Lean production optimization increasingly depends on the ability to coordinate production planning, machining operations, process variability, measurement uncertainty, and real-time manufacturing information within an integrated operational environment. SAP Production Planning (SAP PP) provides structured planning and execution capabilities, while SAP Digital Manufacturing can be positioned as a real-time manufacturing layer for connecting shop-floor execution with production-planning decisions. This research develops a conceptual framework for integrating SAP PP and SAP Digital Manufacturing to strengthen lean production through uncertainty-aware planning, production visibility, process reliability, and continuous feedback. The methodology synthesizes the provided literature on measurement uncertainty, machining reliability, tool dynamics, surface roughness, fuzzy estimation, Monte Carlo simulation, and uncertainty propagation. The analysis demonstrates that lean optimization cannot be reduced to inventory minimization or scheduling efficiency because uncertainty in machining and measurement directly affects production reliability, quality, throughput, and planning stability. The proposed framework therefore incorporates uncertainty characterization, real-time shop-floor feedback, reliability-oriented production planning, and iterative decision correction. The findings indicate that integration can reduce the disconnect between planned production parameters and actual manufacturing conditions, particularly where machining variability influences quality and delivery performance. The study also identifies limitations associated with uncertainty modeling, data quality, system integration complexity, and the absence of empirical implementation data. The resulting framework provides a theoretical basis for connecting lean principles with digital production planning and manufacturing execution.