## 3 Existing Solutions  
Several assembly solutions for parts manufactured with composite materials have been envisaged such as direct threading, bonding, and metallic inserts.  
* Direct threading: Direct threading is generally used for metallic component assembly. Several studies were conducted to study the effect of drilling and tapping parameters on the tools and the resulting force in the assembly of composite materials. [PERSON] et al. [12] studied the effect of tapping parameters, namely, cutting speed and tap coatings, on thread quality. They evaluated the required thrust force and the tapping and tightening torques to drill and tap parts manufactured with composite materials. It was found that coated taps used at a speed of 15 m/min improved thread quality. They allowed a lower tightening torque and thrust. Coated taps reduced the coefficient of friction during the tapping process and protected the tool from wear. This fact was also illustrated by [PERSON] et al. [13], who found that the thrust force increased significantly with tool wear during the drilling process, causing delamination of the carbon fiber composite material. A failure to adjust tool parameters led to increased wear of the tool and, hence, an inconsistency in the drill sizes and taps. It was concluded that the tool is highly affected by the number of drills made per tool, which negatively influences the dimensions of the hole. Adding to that, due to the wear of the tools, the thread parameters are inconsistent, which reflects an inconsistency in the allowable forces supported by the assembly. This led to additional research to reduce the tool wear and defects such as burr caused in the drilled material by using hybrid composite material, for example, aramid (C-AFRP) [14]. Other studies are working on applying an Artificial Neural Network to predict the adequate machining parameters such as spindle rotation speed and feed rate [15]. Even though direct threading is commonly used in metallic applications, it still has a major drawback when applied to composite materials. In metallic applications, welding and re-tapping can fix a failure in direct threaded assemblies. However, in composite materials, local reparations are not feasible, which requires a replacement of the whole part;
* Adhesive bonding: [PERSON] et al. [16] explored adhesive bonding to assemble parts manufactured with carbon composite materials. This method requires extensive surface preparation and cleaning. It allows for a large stress-bearing area and a uniform distribution of stresses. However, there are drawbacks: (i) Assessing the bond area is impractical since the procedure does not allow for visual examination of the assembled surface; (ii) Extensive surface preparation is required to ensure an adequate bond. In fact, successful bonding is characterized by an adequate choice of adhesive material, the cleanliness of the surface, a good joint design that maximizes the bonding surface without altering the mechanical design, and a curing temperature that should be compatible with the adhesive material; (iii) Long curing times might be needed, with presses to hold the assembly together; otherwise, parts should be placed in autoclaves or ovens to decrease the curing time. These processes are time consuming and costly;
* Metallic inserts: The metallic thread inserts shown in Figure 4 are suitable for non-permanent assembly solutions. They are mainly used for plastic, wood, or aluminum assembly applications. These inserts have external knurls to grip the material of the part they assemble. Replacing these described inserts will eventually result in permanent deformation of the parts, compromising the interchangeability requirement. Additionally, the external knurls cause irregular deformations that compromise the leak tightness requirement in a hydraulically integrated component.