BLOCK SCHEDULING - THE FIRST STEP .byBrian Willcox CFPIM of Action MRPII |
| When companies implement an MRPII system, they usually want to introduce detailed or operation scheduling. The MRP module calculates the net requirement for each part by date, and then creates a planned order to satisfy the need taking into account both the ordering rules and the order modifiers. In addition, it deducts the manufacturing lead time held in the item master file from the completion date to establish the order release date (order start date). |
OPERATION SCHEDULINGMRP has now done it's thing, it has planned the future manufacturing programme for us, telling us when each order must be started and finished. What it does not do, is to tell us when each operation is to be performed at the applicable workcentre. To do this the shop floor control module takes each order and breaks it down by operation, as per the information held in the routing file. In a fully operational system the set-up and run time for each operation is held in the routing file. It takes the unit run time and multiplies it by the order quantity and then adds the set-up time to it. This gives the hours that will be consumed on the machine. The set-up and total run time of one operation is separated from the next by the insertion of queue and move time, taken from the workcentre file. The result is a detailed plan for the order, scheduled by operation. From this information, a load per time period, per workcentre, can be established. (Capacity Requirements Planning). |
| Unfortunately, at the time of implementing, most companies either don't have standard times or the ones they do have are known to be inaccurate and bad information is worse than no information. Perhaps the first question we should ask ourselves is, do we really need detailed operation scheduling? |
| There are three main
benefits obtained from operation scheduling -
1. As each operation has a start and completion date, it is now possible to verify if the order is progressing to plan and to give realistic delivery dates to customers subject to sufficient capacity being available. 2. When several orders are at a workcentre it is possible to sequence the work by using one of the many priority rules which depend upon due dates to calculate the individual order's current priority. 3. It permits capacity requirements planning to be performed as there is a planned date for the operation at a specific workcentre. For accurate operation scheduling to
be possible, standard times are required. One thing we do know is that you
can't delay an implementation while you do studies to establish the
standard times. The one alternative you have is to start off with
block scheduling. |
BLOCK SCHEDULINGBlock scheduling uses the principle of allowing an elapse time (a block of time) for each operation. This includes queue, set-up, run, wait and move time for the workcentre, irrespective of the part to be made or the quantity. It is an estimate based on past experience. The advantage is that once you have agreed on a block of time per workcentre, the routings can be created quickly. Typically the times would be, 5 days for stores picking, 3 days for drilling, 3 days for welding, or whatever is applicable for your own industry. This allows the works orders to be scheduled by operation. The disadvantages are that it is not usable for capacity planning, establishing work loads or efficiency reporting as the elapse time included more than the time spent on the machine, and it isn't accurate anyway. In addition, because of the general rules used, the lead times tend to be excessively long, resulting in a high WIP and the costs and disadvantages that go with it. |
| I may have painted too
black a picture. If you don't have standard times and you want to
introduce operation scheduling, block scheduling is the way to start.
Remember, it is quick and simple to apply but does not support capacity
requirements planning. It will never be as accurate as scheduling with
standard times. A point worth considering is that operation time (set-up
and run time) is usually only 5% of the manufacturing lead time, the major
part being queue time. As always in the real world, it's a trade-off. If
you don't want to establish standards because of the time and cost
involved, accept that your WIP will always be that bit
higher.
Some companies use block scheduling as an interim stage to using standard times. They start off with block scheduling and gradually change over to standard times. If you do the top 20% of the items first, they will usually take care of about 80% of the work load and then the scheduling becomes progressively more accurate and a limited form of work load calculations can be made. Other companies stay with block scheduling as it provides the means to schedule and track the WIP and allows priority rules to be applied. Sequencing and priority systems are a
subject to themselves, and will be examined as a separate
subject. |
| December 97 |
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