Although many investigators support the instructor-assigned teams, in the usual teaching practice in project-based learning environments, the formation of the team for the project is often left to the self-selection of the students. The work of this thesis, of experimental exploratory nature, was about engineering students’ teaming-effectiveness, after working in project teams instructor-assigned, based on have role diversity in the team by applying a team role’s questionnaire, and also, after receiving specific instruction about teamwork. The participants in this study were 77 third-year engineering students attending a manufacturing processes course during 2018-01 semester. Teams of 4 to 5 students developed a course project throughout the semester. A questionnaire was implemented to obtain, confidentially, an assessment of teamwork, a self-assessment and a peer assessment of the work of each one of their colleagues. Teams were asked to write a process analysis (reflections on project management, on the individual and group learning processes, and recommendations for their future work in teams). This study was done with a qualitative approach. The salient themes found were: interdependence and team structure, shared mental models, communication inside the team, conflict management, social loafing and scheduling conflicts. The constructs of team spirit and social support were no detected. As far as the author know, there is scarce research on the results of the team-forming method selected, based on mixing the roles that spontaneously assume the students, when developing team projects in engineering education settings.