## 3 Methodology
### Assessment Criteria for the Assembly Sequence  
The proposed tool, based on artificial neural networks, has the objective to support the determined sequence for manual assembly (although it is also possible to apply it, albeit after modifications, to an automated process). It was assumed that at the current stage of research it is used in a specific mechanical production company, where the conditions of the assembly process for newly introduced products are subject to ASP analysis, and the processes implemented were used to teach the network. This applies to issues related to, for example, the available machine park, production organization, process control and supervision, or the level of training of employees, especially in the aspect of manual assembly.  
The following assembly sequence evaluation criteria were used as input to the process:  
* Number of tool changes for the respective assembly sequence.  
This criterion indicates the number of tool changes during assembly operations. Operation constitutes the main structural element of a technological assembly process. In  
Figure 3: The graphic concept of the proposed method of prediction of assembly time.  
this work, operations should be understood as, for example, activities such as riveting, drilling, fitting, and screwing, which are related to changing tools. Depending on the type of parts to be installed, the required tools can be assigned to them in a simple manner, from the set of tools utilized in the considered assembly process.
* The number of changes in assembly direction for the respective assembly sequence. It is the most frequent optimization criterion in ASP. This criterion is connected with the direction in which the parts are attached during their assembly. There are 6 main assembly directions, along the 3 main axes: \(\pm\) X, \(\pm\) Y, and \(\pm\) Z.
* Number of stable and unstable units for the specific assembly unit. Stability criterion determines the number of stable and unstable units for a particular assembly sequence. We assume that a stable unit is such a unit that remains in an assembled state, regardless of the force applied to it. The applied forces may be the force of gravity or the forces associated with the movement of parts or an assembly unit.  
We justify the adoption of these criteria for the evaluation, among others, with the fact that, as one of the few, they can be automatically obtained from the CAD assembly model, although it is also assumed that the data can be completed manually.  
The purpose of the system is to assist in the estimation of time for all acceptable sequences under constructional constraints (i.e., feasible ones) and thus enable the selection of the most favorable one under existing manufacturing conditions. Under these evaluation criteria it is the sequence with the lowest number of tool changes, the smallest number of changes in assembly directions, and the smallest possible number of unstable states that will likely be indicated as the most favorable one; however, it is practically impossible to obtain such values with these criteria for a single sequence. This is related, for example, to the weights of individual criteria in relation to the specific assembly process.