutting with abrasive water jet (AWJ) is a non-conventional machining process which uses high speed water jet for accelerating very hard abrasive grains and thus enabling the removal of workpiece material. While the technology has many advantages like cutting of wide variety of materials independent of their material properties and absence of heat affected zone, it is limited by poor accuracy in comparison to other machining processes like laser, WEDM or milling [1]. The problem lies in the geometrically non-defined and flexible tool as it has a multiphase structure and it disintegrates in the air atmosphere, which is difficult to analytically analyze [2,3]. Another source of inaccuracy is the specific surface texture and tapered cut which is characteristic for this type of process. While there is no current solution for the surface, the taper problem is currently solved by tilting the cutting head [4,5]. The other also significant problem is determining the dimensions or the diameter of the AWJ as these data are used for setting the tool offset while cutting. Currently this is done by using the radius of the focusing tube as a tool offset. While this is good for when the focusing tube is new and if we are satisfied with the accuracy up to 0:1 mm, it is not good after the tube starts to wear or we want to achieve machining accuracies less than 0:1 mm. That is why a method is needed which would measure the AWJ diameter at the stand-off distance from the nozzle exit. The sometimes used solution is to carry a test cut in
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E-mail address: [email protected] (H. Orbanic´). URL: http://www.fs.uni-lj.si/lat (H. Orbanic´). 0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2009.05.008
the case that better accuracy is needed and measure the kerf dimensions. This procedure is not practical from the point of material consumption and the time needed to get a good result for different cutting directions of the jet. That is why an instrument is needed which would directly measure the AWJ diameter on the machine itself prior to the machining and thus accordingly correct the tool offset in the machine controller. Because of very aggressive environment inside of the AWJ different techniques were used in the past in order to measure the condition of the jet. One of the methods is measuring kinetic energy or velocity of the jet by laser Doppler anemometry (LDA) [7] or the laser transit anemometry [6]. The measurements have to be made in the laboratory conditions and are not very practical. More practical is to measure the force of the jet. Momber and Kovacevic [8] have measured the impact force of water jet and AWJ perpendicular on a plate in order to determine the velocity of the jet. The last methods used were optical methods where the jet is observed by a CCD camera [9], where the shape or the diameter of the jet is optically measured. The problems of optical methods are discerning between the compact body of the jet and the droplets which surround it as the jet is not a solid tool. That is why also methods like laser micrometers are difficult to implement as it is in the case of a solid tool [10]. In this research, a new measuring method that utilizes a cantilever load cell for measuring the diameter of the water and abrasive water jet is presented. The device measures the jet force as the jet passes over the edge of the wear resistant probe mounted on the load cell. The device is robust and resistant to moisture and residual abrasive effects. The only major problem is the wear of the probe when the AWJ diameter is measured. By using better wear resistant materials for the probe will enable
ARTICLE IN PRESS H. Orbanic´ et al. / International Journal of Machine Tools & Manufacture 49 (2009) 843–849
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implementation of the device on a commercial AWJ cutting machine. The purpose of this paper is to describe the principle and structure of the measuring device, assessment of its accuracy and performing test measurements on AWJ.
2. The measuring principle and instrument setup The configuration of the device for measuring the diameter of the jet is shown in Fig. 1. The device consists of a load cell with a fixed wear resistant probe. The cell is mounted through a fixture on the AWJ machine catcher wall. The load cell Vishay Transducers 1022 is a cantilever type with binocular shape intended for measuring of axial forces. It has an IP66 protection against dust and moisture and can measure maximum loads of 7 kg at the rated output of 2.002 mV/V and total error less than 0.02%. The compensated temperature range is between 10 and 40 C. The probe has to be from a hard material with well-defined edges. This is why a carbide insert used for turning was used. The photograph of the device placed on the cutting table of an OMAX 2652A AWJ cutting machine is shown in Fig. 2. The measurement method for measuring the jet diameter was conceived so that the impact force of the jet would be measured while it passes over the edge of a probe mounted on a load cell as it is shown in Fig. 3. The jet is moving with a constant velocity v and when it touches the probe, the measured force starts to rise. When the whole diameter of the jet has passed over the edge of the probe the force ceases to rise. If the time t d when the force starts and when it ceases to rise is known, the diameter of the jet d can be determined by multiplying the time with the traverse velocity of the jet (Eq. (1)) d ¼ v td .
jet traverse velocity v nozzle
part 1: force is zero, the jet is approaching the probe, part 2: force is rising, the jet is passing over the edge of the
F
F
(1)
The important part of measurement process is determining the correct start and end of time interval of jet passing over the edge. The logical thing would be to look for the significant change in gradient of the curvature. But by doing this the result can be overestimated. This is because the water droplets, which move around the core of the jet, produce the rise in the measured force. To eliminate this, a method was developed which first cuts off the influence of droplets and also automates the diameter measurement process. This is done by dividing the signal on three parts and fitting a line over each part. The three parts are shown in Fig. 4 and are the following:
probe,
Fig. 2. Photograph of the jet diameter measurement device.
probe
td
v
d = v · td Fig. 3. The principle of measuring jet diameter by measuring impact force.
part 3: force has stabilized, the whole jet is on the probe.
Fig. 1. The configuration of the jet diameter measurement device.
Fig. 4. The three line method.
t
ARTICLE IN PRESS H. Orbanic´ et al. / International Journal of Machine Tools & Manufacture 49 (2009) 843–849
SSE ¼
n X
ðF^ i F i Þ2 ,
1 v = 0.5 mm/s v = 1 mm/s v = 1.5 mm/s v = 2 mm/s
0.9 0.8 0.7
Normalized SSE
The function for the first line is easy to determine as it is put simply as F n1 ¼ 0, where F n is the normalized measured jet impact force. The second line was fitted over the upper 10% of the signal with the function F n2 ¼ k2 x þ n2 . This has to be done because the measured force fluctuates once the jet is on the probe. The third line in the form of F n3 ¼ k3 x þ n3 is fitted over the slope of the signal but before the fitting is performed, the size of the part between points A1 and A2 in Fig. 4 has to be determined in order to get the most accurate result. It was decided to position the line in the middle of the signal, where the size of the slope which will be considered was determined by a sum of square errors (SSE) method as defined in the following equation:
845
0.6 0.5 0.4 0.3 0.2
(2)
i¼1
0.1
where F^ i is the fitted line, F i is the measured force signal on the chosen part of the slope and n the number of samples. The determination of the size or the percentage of signal A in Fig. 4 will be described in Section 3.1.
0 0
10
20
30 40 50 60 Middle part of signal (%)
70
80
90
Fig. 5. The SSE analysis of the line fitting on the slope of the force signal.
3. Experimental verification The experimental work performed in this research has included (1) the determination of the part of signal used for the line fitting on the slope of the signal, (2) determination of the maximum traverse velocity for obtaining useful measurements, (3) determination of the accuracy of the implemented device and (4) testing of the device at measuring the diameter of the AWJ. Although the main purpose of this research is to experimentally verify the performance of the device when measuring the diameter of AWJ, the first three parts of experimental verification were performed by using only pure water jet because of the wear issues of the probe. This enabled us to test the measurement repeatability of the device before moving to the measuring of the AWJ diameter. Wear of the probe is a major issue, which will be dealt with in the next step of research.
3.1. Determination of the slope size For this part of experiment a 0.25 mm diameter water nozzle was chosen and the water pressure was 300 MPa. Other combinations of orifice and water pressure could be used in this case but this would only change the flow conditions and would not significantly influence the results of the measurement. Four different traverse velocities were chosen whose values are 0.5, 1, 1.5 and 2 mm/s. The chosen values were low because there was a concern that higher velocities might cause the response of the instrument to change from a ramp to a step response. This was later shown to be true when the traverse velocity limit was determined. For each velocity the force was measured when the jet passed over the edge of the probe. Each signal was then divided into areas as shown in Fig. 4. In order to get the best fit of the line over the slope, 15 different sized parts between 5% and 95% of the signal around the middle (distance A in Fig. 4) were selected. For each part the slope was fitted with a line and SSE was calculated. The results for the normalized SSE for all four traverse velocities are shown in Fig. 5. It can be seen that if more than 60% of the signal is taken for the fitting of the third line, the SSE starts to significantly increase. That is why it was decided that in order to get the most accurate measurement result for this method, the middle 60% of the signal have to be chosen as a part where the third line will be fitted on the slope of the signal.
Table 1 Traverse velocities used for measuring the times needed for the jet to pass over the edge of the probe. dn (mm)
v (mm/s)
0.1 0.25
0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 2.5 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5
3.2. Traverse velocity limit During the extensive testing of the measuring device it was shown that the traverse velocity has a significant effect on the measurement result. It is desired that the velocity is as high as possible so that the measurement would be performed in short time. On the other hand there was a concern that at higher velocities the response of the instrument could change from a ramp to a step response. This would cause for the measured time for passing of the jet over the probe edge at certain velocity to always have the same value. As the measured diameter is linearly dependent of traverse velocity this would cause the result of the measurement to increase with increasing of velocity. In order to prevent this a set of measurements has been made to determine the maximal traverse velocity where the transition happens. For this experiments two different diameters dn of water nozzle (0.1 and 0.25 mm) were chosen, where the times needed for the jet to pass over the edge of the probe t d were measured. The experiments were performed at water pressure of 300 MPa, the stand-off distance from the nozzle exit hf was 9 mm and for each nozzle 10 different velocities shown in Table 1 were chosen. It was assumed that for a larger nozzle diameter the jet passing times will be longer and thus higher velocities can be used. This is why a different range of velocities was used for the 0.25 mm nozzle. The measurement for each velocity was repeated four times. The chosen stand-off distance was the minimal possible distance where the measurement was still possible with the used cutting head setup. The results of this measurement are shown in Fig. 6. It can be seen that by increasing the traverse velocity the time t d is approaching the value of 0.1 s. It can also be seen that the same happens with the smaller nozzle diameter only this time at the lower velocity. It is believed that this limit is connected to the properties of the instrument, i.e. its response time. So by using
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t d (s)
1.5 1 0.5
td = 0.1 s
0 0
0.5
1
1.5
2
2.5
v (mm/s)
t d (s)
1.5 1 td = 0.1 s
0.5 0 0
0.5
1
1.5 2 v (mm/s)
2.5
3
3.5
Fig. 6. The times needed for the jet to pass over the edge of the probe for different traverse velocities: (a) dn ¼ 0:1 mm and (b) dn ¼ 0:25 mm.
even higher velocities the response time would be always the same and the diameter measurements would not give a correct result. By analyzing the obtained data it was established that the limit t d is dependant of the traverse velocity and the jet diameter. Thus a rule for traverse velocity limit was devised in order to obtain usable results. The rule is in the form of vlim
dn ¼ , ta
(3)
where vlim is traverse velocity limit and t a the limit value of td . In the case of nozzle diameters 0.1 and 0.25 mm the calculated vlim is 1 and 2.5 mm/s, respectively. All future measurements should then be performed by traverse velocities lower than the vlim for the given nozzle diameter.
3.3. The measurement accuracy of the device In order to determine the measurement accuracy and uncertainty of the device a series of experiments were performed in that direction. Because of the relation used for measuring jet diameter in Eq. (1) it is first necessary to determine the accuracy of the set traverse velocity on the machine where the measurement is performed. After that the accuracy of the whole device can be determined. The linear velocity on the water jet machine OMAX 2652A was measured by using a positioning laser beam device Keyence LV-H32, which detects when the laser beam goes over the part edge. If the laser device is mounted on the cutting head and the beam is driven by a constant velocity over a block of known length, the time needed for laser device to detect the edges of the block can be measured. By knowing the length of the block and the time needed to pass over it, a linear velocity can be calculated. The velocity measurements were performed for two velocities 0.5 and 2 mm/s and in two directions, which are parallel to X- and Y-axis of the machine. For each velocity and direction 20 repetitions were made. The length of the block was measured by an optical measuring microscope and was 30:02 0:01 mm. The results of the measurements are shown in Table 2 where
Table 2 The difference between the measured and set traverse velocities for X- and Y-axis of the OMAX 2652A water jet cutting machine. X-axis
Y-axis
v (mm/s)
0.5
2
0.5
2
vavg
0.49 0.0001 0.01
1.95 0.0004 0.05
0.47 0.001 0.03
1.89 0.005 0.1
sv nv
the average velocity vavg, standard deviation sv and the discrepancy from the set velocity on the machine nv are shown. It can be seen that the measurement is highly repeatable and that the discrepancy between the measured and the set traverse velocities is larger at higher velocity. Also there is a discrepancy between two axes where the difference between X- and Y-axis is approximately twice the size. It can be concluded that the accuracy of set traverse velocity is worse for Y-axis or if the velocity is increased. This would indicate that using lower traverse is more accurate if the velocity is not previously measured. The measured velocities can then be used in the next step to calculate the measured jet diameter. The measurements of water jet diameter were performed for the same velocities and directions as in the former case. Ten repetitions were performed for nozzle diameter of 0.25 mm at water pressure of 300 MPa. Altogether this gave 40 measurements. The result of these measurements was the time td which is then multiplied by the measured traverse velocities shown in Table 2 in order to obtain the diameter of the jet d (Eq. (1)). Table 3 shows the measurement results for each direction and velocity used, first in the form of td and then after it is multiplied by v also as a diameter of the jet d. The repeatability of the measurements is quite good as the standard deviations for all measurements were around 0.01 mm. The diameters at higher velocities were bigger which was expected. It was also expected that after the calibration of the traverse velocities the diameter in both directions should be similar because of the
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Table 3 Measured jet diameter in X- and Y-axis.
Table 4 The combined and expanded uncertainty analysis results.
X-axis
Y-axis 2
X-axis
v (mm/s)
0.5
Measurement
t d (s)
d (mm)
t d (s)
d (mm)
t d (s)
d (mm)
t d (s)
d (mm)
1 2 3 4 5 6 7 8 9 10
0.57 0.52 0.55 0.60 0.58 0.53 0.56 0.60 0.55 0.54
0.28 0.25 0.27 0.29 0.29 0.26 0.28 0.29 0.27 0.26
0.16 0.16 0.16 0.15 0.16 0.16 0.16 0.17 0.15 0.16
0.3 0.3 0.31 0.3 0.32 0.31 0.3 0.33 0.3 0.31
0.53 0.52 0.52 0.53 0.55 0.54 0.51 0.51 0.55 0.54
0.25 0.25 0.25 0.25 0.26 0.25 0.24 0.24 0.26 0.25
0.15 0.15 0.14 0.14 0.15 0.15 0.14 0.15 0.15 0.14
0.28 0.28 0.27 0.27 0.28 0.28 0.27 0.28 0.28 0.27
Average
0.55 0.03
0.27 0.01
0.16 0.01
0.31 0.01
0.53 0.01
0.25 0.01
0.15 0.01
0.28 0.01
s
0.5
2
round nozzle. The difference thus shows that the jet has a slightly elliptical shape. The overall uncertainty of the instrument is evaluated through the measured diameter of the jet expressed in Eq. (1) for the traverse velocities of 0.5 and 2 mm/s and both axes. In order to take into account the uncertainty sources ten measurement sets for the time t d and 20 measurement sets for velocity measurements were acquired for each setup. For each set of measurements, both mean and standard deviation are calculated, then the type A standard measurement uncertainty is calculated by
s
u ¼ pffiffiffi , n
(4)
where u is the standard measurement uncertainty, s the standard deviation and n the number of measurements. After that the combined standard uncertainty uc ðdÞ is evaluated using the uncertainty propagation theory [11], according to the equation: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 @d @d @d @d uc ðdÞ ¼ uðtd Þ þ uðt d Þ uðvÞ þ 2 uðvÞ R @td @v @t d @v sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
¼
st
d vavg pffiffiffiffiffiffi ffi nt d
2
s
v þ t¯ d pffiffiffiffiffi nv
847
2
st
sv
d þ 2 pffiffiffiffiffiffi ffi pffiffiffiffiffi vavg t¯ d R, nt d nv
(5) where std and sv are the standard deviations, t¯ d the mean jet passing time, ntd and nv the number of repetitions for jet passing time and traverse velocity measurements, and R the correlation coefficient. The part of combined uncertainty dealing with correlation is necessary because there is a correlation between t d and v evident from Fig. 6. Based on these data R was calculated for both orifices and was 0:67 for 0.1 mm orifice and 0:72 for 0.25 mm orifice. The negative R shows anti-correlation, i.e. the negative trend of t d if v is increased. If the data from Table 3 are used for all four combinations of velocities and axes, the combined uncertainties as presented in Table 4 are obtained. The expanded uncertainty was determined as a standard uncertainty multiplied by a coverage factor k ¼ 2, which is for normal distribution associated with a confidence level of 95%. During the rounding of the expanded uncertainty we decided to have only one important cipher, which means that all ciphers right of the place of rounding are rounded up. It can be seen that uncertainty increases with the traverse velocity, which would imply to perform measurements of the diameter at lower velocity. Also noticeable is the difference between both axes, where X-axis has greater uncertainty at lower velocity. This is
Y-axis
v (mm/s)
0.5
2
0.5
2
uc ðdÞ (mm) 2 uc ðdÞ (mm)
0.0046 0.01
0.0062 0.02
0.0015 0.003
0.006 0.02
Table 5 The measured error of the instrument. X-axis
Y-axis
v (mm/s)
0.5
2
0.5
2
d¯ (mm) Absolute error
0.27
0.31
0.25
0 .28
0.02
0.06
0.00
0.03
caused as a consequence of bigger standard deviation std for this particular case seen in Table 3. Such fluctuations were not observed for other data sets which imply on some random disturbances during the experiment. In the case of determining the accuracy of the instrument it was assumed (1) that the jet is round and (2) that the true value of the measured jet diameter should be the nominal diameter of the used nozzle, which is 0.25 mm. The error of measurement is calculated in Table 5 as the absolute difference between the true value and the average measured results. Again similar results as in the case of uncertainty are obtained as it is shown that measuring with lower traverse velocities provides more accurate measurement which on the other hand take longer time to perform. Noticeable is also better accuracy in the Y-axis. The difference in accuracy can also be argued as the consequence of the elliptical shape of the jet.
3.4. AWJ diameter measurement The main purpose of the described instrument is to measure AWJ diameter as it is usually used more for cutting material than the pure water jet. The major difficulty in using this method for measuring diameter is the wear of the probe by the abrasive erosion. AWJ can machine almost any material as long as its hardness is lower than the hardness of the used abrasive. That is why the probe has to be made from as hard as possible material. On the other hand it must not be too brittle as the passing of the jet can cause the crumbling of the edge of the probe. For this phase of experiments two different focusing tubes with the nominal diameter of 0.8 mm were used. The difference between them is in the usage period, where one was brand new (Fig. 7(a)) while the other one was used for approximately 10 h of machining (Fig. 7(b)). The difference was established by measuring the exit diameter of the nozzle by an optical method where the relative error of measurement was less than 1%. The new focusing tube diameter was measured to be 0.8 mm and the used one 0.86 mm. The AWJ diameter was measured at stand-off distance hf ¼ 2 mm as it is shown in Fig. 8(a). The water pressure was 300 MPa and the used abrasive was GMA garnet mesh #80 (mean size of abrasive grains is 0.19 mm) at the mass flow rate of 5.6 g/s. For the probe a turning carbide insert ISCAR WPEB 060400L08 was used. The jet was passing the probe edge at a 901 angle as it is shown in Fig. 8(b). The diameter was measured in X- and Y-axis of the machine. The measurements were repeated
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Fig. 7. (a) New focusing tube df ¼ 0:8 mm and (b) used focusing tube df ¼ 0:86 mm.
engraving on the insert focusing nozzle 90° hf
probe
turning insert i.e. probe
Fig. 8. (a) The position of measuring the AWJ diameter, i.e. the stand-off distance of the focusing tube from the probe. (b) The travelling path of the AWJ over the probe.
only three times because of the limitations caused by the probe wear. The AWJ causes the surface of the probe to be engraved which then causes the distortion of the measurement as the probe edge is not the same as it was on the start. In Fig. 9(a) a new cutting insert can be seen, while in 9(b) the insert after all 12 measurements were performed is shown. The magnified damage on the insert can be seen in 9(c). The wear of the probe was consequently measured in order to establish its influence on the end result. The measurement results are presented in Fig. 10. Except for the first 0.05 mm from the insert edge, the measured depth of the engraving after three passes of the AWJ was 0.13 mm. Where the starting contact between the AWJ and the insert is, i.e. 0.05 mm from the edge, the wear was 0.32 mm deep. This wear causes the additional rounding of the insert edge and can cause the distortion of subsequent measurements. The traverse velocity selected for these experiments was 3 mm/s. Because the wear issue plays a significant role in this experiment it was decided to take a slightly bigger velocity than in previously performed experiments in order to reduce the wear of the probe. The chosen velocity is still agreeable according to the condition given in Eq. (3) as the maximum allowable velocity for jet diameter of 0.8 mm is 8 mm/s. The combined uncertainty has increased and is estimated based on the standard deviations for each axis presented in Table 6 to be approximately 0:03 mm. The expanded uncertainty for a confidence level of 95% is in this case 0:06 mm. Table 6 shows the results of measuring AWJ diameter df for both focusing tubes and the calculated jet roundness S defined by the following equation: S¼
dfX , dfY
(6)
where dfX and dfY are AWJ diameters measured in X- and Y-axis, respectively. Presented are three repetitions with calcu-
lated average value and standard deviation sd . The measured results are close to the nominal diameter of the focusing tube. The measured diameter in X-axis is mostly bigger than in Y-axis except for the case of the first pass of the used focusing tube. That is why the resulting roundness S shows a slightly elliptically AWJ shape in the X-axis. It was assumed that the most accurate measurement should appear at the first pass although second and third pass give quite similar results. Further passes were not performed as it was thought that additional wear would prevent us from obtaining accurate results. Taking into account that the AWJ is not geometrically defined tool a lot of variables influence the fine measurement of the jet diameter. The distribution of abrasive grains inside of the jet and their variable size can cause the variable wear of the probe edge. As it is important to sense when the jet starts to pass the probe edge, the wear of the edge can cause slower rising of the gradient of the force signal and thus change the outcome result. Taking this into account together with the uncertainty of the traverse velocity of the cutting head we conclude that if only one pass is made over the probe edge, the device for this particular probe should be able to measure the AWJ diameter with accuracy of 0:02 mm. Of course additional tests should be made on new cutting inserts or a new even more wear resistant material should be made available for the probe. As it was mentioned before harder materials can be used but are on the other hand hard to machine and also very brittle which causes them to chip when the AWJ passes over them.
4. Conclusions An instrument capable of measuring water and abrasive water jet diameter has been developed. This paper described the structure of the device and the method for determining the diameter of the jet. Measurement experiments show the validity of the proposed method and its accuracy. The results are summarized as follows: 1. A method for determining the diameter of the jet from the measured force as the jet passes over the edge of a probe mounted on a load cell. The proposed method was verified by experiments. 2. The uncertainty and accuracy of the proposed device were established by measuring pure water jet diameter. The measurement result is dependent of the traverse velocity of the jet passing the edge of the probe. There is a maximum velocity where usable measurements are obtained. By increasing the traverse velocity the time of the measurement decreases but on the other hand the uncertainty becomes bigger and accuracy gets worse.
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Fig. 9. (a) Cutting insert before and (b) after the performed measurements. (c) The wear of the cutting insert.
0.13 mm
0.32 mm
0.05 mm
Fig. 10. The worn edge of the cutting insert after the third pass of the AWJ: (a) top view and (b) front view.
References
Table 6 The results of measuring AWJ diameter for the new and used focusing tube. 2
3
Average
sd
New focusing tube 0.79 dfX (mm) 0.74 dfY (mm) S 1.06
0.81 0.74 1.09
0.82 0.78 1.05
0.81 0.75 1.07
0.02 0.02
Used focusing tube 0.81 dfX (mm) 0.83 dfY (mm) S 0.97
0.82 0.77 1.06
0.89 0.79 1.12
0.84 0.8 1.05
0.04 0.03
Pass
1
3. The measuring of AWJ diameter showed promising results as they were close to the nominal diameter of the focusing tube. The major issue was the wear of the measuring probe which distorts further measurements on the same spot on the probe. Despite this it was estimated that the measurement could be performed with accuracy in the range of 0:02 mm.
Acknowledgment This work is supported by the ‘‘Multi-Material Micro Manufacture: Technology and Applications (4M)’’ Network of Excellence, Contract number NMP2-CT-2004-500274 within the EU 6th Framework Program.
[1] H. Orbanic, M. Junkar, Analysis of striation formation mechanism in abrasive water jet cutting, Wear 265 (5/6) (2007) 821–830. [2] H. Liu, J. Wang, N. Kelson, R.J. Brown, A study of abrasive waterjet characteristics by CFD simulation, Journal of Materials Processing Technology 153–154 (2004) 488–493. [3] I. Bajsic, B. Blagojevic, Disintegration of flat liquid jets, Strojniski vestnik, Journal of Mechanical Engineering 39 (3/4) (1993) 75–93. [4] M. Knaupp, A. Meyer, G. Erichsen, M. Sahney, C. Burnham, Dynamic compensation of abrasive water jet properties through 3-dimensional jet control, in: Proceedings of 16th International Conference on Water Jetting, BHR Group, Aix-en-Provence, France, 2002, pp. 75–89. [5] J. Zeng, J. Olsen, C. Olsen, B. Guglielmetti, Taper-free abrasive waterjet cutting with a tilting head, in: Proceedings of the 2005 WJTA American Water Jet Conference, WJTA, Houston, TX, 2005, p. 7A-2. [6] W.L. Chen, E.S. Geskin, Jet Cutting Technology, Chapter Measurements of the Velocity of Abrasive Waterjet by the use of Laser Transient Anemometer, Elsevier, London, 1991, pp. 23–36. [7] K.F. Neusen, T.J. Gores, T.J. Labus, Measurement of particle and drop velocities in a mixed water jet using a forward-scatter LDV system, in: Proceedings of 11th International Conference on Jet Cutting Technology, BHR Group, Kluwer Academic Publishers, Dordrecht, 1992, pp. 63–74. [8] A. Momber, R. Kovacevic, Energy dissipative processes in high speed watersolid particle erosion, in: Proceedings of ASME Heat Transfer and Fluids Engineering Division, New York, pp. 243–356. [9] R. Kovacevic, Development of an Opto-Electric Sensing System to Monitor the Abrasive Waterjet Nozzle Wear, PED, vol. 44, pp. 9–16. [10] H.S. Lim, S.M. Son, Y.S. Wong, M. Rahman, Development and evaluation of an on-machine optical measurement device, International Journal of Machine Tools & Manufacture 47 (2007) 1556–1562. [11] Guide to the Expression of Uncertainty in Measurement, ISO, 1993.
Contents lists available at ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
An instrument for measuring abrasive water jet diameter H. Orbanic´ , M. Junkar, I. Bajsic´, A. Lebar University of Ljubljana, Faculty of Mechanical Engineering, Askerceva 6, SI-1000 Ljubljana, Slovenia
a r t i c l e in fo
abstract
Article history: Received 10 March 2009 Received in revised form 21 May 2009 Accepted 27 May 2009 Available online 6 June 2009
In order to improve the accuracy of abrasive water jet (AWJ) machining the precise value of the jet diameter has to be known. Because of an aggressive environment caused by high velocity abrasive grains, the diameter is not easily measured. That is why a measuring device consisting of a load cell and a wear resistant probe was developed. The device measures the force of the jet while it passes over the edge of the probe. If the feed rate of the jet is constant and the time needed for jet to pass is known, the diameter can be determined. Because of probe wear issue several preliminary tests were made with water jet only in order to determine the measuring uncertainty and accuracy of the device. In the end the measurement of the AWJ was performed for two different focusing nozzles of different diameters. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Abrasive water jet Jet diameter Jet force Measurement uncertainty Measurement accuracy
1. Introduction Cutting with abrasive water jet (AWJ) is a non-conventional machining process which uses high speed water jet for accelerating very hard abrasive grains and thus enabling the removal of workpiece material. While the technology has many advantages like cutting of wide variety of materials independent of their material properties and absence of heat affected zone, it is limited by poor accuracy in comparison to other machining processes like laser, WEDM or milling [1]. The problem lies in the geometrically non-defined and flexible tool as it has a multiphase structure and it disintegrates in the air atmosphere, which is difficult to analytically analyze [2,3]. Another source of inaccuracy is the specific surface texture and tapered cut which is characteristic for this type of process. While there is no current solution for the surface, the taper problem is currently solved by tilting the cutting head [4,5]. The other also significant problem is determining the dimensions or the diameter of the AWJ as these data are used for setting the tool offset while cutting. Currently this is done by using the radius of the focusing tube as a tool offset. While this is good for when the focusing tube is new and if we are satisfied with the accuracy up to 0:1 mm, it is not good after the tube starts to wear or we want to achieve machining accuracies less than 0:1 mm. That is why a method is needed which would measure the AWJ diameter at the stand-off distance from the nozzle exit. The sometimes used solution is to carry a test cut in
Corresponding author. Tel.: +386 1 4771724; fax: +386 1 4771759.
E-mail address: [email protected] (H. Orbanic´). URL: http://www.fs.uni-lj.si/lat (H. Orbanic´). 0890-6955/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmachtools.2009.05.008
the case that better accuracy is needed and measure the kerf dimensions. This procedure is not practical from the point of material consumption and the time needed to get a good result for different cutting directions of the jet. That is why an instrument is needed which would directly measure the AWJ diameter on the machine itself prior to the machining and thus accordingly correct the tool offset in the machine controller. Because of very aggressive environment inside of the AWJ different techniques were used in the past in order to measure the condition of the jet. One of the methods is measuring kinetic energy or velocity of the jet by laser Doppler anemometry (LDA) [7] or the laser transit anemometry [6]. The measurements have to be made in the laboratory conditions and are not very practical. More practical is to measure the force of the jet. Momber and Kovacevic [8] have measured the impact force of water jet and AWJ perpendicular on a plate in order to determine the velocity of the jet. The last methods used were optical methods where the jet is observed by a CCD camera [9], where the shape or the diameter of the jet is optically measured. The problems of optical methods are discerning between the compact body of the jet and the droplets which surround it as the jet is not a solid tool. That is why also methods like laser micrometers are difficult to implement as it is in the case of a solid tool [10]. In this research, a new measuring method that utilizes a cantilever load cell for measuring the diameter of the water and abrasive water jet is presented. The device measures the jet force as the jet passes over the edge of the wear resistant probe mounted on the load cell. The device is robust and resistant to moisture and residual abrasive effects. The only major problem is the wear of the probe when the AWJ diameter is measured. By using better wear resistant materials for the probe will enable
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implementation of the device on a commercial AWJ cutting machine. The purpose of this paper is to describe the principle and structure of the measuring device, assessment of its accuracy and performing test measurements on AWJ.
2. The measuring principle and instrument setup The configuration of the device for measuring the diameter of the jet is shown in Fig. 1. The device consists of a load cell with a fixed wear resistant probe. The cell is mounted through a fixture on the AWJ machine catcher wall. The load cell Vishay Transducers 1022 is a cantilever type with binocular shape intended for measuring of axial forces. It has an IP66 protection against dust and moisture and can measure maximum loads of 7 kg at the rated output of 2.002 mV/V and total error less than 0.02%. The compensated temperature range is between 10 and 40 C. The probe has to be from a hard material with well-defined edges. This is why a carbide insert used for turning was used. The photograph of the device placed on the cutting table of an OMAX 2652A AWJ cutting machine is shown in Fig. 2. The measurement method for measuring the jet diameter was conceived so that the impact force of the jet would be measured while it passes over the edge of a probe mounted on a load cell as it is shown in Fig. 3. The jet is moving with a constant velocity v and when it touches the probe, the measured force starts to rise. When the whole diameter of the jet has passed over the edge of the probe the force ceases to rise. If the time t d when the force starts and when it ceases to rise is known, the diameter of the jet d can be determined by multiplying the time with the traverse velocity of the jet (Eq. (1)) d ¼ v td .
jet traverse velocity v nozzle
part 1: force is zero, the jet is approaching the probe, part 2: force is rising, the jet is passing over the edge of the
F
F
(1)
The important part of measurement process is determining the correct start and end of time interval of jet passing over the edge. The logical thing would be to look for the significant change in gradient of the curvature. But by doing this the result can be overestimated. This is because the water droplets, which move around the core of the jet, produce the rise in the measured force. To eliminate this, a method was developed which first cuts off the influence of droplets and also automates the diameter measurement process. This is done by dividing the signal on three parts and fitting a line over each part. The three parts are shown in Fig. 4 and are the following:
probe,
Fig. 2. Photograph of the jet diameter measurement device.
probe
td
v
d = v · td Fig. 3. The principle of measuring jet diameter by measuring impact force.
part 3: force has stabilized, the whole jet is on the probe.
Fig. 1. The configuration of the jet diameter measurement device.
Fig. 4. The three line method.
t
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SSE ¼
n X
ðF^ i F i Þ2 ,
1 v = 0.5 mm/s v = 1 mm/s v = 1.5 mm/s v = 2 mm/s
0.9 0.8 0.7
Normalized SSE
The function for the first line is easy to determine as it is put simply as F n1 ¼ 0, where F n is the normalized measured jet impact force. The second line was fitted over the upper 10% of the signal with the function F n2 ¼ k2 x þ n2 . This has to be done because the measured force fluctuates once the jet is on the probe. The third line in the form of F n3 ¼ k3 x þ n3 is fitted over the slope of the signal but before the fitting is performed, the size of the part between points A1 and A2 in Fig. 4 has to be determined in order to get the most accurate result. It was decided to position the line in the middle of the signal, where the size of the slope which will be considered was determined by a sum of square errors (SSE) method as defined in the following equation:
845
0.6 0.5 0.4 0.3 0.2
(2)
i¼1
0.1
where F^ i is the fitted line, F i is the measured force signal on the chosen part of the slope and n the number of samples. The determination of the size or the percentage of signal A in Fig. 4 will be described in Section 3.1.
0 0
10
20
30 40 50 60 Middle part of signal (%)
70
80
90
Fig. 5. The SSE analysis of the line fitting on the slope of the force signal.
3. Experimental verification The experimental work performed in this research has included (1) the determination of the part of signal used for the line fitting on the slope of the signal, (2) determination of the maximum traverse velocity for obtaining useful measurements, (3) determination of the accuracy of the implemented device and (4) testing of the device at measuring the diameter of the AWJ. Although the main purpose of this research is to experimentally verify the performance of the device when measuring the diameter of AWJ, the first three parts of experimental verification were performed by using only pure water jet because of the wear issues of the probe. This enabled us to test the measurement repeatability of the device before moving to the measuring of the AWJ diameter. Wear of the probe is a major issue, which will be dealt with in the next step of research.
3.1. Determination of the slope size For this part of experiment a 0.25 mm diameter water nozzle was chosen and the water pressure was 300 MPa. Other combinations of orifice and water pressure could be used in this case but this would only change the flow conditions and would not significantly influence the results of the measurement. Four different traverse velocities were chosen whose values are 0.5, 1, 1.5 and 2 mm/s. The chosen values were low because there was a concern that higher velocities might cause the response of the instrument to change from a ramp to a step response. This was later shown to be true when the traverse velocity limit was determined. For each velocity the force was measured when the jet passed over the edge of the probe. Each signal was then divided into areas as shown in Fig. 4. In order to get the best fit of the line over the slope, 15 different sized parts between 5% and 95% of the signal around the middle (distance A in Fig. 4) were selected. For each part the slope was fitted with a line and SSE was calculated. The results for the normalized SSE for all four traverse velocities are shown in Fig. 5. It can be seen that if more than 60% of the signal is taken for the fitting of the third line, the SSE starts to significantly increase. That is why it was decided that in order to get the most accurate measurement result for this method, the middle 60% of the signal have to be chosen as a part where the third line will be fitted on the slope of the signal.
Table 1 Traverse velocities used for measuring the times needed for the jet to pass over the edge of the probe. dn (mm)
v (mm/s)
0.1 0.25
0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 2.5 0.2, 0.4, 0.6, 0.8, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5
3.2. Traverse velocity limit During the extensive testing of the measuring device it was shown that the traverse velocity has a significant effect on the measurement result. It is desired that the velocity is as high as possible so that the measurement would be performed in short time. On the other hand there was a concern that at higher velocities the response of the instrument could change from a ramp to a step response. This would cause for the measured time for passing of the jet over the probe edge at certain velocity to always have the same value. As the measured diameter is linearly dependent of traverse velocity this would cause the result of the measurement to increase with increasing of velocity. In order to prevent this a set of measurements has been made to determine the maximal traverse velocity where the transition happens. For this experiments two different diameters dn of water nozzle (0.1 and 0.25 mm) were chosen, where the times needed for the jet to pass over the edge of the probe t d were measured. The experiments were performed at water pressure of 300 MPa, the stand-off distance from the nozzle exit hf was 9 mm and for each nozzle 10 different velocities shown in Table 1 were chosen. It was assumed that for a larger nozzle diameter the jet passing times will be longer and thus higher velocities can be used. This is why a different range of velocities was used for the 0.25 mm nozzle. The measurement for each velocity was repeated four times. The chosen stand-off distance was the minimal possible distance where the measurement was still possible with the used cutting head setup. The results of this measurement are shown in Fig. 6. It can be seen that by increasing the traverse velocity the time t d is approaching the value of 0.1 s. It can also be seen that the same happens with the smaller nozzle diameter only this time at the lower velocity. It is believed that this limit is connected to the properties of the instrument, i.e. its response time. So by using
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t d (s)
1.5 1 0.5
td = 0.1 s
0 0
0.5
1
1.5
2
2.5
v (mm/s)
t d (s)
1.5 1 td = 0.1 s
0.5 0 0
0.5
1
1.5 2 v (mm/s)
2.5
3
3.5
Fig. 6. The times needed for the jet to pass over the edge of the probe for different traverse velocities: (a) dn ¼ 0:1 mm and (b) dn ¼ 0:25 mm.
even higher velocities the response time would be always the same and the diameter measurements would not give a correct result. By analyzing the obtained data it was established that the limit t d is dependant of the traverse velocity and the jet diameter. Thus a rule for traverse velocity limit was devised in order to obtain usable results. The rule is in the form of vlim
dn ¼ , ta
(3)
where vlim is traverse velocity limit and t a the limit value of td . In the case of nozzle diameters 0.1 and 0.25 mm the calculated vlim is 1 and 2.5 mm/s, respectively. All future measurements should then be performed by traverse velocities lower than the vlim for the given nozzle diameter.
3.3. The measurement accuracy of the device In order to determine the measurement accuracy and uncertainty of the device a series of experiments were performed in that direction. Because of the relation used for measuring jet diameter in Eq. (1) it is first necessary to determine the accuracy of the set traverse velocity on the machine where the measurement is performed. After that the accuracy of the whole device can be determined. The linear velocity on the water jet machine OMAX 2652A was measured by using a positioning laser beam device Keyence LV-H32, which detects when the laser beam goes over the part edge. If the laser device is mounted on the cutting head and the beam is driven by a constant velocity over a block of known length, the time needed for laser device to detect the edges of the block can be measured. By knowing the length of the block and the time needed to pass over it, a linear velocity can be calculated. The velocity measurements were performed for two velocities 0.5 and 2 mm/s and in two directions, which are parallel to X- and Y-axis of the machine. For each velocity and direction 20 repetitions were made. The length of the block was measured by an optical measuring microscope and was 30:02 0:01 mm. The results of the measurements are shown in Table 2 where
Table 2 The difference between the measured and set traverse velocities for X- and Y-axis of the OMAX 2652A water jet cutting machine. X-axis
Y-axis
v (mm/s)
0.5
2
0.5
2
vavg
0.49 0.0001 0.01
1.95 0.0004 0.05
0.47 0.001 0.03
1.89 0.005 0.1
sv nv
the average velocity vavg, standard deviation sv and the discrepancy from the set velocity on the machine nv are shown. It can be seen that the measurement is highly repeatable and that the discrepancy between the measured and the set traverse velocities is larger at higher velocity. Also there is a discrepancy between two axes where the difference between X- and Y-axis is approximately twice the size. It can be concluded that the accuracy of set traverse velocity is worse for Y-axis or if the velocity is increased. This would indicate that using lower traverse is more accurate if the velocity is not previously measured. The measured velocities can then be used in the next step to calculate the measured jet diameter. The measurements of water jet diameter were performed for the same velocities and directions as in the former case. Ten repetitions were performed for nozzle diameter of 0.25 mm at water pressure of 300 MPa. Altogether this gave 40 measurements. The result of these measurements was the time td which is then multiplied by the measured traverse velocities shown in Table 2 in order to obtain the diameter of the jet d (Eq. (1)). Table 3 shows the measurement results for each direction and velocity used, first in the form of td and then after it is multiplied by v also as a diameter of the jet d. The repeatability of the measurements is quite good as the standard deviations for all measurements were around 0.01 mm. The diameters at higher velocities were bigger which was expected. It was also expected that after the calibration of the traverse velocities the diameter in both directions should be similar because of the
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Table 3 Measured jet diameter in X- and Y-axis.
Table 4 The combined and expanded uncertainty analysis results.
X-axis
Y-axis 2
X-axis
v (mm/s)
0.5
Measurement
t d (s)
d (mm)
t d (s)
d (mm)
t d (s)
d (mm)
t d (s)
d (mm)
1 2 3 4 5 6 7 8 9 10
0.57 0.52 0.55 0.60 0.58 0.53 0.56 0.60 0.55 0.54
0.28 0.25 0.27 0.29 0.29 0.26 0.28 0.29 0.27 0.26
0.16 0.16 0.16 0.15 0.16 0.16 0.16 0.17 0.15 0.16
0.3 0.3 0.31 0.3 0.32 0.31 0.3 0.33 0.3 0.31
0.53 0.52 0.52 0.53 0.55 0.54 0.51 0.51 0.55 0.54
0.25 0.25 0.25 0.25 0.26 0.25 0.24 0.24 0.26 0.25
0.15 0.15 0.14 0.14 0.15 0.15 0.14 0.15 0.15 0.14
0.28 0.28 0.27 0.27 0.28 0.28 0.27 0.28 0.28 0.27
Average
0.55 0.03
0.27 0.01
0.16 0.01
0.31 0.01
0.53 0.01
0.25 0.01
0.15 0.01
0.28 0.01
s
0.5
2
round nozzle. The difference thus shows that the jet has a slightly elliptical shape. The overall uncertainty of the instrument is evaluated through the measured diameter of the jet expressed in Eq. (1) for the traverse velocities of 0.5 and 2 mm/s and both axes. In order to take into account the uncertainty sources ten measurement sets for the time t d and 20 measurement sets for velocity measurements were acquired for each setup. For each set of measurements, both mean and standard deviation are calculated, then the type A standard measurement uncertainty is calculated by
s
u ¼ pffiffiffi , n
(4)
where u is the standard measurement uncertainty, s the standard deviation and n the number of measurements. After that the combined standard uncertainty uc ðdÞ is evaluated using the uncertainty propagation theory [11], according to the equation: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 @d @d @d @d uc ðdÞ ¼ uðtd Þ þ uðt d Þ uðvÞ þ 2 uðvÞ R @td @v @t d @v sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi
¼
st
d vavg pffiffiffiffiffiffi ffi nt d
2
s
v þ t¯ d pffiffiffiffiffi nv
847
2
st
sv
d þ 2 pffiffiffiffiffiffi ffi pffiffiffiffiffi vavg t¯ d R, nt d nv
(5) where std and sv are the standard deviations, t¯ d the mean jet passing time, ntd and nv the number of repetitions for jet passing time and traverse velocity measurements, and R the correlation coefficient. The part of combined uncertainty dealing with correlation is necessary because there is a correlation between t d and v evident from Fig. 6. Based on these data R was calculated for both orifices and was 0:67 for 0.1 mm orifice and 0:72 for 0.25 mm orifice. The negative R shows anti-correlation, i.e. the negative trend of t d if v is increased. If the data from Table 3 are used for all four combinations of velocities and axes, the combined uncertainties as presented in Table 4 are obtained. The expanded uncertainty was determined as a standard uncertainty multiplied by a coverage factor k ¼ 2, which is for normal distribution associated with a confidence level of 95%. During the rounding of the expanded uncertainty we decided to have only one important cipher, which means that all ciphers right of the place of rounding are rounded up. It can be seen that uncertainty increases with the traverse velocity, which would imply to perform measurements of the diameter at lower velocity. Also noticeable is the difference between both axes, where X-axis has greater uncertainty at lower velocity. This is
Y-axis
v (mm/s)
0.5
2
0.5
2
uc ðdÞ (mm) 2 uc ðdÞ (mm)
0.0046 0.01
0.0062 0.02
0.0015 0.003
0.006 0.02
Table 5 The measured error of the instrument. X-axis
Y-axis
v (mm/s)
0.5
2
0.5
2
d¯ (mm) Absolute error
0.27
0.31
0.25
0 .28
0.02
0.06
0.00
0.03
caused as a consequence of bigger standard deviation std for this particular case seen in Table 3. Such fluctuations were not observed for other data sets which imply on some random disturbances during the experiment. In the case of determining the accuracy of the instrument it was assumed (1) that the jet is round and (2) that the true value of the measured jet diameter should be the nominal diameter of the used nozzle, which is 0.25 mm. The error of measurement is calculated in Table 5 as the absolute difference between the true value and the average measured results. Again similar results as in the case of uncertainty are obtained as it is shown that measuring with lower traverse velocities provides more accurate measurement which on the other hand take longer time to perform. Noticeable is also better accuracy in the Y-axis. The difference in accuracy can also be argued as the consequence of the elliptical shape of the jet.
3.4. AWJ diameter measurement The main purpose of the described instrument is to measure AWJ diameter as it is usually used more for cutting material than the pure water jet. The major difficulty in using this method for measuring diameter is the wear of the probe by the abrasive erosion. AWJ can machine almost any material as long as its hardness is lower than the hardness of the used abrasive. That is why the probe has to be made from as hard as possible material. On the other hand it must not be too brittle as the passing of the jet can cause the crumbling of the edge of the probe. For this phase of experiments two different focusing tubes with the nominal diameter of 0.8 mm were used. The difference between them is in the usage period, where one was brand new (Fig. 7(a)) while the other one was used for approximately 10 h of machining (Fig. 7(b)). The difference was established by measuring the exit diameter of the nozzle by an optical method where the relative error of measurement was less than 1%. The new focusing tube diameter was measured to be 0.8 mm and the used one 0.86 mm. The AWJ diameter was measured at stand-off distance hf ¼ 2 mm as it is shown in Fig. 8(a). The water pressure was 300 MPa and the used abrasive was GMA garnet mesh #80 (mean size of abrasive grains is 0.19 mm) at the mass flow rate of 5.6 g/s. For the probe a turning carbide insert ISCAR WPEB 060400L08 was used. The jet was passing the probe edge at a 901 angle as it is shown in Fig. 8(b). The diameter was measured in X- and Y-axis of the machine. The measurements were repeated
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Fig. 7. (a) New focusing tube df ¼ 0:8 mm and (b) used focusing tube df ¼ 0:86 mm.
engraving on the insert focusing nozzle 90° hf
probe
turning insert i.e. probe
Fig. 8. (a) The position of measuring the AWJ diameter, i.e. the stand-off distance of the focusing tube from the probe. (b) The travelling path of the AWJ over the probe.
only three times because of the limitations caused by the probe wear. The AWJ causes the surface of the probe to be engraved which then causes the distortion of the measurement as the probe edge is not the same as it was on the start. In Fig. 9(a) a new cutting insert can be seen, while in 9(b) the insert after all 12 measurements were performed is shown. The magnified damage on the insert can be seen in 9(c). The wear of the probe was consequently measured in order to establish its influence on the end result. The measurement results are presented in Fig. 10. Except for the first 0.05 mm from the insert edge, the measured depth of the engraving after three passes of the AWJ was 0.13 mm. Where the starting contact between the AWJ and the insert is, i.e. 0.05 mm from the edge, the wear was 0.32 mm deep. This wear causes the additional rounding of the insert edge and can cause the distortion of subsequent measurements. The traverse velocity selected for these experiments was 3 mm/s. Because the wear issue plays a significant role in this experiment it was decided to take a slightly bigger velocity than in previously performed experiments in order to reduce the wear of the probe. The chosen velocity is still agreeable according to the condition given in Eq. (3) as the maximum allowable velocity for jet diameter of 0.8 mm is 8 mm/s. The combined uncertainty has increased and is estimated based on the standard deviations for each axis presented in Table 6 to be approximately 0:03 mm. The expanded uncertainty for a confidence level of 95% is in this case 0:06 mm. Table 6 shows the results of measuring AWJ diameter df for both focusing tubes and the calculated jet roundness S defined by the following equation: S¼
dfX , dfY
(6)
where dfX and dfY are AWJ diameters measured in X- and Y-axis, respectively. Presented are three repetitions with calcu-
lated average value and standard deviation sd . The measured results are close to the nominal diameter of the focusing tube. The measured diameter in X-axis is mostly bigger than in Y-axis except for the case of the first pass of the used focusing tube. That is why the resulting roundness S shows a slightly elliptically AWJ shape in the X-axis. It was assumed that the most accurate measurement should appear at the first pass although second and third pass give quite similar results. Further passes were not performed as it was thought that additional wear would prevent us from obtaining accurate results. Taking into account that the AWJ is not geometrically defined tool a lot of variables influence the fine measurement of the jet diameter. The distribution of abrasive grains inside of the jet and their variable size can cause the variable wear of the probe edge. As it is important to sense when the jet starts to pass the probe edge, the wear of the edge can cause slower rising of the gradient of the force signal and thus change the outcome result. Taking this into account together with the uncertainty of the traverse velocity of the cutting head we conclude that if only one pass is made over the probe edge, the device for this particular probe should be able to measure the AWJ diameter with accuracy of 0:02 mm. Of course additional tests should be made on new cutting inserts or a new even more wear resistant material should be made available for the probe. As it was mentioned before harder materials can be used but are on the other hand hard to machine and also very brittle which causes them to chip when the AWJ passes over them.
4. Conclusions An instrument capable of measuring water and abrasive water jet diameter has been developed. This paper described the structure of the device and the method for determining the diameter of the jet. Measurement experiments show the validity of the proposed method and its accuracy. The results are summarized as follows: 1. A method for determining the diameter of the jet from the measured force as the jet passes over the edge of a probe mounted on a load cell. The proposed method was verified by experiments. 2. The uncertainty and accuracy of the proposed device were established by measuring pure water jet diameter. The measurement result is dependent of the traverse velocity of the jet passing the edge of the probe. There is a maximum velocity where usable measurements are obtained. By increasing the traverse velocity the time of the measurement decreases but on the other hand the uncertainty becomes bigger and accuracy gets worse.
ARTICLE IN PRESS H. Orbanic´ et al. / International Journal of Machine Tools & Manufacture 49 (2009) 843–849
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Fig. 9. (a) Cutting insert before and (b) after the performed measurements. (c) The wear of the cutting insert.
0.13 mm
0.32 mm
0.05 mm
Fig. 10. The worn edge of the cutting insert after the third pass of the AWJ: (a) top view and (b) front view.
References
Table 6 The results of measuring AWJ diameter for the new and used focusing tube. 2
3
Average
sd
New focusing tube 0.79 dfX (mm) 0.74 dfY (mm) S 1.06
0.81 0.74 1.09
0.82 0.78 1.05
0.81 0.75 1.07
0.02 0.02
Used focusing tube 0.81 dfX (mm) 0.83 dfY (mm) S 0.97
0.82 0.77 1.06
0.89 0.79 1.12
0.84 0.8 1.05
0.04 0.03
Pass
1
3. The measuring of AWJ diameter showed promising results as they were close to the nominal diameter of the focusing tube. The major issue was the wear of the measuring probe which distorts further measurements on the same spot on the probe. Despite this it was estimated that the measurement could be performed with accuracy in the range of 0:02 mm.
Acknowledgment This work is supported by the ‘‘Multi-Material Micro Manufacture: Technology and Applications (4M)’’ Network of Excellence, Contract number NMP2-CT-2004-500274 within the EU 6th Framework Program.
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