Forward Scheduling
A Preactor system will NOT plan jobs in the past. So a forward schedule starts at TODAY and takes each job by priority and puts it on the planning board in the first available slot. Depending on the availability of machines due shift patterns and other more important jobs already planned on the required machine.
So a forward plan tells you:
The resulting schedule Gantt chart will show jobs bunched up to the left hand side of the timeline.
So it tends to increase WIP. And makes it difficult to get sensible promise dates for new orders because just adding them onto the schedule could result in the new order being late and an order placed weeks ago being very early. Maybe the orders have the same required date and BOTH could be on time if the whole system is re-planned!
Backward Scheduling
The Sage 1000 MRP program is a backward schedule!
It works back from the top level required date on a Sales order or forecast, calculating start / end dates for each level of BOM, using the lead times.
Often this suggests start dates that are in the past, even though the MRP program has no restriction on machine / people capacity.
With a Preactor finite capacity schedule this could be even “worse”. Preactor only lets one job be planned on each machine at a time, so if there is a long bottleneck on a resource then it too will try to plan things in the past. But Preactor deals with this in a different way to MRP, because it refuses to schedule the operations that would need to run before today on a backward schedule.
Leaving the planner to do something about them.
The resulting schedule Gantt chart will show jobs more to the right hand side of the timeline, and you will be able to see gaps in the middle where new orders can be placed without affecting the expected dates of existing orders.
A Good Compromise
Just one idea, there are others:
Run a FORWARD schedule
Let Preactor highlight the EARLY operations
Un-schedule the early orders (or those more than say 5 days early)
Re-schedule them with a BACKWARD run
If the ops were EARLY on a FORWARD, they are bound to still FIT on a BACKWARD, without going into the past!
The end result being a plan where urgent or potentially late orders are planned to run ASAP, the rest are planned to finish when they are needed.
So they don’t start until they really need to , so WIP is reduced and MRP for purchasing can be set to only bring material in on dates where it is actually needed.
Reducing stock holding costs and freeing up cash and warehouse space!
The plan will also show gaps where new orders can be added.