**4. Machining Tools**

There are several different types of tools used in the machining operations on composites and multi-materials, depending on crucial factors, namely, the process and the material to be machined. According to these, a set of variables has to be chosen, such as the tool material, geometry or need for a coating, from which a large range can be employed. These factors will result in determined cutting forces and torques, which will impact the base material, considering its final roughness or the existence of delamination, and the tool, in particular its wear mode, in a specific way.

In order to optimize the machining process, the desirable mechanical properties for a cutting tool material are small grain size, to produce a sharp cutting edge, good toughness, to keep the sharp cutting edge without chipping or deformation under the dynamic action of the cutting forces, and high hot hardness, to provide a great resistance to abrasive wear under elevated cutting temperatures. In terms of thermal properties, since the temperature rises abruptly during the process, the tool needs to possess a good thermal conductivity to remove heat from the cutting zone, thermal stability, to maintain integrity at cutting temperatures and low chemical affinity to the workpiece material [57].

The preferable tools in the literature for drilling composite-metal stacks are carbide tools such as tungsten carbide, due to their high strength and wear resistance compared to HSS/HSS-Co tools. These also are among the ones most used, alongside Poly-Crystalline Diamond (PCD) tools [86].

#### *4.1. Tool Geometry*

When referring to the geometry of the tools, each machining operation has a specific type of geometry, the most fit to perform a specific kind of work. In the drilling case, there are two important angles to be considered, namely, the point angle and the helix angle. The standard helix angle for most drills is 30◦; however, in spite of most drills coming with a 118◦ drill point angle, when it comes to drilling composites, it is recommended to use a drill bit with a point angle between 130◦ and 140◦, values also used in aluminum drilling. A cutting tool with large helix angle, usually higher than 24◦, allows a quick chip evacuation, whereas large point angles improve chip removal and reduce burr formation. The increment of these two parameters, seen in Figure 12, also minimizes surface roughness [87]. Additionally, this figure presents the various drill geometries for specific drilling operations.

**Figure 12.** Different tool geometries used in drilling: (**a**) standard twist drill, (**b**) step drill, (**c**) Brad point drill bit, (**d**) straight flute drill bit (dagger) and (**e**) multifaceted drill bit (adapted from [54]).

The step drill is one of the drilling strategies to avoid material damage around the hole, for example delamination, as it first inflicts a primary hole, with a smaller diameter, and then it drills the intended size, so as the contact with the substrate is smoother [88].

Apart from the already exposed geometries, tools with special designs have been proposed to decrease the probability of delamination and obtain better productivity. In this matter, double helix tools minimize axial forces by utilizing two opposite helix angles [89]. Double point and multi-facet drills were also developed, producing lower thrust forces, due to an additional cutting edge [90]. In the case of milling, the multi-tooth milling cutter could significantly minimize milling defects for CFRP and decrease cutting forces [91].

When there is a need to measure the temperature reached in drilling processes, a thermocouple can be inserted into the drill bit lubricant-cooling fluid holes, as shown in Figure 13. The thermocouples do not necessarily have to be positioned in the same place for every case, meaning that one device may be in the sharp edge and other in the flank of the drill. This is a more complex procedure, but this way a more complete analysis of the temperature reached by the tool can be achieved, with information from different areas, each of which is subjected to a specific stress and, therefore, temperature [92].

**Figure 13.** Thermocouples in the drill (adapted from [93]).

This set-up allows the measurement of the temperatures arising in the tool during the drilling. Thermocouples may also be inserted inside the multi-material: one in the composite laminate and other in the metal sheet. Although in the last years, alternative approaches to measure the temperature during the machining processes have been proposed, less invasive and relying on no-contact methods, such as infrared pyrometers or infrared cameras, the thermocouples are still the most effective method [53].
