As expert system technology moves to the factory floor, certain problems naturally arise. Knowledge representation (1)* and acquisition (2) have been stated as major stumbling blocks, which may be true in a general sense. For a specific application, however, representation and acquisition are a minor hurdle compared to the wall of data acquisition and filtering. Expert systems are high-level programs which require high-level concepts to manipulate. Raw data from the factory floor is unsuitable for an expert system and must be filtered and combined into a more appropriate form. In this paper we examine building an expert system for arc welding thin metal plates, and the difficulties in providing the expert system with appropriate high-level concepts.