I am in a factory with many manual (non-machine) operations. Process time variability for most operations is high, leading to highly variable cycle times at the part level. There are efforts to reduce process time and variability, but the current state is highly variable and will likely always be at least higher variability than a machine-based operation. Scheduling is in a bad state and items are frequently late to internal plans.
Orders are entered in SAP based on their due dates, then subassemblies are planned backwards from there based on planned cycle times stored in SAP. Setting those planned cycle times is where I'm looking for the best approach.
The factory has a historic practice of setting planned cycle times in SAP to recent average cycle times. This approach doesn't seem like a good one - wouldn't you be planning to be late roughly half the time if SAP is planning orders to your average cycle time?
My leading thought is that aiming for a service level such as 95% on-time delivery for the deliverable part would lead to very high service level requirements for the subassemblies, meaning planned cycle times would be set very high in comparison to usual practice so that the parts have a high chance of coming out on time. This would cause WIP levels to rise and more space to be used in the factory. However, on-time delivery should improve.
Looking at one example, a part may come out in 2 days or up to 20 with high variability over each part completion. To meet service levels, I may plan in SAP for this part to take, say, 18 days, which would usually not be needed but would be a level which would give us higher confidence in finishing on time.
Additionally, is it better practice to use data on recent history or try to forecast cycle time instead? There are so many factors that can inflate cycle time - vendor shortages, defect trends, poor performance, etc. that feel unpredictable.
Curious if anyone has some good practices for planning cycle times in manual labor, high variability factories.