Within a typical global manufacturing company the product development process uses huge amounts of knowledge related to manufacturing and design activities. The author identified a need to create structures and methods that readily manage and maintain such knowledge to assure the long-term use of their Knowledge Based systems and improve the company's competitiveness. Modeling manufacturing knowledge is a requirement to design a Decision Support System. This research defines a Manufacturing Knowledge Model with the ability to store and manage various types of knowledge using a knowledge maintenance method. Major emphasis was placed on process knowledge and resource knowledge to support process planning decisions. This work also explains the relevance of using different types of knowledge categorization, according to the field of Knowledge Management. Following the Knowledge Maintenance Life Cycle described here as a method to maintain knowledge, it was possible to capture new and valuable machining knowledge using different types of representations. An experimental software based on Object Oriented programming is presented to demonstrate the research ideas.