This study was carried out experimentally to investigate the effect of various laser parameters on the overall performance of the LBMM-µEDM process. Initially, sub-millimetre holes were drilled using the LBMM method with a programmed diameter of 200mm, which was later fine- finished using the µEDM operation. The LBMM drilling task was performed using a desktop fibre laser machine (Ytterbium Doped) machine (rated maximum power 20 W, actual maximum power was ~ 16.7 W, measured using Gentec prento laser power meter), as shown in Fig. 1 (a). The laser that is used in this machine is a pulsed laser with a wavelength of 1060 nm. The focal length of this laser is 200 mm. An X/Y galvanometer scanner (maximum scan angle ± 15°, and resolution 12 µrad) can control the laser beam to create the various patterns on the workpiece. Figure 1(b) shows the scanning strategy followed during the LBMM process to make the pilot holes. The line spacing between each scan line was 10 mm, as indicated in Fig. 1(b). The whole laser system, including the galvanometer, can be controlled using an integrated graphical user interface (GUI). Three parameters can be controlled in the LBMM system, namely the laser power, the pulse frequency and the laser scanning speed, for a feature with fixed geometry. Following the LBMM drilling operation, the workpiece and the fixture were transferred to the µEDM machine (DT110, Fig. 1 (c)) from Mikrotools Pte. Ltd. The µEDM is a CNC programmable machine that has a positional accuracy of ± 1 µm per 100 mm of travel length. The programming resolution of this machine is 0.1 µm. The EDM power supply of this machine is an RC pulse generator. The stray capacitance of the system was measured by monitoring the single pulse discharge energy which was found to be 0.6 nF. The material used for this study was Stainless Steel (SS304) with a thickness of 0.2 mm. Table 1 describes the laser and µEDM parameters that have been used for this research. It is to note that µEDM parameters were kept constant whereas the laser parameters were varied in four stages with a full factorial experimental design.
Table 1
Various parameters for laser and µEDM
Parameter
|
Laser
|
µEDM
|
Average Power (%)
|
15,40,65,90
|
-
|
Measured Average Power (W)
|
6.4,9.4,12.5,15.5
|
|
Laser spot diameter (mm)
|
40
|
|
Laser pulse duration (ns)
|
100
|
|
Pulse Frequency (kHz)
|
5,10, 15, 20
|
-
|
Voltage (V)
|
-
|
80
|
Capacitor (nF)
|
-
|
1
|
Stray capacitance (nF)
|
|
0.6
|
Laser scanning speed (mm/s)/ µEDM feed speed (µm/s)
|
50, 500, 950,1400
|
5
|
Set diameter of the hole for Laser operation (µm)
|
200
|
-
|
Electrode diameter for µEDM (µm)
|
-
|
500
|
Loop count (Nos)
|
75
|
-
|
µEDM electrode rotational speed (RPM)
|
-
|
500
|
µEDM electrode material
|
-
|
Tungsten (W)
|
At first, a square array of pilot holes was drilled with different laser parameters as per the experimental design. The time needed to drill each hole by the LBMM process was recorded using a stopwatch (resolution was 1/100th of a second). In the second phase, fine machining was performed on the µEDM (DT110) machine. Schematically the overall sequential machining process is described in Fig. 2. To carry out the fine machining by µEDM, a 500 µm tungsten tool was used. To maintain the tungsten tool's positional accuracy above the laser-drilled holes, an on-machine measurement (OMM) system was developed that consists of a variable lens optical microscope equipped with a high-resolution digital camera. As described in Fig. 2, the OMM was mounted adjacent to the µEDM spindle.
The method that was followed to position the tungsten tool above the LBMMed holes accurately is described here. At first, the sample workpiece was firmly attached with the fixture and arrays of LBMMed holes were machined. Following this, the whole fixture and the workpiece were transferred to the µEDM (DT110) machine, and a reference hole was machined (using the µEDM process) near the arrays of LBMMed holes. The reference hole coordinates were made known to us from the linear scale feedback of the µEDM machine. Next, with the help of the camera (crosshair mark) and linear scale feedback the deviation of X and Y coordinates for each LBMMed hole from the reference hole was measured and subsequently added to the reference hole's coordinates. Thus, the tungsten tool was positioned right on the top of the centre of the LBMMed holes for fine machining with acceptable accuracy (theoretically the accuracy is within 2 µm). To evaluate qualitatively the accuracy of the EDM tool's positioning method over the LBMMed holes, the mEDM operation was carried out on the LBMMed hole partially, and an optical image was captured (Fig. 3). It is quite clear from Fig. 3 that the mEDM tool could be positioned on top of the pilot holes with adequate accuracy using the developed OMM system. Next, the time needed to drill each hole (by mEDM) was recorded from the machine directly without the need of a stopwatch.
The discharge current amplitude and frequency was measured using a current probe (Tektronics: Tek CT1, sensitivity 5mV/mA) as it was attached with an oscilloscope (RIGOL). During the EDM process, a short circuit may occur due to metal to metal contact, which causes no material removal [30]. When short circuit happens (which is detected by the controller of the DT110 machine by monitoring the electrical characteristics of the mEDM process) the machine creates a buzzer sound and causes the tool's forward motion to be reversed until an open circuit is detected again. The buzzer sound was used to count the total number of short circuits' occurrence during each mEDM operation. It is to note that the DT110 machine has the capability of detecting a conductive surface as soon as it touches it in X, Y or Z direction. In order to measure the vertical tool wear, the tungsten tool was moved in the negative Z-direction to detect a predefined reference surface. Once the surface was detected, the Z coordinate was recorded. This operation was carried out before and after the machining of each hole. Subsequently, the difference between the two Z coordinates was calculated to find the vertical tool wear. For, µEDM operation, each hole the bottom portion of the tungsten tool was flattened using a process called reverse EDM to ensure a similar tool shape for machining all the holes [31].
The characterization of the machined samples was carried out using the scanning electron microscopy (SEM: JSM-IT100 InTouch Scope™), and the characterization was conducted two times, first after the LBMM operation and finally after the µEDM finishing. Once all the SEM images were captured, a third-party image analysis software (ImageJ) [32] was used to measure the holes' entry and exit areas (both for the LBMMed and the µEDMed holes). The volume of material removed during the LBMM process is a crucial parameter to be measured for this study. This measurement is complicated as LBMMed holes does not have a defined circular shape and some laser parameters used in this study was unable to penetrate a thru-hole on the workpiece. Figure 4 shows the measurement example using the image analysis software (ImageJ) [32]. Once holes' areas (entry area Fig. 4 (a) and Fig. 4 (c), exit/bottom area: Fig. 4(b) and Fig. 4 (d)) and slanted height (Fig. 4 (e)) were measured, then the removed volume by the LBMM process each hole was calculated. The hole that was formed by the LBMM process resembles the shape of a cone whose top has been sliced off. If the entry area of the hole is B1 and the exit area of the hole is B2, and the depth of the hole is h, then the formula for volume(V) of material removed by LBMM is given by Eq. (1).
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It was not possible to measure the depth (h) directly for the holes which were not fully penetrated due to the unavailability of the cross-sectional image of such holes. So, the formula for h in terms of the slanted height was derived as equation (2) which was used for the holes like that for Fig. 4(e). However, for the case of fully penetrated holes like Fig. 4 (a) and Fig. 4 (b) the thickness of the workpiece was taken as h.
![](data:image/jpeg;base64,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)
The above-mentioned method of measuring the removed volume during the LBMM process was accurate enough to provide the average trend of material removal by the said process with the variation of laser power and scanning speed.