**2.1. Tool wear monitoring**

The assembly of FRP components for the aeronautical industry is generally performed by means of mechanical joining processes like riveting, which offer higher performance and less challenges compared to welding and adhesive joining techniques. As a consequence, hole making is a central process since a large number of holes is required to allow for riveting of aircraft components. Mechanical drilling using conventional or innovative drill bits is the most commonly employed hole making process for FRP components, although alternative nontraditional machining processes, such as laser and water-jet machining, have been developed in the last years [2, 3].

Despite the large application of FRP mechanical drilling processes, still efforts are required to optimize them since tool wear typically develops very fast, and severe damages can be easily

Numerous critical defects such as geometric/dimensional errors, entry/exit delamination, interlaminar delamination, fiber pullout, uncut fibers, spalling, and cracking and thermal damage can be generated by drilling of FRP laminates [8–11]. Drilled holes of low quality result in out-of-tolerance assembly and long-term weakening of structural properties, which are not acceptable in the aeronautical sector. Tight geometrical/dimensional tolerances and surface integrity need to be met as they are a key requirement to guarantee the functionality

In the last years, several research studies have investigated the role of drilling process parameters, including cutting speed, feed rate, drill bit geometry, and composition, on the output product quality, with particular reference to carbon fiber-reinforced plastic (CFRP) laminates,

Different types of tools, distinguished by diverse geometry and material, have been investigated for FRP composite drilling. A complete analysis of delamination produced by drills of different geometry, including traditional twist drills and innovative drills as candle stick drill, saw drill, core drill, and step drill was reported in [13]. To reduce the high wear rate of the sintered carbide drills, TiN and DLC coatings were employed in [14] for drilling of CFRP laminates, studying material damage, thrust force, and torque generated during processing. The experimental results showed that tool wear or damage was not significantly improved by using coatings. In [15], the performance of uncoated and diamond-coated carbide tools was investigated: the diamond coating provided significantly better results achieving a tool life 10–12 times higher than uncoated carbide drills and much higher cutting speeds (170 m/min against 56 m/min).

Drilling of CFRP laminates was also investigated in [16] to identify the most suitable drilling parameters satisfying the hole quality requirements, including surface integrity and roughness, hole roundness and diameter error, showing that the thrust force and the delamination

The studies in [17] on high speed drilling of CFRP laminates using K20 carbide drill bits under different drilling parameters (spindle speed and feed rate) showed that feed rate has a major influence on thrust force, push-out delamination, and hole diameter, whereas spindle speed

A thorough study on the cutting mechanism and the influence of cutting parameters on delamination in CFRP drilling was presented in [18], showing that thrust force is highly

generated on the workpiece, affecting material integrity and surface quality [4–7].

which are the most commonly utilized in aerospace applications [1, 12].

damage were in agreement with the tool wear zones.

is one of the key factors of the drilled hole roundness.

of the assembled components.

68 Characterizations of Some Composite Materials

In the aeronautical industry, the practice for CFRP/CFRP stack drilling is typically based on manual drilling processes, where tools are replaced largely in advance to avoid any risks of material damage due to early tool failure, since severe geometrical and dimensional tolerances need to be met, and surface integrity is crucial. To fully exploit the entire tool life and increase the productivity of the aeronautical industry through a higher automation of drilling processes able to preserve the integrity of the workpiece, a reliable on-line tool condition monitoring procedure is required [21].

With the aim to perform tool condition monitoring in drilling of CFRP/CFRP stacks, different methodologies have been developed [21, 22, 24]. Such methodologies are based on the employment of multiple sensor monitoring systems for the acquisition of thrust force and torque sensor signals to be used for tool wear estimation. The different procedures of advanced sensor signal processing and feature extraction implemented in the time and frequency domain are described in the following sections. On the basis of the features extracted from the force and torque sensor signals, it is possible to develop an artificial neural network (ANN)-based cognitive paradigm for pattern recognition with the aim to find correlations between the extracted sensor signal features and tool condition [21, 22, 24].
