Automated 1st Order Tolerancing: Schema Generation

Tolerances are specified by a designer to allow reasonable freedom to a manufacturer for imperfections and inherent variability without compromising performance. A group of tolerance classes, tolerance values and datums specified in design that control the variations in a part to be manufactured, is called a tolerance scheme. It takes knowledge and experience to create a good tolerance scheme. It is a tedious process driven by the type of parts, their features and controls needed for each one of them. In this paper, we investigate the development and implementation of 1st order automated Geometric Dimensioning and Tolerancing (GD&T) schema generation. Prior to schema generation, some assembly information such as presence of assembly features and pattern features in the given assembly CAD file is required. Mohan et al [6] proposed and implemented 3 preprocessing modules which provides required assembly information. Rao [8] developed and reported 5 experiential GD&T rulesets that could help the process of auto-tolerancing. Haghighi et al [5] proposed a procedure for automating 1st order GD&T schema generation and value allocation based on the provided assembly information and using GD&T rulesets. In this paper we present the development and implementation of 1st order GD&T schema generation. The output of this toolset is a complete GD&T schema for given assembly without tolerance values. Biswas et al [12] proposed and developed a toolset for tolerance value allocation and analysis. Once GD&T schema is generated and tolerance values are allocated, recommended GD&T is translated into STEP AP242 file format. Venkiteswaran et al [13] developed a module that reads the nominal geometry in STEP AP203 and GD&T information in CTF format and translates it to STEP AP242. Combining all these 3 modules with pre-processing modules leads to the completion of 1st order auto-tolerancing.