Even the most powerful computers with software that can solve problems can become quickly overwhelmed when balancing multiple jobs and limited resources. But in scheduling, one of the fundamental challenges is juggling the conceptual world of mathematics and the tangible world of job-shop manufacturing – and producing jobs in the shortest amount of time.
The main question that needs to be answered is always this: What is the best way to complete the work that needs to be done in the quickest time period? In job-shop scheduling, two separate groups of people are in play. Mathematicians see the problems from the ivory towers, while the management team tries to meet production schedule demands on the ground.
Oftentimes, the disconnect between these two groups causes delays and confusion in scheduling jobs and tasks. The mathematicians are focused on solving the math-based issues using determining devices, like a Turing machine. Even though there is a strong focus these days on solving issues with technology and devices, the jury is still out as to whether a computer can solve job-shop scheduling problems when multiple machines are involved. Frequently, job-shop scheduling that involves three or more workstations is labeled as NP-complete, which creates problems that take an extremely long time to calculate, and in turn delay the scheduling process.
The second group — the ground-level managers and workers — is interested in practical answers to job-shop scheduling issues – answers and solutions that will help achieve the goal of completing work more efficiently in the field.
Overcoming the Challenges
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Connecting these two groups using visual scheduling models is an easy way to reduce confusion and increase mutual understanding of the jobs on the shop schedule. Using an annotated Logical Diagram Method (LDM) simplifies the task of visualizing activity paths and highlights resource contentions.
Implementing Critical Path Method (CPM) scheduling often doesn’t yield the desired solutions and answers to scheduling issues. Many times, when tasks with overlapping resources or resource constraints are inputted into a CPM system, the system creates unobtainable time schedules. However, by manipulating the inputs and setting constraints for the multiple jobs, the software is able to develop schedules with obtainable timeframes.
A more superior method for dealing with job-shop scheduling issues is Graphical Path Method (GPM), which creates a graphical model of the planned production schedule. The visual model allows for real-time feedback and incorporates ongoing schedule changes and evolutions through the use of LDM. With GPM, the scheduler selects the deadlines and milestone dates, and the jobs are scheduled within these parameters to meet the date goals.
A key difference between CPM and GPM is that GPM allows for float, or drift days. The float or drift is the number of days an activity can be moved to earlier or later dates on the schedule without affecting overall milestones or up-stream activities.
Recognizing the importance of flexibility and evolution in project planning and job-shop scheduling is critical to project success. Utilizing Critical Chain Scheduling (CCS) also helps participants recognize the role of resources for a project. Additionally, CCS gives project managers the ability to control float or drifts days to ensure efficient and successful activity and project completion. CCS inputs the maximum float days as determined by subject matter experts, in order to estimate activity durations to only a 50 percent probability of on-time completion. Typically, the end result is a schedule with more float time than is needed, but project managers still control the float and drift days to determine the velocity of the project.
Embracing a flexible scheduling model is optimal for creating schedules that can easily handle the ongoing changes, bumps in the road and pop-up issues that all projects encounter. Visualizing the overall project, including all activities, tasks and resources, also makes scheduling more intuitive and easier for all stakeholders to grasp – from leadership to ground-level managers. The ability to be mobile, quick-to-respond and visual is the key to future of job-shop scheduling.