bottlenecks
times and run times. Whereas MRPII assumes that a machine can always work at capacity, OPT accepts that the actual capacity is affected by statistical fluctuations and a dependence on previous operations to supply product for processing. This makes OPT can be more realistic in scheduling than MRPII by taking this into account and also allowing for improvement in times and routing.
The OPT rules
OPT is based on a set of rules:
1. Balance the flow, not the capacity.
2. Let bottlenecks determine usage of the non-bottlenecks and do not seek machine utilisation. If a resource is activated when output cannot get through the constraint then all it produces is inventory.
3. Utilisation and activation of a resource are not the same thing. Activation is when a resource is working but utilisation is when it is working and doing useful work. Producing stock for inventory is not useful work.
4. An hour lost at a bottleneck is an hour lost for the whole system and cannot be recovered and an hour saved at a non-bottleneck is a mirage.
5. Bottlenecks govern both throughput and inventory.
6. A transfer batch is not necessarily equal to a process batch, i.e. because you cut 20 frames at a time on the optimiser saw it does not mean that you have to push them all to the welder at once. Break the process batch (20 frames) down into smaller process batches (1 order).
7. Process batches should be variable and not fixed.
Bottlenecks beat out the pace like a drum and should be protected from interruptions such as breakdowns, quality, labour shortages etc. This can be achieved by building in time buffers that are a focus for process improvements. The other operations are synchronised to the bottleneck operation and work is pulled through as if it were on a rope.
OPT rarely requires large investment in machinery but seeks to improve the flow of the product and get inventory moving to show that production area as the real profit maker for the company.
For and Against OPT
For
1. Easy to start at practical level and easily understood by the shop floor
2. Quickly targets areas of concern such as bottlenecks, quality and inventory.
3. Gives quick results and financial feedback.
4. Suitable for discrete, batch and process industries.
Against
1. Challenges traditional cost accounting.
2. Requires accurate database and complex computer process modelling for the full form.
Summary
OPT is a philosophy for running the business rather [next page]