Why Job Sequencing of Manufacturing Activities are Necessary? Make Your Production Planning Robust with Proven Techniques!
Abstract
- FIFO - First In, First Out.
- LIFO - Last In, First Out.
- SPT - Shortest Processing Time.
- LPT - Longest Processing Time.
- EDD - Earliest Due Date. .
- LS - Least Slack.
- SIPT - Shortest Imminent Processing Time.
- LIPT - Longest Imminent Processing Time
- LS (Least Slack).
Sequencing Lets You Account For Everything
With proper process and task sequencing, you’re less likely to skip steps.
Having an overview of your project will help you make sure you don’t leave anything out. It will also allow you to make sure you’re on track and hitting your goal targets along the way.
Every project has objectives and proper sequencing helps you meet them.
Sequencing Helps You Focus On What Matters & Plan For Setbacks
Some objectives matter more than others. Sequencing helps you plan your project and plan for setbacks.
“Surprisingly, many large and well-known companies have reactive planning processes. But reactivity – as opposed to proactivity – can often cause projects to go into survival mode.
Being able to place emphasis on certain parts of the project lifecycle is a benefit of proper sequencing.
Sequencing Leads To Faster Learning
A good training model will have a set of steps or lessons that need to take place. These training tools, modules and tasks require a certain order of learning. This is because each lesson sets up the next.
Sequencing Lets You Plan For Downtime & Setbacks
Sometimes you just have to wait.Hurry up and wait is a humorous phrase that typically describes a situation where you have to rush to do something on time, only to have to wait for the next phase to begin.
An example of this would be if a client wants a report done ASAP, but once it’s complete, they take weeks to review it.
Knowing the sequence of your tasks takes into account this tedious aspect of business. If you already know there will be a lull in between project stages, you can plan accordingly.
Rather than waste the opportunity, you can plan to spend downtime working on aspects of the project that might need a second look.
From fitness to finance, in every industry, sequencing matters. Numbers, steps and stages, when tackled in the correct order can make all the difference.
Job Sequencing Rules
There are many Job Sequencing Rules that can be used. Here are some of them:
Earliest Due Date - A common job sequencing technique is to prioritize jobs that have the earliest need date. This can also be referred to as ‘Due Date Assignment’ and it places a high priority on processing jobs with early dues dates in an effort to complete all jobs on time. This technique allows you to measure job shop quality performance through the number of late jobs, average tardiness across late jobs, and average tardiness among all jobs. Although the goal of this technique is to ensure that all jobs are completed before their due date, there can be some inefficiencies in the schedule due to frequent changeovers.
Longest Processing Time - The longest processing time methods assigns highest priority to jobs with the longest processing time. When scheduling longer jobs at the beginning of the schedule, the schedulers can reduce a large amount of much more time consuming jobs at the end of the job schedule. This form of job sequencing is extremely beneficial to manufacturers as it ensures that long jobs have time to complete before they are needed. This job sequencing technique relies on knowing the processing length for each operation to determine which jobs actually take the longest.
Shortest Processing Time - Another common method of job sequencing that is based on completion time is the shortest processing time method. This method assigns highest priority to jobs with the shortest processing time. Similar to the longest processing time job sequencing method, this requires a time estimation for each operation of the job. The shortest processing time can effectively reduce the average flow time and minimize the mean waiting time for jobs.
First-Come, First Serve - The first-come, first-served sequencing method processes orders in the order of their arrival at their production facility or resource. The arrival time is a key component and factor within the job sequencing rule, which is what separates it from methods such as longest processing time and shortest processing time. This method is commonly used in production facilities as it is extremely easy to implement and there is no estimation required for the processing time length.
Least Setup Hours - This rule is often used in cases where setup or changeover time is incurred when changing from one type of material, dye, or other attribute to another. This is also referred to as ‘sequence-dependent setup time’. This rule will create a schedule where there is the least amount of setup hours required, which is usually achieved by grouping jobs with like attributes together.
Sequencing types can be classified as follows-
- Two jobs ‘m’ machine case
- ‘n’ jobs one machine case
- ‘n’ jobs ‘m’ machine case
- ‘n’ jobs two machine case
Sequencing problems can be classified as follows-
- Two jobs ‘m’ machine case
- ‘n’ jobs one machine case
- ‘n’ jobs ‘m’ machine case
- ‘n’ jobs two machine case
Solutions of these sequencing problems depend on a number of factors like-
- Number of machines which are there in that particular machine shop.
- Number of jobs which are scheduled.
- Whether this manufacturing facility type is fast shop or slow shop.
- The specific criteria based on which the scheduling alternatives are assessed.
- Whether the jobs arrive on static or dynamic manner at the facility.
With every increase in the number of machines (m) and the number of tasks (n), these sequencing problems get tougher. In reality, no most advantageous solutions exist in real for sequencing problems having larger ‘m’ and ‘n’. Thus, for effective scheduling problems, it is simulation technique which proves to be the most efficient solution.
The following types of manufacturers commonly utilize production sequencing:
- Automotive manufacturers
- Industrial equipment manufacturers
- High tech product manufacturers
- Consumer electronics manufacturers
- Aerospace manufacturers
- Defense equipment manufacturers
- Agricultural machinery manufacturers
Basically, any form of manufacture in which a discrete item is being produced - whether this is a complete product or simply a replacement part - will benefit from sequencing. If manufacturers are producing undifferentiated commodities such as processed grain or refined oil, sequencing rules are not so readilyapplicable.
Manufacturers can realize the following benefits with the right approach to production sequencing:
Reliable Delivery Timeframes
Manufacturers are able to fully comply with due dates and can prioritize orders to meet tight timeframes.
Changeover Reductions
Unproductive changeovers between machines and workstations are significantly reduced.
Bottleneck Identification and Elimination
Bottlenecks in the manufacturing process can be swiftly identified using production data, and then these bottlenecks can be re-routed and eliminated during execution.
Better Throughput
With the right approach to production sequence, the rate of throughput is increased and products are completed more efficiently.
Sustainable Work in Progress (WIP) Inventory
Sequencing can prevent the inventory of WIP jobs from getting out of hand, giving teams the power to prioritize key jobs in order to push products through to completion.
Optimized Work Station and Machine Utilization
Multi-zone Sequencing gives manufacturers the power to ensure that workstations and equipment are utilized efficiently and effectively, reducing the levels ofunproductive time in the process.
While FCFS can be very advantageous to an organization seeking simplicity in its order scheduling processes, it does have a few drawbacks that must be considered. Because FCFS is so simple, it does not offer some of the more complex features of other order scheduling methods. We will discuss these drawbacks, including; long waiting time, favored CPU over I//O processing, lower device utilization, and incompatibility with time-sharing systems.
Long Waiting Time
A reasonably significant disadvantage of FCFS is its long wait times. Because FCFS is a non-preemptive CPU scheduling algorithm, it exclusively processes orders in the order they arrive. This means that a subsequent order cannot begin processing until the order before has finished executing. Once a process has been allocated to the CPU, it will never release the CPU until it is completed. This means that if the first order placed has a considerable burst time, all orders behind it are forced to wait for its completion, no matter how small their burst times may be. Additionally, once an order is placed and begins execution, it cannot be stopped. This means that an order that needs to be canceled or adjusted cannot be if already allocated to the CPU. In a world that demands flexibility, this is not ideal.
Lower Device Utilization
Because FCFS is so simple, it tends not to be very efficient. This goes hand-in-hand with extended waiting times. If the CPU is preoccupied with one lengthy order process, all other orders are left sitting idle, and this can cause a significant backup. Because the CPU can only handle one order at a time, FCFS utilizes a minimal portion of your system’s capabilities, rendering it very inefficient.
Favors CPU Over I/O
Another disadvantage is that FCFS highly favors CPU over I/O. This means that the algorithm is more compatible with central processing units (CPU) over input/output (I/O) systems. For organizations utilizing I/O systems, this could be a deterring attribute.
Not Ideal for Time-Sharing Systems
Finally, the FCFS algorithm can be particularly troublesome for time-sharing systems. For time-sharing systems, it is essential that each user get a share of the CPU at regular intervals. Because FCFS is a non-preemptive system, this is not always feasible. Again, it could leave dependent systems idle and waiting, which is not ideal for keeping operations running smoothly and efficiently.
First-come-first serve order scheduling can be highly beneficial for organizations looking for a quick and easy solution to order processing. Because the concept is so simple, it is very user-friendly, allowing team members of all levels of familiarity to work with the system and conduct its functions. But, organizations must understand the full capabilities of FCFS before implementing, and this includes its limitations. Simplicity can result in inefficiency, which brings wait times and lowered device utilization. Before choosing to implement FCFS in your organization, you must understand all of its features and determine if it is a good fit for your needs. Order scheduling is a necessary and vital process for all organizations. Finding the right solution for you is key to achieving efficiency.
1. FIFO - First In, First Out
Under FIFO, the first piece that is inputted into the process is also the first outputted from the process. Pieces are not held up at each individual phase of the manufacturing procedure and are instead moved through the phases in chronological order from first arrival onwards.
2. LIFO - Last In, First Out
LIFO is essentially the opposite of FIFO. Under this rule, the last piece to be inputted is the first to be outputted, which means all preceding pieces are held up in the production process to give priority to this last input. This method can be used to accelerate the manufacture of a specific product that has been delayed in the preparation phase, but it is rarely deployed.
3. SPT - Shortest Processing Time
Using the SPT rule means prioritizing pieces that can be produced and finished most quickly. Adopting this method allows products to progress through the manufacturing processin a more logical and streamlined manner, reducing queues and waiting times ahead of each phase.
4. LPT - Longest Processing Time
LPT is essentially the inverse of SPT, giving priority to products that will take the longest time to process. The aim of this rule is to reduce changeovers between different machines, increasing machine utilization in factories.
5. EDD - Earliest Due Date
This method gives the priority to products that are due to be completed soonest, in other words, the most urgent jobs. With this method, teams try to ensure that no jobs miss their individual deadlines.
6. LS - Least Slack
LS prioritizes items that have the least disparity between deadline date and expected processing time. When these items are completed, there will be very little time left over before the deadline, and so LS aims to complete these jobs first.
7. SIPT - Shortest Imminent Processing Time
SIPT is similar to SPT but prioritizes items with the shortest individual processing time.
8. LIPT - Longest Imminent Processing Time
LIPT is the inverse of SIPT and works in a similar way to LPT. Priority is afforded to items with the longest individual processing times.
9. LWQ - Least Work Next Queue
LWQ gives priority to machines, workstations or phases of production that have the smallest queue. The idea is that these queues can be eliminated soonest, minimizing delays on the next phases of manufacture.
10. CR - Critical Ratio
The Critical Ratio is defined by dividing the total remaining time until the deadline by the total remaining production time. Priority is given to items in the production cycle with the smallest critical ratio. CR brings together data from theEDD and SPT sequencing rules, combining these to provide a set of production priorities. Like DLS below, this is a dynamic method, and the Critical Ratio will change over time.
11. DLS - Dynamic Least Slack
DLS is another dynamic rule, like CR listed above. Under DLS, production is prioritized based on the difference between the deadline date and expected processing time - in the same way as LS. However, DLS adds dynamism to the calculation and will change over time as deadlines draw nearer. This makes it more difficult to implement than LS but also provides more accuracy during scheduling. The idea is to prevent delays and to ensure that no production job misses its deadlines.
While it may not be intuitive, the two processes are closely aligned, and in fact, production sequencing is a subset of the production scheduling discipline.
Sequencing focuses on one particular area - the order and prioritization of production processes. In this sense, it fits neatly within the wider scope of production scheduling, which deals with the specifics of deadlines, timeframes and timestamps. This is why production scheduling is considered to be more complex than sequencing
How to Sequence Activities in a Project?
Everybody subconsciously creates sequence activities in their day-to-day lives. We plan weekly trips to the store, clean and tidy our homes, we run errands — the list goes on. In this way, we are all familiar with basic project management skills.
When we can identify sequential activities in our day-to-day life, it’s much easier to then spot them in the context of a complex project. So, let’s define sequential activities in terms use of something we all do and learn how we can relate it to project management.
What Are Sequential Activities?
Sequential activities are tasks or duties that must be completed, or at least started, in order. For example, if you’re making spaghetti, you must first fill a pot with water, then boil the water and then add the pasta. These three activities are directly related and they must happen in this exact sequence to result in cooked pasta.
What Is the Purpose of Sequencing Activities?
Whether you’re making a hypothetical spaghetti dinner or planning the construction of a new building, you will need to complete activities in order to arrive at the goal.
The question is: does it really matter what order these activities take place in if the end result is the same? The answer is a resounding yes! Sequencing tasks is an important part of project task management because it helps you make the most out of your resources and help you identify critical tasks and prioritize them.
An Example of an Activity Sequence
Back to the spaghetti analogy. In this scenario, you should put the water on to boil before beginning to chop the garlic. Sure, it wouldn’t ruin the meal to chop the garlic before boiling the water, but this way you are making the most of your time and energy.
Let’s take this analogy further and apply it to how a chef runs a kitchen. In this case, each dish is a task made up of activities. When someone orders a certain dish, the activities must be executed to complete the task.
The tricky part is, that there are multiple different tasks being completed at once, which means even more activities are taking place at the same time. If you aren’t careful, the necessary activities for one task can get in the way of those for another.
When you apply this concept to a larger-scale project with more resources and constraints, activity sequencing goes from advantageous to absolutely crucial. So, how exactly do you go about sequencing activities?
How to Sequence Activities for a Project?
In order to sequence activities, you must define the relationship between those activities. There are many different ways to do this, but today we’ll cover the
Precedence Diagramming Method.
The Precedence Diagramming Method uses a type of network diagram in order to show the activities which must be completed, how they’re connected and what sequence the work must follow. This method illustrates these things by representing activities with boxes (or nodes) and dependencies with arrows connecting them.
Activity Sequencing Steps
The Precedence Diagramming Method can seem complex at first glance, but really it can be broken down into a few simple steps. Let’s assume you already have an activity list and go from there.
Define Activity Attributes: When sequencing activities, you must have all the details about each activity. You must keep organized and track of these details. Defining activity attributes involves naming each activity, assigning it a number, writing a short description, etc. Each activity must have all the same attributes defined and noted in order to sequence them accurately. This legwork makes all the difference.
Determine Constraints: Any task will come with constraints — things that limit it. These things can be money, manpower, time and a number of other things. As with defining activity attributes, knowledge is power, and determining every possible constraint is the only way to sequence activities in such a way as to not breach these constraints.
Identify Dependencies: In order to accurately sequence activities, correctly identifying dependencies is, perhaps, the most important part of the process. Without knowing exactly how tasks are related, there is no way to sequence them appropriately. In fact, without the knowledge of task dependencies, it’s all too easy to schedule tasks incorrectly and set the entire project up for failure.
These relationships can also be referred to as dependencies, of which there are four different types.
Finish-to-Start (FS): In a Finish-to-Start dependency, Task A must be completed before Task B can start.
Finish-to-Finish (FF): In a Finish-to-Finish dependency, Task A must be completed before Task B can be completed.
Start-to-Finish (SF): In a Start-to-Finish dependency, Task B cannot be completed before Task A has started.
Start-to-Start (SS): In a Start-to-Start dependency, Task B cannot start before Task B has started.
What's the Best Practices When Sequencing Activities?
Although creating an activity sequence can seem rather intuitive, there are certain best practices that should be followed:
Create a Complete Task List Before You Sequence Activities
When you’re working on a project, new, unexpected tasks will naturally crop up along the way. But, the best way to begin sequencing activities is to work with all the information you have at the time. Even if you know a task will probably produce more tasks down the road, don’t hesitate to factor it into your task list. Sequencing activities begin as the best possible rough draft and are cleaned up along the way.
Consider Your Team’s Workload
You must identify constraints while sequencing activities. One often overlooked constraint is a team member’s individual workload and time. There are only so many hours in the day and only so much a person can have on their plate at one time. Neglecting to keep this in mind can lead to setbacks, burnout and activities taking much longer than expected.
Know the Difference Between Mandatory vs. Discretionary Constraints
In order to accurately sequence activities, you must keep in mind there isn’t just one kind. Mandatory constraints and discretionary constraints are very different, and both must be identified. Mandatory constraints are concrete and cannot be worked around. Discretionary constraints, on the other hand, are flexible and team members can decide on them while scheduling the project.
Don’t Neglect Task Constraints
As you can imagine, neglecting task constraints is one of the most detrimental mistakes you can make when sequencing activities. But, it is equally important to take the project management triangle into account. This triangle consists of the three most impactful constraints: scope, time and cost.
Sequencing activities is a very involved process, and it’s not difficult to forget the broader constraints of the entire project. Keeping these things top of mind can make all the difference between project success and project failure.
How Project Manager Helps With Activity Sequences
In order to ensure that all your tasks are sequenced properly with appropriate dependencies, we recommend you make use of project management software. ProjectManager is an award-winning project management software that lets you create, detail and sequence activities and view these activities in the way that best suits your project.
- FIFO - First In, First Out.
- LIFO - Last In, First Out.
- SPT - Shortest Processing Time.
- LPT - Longest Processing Time.
- 5. EDD - Earliest Due Date. .
- LS - Least Slack.
- SIPT - Shortest Imminent Processing Time.
- LIPT - Longest Imminent Processing Time.
References
1."Introduction to Sequencing and Scheduling" by K R Baker
2."Machine Scheduling Problems, Classification, Complexity and Computations" by A H G Rinnooy Kan
3."Maintenance Planning and Scheduling Handbook" by Doc Palmer and Richard Palmer
4."Just-In-Time Scheduling" by Joanna Jzefowska
5."Schedule-Based Modeling of Transportation Networks: Theory and Applications" by Nigel H M Wilson and Agostino Nuzzolo
6."Behavioral Operations in Planning and Scheduling" by Wilson
7."Scheduling: Theory, Algorithms, and Systems" by Michael L Pinedo
8."Process Planning And Scheduling For Distributed Manufacturing" by Lihui Wang and Weiming Shen
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