Production Scheduling – Forward Order
At first hand, most schedulers are thinking they have to meet a due date, so therefore they are thinking that they need to Reverse Schedule from that due date. However, in reality, this is not how most operations actually work. Most operations will continue to keep everyone busy, they continually load the shop with orders as the orders come in with an eye on the original estimated due date that might have been promised to customer. Orders are worked in Routing order from first to last, in the example below, this is Workcenter 10, then 20, and finally 30.
Depending on order complexity along with Plant capacity and material constraints, this can result in one of three outcomes:
- Order is completed before the promised due date
- Order is completed on due date
- Order is completed past the due date.
Here is an example of an order that takes 7+ days and finishes early (before Due Date).
Production Scheduling – Reverse Order
Reverse order is exactly that, you start with the Due Date as the date the last Workcenter must finish. Then, you back track in time to determine when the last workcenter must start. And so, the start date for workcenter 30 in our example becomes the end date for workcenter 20, then workcenter 20s’ cycle time is subtracted from end date to find start date, and so forth until you have calculated start date for JOB as well as work-in-process dates for intermediate steps.
Similar with Forward scheduling, we have 3 possible outcomes:
- Order starts today and ends on due date (this is the only case where Forward and Reverse scheduling are exactly the same).
- Order start is delayed. As depicted in diagram below.
- Order should have started BEFORE today. Which, in most cases, is impossible.
Forward Production Scheduling vs Reverse – Making the Choice
In summary, there are a couple of easy decisions on what direction makes the most sense.
- If it is imperative to always front load the shop, and you can live with orders being finished before promise date, you should always forward schedule.
- If the time to complete an order is less that due date minus today, and if you do NOT want the burden (space, cost, etc.) of holding completed order until customer ready to receive, THEN you should reverse schedule and perhaps include some buffer time. Buffer time is simply adding some extra time, say a day or two, which allows you to accommodate other changes/exceptions to working schedule without having to fuss with the system.
- If you have both make to stock and make to order production, you might consider forward scheduling make to order first, then fill in the remaining production capacity with make to stock in reverse order.
Now, for all your other cases, it is not a simple decision. Some of your customers are demanding you deliver by the date THEY need the products, without factoring in your standard or dynamic lead times. How can you accommodate them? As we see in example above, if you use Forward Production Scheduling, where you put in a start date then explode the Bill-Of-Material (BOM), checking material constraints, along with Routing, checking available capacity against current production scheduled, and the scheduling system generates a viable finish date — which MAY be beyond the customers due date. If you use Reverse Production Scheduling, you enter in a DUE DATE and the production scheduling system reverse explodes BOM and routing, generating the start date and all materials, and workcenters, but this may be a start date before today. Where does that leave you? How about considering both directions?
As mentioned above we can, reverse schedule, to remain Lean and order materials Just In Time (JIT) but establish a time-fence that will prevent the system from scheduling in the past.
Now I can see exactly what has to be done to make due dates:
- I need to run overtime for Steve and Dave next Thursday and Friday
- I need to borrow inventory from Production Order #MO234 that is on hold
- I have to move scheduled maintenance on Machine #7 on Wed to Tues
Now, we can compare this with a forward schedule, from today, with a forced finish due date since this is how we work in order to keep everyone busy. Now, I get the same optimized production schedule with the same to-do list and everyone is busy. For this particular situation, I am fine with trading the chance of carrying inventory longer (from pulling production up) in exchange for optimizing my workforce.