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"Why does MRP make
impossible promises?"
Q. I
work in sales. One of my friends who works in production control at my
company told me that "MRP (the Material Requirements Planning software)
accepts new promise dates that are already past-due on the day they are
promised, and MRP also loads the shop to 'infinite capacity,' without regard to
any realistic capacity." This remark disturbed me very much. No
wonder we have scheduling problems! Why would anybody use a scheduling
software that is so crazy that it promises things that are already past due, and
assumes infinite capacity?
A.
Let's take this one point at a time. MRP is a generic name for a
scheduling discipline. There are many, many MRP software products under
different brand-names. Every product that we know of is capable of
accepting an order promise past-due. Here's why:
MRP logic starts with the desired completion date of a
finished-item, then works backward on the calendar to figure out when to start
final assembly, when to start component production, when to receive raw
material, etc. MRP sorts all the upstream activities so that they get
scheduled in the same priority order dictated by the human beings who put in the
dates in the first place. To get all these priorities in the demanded
sequence, MRP must be the obedient servant and take the dates the humans put in
it. MRP doesn't make impossible promises, human beings do! Only the
humans can decide whether to promise something "yesterday," which
doesn't make sense, but is done all the time. MRP can't negotiate with a
customer, only a human being can.
Now, for the remark about MRP loading to "infinite
capacity." This is an "old chestnut" half-truth about MRP.
MRP will let you load work-centers to more than 100% of capacity, that's
true. However, MRP provides plenty of capacity reports which the humans
are supposed to read and act on. MRP can't hire more people, approve
overtime, or buy another machine -- that's a human being's job! Again, MRP
is the obedient servant -- it reports the facts to humans who are supposed to
make the decisions. Your calculator will let you run a negative balance in
your checkbook -- but it isn't your calculator's fault! Your calculator
can't make you stop spending more than you earn. It only reports back what
you put in it, just like MRP.
It is very unlikely that scheduling problems in your
company are due to faulty logic in MRP software. The responsibility
remains with the living people whose job it is to work together, make realistic
promises, and adjust physical capacity.
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