MODEL BASED DEFINITION: FINALLY, THE ENGINEERING DRAWING KILLER?
Editor: Bohemia, Erik; Kovacevic, Ahmed; Buck, Lyndon; Brisco, Ross; Evans, Dorothy; Grierson, Hilary; Ion, William; Whitfield, Robert Ian
Author: Garland, Nigel Patrick; Wade, Russell; Glithro, Richard; Palmer-Smith, Sarah
Institution: Bournemouth University, United Kingdom
Section: Innovation 1
DOI number: https://doi.org/10.35199/epde2019.69
The death of the Engineering Drawing has been foretold many times since the first CAD systems began automating the technical product specification (TPS) process. As CADCAM technology advanced it became routine to drive both manufacturing operations and metrology of complex parts directly from the geometrical model with only variance derived from the drawing. This solution can, however, lead to problems of precedent with the drawing considered the master document.
Alongside the integration of operations there has been continual development of Engineering Drawing practice (TPS) through International Standards. However, despite Geometrical Tolerancing being incorporated into British Standards since 1953 (BS308:1953) it has been widely ignored within UK Higher Education. The lack of effective teaching in this area leaves many graduates un-prepared for the demands of industry and disadvantaged compared to their international counterparts.
More recently, the 3D CAD environment has provided the backdrop for application of specification directly to the model through Model Based Definition (MBD) or Product Manufacturing Information (PMI) rather than within an Engineering Drawing. These methods lead to significant advantages in the transfer of requirement between organisations and processes such as translation from Model to Specification, Manufacture and Metrology per concepts such as Industry 4.0. Furthermore, these tools overcome the problem of precedence, since there is no drawing.
The emergence of GPS as the dominant method for specification lends itself ideal for conveying through PMI/MBD methodologies since the model itself is the TED and only the features of size require dimensioning with the remaining surface variance being controlled through Geometrical Tolerances. However, the widespread use of these methods is currently limited by interoperability, user knowledge, standards compliance and learning material.
A new unit (Engineering Design Tools) was developed to instil knowledge and understanding of MBD to Design Engineering students. The students had limited understanding of GPS and TPS from level 4 but no knowledge of MBD. However, the same four step procedure developed for level 4 was applied within the MBD environment: Understand system functionality; identify and classify functional surfaces; establish kinematic relationships of functional surfaces; control of remaining surfaces.
Students were asked to design a unique two stage gearbox; each was provided a different combination of power rating, input speed and ratio. Students developed the specification of the gears, shafts and key-ways before the modelling of any components in the CAD environment. With a sound understanding of gearbox functionality developed through the design phase and practical support tutorials they were introduced to MBD functionality and various layup methods of features of size, fits, and geometrical tolerances. Students were then introduced to basic stack-up analysis techniques to evaluate the projected alignment of gears, hence control of backlash. Again, by focusing upon the functionality of the whole system, students developed understanding of the impact of tolerance selection.