## 2 Materials and Methods  
### Experimental Part  
The experiments carried out to determine the drilling torque and thrust force used the methodology presented by [PERSON] and [PERSON] [19]. Two sets of drills were used: four flat drills and four helical drills (Figure 1). All eight drills had a drilling diameter of 10 mm. The drills were sharpened with four values of tip angle (2 xr = 30\({}^{\circ}\), 60\({}^{\circ}\), 90\({}^{\circ}\), 120\({}^{\circ}\)) and with the same clearance angle \(\alpha\) = 20\({}^{\circ}\).  
Eighty samples of beech plywood (Figure 2), with a thickness of 18 mm, were used for processing. The rotation speed of the drills was \(n\) = 3000 rpm. Four feed speeds were used, _vf_ = 0.6, 1.8, 3.0 and 4.2 m/min. Forty samples were processed with the flat drills, and the other forty with the helical ones. Each group of forty samples was divided into four groups of ten samples, each such group being processed with one of the four feed speeds.  
Drilling was performed on an ISEL GFV/GFY CNC machining center. The measured parameters were the thrust force and the active power consumed by the spindle motor during drilling. The thrust force was measured with a device consisting of three HBM force transducers, type S2 (nominal force: 500 N). The measurement of the active power consumed by the spindle motor was performed by a Camille Bauer Sineax P530/Q531 transducer for active and reactive power. The equipment connection scheme is shown in Figure 3.  
For data recording, a DAQ Board Keithley Model KUSB-3108 was used with the help of the Keithley KUSB QuickDataAcq software v.1.5.5.0. Data recording was carried out with a frequency of 100 values/s simultaneously on all four channels: three for force sensors and one for the active power.  
The processing of the experimental data was carried out with the help of Microsoft Excel under Windows 10. To begin with, a filtering of the data was performed with a fourth-order Butterworth digital filter, in order to remove the parasitic components of the signal.  
To determine the thrust force \(F_{T}\), the data from the three force sensors were summed 3 by 3.  
Figure 1: The drill bits used in the experiments: (**a**) flat drills; (**b**) helical drills.  
Figure 3: Connection diagram of the equipment for measuring the dynamic parameters of the drill.  
Figure 2: The plywood samples used during drilling experiments.  
To determine the drilling torque \(T_{D}\), the data from the measurement of the active power consumed by the spindle motor were transformed using Formulas (1) and (2).  
\[P_{D}=P_{T}-P_{0} \tag{1}\]  
where:  
\(P_{D}\) is the power consumed only for the actual drilling, in W;  
\(P_{T}\) is the total active power consumed by the spindle motor during drilling (and measured), in W;  
\(P_{0}\) is the power consumed during idle running, in W.  
As known, in rotational motion the mechanical power \(P_{D}\):  
\[P_{D}=T_{D}\cdot n\]  
where:  
\(T_{D}\) is the torque, in Nm;  
\(n\) is the rotational speed, in rad/s.  
Therefore, the torque is: \(T_{D}=\frac{P_{D}}{n}\left[Nm\right]\) if \(P_{D}\) is given in [W] and \(n\) in [rad/s].  
However, if the rotational speed \(n\) is given in [rpm], we must transform the equation according to the new measurement units.  
So:  
\[1\frac{rot}{min}=\frac{1 rot}{60\text{s}}=\frac{2\pi rad}{60\text{s}}=\frac{2\pi} {60}\cdot\frac{rad}{s}\cong\frac{1}{9.55}\frac{rad}{s}\]  
Therefore, if \(P_{D}\) is given in [W] and \(n\) in [rpm]:  
\[T_{D}=9.55\frac{P_{D}}{n}\ \left[Nm\right] \tag{2}\]  
where:  
\(n\) is the rotation speed of the drill, in rpm.  
For further processing and analysis, the maximum values of both thrust force F\({}_{\text{T}}\) and drilling torque \(T_{D}\) were taken into account.