完成 and 完遂: a finished creation and carrying out a mission
Compare completion of a bridge or model with carrying out a mission or plan, preserving the remaining part and the cooperation required.
The state of a creation with 完成
完成 (かんせい) refers to completion of something being made. The example 「半年かけて木造の橋が完成した。」 is translated as “The wooden bridge was completed after six months of work.” The wooden bridge reaches completion after six months of work.
The example 「模型の完成まであと一つ部品が必要だ。」 is translated as “We need one more part to complete the model.” The model needs one more part for completion, so it is not yet complete.
Carrying out a task with 完遂
完遂 (かんすい) refers to carrying out a mission or plan to the end. The example 「調査隊は困難な任務を無事に完遂した。」 is translated as “The survey team successfully completed its difficult mission.” Read the expedition's successful completion of its mission.
The example 「計画の完遂には各部門の協力が欠かせない。」 is translated as “Cooperation among all departments is essential to completing the plan.” Cooperation between departments is required to carry out the plan. That requirement does not report completed cooperation or accomplishment.
What is complete and what is accomplished
完成 concerns the completion state of the bridge or model; 完遂 concerns carrying a mission or plan through. Identify what reaches completion and what task is accomplished rather than dividing subjects into things and people.
The model needs one more part for completion, while the plan requires cooperation from all departments. Separate the remaining part and required cooperation from the completed bridge and mission.
What do we identify for 完成 in the model needing another part and 完遂 in the plan needing cooperation?
Show the answer and why
For the model, identify the creation's completion state and remaining part. For the plan, identify the work to carry through and the cooperation required. The model remains incomplete, and the plan sentence states a requirement rather than a completed result.