Finite Capacity Scheduling in Job Shops

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An Important Role of Finite Capacity Scheduling in Job Shop Production Management

Many job shops are engaged in order-driven, high-variety production which is more complex than a production line that makes a variety of products one after another and sends them to final goods inventory. Most job shop managers will agree that they find it difficult to manage such production but somehow manage it mostly by firefighting. These shops have no prediction of process requirements, quantities and receiving times of customer orders and they find it necessary to make products only against received orders.

Scheduling Solutions to Overcome Difficulty with Production Management:

Major problems faced by job shops in make-to-order (MTO) production include:

Quoting a rational lead time for each customer order
Finding an optimal material release time for each work order
Handling rush orders, order priority changes and changes made by customers after placing orders
Proactive capacity planning for dealing with variation in workload.

These problems arise mainly because the nature of workload consisting of different products continuously changes over time and bottlenecks keep changing as a consequence. No method that ignores prediction of work flow (the progress of work orders) on shop floor for given workload, order completion times and bottleneck formations can effectively support efficient management of complex production in job shops. Capability to make such prediction greatly supports efficient, proactive production management. Scientific scheduling solutions based on rigorous calculations provide such predictive mechanism.

A Narrow View of Production Scheduling:

Scheduling is one of the most common functions across job shops. But, some people have a narrow perception of potential benefits of production scheduling in job shops and treat scheduling as a simple commonsense exercise. There is no formal training for scheduling complex production in job shops. For many shop floor people, scheduling is merely a mechanism for timing of operations over short time intervals with feasible resource assignment. It is better to view a production schedule as a guideline for production control rather than a definite thing on shop floor. In some factories, people develop operations-level schedules over short time intervals for individual work centers or departments without a comprehensive view of job progress across the system over time and without any prediction of job completion timesand moving bottlenecks, while struggling to synchronize individual schedules. Revision of such schedules can be laborious and time consuming for a factory consisting of several work centers or departments. They do not see scheduling as a powerful function in support of overall production management in the plant.

To generate detailed, operations-level production schedules with the narrow objective, people are currently using Excel applications, custom software applications, project management software, scheduling modules of ERP software and off-the-shelf scheduling software. They however find it cumbersome, laborious and time consuming to revise a detailed operations-level schedule in response to significant changes in the system unless they use a powerful software tool that properly addresses all major constraints including finite capacity of resources.

Finite Capacity Scheduling (FCS) and Proper Implementation of Schedules:

Scheduling production at the level of operations cannot be easily done in a meaningful manner without taking into account the finite capacity of resources, that is, without using finite capacity scheduling (FCS) logic. A detailed, operations-level production schedule generated by FCS explicitly contains scheduled start and finish times of operations. FCS requires a lot of computations which are quite laborious and time consuming for manual FCS implementation. Therefore, for all practical purposes, FCS is usually implemented on computer with the help of software. There is some criticism about poor practical value and futility of finite capacity scheduling software on shop floor. The factors supporting the criticism include the following.

Selection of unsuitable software for production scheduling
Weak scheduling logic in the software and poor quality of schedules generated by the software
Incorrect or improper configuration of the software
Problems with availability of input data for the software
Cumbersomeness and unfriendliness of the software for users
Lack of training on the software
Lack of interest to use the software
Improper implementation of schedules on shop floor.

Free, Fully Functional, 60-Day Trial Copies of FCS Software:

Very often, selection of software for job shop scheduling becomes a failure primarily because the software is not evaluated even for a few days on shop floor with real production data. The popularity and marketing power of vendors and customer reviews may not really give any guarantee for software success in a specific job shop. Software selection based mostly on price without any concern for ROI can also lead to a wrong decision. Some scheduling software buyers get impressed by user interfaces and features without much idea of how strong the hidden schduling logig is. Only evaluation with real data can provide such guarantee. For this reason, Optisol offers job shops free, fully functional, 60-day trial copies of its scheduling software.

Meaningful Implementation of Schedule from FCS Software:

Some people try to implement operations-level production schedules the way we implement class room schedules, schedules for events like games, meetings, etc. and schedules for transport carriers like buses, trains and flights etc. where scheduled start and finish times of activities are implemented. The uncontrollable natural variation in the production system makes such implementation impossible unless a lot of buffer times are incorporated into the detailed operations schedule to account for the variation in the system. If buffer times are created in the schedule to facilitate such time-specific implementation, they can reduce throughput, overall resource utilization and consequently the practical value of the schedule.

There is however an effective way to incorporate buffer times in an FCS schedule and implement the schedule on shop floor in a sensible manner. Many FCS software developed decades ago do not account for natural variation in the system. The FCS software, Schedlyzer judicially creates buffer times before bottleneck operations as shown in the chart below. For a better description of the chart, read the web page, An Illustration of Lean Production.

The buffer times in Schedlyzer are not created to enable the implementation of scheduled start and finish times of operations on shop floor. They are expected to absorb the uncontrollable natural variation in the system and ensure that scheduled order completion times and schedules on bottlenecks are more dependable.

Summary Gantt Chart of Optimal Schedule:

In order to implement the schedule on shop floor in a meaningful manner, we have to convert the operations-level production schedule into operation sequences (dispatch lists) for resources by ignoring the scheduled start and finish times of operations and see that every resource follows its dispatch list as much as possible. It is quite convenient to implement resource dispatch lists under the influence of uncontrollable natural variation in the system. For making implementation easier and more practical, the resource dispatch lists can be revised periodically by updating the detailed operations-level schedule.

Creation of finite capacity schedules with judicious insertion of buffer times and implementation of regularly updated, short-term resource dispatch lists will make finite capacity scheduling approach practically meaningful.

Multiple Benefits of FCS:

A good finite capacity scheduling solution can yield several benefits. These benefits include:

Predicting work order progress on shop floor, bottleneck formations and work order completion times
Finding minimal achievable lead times for customer orders based on the existing workload and committed orders
Finding an optimal time for material release for each work order
Handling rush orders and order priority changes efficiently
Reliable what-if analysis of production
Proactive capacity planning for dealing with variation in workload
Generation of short-interval production schedules to help and guide shop floor people

Summary:

Production schedules generated by FCS software can be effectively implemented on shop floor in a sensible manner. Good FCS software supports intelligent decision-making in job shop production management by providing useful prediction of workflow, order completion times and bottleneck occurrences and facilitating what-if analysis and proactive capacity planning.

FCS software can help (a) Salespersons in determining rational lead times for orders, (b) Production managers in predicting workflow, doing what-if analysis and proactive capacity planning and taking intelligent decisions for efficient production control and management and (c) Shop floor people in prioritizing operations for resources at any time. FCS software serves as an intelligent decision support tool in controlling and managing complex production in job shops. Production managers with basic exposure to computers can simulate the work flow in their production using good FCS software like Schedlyzer without any knowledge and prior experience in discrete event simulation.

FCS software requires a reasonable amount of information about quantities, priorities, due dates and routings (including resource requirements) of orders and resource available times. Nowadays, most ERP systems easily maintain the information as required by FCS software. For more details of FCS, refer to the book, Finite Capacity Scheduling: Optimizing a Constrained Supply Chain.

Prasad Velaga, Ph.D. (Scheduling)
Optisol
College Station, TX

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