Relation Type between down straem and up stream of assemllly line
I am working on a project which contains an assembly line. This assemble includes four processes, where 80 items would be produced. I supposed each step as a task in MSP 2007, and set each duration as the calculation of working on all items for each process. , since these processes (tasks) are doing series, when prcoess 1 (task 1) prepares 10 items, process 2 (task2) could be begun, so I set the predessesor of task 2 as start to strat with taks 1 besides lag 10 min, supposing 1 min for each item (i.e., SS1+10)
But the problem it is: duration of working on each item of task 2 is less than the duration of working on each item of task 1, so task 2 would be finished sooner than task 1! While it is not logical, because as a regular assembly line this could not be possible! In other words, an item of taks 2 needs finished item of task 1. So I wondering what type of relation I should set between tasks of an assembly line to suppose up streams need down stream but could be started after some items in down streams are prepared to proceed.
I would be thankful, if you could please reply this post with your percious answers.