SEQUENCING AND PRIORITY SYSTEMS .
by
Brian Willcox CFPIM
of
Action MRPII
Scheduling Methods
Limiting or critical operation scheduling, or as it is more commonly known, bottle neck scheduling. The logic for this method is that if you can schedule the work through the bottle necks, the rest will look after themselves.Single machine sequencing is a method used when a work centre consists of only one machine. This method allows the planner to prioritise the work to achieve certain benefits, like to reduce the number of late jobs, or to get the parts through that are holding up a major assembly from being shipped.
Flow shop sequencing, also know as the Johnson's Method. Theoretically it is suitable for scheduling various jobs through two different machines when all the work flows in the same direction. The planner's responsibility is limited to launching the required work within the capacity restraints and the shop supervision is responsible for planning the sequence within the due date requirements to operate most efficiently.
The basic concept of operation scheduling is to establish start and completion dates for each operation. Sequencing goes into more detail by endeavouring to sort out the order that work is to be performed within the scheduled dates. This obviously leads us into the subject of priority rules.
Back in the real world I often see the
way many companies tackle this problem. They use coloured stickers. The
concept is fine, you put a sticker on the jobs that are the priorities as
defined at the production meeting. Unfortunately priorities change, but no
one gets round to taking off the stickers that are no longer applicable.
The result is that we go for a different colour and now stick these on to
the current priorities. The message is, if you have an operation
scheduling system, make it work for you. Update the priorities on the
system as they occur and let the daily dispatch list give the shops the
priority information. The one point that still needs looking at is how do
we tell the system how to set the priorities. Some packages have a field
called priority, but I have yet to find out the logic it uses and I have
yet to find some one that uses it with worthwhile results. (I'd be pleased
to hear from any one who does use it, and gets some benefit from
it.)
Priorities
One of the statements made about an MRPII system is that it is a priority planning and control system. Changes in priorities lead to rescheduling orders, which then cause changes to the sequence the work is required from the shops.Priority control is the process of
making the plan happen and the recognised tool for doing this is the daily
dispatch list. There are various recognised priority rules which use the
basic planning information in different ways, to establish the final
detailed sequence for the jobs to be worked on.
Critical Ratio
The critical ratio priority rule is defined by APICS as a dispatching rule which calculates a priority index number by dividing the time to due date remaining by the expected elapsed time to finish the job.
Typically ratios of less then 1.0 are behind, ratios greater than 1.0 are ahead and a ratio of 1.0 is on schedule.
Time remaining is the time from now to
when the order should be completed. Work remaining is the time required to
perform the planned activities. This includes the planned queue, wait and
move time (interoperation time).
By using this ratio the jobs furthest behind
are brought to the top of the priority list.
Start Date
The job with the highest priority on the work centre is the one with the earliest start date.The objective is to start and complete
work in the sequence it was planned. This will bring the components
together so that assembly kits can be issued to manufacturing without
shortages. It is working to the plan.
Due Date
The job with the highest priority on the work centre is the one with the earliest due date. This rule can be applied to either the operation or the order due date. The objective is to meet the customer due date.Planned Slack Time
In simple terms planned slack time is the time which has been built into the scheduling of an order in between each of the operation times, the majority of which is queue time. Slack time is used in several sequencing techniques.Slack time rule
This is defined as a dispatching rule which directs the sequencing of jobs based on;(days left x hours/day) - standard hours left = priority,
eg (5 x 8) - 12 = 28.
The job with the highest priority is
the job with the lowest remaining slack time.
Queue Ratio (Compression)
A simple definition is the remaining slack time divided by the remaining planned queue time.
Jobs with the smallest queue ratio have the highest priority.
Shortest Processing Time
APICS defines the shortest process time rule (SPT) as a dispatching rule which directs the sequencing of jobs in ascending order by processing time. This means that the highest priority job is the one with the shortest operation time. The result is that the small quick jobs move fast and the jobs with a long operation time can stay indefinitely. Although it does not prioritise work to meet customer delivery dates it can be used to move work quickly onto a starving downstream work centre.Non-Dispatch List Environments
In the traditional job shop environment, the dispatch list shows all the jobs to be started, lists the work in priority sequence and states the operation's start and completion date.When work cells are introduced, or
where product is produced in a flow process (continuous production) or in
high quantities over a period of time (repetitive manufacture), work is
produced to a schedule and not to works orders scheduled by operation. In
these cases, the dispatch list is not appropriate and priority control and
dispatching is controlled at the parent code by using a schedule with a
run rate.