The final product quality is determined by cumulation, coupling and propagation of product quality variations from all stations in multi-stage manufacturing systems （MMSs）. Modeling and control of variation propagation is essential to improve product quality. However, the current stream of variations （SOV） theory can only solve the problem that a single SOV affects the product quality. Due to the existence of multiple variation streams, limited research has been done on the quality control in serial-parallel hybrid multi-stage manufacturing systems （SPH-MMSs）. A state space model and its modeling strategies are developed to describe the multiple variation streams stack-up in an SPH-MMS. The SOV theory is extended to SPH-MMS. The dimensions of system model are reduced to the production-reality level, and the effect and feasibility of the model is validated by a machining case.
