2.1. Experimental Setup
Micro-EDM drilling experiments are carried out on a SARIX
® SX-100-HPM machine tool (Sarix SA, Sant’Antonino, Switzerland) in order to obtain the required datasets for testing the data-driven approaches for tool wear prediction. The machine tool is equipped with a PULSAR power supply from SARIX
® and a process monitoring system, which is embedded in the control unit of the machine tool. This system allows for monitoring and recording of several quantities during the drilling process, such as the amount of discharge pulses and the average gap voltage. Regarding the architecture and operation of this process monitoring system, further details can be found in the published previous work [
17].
Small plates of Ti6Al4V alloy of 2 mm thickness, 6 mm width and 70 mm length are used as workpiece. Ti6Al4V is selected as workpiece material since it is widely used, especially in the aerospace and biomedical sectors [
5,
18,
19]. Brass tubes provided by SARIX
® (outer diameter: 350 µm, inner diameter: 130 µm) are used as tool electrode, which means that the aspect ratio of the micro holes is approximately 6. Hydrocarbon oil (HEDMA
® 111) is applied as dielectric liquid. In particular, side flushing and internal flushing through the tubular tool are applied simultaneously.
Brass tubular tools of small diameter are rather flexible. For this reason, a tool guiding system is used to reduce the run out of the tool electrode when approaching the workpiece. This guiding system consists of a ceramic tool guide and a guide holder, as shown in the magnified view in
Figure 1. During the experiments, the guide holder is positioned such that the ceramic guide is above the top surface of the workpiece at a distance of approximately 2 mm.
In order to drill holes of constant depth, the micro-EDM drilling process is continued until the breakthrough of the tool electrode through the workpiece is observed. The Ti6Al4V plates are clamped as cantilevers to facilitate observation of this breakthrough. A Dino-Lite
® Edge AM4115ZT digital microscope (AnMo Electronics Corporation, New Taipei, Taiwan) is used to monitor the drilling process, which is interrupted as soon as sparks are visually observed at the bottom surface of the workpiece. This allows to interrupt the drilling process before the tool has fully broken through, avoiding the instabilities in the discharging process occurring during the breakthrough stage [
20,
21], as these are not the subject of this research.
2.2. Data Collection
In order to obtain a heterogeneous dataset for training the regression models, the processing parameters applied in the micro-EDM drilling experiments are varied following a two-level full factorial Design of Experiments (DoE). Five processing parameters are varied, for a total of 32 experimental runs: pulse-on time (Ton), open voltage (Uo), charging frequency of the pulse generator (F), gain factor of the servo control loop (K) and reference gap voltage (Ue). The 32 experimental runs are performed in random order. The number of experimental runs is limited to only 32 experiments since the goal of this study is to compare the performance of different predictive models rather than showing the highest prediction accuracy, which can be achieved by each predictive model. It is believed that the accuracy of the predictive models would improve by increasing the number of experimental runs and therefore the amount of training data.
The five processing parameters in the DoE are selected so as to include variations to some of the main factors influencing the material removal process in micro-EDM drilling, namely the energy input per discharge (
Ton,
Uo), the cycle frequency of the power supply system (
F) and the settings of the servo feed control system (
K,
Ue). The levels of each parameter are set so as to cover a wide process window. The choice is carried out based on the recommendations from the machine vendor and previously-reported research work, which is carried out using a similar combination of tool and workpiece materials [
22,
23,
24].
Table 1 is a summary of the factors and levels of the DoE.
In all experimental runs, a constant tool rotation speed equal to 750 rpm is set and negative polarity is applied to the tool electrode. The energy index (
E) and current index (
I) are kept constant and equal to 301 and 70 respectively. According to the machine vendor, the energy index is the parameter determining the shape of the discharge pulses. The chosen energy index provides triangular-shaped pulses. When this energy index is selected, the current index can be used as a parameter to regulate the pulse peak current as shown in one of our previous studies [
21]. Both the energy index and current index can vary the energy input per discharge pulse. In this study, the values of
E and
I are kept constant since the effects of variations to the discharge energy are already taken into account into the DoE by varying other processing parameters (
Ton,
Uo).
A second dataset is recorded for evaluating the prediction accuracy of the regression models at experimental conditions which are different from the ones applied when recording the training data. In this case, 10 experimental runs are performed by randomly varying the processing parameters within the process window defined by the DoE. In other words, this means that the five processing parameters that are included in the DoE are set randomly to a value between the minimum (Level − 1) and maximum (Level + 1) levels applied in the DoE. The processing parameters applied in each of these 10 experimental runs are listed in
Table 2. All other processing parameters are kept constant and set as mentioned above.
During all experiments, three quantities are monitored and recorded using the monitoring system embedded in the control unit of the SARIX
® machine: the average amount of normal discharge pulses per time unit (
fp), average tool feed rate (
vf), and average gap voltage (
u). These quantities are recorded in order to be used as tool wear and material removal predictors since they are highly correlated to the wear of the tool electrode. This has been demonstrated by numerous experimental and theoretical studies in literature, for instance references [
17,
25,
26,
27,
28]. The values of
fp,
vf, and
u are computed using an averaging window corresponding to steps of 50 µm in drilling depth, similarly to what was done in [
17,
29]. The value of 50 µm refers to the nominal drilling depth as read by the
z-axis encoder of the machine tool. Due to the longitudinal wear of the tool electrode, it does not correspond to the depth of the hole being drilled. The computed values of
fp,
vf, and
u at every drilling step are automatically stored into a comma-separated value (.csv) text file.
The material removal rate (MRR) and tool wear rate (TWR) for each experimental run are computed at the end of the micro-EDM drilling experiments. The MRR is calculated as the ratio between the volume of material removed from the workpiece (
Vw) and the total machining time (
T), approximating the holes to a cylinder:
where
H is the thickness of the workpiece and
Din is the inlet of a drilled hole, which is measured by means of a VideoCheck HA coordinate measuring machine (Werth Messtechnik GmbH, Gießen, Germany) in optical mode. Similarly, the TWR is computed as the ratio between the volume of material removed from the tool (
Vt) and the total machining time (
T). To compute
Vt, the nominal inner (
dt) and outer (
Dt) diameters of the brass tubular tool are used. As shown in Equation (2),
Vt is calculated by subtracting the workpiece thickness (
H) from the monitored value of the drilling depth when the breakthrough is observed (
Zb).