## 2 Materials and Methods  
Polyurethane (PUR) foam blocks (Sawbones, Vashon Island, WA, USA) used in the present studies simulate the clinical conditions encountered during implant bed preparation. Artificial bone blocks made from solid rigid polyurethane foam (Figure 1) are similar to human D1 bone i.e., primarily dense cortical bone according to [PERSON]'s classification [20] and offer uniform and consistent physical properties that eliminate the variability encountered when testing with human cadaver bone. For this experiment foam blocks with a thickness of 10 mm, a width of 15 mm, a length of 15 mm, and density of 0.64 g/cm\({}^{3}\) (40 pounds per cubic foot = 40 pcf) were used.  
The experiment was carried out with eight different drills from the same manufacturer (Ihde Dental, Gommiswald, Switzerland), i.e.:  
_Group 1_: Conical drills with (drill 1) and without (drill 2) a hole for standard implants with conical core.  
_Group 2_: Cylindrical drills \(\mathcal{O}\) 2.0 mm (drill 3), \(\mathcal{O}\) 2.5 mm (drill 4) and with cutting disc \(\mathcal{O}\) 7 mm (drill 5).  
_Group 3_: Cutting discs \(\mathcal{O}\)8 mm (drill 6), \(\mathcal{O}\)9 mm (drill 7) and \(\mathcal{O}\)10 mm (drill 8) as presented in Figure 2.  
Figure 1: The polyurethane (PUR) block used for drills testing.  
The experimental setup, as shown in Figure 3, consisted of the surgical drilling machine (Implantmed, W&H, Burmoos, Austria) fixed in a three- axis movement CNC machine. A PUR block with thermocouples to detect the temperature changes inside the foam was fixed on a table in the same horizontal axis as the surgical drilling machine (drills 1 and 2) and in the same drilling direction. An IR camera (FLIR T1020, FLIR Systems, Wilsonville, OR, USA) was mounted in front of the PUR block to record the temperature at the cutting edge of the experimental drills while perforating the PUR block.  
Figure 3: Experimental setup: 1—Infrared (IR) camera with PC connection; 2—PUR block with installed thermocouples connection to PC; 3—measured drill; 4—W&H surgical drilling machine; 5—drilling machine fixation and three axis movement; 6 and 7—PC for drill movement controlling and recording data from IR camera and thermocouples.  
Figure 2: Specific drills for the experimental investigation.  
In this study, a load of 2 kg was applied, in accordance with the procedures described by [PERSON] et al. [21]. For the cutting disc measurement, drills 3, 4, 5, 6, 7 and 8 were installed in the vertical axis with the rotary function of the CNC machine, as shown in Figure 4. In front of the PUR block the FLIR T1020 HD thermal camera was mounted to record the temperature in the cutting edge while drilling [18]. The experiment was done at three different rotational speeds of 800, 3000 and 5000 rpm. Each measurement combination of the drill type and rpm was performed only with cooling, for which the surgical drilling machine cooling system was used with water as a cooling medium. Cooling water was showered on the drill or cutting disc while performing experiment. Three holes were drilled with each drill to test the samples of the polyurethane foam.  
Drills 1 and 2 were used in horizontal axis 2 mm/30 mm/min axial movement of a drill (drilling) and then the load was reduced by moving the drill 1 mm back (with an axial movement of a drill 500 mm/min). Using this interrupted drilling technique it was possible to drill through the PUR block. According to some researchers one of the benefits of this type of interrupted drilling protocol is lower heat production [22]. Interrupted drilling protocol 5 mm/min was also used for drills 3, 4, 5, 6, 7 and 8 (drilling was 2 mm by 5 mm/min and then 1 mm back by 500 mm/min).