## 3 Results and Discussion  
### Bushing Height and Wall Thickness  
Bushing height and wall thickness values are crucial geometrical dimensions, affecting the outcomes of the TFD operations. While bushing height affects the connecting length and strength by means of achieving the numbers of the threading teeth, the depth of the threading tooth is affected  
Figure 4: Beakers, pipet, and samples, whose volumes were measured. Figure 5: Geometrical properties of samples. by the bushing wall thickness. However, these dimensions have been barely investigated in the open literature. Nevertheless, in the literature, it is established that the bushing height is about 2-3 times the frictional thermal drilled material thickness. However, this idea may be valid for thin sheet metal materials, thinner than 2 mm, but it does not seem to be valid for sheet materials thicker than 2 mm. In TFD operations, it is considered that the most influential parameter on bushing geometrical dimensions is material thickness. However, there are other crucial parameters, such as hole diameter, spindle speed, and feed rate, having a significant effect on these dimensions. It is because the volume of the evacuated material from the thermal frictional drilled material shapes the bushing. Therefore, the material thickness (t) and hole diameter (\(\mathcal{O}\)d) are more influential parameters than the selected spindle speed and feed rate, on the shaping of bushings. Although the effects of spindle speed and feed rate on the geometrical dimensions of the bushing shape are less than those of the material thickness and hole diameter, they may cause the generation of cracks and petal formation, especially in TFD of brittle sheet materials due to rotating and proceeding motions of the conical tool. The experimentally gained bushing shapes for different t and d values are given in Figure 6. As shown, it can easily be seen that the cracks and petal formations on the bushings increased with higher d and lower (t) values. When the (t) value exceeded 6 mm, cracks and petal formations were  
Figure 6: Bushing shape according to material thickness and hole diameter. almost completely eliminated under the experimental conditions; 4 and 6 mm material thicknesses were found to be appropriate only for hole diameters of 5 and 10 mm according to cracks and petal formation criteria, while 2 mm material thickness was not appropriate. Therefore, in the TFD of sheet materials, 2 mm in thickness, hole diameters, smaller than 5 mm, are not recommended to be selected. The effects of TFD parameters on the bushing height and wall thickness are given in Figure 7a,d. Figure 7a tells us the bushing height has a tendency to increase linearly with the hole diameter (d) for all values of material thickness. A negligible difference between 2 to 10 mm thicknesses was observed. However, the bushing wall thickness showed a nonlinear alteration with changing hole diameter from 5 mm to 20 mm, and material thickness from 2 mm to 10 mm, as demonstrated in Figure 7c,d. Bushing wall thickness values remained constant for material thicknesses greater than 6 mm for all hole diameters. However, it showed a linear change for 2, 4, and 6 mm thickness with selected 5, 10, and 15 mm hole diameters. Our findings are not in agreement with the literature, which states a 2-3 times relationship between the bushing height and the thickness of the material [3; 4; 5; 6; 13; 14]. According to the graphs in Figure 7, the bushing height was linearly influenced by hole diameter, while the effect of material thickness did not have an important influence. For the implemented experimental procedure, four kinds of hole diameters and five kinds of material thicknesses values were selected. Consequently, the recorded results for bushing height (h\({}_{\text{a}}\)) and bushing wall thickness (t\({}_{\text{w}}\)) depended on material thickness (t) and hole diameter (d).