In situ machining of large parts using mobile machines that are moved to part location(s) allows these parts to be machined, repaired and maintained directly on the site of operation. However, every new part and location that the machine is moved to results in different boundary conditions for the machine-part system which singularly influences the dynamics of the combined system and makes for difficult planning of first-time right machining solutions. To facilitate the planning of in situ machining solutions, this paper presents a dynamic substructuring framework that can predict assembled system dynamics by combining measured and/or modelled substructural response of the individual components under varying base/part/contact/configuration characteristics. Substructural coupling is formulated in the frequency and the continuous-time domains for representative examples of the mobile machine being connected to two different part/base models. The frequency based substructuring approach is used to predict tool point dynamics, results of which are instructive for designing of machining strategies to ensure stable productive cutting conditions. State-space substructural coupling predicts the changing nature of the plant model under the varying influences, which provides guidelines for designing appropriate control strategies and selection of appropriate CNC control parameters. Methods presented aid establishment of experimental guidelines for planning of first-time right in situ machining solutions.