The ease with which 3D CAD models may be modified and reused are two key aspects that improve the design-intent variable and that can significantly shorten the development timelines of a product. A set of rules are gathered from various authors that take different 3D modelling strategies into account. These rules are then applied to CAD strategic-knowledge learning methodology and included in 3D CAD modelling exercises for students following the degree in mechanical engineering at the University of Burgos (Spain). The experiment was conducted in two groups with a total of 75 students. The designintent rules were introduced in the different exercises that the teacher explained in both the theoretical and the practical classes. In addition, a summary of the different design rules in each of the practical exercises was explained in the practical classes in only one of the groups. The experimental results, reported in this paper, tested the influence of these summaries on overall improvements in 3D modelling and on the design-intent variable, which is subdivided into four sections: skeleton, structures, alterations and constraints. The use of the summaries of the design intent rules led to statistically significant improvements in 3D modelling in the experimental group, in comparison with the group of students to whom those summaries were not explained. KEYWORDS:Strategic knowledge; 3D CAD learning; reuse and alteration of the CAD model; design intent; 3D design rules.

An Interactive Learning Management System (ILMS) is presented, which functions as a webbased Spatial Visualization Ability (SVA) learning support tool for students of engineering graphics and as a management tool for teachers to track student learning. This software is designed to fill the gaps in student knowledge, giving them more uniform spatial visualization abilities when enrolling on University Engineering degrees. The ILMS_SVA consists of: 1) a Content Management System (CMS); 2) a preliminary level assessment test; 3) a web-based tool for exercise management and self-assessment incorporating a 3D viewer that functions as an interactive tutorial (IT), allowing the manipulation of 3D objects in every exercise; 4) a database. It is designed for three types of users (student, teacher and administrator), and has been validated with engineering graphics students at the University of Burgos (Spain) by means of experimental trials in the classroom and a user satisfaction survey, over two academic years. The results indicate that use of this tool improved SVA among students generally and was even of greater effectiveness for those students that accessed engineering courses with no prior knowledge of Technical Drawing.

A simulation of tolerance analysis in assemblies using Sigmund CAT (Computer Aided Tolerancing) software is validated through the example of an automobile locking device. Simulation with CAT, applying criteria on both the statistical distribution and the rivet pin position in the hole used in the example, will allow us to predict the functional dimension tolerances in these assemblies with greater accuracy in the preliminary design phase. These tolerances will subsequently define the manufacturing specifications. The statistical distribution, in the example, that best fits the overall set of tolerances, is the triangular distribution followed by the normal distribution; the position of the rivet pin axis in its hole is off-centre by 53% with regard to its maximum value.

In CAD modeling, there is no one general standardized teaching-learning methodology. We use the strategic-learning methodology, maintaining design intent, fully aware that it is necessary to modify CAD models for their reuse. Questions concerning the thought processes of students when modeling with CAD and the strategies that they choose that best maintain design intent arise in the course of using the 3D modeling programs. Our aim here is to determine the importance of adaptive expertise in the results of CAD models and, particularly, in one of its constructs: design intent. To do so, CAD-based experimentation took place over two years with 78 third-year students in the first year and with 53 third-year students in the second year from the subject module of Graphic Engineering, on the Degree in Mechanical Engineering of the University of Burgos. At the start of the year, the students conducted a survey to measure adaptive expertise. Subsequently, in the first year of experimentation, the students prepared various CAD models and the design intent was evaluated in one of them (a connecting rod or conrod), broken down into the skeleton, the structure, the modifications and the constraints. In the conrod exercise, the students also completed a questionnaire both before and after designing their models, which were analyzed to detect the thought processes and the strategies that they had applied. In the second year of experimentation, design intent was incorporated in various exercises at the beginning of the year, in addition to the conrod. The main conclusion is that the correct division of the part into its pieces and adaptive expertise improved the results in relation to design intent in the CAD.

El incremento de capacidad de visión espacial para los alumnos del grupo E sin conocimientos previos de dibujo técnico no es mayor, de forma significativa, que el de los alumnos del grupo T sin conocimientos previos de dibujo técnico. Pero existen menos diferencias, de forma significativa, entre los alumnos del grupo E, que tienen y no tienen, conocimientos previos de dibujo técnico, que entre los alumnos del grupo tradicional en las mismas circunstancias.