Different authors have proposed a method to define geometric and dimensional tolerances based on the assembly diagram of a mechanical device. While some recommend a sequential tolerancing analysis to examine the mating conditions of parts within assemblies, determining geometric tolerances often depends on heuristic processes. This paper introduces an algorithm that systematises the functional analysis related to these tolerances. To demonstrate the algorithm’s efficacy, we have applied it to a lathe tailstock centre as a preliminary case study. The proposed algorithm has proven to be effective in determining the geometric and dimensional tolerances in the mechanical device under study together with the standards or specifications of the parts incorporated in the device manufactured by third parties.