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Are your schedulers primed for success or set up for failure?

If your schedulers are ignoring start dates set at the planning level, missing due dates, and generally producing excessive amounts of WIP, it may not be their fault. Given the way planning and scheduling decisions feed into and affect each other, scheduling problems may well begin at the planning level.

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Interaction between production planning, scheduling and execution

Many planning solutions provide schedulers with a ‘target date’ – the date by which an operation must be completed in order to meet a delivery date. This approach gives schedulers the freedom to choose the best orders for a sequence, while taking the due dates of each operation into account. The drawback, of course, is that schedulers may schedule an operation too early and produce unnecessarily high levels of WIP.

One way around this problem is to provide schedulers with both a target date and an earliest start date. When done correctly, this approach helps schedulers create a good sequence, meet due dates and keep WIP under control.

So how should the planning level determine the earliest start of an operation?

A possible first step would be to subtract the anticipated production duration from the target date to arrive at the latest possible start date. The problem here is that no scheduler in his or her right mind is going to take this interval seriously: It simply doesn’t give schedulers enough room to respond to changing circumstances, for example by expediting or delaying operations.

To arrive at a realistic interval, the planning department should apply a time buffer to the latest possible start date. The earliest start is then determined as follows: Target date – post-processing duration – production duration – time buffer = Earliest start date.

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Determining the earliest start date

Let’s refer to this time buffer as the ‘scheduling space’. Getting this space right is critical. Provide schedulers with too ‘generous’ a space and you run the risk of creating excessive WIP, under utilizing resources and increasing lead times. Give them a scheduling space that’s too ‘tight’ and your delivery performance suffers.

So how should you determine the scheduling space?

As the scheduling space should reflect the minimum lead time between two subsequent operations, your first step is to determine the minimum technically feasible lead times between operations by analyzing historical production data.

Next, increase the scheduling space for resources that require setup as the scheduler will need more orders to choose from to create the best possible sequence.

Finally, fine tune the time buffer to take your desired WIP levels into account. If you would like to increase WIP in front of a machine, increase the scheduling space of the previous operation.

Here are a few points about the scheduling space that you should bear in mind when determining it:

• It’s resource-specific

The scheduling space must be generous enough to allow for the creation of a good sequence, and this varies from resource to resource. For example, long sequences are usually created for steel casters, making it impossible to predict where an order will end up in a heat sequence. The scheduling space should reflect this uncertainty.

• It can be order-specific

It can be commercially interesting to offer shorter lead times to VIP customers. This should then be reflected in a tighter scheduling space.

• It can be material-specific

It makes sense to increase the scheduling space for an operation that requires a rarely used material that’s hard to combine with other orders.

• It influences WIP directly

As the target date for a previous operation depends on the earliest start of the next operation, increasing the scheduling space leads to higher WIP in front of the next resource. You can therefore use the scheduling space to keep your WIP under control.

Are your schedulers getting the right kind of scheduling space? What’s the best way to calculate the appropriate time buffer? Let’s discuss.