PRIORITIES WITH MRPbyBrian Willcox CFPIM of Action MRPII | |
| One
of the statements made about material requirements planning (MRP) is that
it is a priority planning and control system. First we will look at the
logic used to plan priorities. | |
Priority Planning |
Priority
planning is the function of determining when material is required. When
using an MRP system, requirement dates are established into the future by
the use of planned orders. The planned order release date (when the job is
due to be started) determines the requirement date of its components. MRP
plans the start date by deducting the lead time from the completion date.
Depending on the software package, the lead time for an item may be fixed,
on the item master file, or may vary depending on the batch size,
calculated from the standard times held on the routing file. The fixed
lead time is more common.
MRP works at the order level only and
calculates the order priorities with the order due date as the key
information. From this, the capacity requirements planning module or the
production activity control module, with the use of the routings, breaks
down the orders into operations and schedules them using data from both
the routing and the work center file. The individual operations start and
completion dates are calculated by back scheduling from the order
completion date. Various priority methods are used and the most common are
operation "due date" or "start date" sequence. It is the operation start
and completion dates which appear on the daily dispatch list and it is the
operation completion date which is used to measure if production is
“performing to plan”. |
Relative and Absolute Priority |
These are two
terms which are used in the text books. Relative priority is really a sequence. MRP relates the priority of one job to another and it is from this information that the work to be performed is sequenced. The absolute priority is a different term altogether and that indicates the date it is actually needed. The relative priority is the sequence and the absolute priority is the actual date it is needed . |
Integrity of Priorities |
The
priorities of all levels of manufacturing and purchased parts is
calculated by MRP by starting with the MPS. If the bills of material,
stock levels and WIP records are correct the dates calculated by MRP will
be valid. When due dates and need dates drift apart, if the rescheduling
messages are actioned the priorities will again be valid. The question is
are they correct? Do we really need a certain sub-assembly on a certain
date or are the system lead times excessive. A typical problem is that
some want safety stock between the levels of manufacture. All this does is
off-set the due date. In other words, it tells us that a part is scheduled
to be completed before it will be needed. Manufacturing personnel will
quickly determine if parts are really needed when the schedules say they
are. If not, the schedules will soon be ignored and it will be back to the
informal system with the shortage list. For the master schedule and in fact the whole priority system to be accepted it must not only be calculated correct (valid), but also truthful - to have integrity. |
Priority Control |
Priority
control is the process of making the plan happen. The tool used is the
"daily dispatch list". This is a list of jobs currently on a work center
sorted in a priority sequence. Start or completion dates are the most
common sequences, but it can be by one of the other priority rules. The
report also shows the orders due to arrive, which are currently on the
previous operation. As priorities change for assemblies, and the planner reschedules them, the system will then either change or advise the planner to revise the due date of the lower level orders. It is then up to the planner to negotiate with the shop manager to obtain agreement so he can enter the required changes. |
Dependencies |
Due to lack
of time, information or understanding, our planning function often keep
pressing the foremen for work because the schedule says so, even though
the part cannot be used. We need to understand how one part is dependent
upon another. The required date for a part is initially established by MRP from the "pick" date of the order on which it will be used. This in MRP terminology is the planned order release date of the parent. Consider also that twenty other parts are to be issued to that order during the picking process. If any one of those twenty parts will not be available for the "pick" date it is pointless having the other nineteen. Thus horizontal dependency is the priority of one part being determined by the availability of another at the same level. Considering the nineteen parts that needed to be rescheduled due to the non-availability of the twentieth, there is another implication. If any of the nineteen are manufactured In-House, the rescheduling of their completion date will alter their pick date. This in turn alters the requirement date of their components. As can be realized, the effect of rescheduling an MPS item can cause rescheduling down through the levels of the bills. Vertical dependency is the concept of
a component's requirement date being dependent upon its parent's pick date
one level above. |
The Planner's Tools |
To assist the
planner to keep control of the priorities of the various works orders in
manufacturing and the placed purchase orders, he has the daily dispatch
list and the supply and demand schedule. MRP also provides facilities
called pegging and order crashing. The planner can also use bottom-up
replanning. Next month we will look at how the planner uses these tools to
make his priorities practical and realistic - to have
integrity. |
| March1998 | |
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