HOW OFTEN MRP?byBrian Willcox CFPIM of Action MRPII |
| I am often asked how
often a company should run MRP. The only answer to that is a question in
return, how dynamic is manufacturing? Ask your manufacturing management.
Is it satisfactory to produce a weekly or a monthly schedule, photocopy it
for everybody to keep as the key document for the rest of the week or
month. These good gentlemen will soon tell you that it is hopeless as
manufacturing changes by the hour or the day. Let face it, manufacturing
is a problem. Having been a scheduler or responsible for scheduling for
more than 15 years I have to admit that the key problem in manufacturing
is the scheduling. Why is scheduling such a problem? Well to start with it
starts with a forecast which as we know is always wrong and that means the
material plan is also wrong. What else causes a material supply problem?
There are several reasons, customers change their mind on what they want,
engineering create change notes, manufacturing scrap parts, the cycle
counters alter the stock balances and of course our suppliers deliver
late. Each of these play havoc with our planned material flow and the
lower level manufacturing schedules. One thing we do know is that our
scheduling system must be able to do two things, one to initially
establish what is needed, and secondly as the real world alters tell us
where that plan has altered so we can make the necessary arrangements to
correct the plan.
Manufacturing is dynamic, it changes by the hour or by the day, therefore for a system to be useful it must be just as dynamic. We cannot accept weekly or monthly schedules any more. When MRP first started the data processing section would prepare for the "monthly run" this was a regenerative run which APICS defines as; an MRP processing approach where the master production schedule is totally re-exploded down through all bills of material to maintain valid priorities. New requirements and planned orders are completely regenerated at that time. The result of this run was monster
printouts for the buyers and materials planners to work to. As the
manufacturing packages have been improved so have the basic MRP runs and
now most packages have a "net change" facility. Net change is defined as
an approach by which the material requirement plan is continually retained
in the computer. Whenever there is a change in the requirements, open
order, inventory status or bill of material, a partial explosion is only
made for those parts affected by the change. |
| Due to the method used in processing a net change run, it is less economical for the same number of records than a regeneration run. Usually, as the volume of change is so small, the net change run will take less than 25% of the time taken by a regeneration run and this means it can be run nightly. The system of course will then generate exception messages on a daily basis. Some planners say they could not handle that volume of exception message as it takes them all week now to process the ones they get. We need to be practical we need to look at this from a different angle. If manufacturing alters by the hour you can guarantee that many of the messages you receive on Monday are no longer correct by Thursday or Friday. When we receive the daily exception messages, at least we know they are correct and if sorted by a predetermined priority the most urgent ones will be handled first. As manufacturing is a real place with real people and normal problems, it is those things that goes wrong in the middle of the week that can not wait for next weeks run. They need attention straight away therefore it is essential that the planner is aware of them and the daily run allows for this. If the planner is unable to action all of the exception messages he receives on a particular day, a revised list sorted again into priority sequence will be produced the next day, and if more urgent things have happened they will be brought to the top of the list. |
| If we get down into shop floor control, the daily "Dispatch List" is only useful if it is daily as it lists the work that is on the section. Not the work that was there last Monday, but the work that is physically on the work centre so the shop supervision know exactly the correct priority of the work they have. |
| We often hear the statement that net change runs are fine but once a week you need to do a regeneration run to tidy the system up. In practice I have never found the need for this as the daily net change run brings the whole situation back into line. This may appear to suggest that a regeneration run is never needed, this is also incorrect. When you make major changes to the MPS, the amount of processing required for all the items below that level is so vast it is more practical to do a regeneration run. A typical cause would be where you add the next months requirement to the MPS and a whole range of parts need to be exploded. My view as a user, is that net change is required nightly and only when we have submitted major changes would we ask for a regeneration run. That would usually have to be at a weekend to allow for the processing time required. |
| If currently you have a package with net change facilities but only do a regeneration run every week, start talking to your DP manager. He will probably try to talk you out of it initially as he envisages hours and hours of running every night. If you have serious problems with your DP manager suggest to him that you do the first net change run the night after you have done the regeneration run because this way it will only need to process a few changes, only those that have happened in the last 24 hour period and he may have a pleasant surprise. Remember, the package is there to serve manufacturing's needs and that it must do to the best of it's ability. |
| July 1997 |
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