The proposed methodological framework concerns axiomatic theory fusion (ATF) of non-additive engineering design theories. ATF includes seven steps: (i) semantic discretization of the composite theories, (ii) deriving epistemological entities by logical/semantic analysis, (iii) establishing and representation of relations among all relevant epistemological entities, (iv) combining the inter- theoretical epistemological entities of the component theories, (v) deriving propositions based on the combined set of epistemological entities, (vi) transcription of the epistemological entities and propositions into a textual/visual theory description, and (vii) validation of the resultant theory in application contexts. The proposed framework makes ATF an effective, content independent methodology for fusing component theories, no matter if they are descriptive, explanatory, predictive or controlling in nature. ATF methodology requires professional comprehension and rigor from the researchers. It is necessary to justify the logical correctness and practical validity of the target theory in the specific application context.