The problem of scheduling the production work centers in a manufacturing plant is one that continues to plague managers. They are under constant pressure to get work orders processed quickly and efficiently. Of particular concern is the frequent need to find a specific work order and rush it though the plant to satisfy a special customer or to keep a salesman happy.
There arises the question of how to prioritize the work-in-process at each work center. This prioritization is needed to keep the work flowing smoothly through the various work centers and meet delivery commitments. The general term for this prioritization is Dispatching rules.
The APICS body of knowledge defines five different, commonly used dispatching rules:
1. First come, first served- jobs are worked on in the order they are received at the work center.
2. Earliest Due Date- jobs are performed at each work center according to the order’s delivery date. The job with the earliest date takes precedence over other orders.
3. Earliest Operation date- jobs are performed according to their operation due date. To use this rule each operation on the job router must have a due date assigned to it. The assignment of a work center due date is usually done with a computer program and a data base of time elements for each operation.
4. Shortest process time- jobs are worked according to their process times. Jobs with the shortest times are worked before those jobs with longer process times. This method ensures that the highest numbers of work orders are moved through a work center.
5. Critical Ratio – This method uses the critical ratio for a job. This ratio is calculated by dividing time remaining on a job by the expected elapsed time to finish the job. This ratio has to be calculated after each work order has been completed at a work center. It is a constantly changing number. If the critical ratio is less than 1.0 the job is behind schedule. When the ratio is greater than 1.0 it is ahead of schedule. When the ratio equals 1.0 the job is on schedule. The work center must prioritize the jobs in its queue by the critical ratio and start with the lowest ratio first.
My experience has shown there to be a 6th rule. This rule is usually employed by work center leaders or production supervisors. The rule is to match jobs that have the same or nearly the same set up. This rule is justified in that less time is lost in making set ups and the work center is more fully utilized and maximizes it run time.
Brace yourself: Each of these rules can work, but no rule works all the time. Each rule has its drawbacks and you must consider each rule based on your style of manufacturing and your customer requirements.
The rule that seems to make the more sense to a production supervisor is actually the worst rule. It is Rule 6. To reduce the need to make set up changes, the supervisor will pull in the next order that uses the existing setup. The logic of reducing total set up changes and maximizing run time takes no account of customer due dates. It may make the work center look efficient, but it will cause shipments to be late and will use up production capacity on work orders that are not needed for customer delivery.
If a supervisor has issues with set up times that are too long, this problem should be addressed with a Set Up Reduction Kaizen. Solve the problem; don’t try to work around it.
What should you do about the need to set rules for processing work orders at work centers? Remember the plant is judged first and foremost by on-time shipments. Use this criterion to set priorities. If a work center is idle, because there are no orders that require its use, move the resources (labor, and if possible machinery) to another work center. Your goal should be to maximize the entire plant, not just a few work centers.
Look at your Lean Manufacturing techniques and your Constraints Management principles to keep work flowing quickly and smoothly through the plant.
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