Most digital manufacturing projects follow the software-development process. However, the application of all the characteristics and processes specific to a digital manufacturing project cannot be generalized to all other engineering projects. Therefore, digital manufacturing project teams typically design and apply process principles that can be customized to individual use cases by project members. Tailoring these principles, including developing and referencing them, is a knowledge-intensive activity that requires the gradual improvement of development processes. This paper proposes a knowledge-oriented digital manufacturing ontology model and a semantic rule-based tailoring system that can derive tailoring strategies from knowledge ontologies via an inference-rule design and engine. The proposed model and system can assist in concrete project implementations based on software-development experience. A practical example is demonstrated through the 3D-modeling digital manufacturing of a hospital kitchen.