This　article　presents　a　reliability-oriented　accuracy　synthesis　framework　for　five-axis　hybrid　kinematic　machining　units　(HKMUs)　that　harmonize　geometric　accuracy,　computational　efficiency,　and　manufacturing　economy.　Current　tolerance　design　methods　struggle　with　the　geometric　error　propagation　inherent　in　parallel-serial　HKMUs,　which　may　lead　to　unreliable　pose　accuracy.　To　address　this,　a　trilayer　architecture　is　proposed:　(1)　a　foundational　geometric　error　model　employing　equivalent　joint　decomposition　and　screw　theory　establishes　matrix-form　error　mappings;　(2)　a　computational　layer　introduces　a　dual-criteria　pose　reliability　algorithm　(position　sphere/orientation　cone)　accelerated　via　fourth-moment-maximum-entropy　integration,　reducing　computational　load　versus　Monte　Carlo　methods;　(3)　an　optimization　layer　formulates　tolerance　allocation　as　a　reliability-constrained　nonlinear　program,　solved　by　a　Proportional-Integral-Differential　(PID)　search　algorithm　(PSA)　to　avoid　local　minima.　Validated　on　a　2PRU&1PRS-2P　HKMU　(＇R＇,　＇U＇,　＇S＇,　and　＇P＇represent　revolute　joint,　universal　joint,　spherical　joint　and　actuated　prismatic　joint,　respectively),　the　framework　quantifies　the　mapping　relationships　between　9　geometric　tolerances　and　16　uncompensatable　key　source　errors　through　the　Small　Displacement　Torsor　(SDT)　method.　Under　allowable　errors　of　0.10　mm　(position)　and　0.02　deg　(orientation),　PSA　achieves　90%　pose　reliability　-　a　203.7%　improvement　over　baseline　-　while　increasing　manufacturing　costs　by　only　11.8%.　The　framework　provides　a　systematic　roadmap　for　designing　economically　viable,　high-reliability　HKMUs　essential　for　precision　manufacturing.　Copyright　©　2025　by　ASME.