The value of comprehensive rationale information for documenting a design has long been recognized. However, detailed rationale is rarely produced in practice because of the substantial time investment required. Efforts to support the acquisition of rationale have focused on languages and tools for structuring the acquisition process, but still require substantial involvement on the part of the designer. This document describes an experimental system, the Rationale Construction Framework (RCF), that acquires rationale information for the detailed design process without disrupting a designer’s normal activities. The underlying approach involves monitoring designer interactions with a commercial CAD tool to produce a rich process history. This history is subsequently structured and interpreted relative to a background theory of design metaphors that enable explanation of certain aspects of the design process. Evaluation of RCF within a robotic arm design case has shown that the system can acquire meaningful rationale information in a time- and cost-effective manner, with minimal disruption to the designer.