*2.3. Equipment Used for the Operation and Evaluation of Drills*

The selected OSD strategy has been the conventional dry type, applied separately to the CFRP and Al plank and the CFRP/Al stack. The stack configuration was CFRP/Al as it is the established machining sequence in the aerospace industry. This sequence aims to minimize the defects produced in the internal faces during the application of one-way assembly (OWA) techniques. The use of lubrication has not been considered, as we are looking to develop an environmentally friendly drilling process. The joining method has been defined in order to avoid possible failures caused by displacements of the plates that compose the stack.

The set values for the cutting parameters have been defined on the basis of other studies and real application cases, and are indicated in Table 1. Two tests were carried out using a Kondia Five 400 5-axis machining centre (Elgoibar, Guipuzcoa, Spain), controlled by a Heidenhain iTNC530 control system (Traunreut, Bavaria, Germany).



Microstructural analysis has been developed using scanning electron microscopy (SEM,) techniques. The compositional analysis of the materials and the cutting tool were carried out by means of EDS (energy-dispersive spectrometry) techniques with analytical capacity. The equipment used for the application of SEM and EDS techniques was the EDAX EDS System (Mahwah, NJ, USA).

For monitoring the shear forces, a dynamometer table model KITSLER© 9255B (Fx, Fy, Fz and Mz) (Kistler Holding AG, Winterthur, Zürich, Switzerland) was used. This instrument is connected to a computer that transfers the obtained data to the computer using Labview software (National Instruments, 2014, Austin, TX, USA) for processing. The sampling rate is 1000 Hz.

One of the analysis parameters studied within the macrogeometric deviations was the dimensional tolerance of the hole diameters. This measurement was carried out with a Mitutoyo three-contact internal micrometre (Mitutoyo Corporation, Kawasaki-shi, Kanagawa, Japan) with a measuring range of 6–8 mm, an accuracy of 0.001 mm and a measurement uncertainty of 2 μm. A total of three were measured sizes at different heights and angles per hole in each material.

#### **3. Results and Discussion**

#### *3.1. Tool Wear during CFRP Drilling*

Through the observation of different images obtained by SEM, abrasion was ratified as the main wear mechanism as a consequence of the continuous cutting action of carbon fibre (CF) [11–13]. Its geometric effects along the tool were visible when the tool surface's own marks disappear (Figure 5a). Other consequences found were erosion of the cutting edges (Figure 5b) and the tool tip (Figure 5c), and the appearance of marks along the periphery of the drill bit (Figure 5d).

**Figure 5.** Location of drill wear by the abrasive action of carbon fibres: (**a**) main edge; (**b**) primary and secondary edge union; (**c**) tool tip; (**d**) peripheral face.

The continuous action of the carbon fibre on the cutting edge of the tool caused its irregularity as a consequence of abrasion [8–10] (Figure 6).

The incipient fusion of the epoxy matrix may cause its dispersed adhesion over the tool. Adhered elements in the tool can incorporate particles or small pieces of CF that interfere with machining, especially when located in chip evacuation channels. A considerable amount of adhered material (Figure 7) was observed, which could be a signal of further deterioration of the matrix in the walls of the drilled material. The absence of resin is the cause of the problematic loss of the fibres junction, with a consequent decrease in the quality of the hole.

**Figure 6.** Detail of irregularities in the cutting edge of the tool.

**Figure 7.** Accumulation of carbon fibre adhered to the tool: (**a**) face rake; (**b**) the area next to the guide surface of the tool.
