Spanning knowledge group boundaries is both a source of and barrier to design performance and innovation. Objects—from prototypes to kanban boards—are frequently used in cross-functional design practice, but their associated outcomes appear varied and dependent not only on the objects themselves but on how, when, and by whom they are used. We conducted a two-year ethnography within a turbomachinery design company to understand how professional engineering designers span knowledge group boundaries to advance their designs and design processes. Our findings identify three roles of objects of collaboration: routinizing cross-boundary interaction, translating information across boundaries, and motivating joint negotiation or discovery. We illustrate two prominent outcomes—the co-discovery of a design risk, opportunity, or bottleneck and the co-design of a joint integrated solution—and describe two object role sequences from which these outcomes seem to follow. These findings are significant because they suggest ways for designers to effectively use objects to span knowledge group boundaries.