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In general each chain has only one weakest link. However imagine you have in your environment the following
situation.
As you notice this is an unusual situation. In one flow there are two operations that takes the same
amount of time and those two operations are both running on the constraint resource.
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For example resource B also takes 12 minutes. We learned that we need to protect
the constraint. How do we protect the constraint in this particular situation ?
And if you want to implement the DBR concept, what about the DRUM?
Scheduling still needs to be done in order not to stop the constraint and in
order to protect the Throughput.
What about BUFFER and ROPE ?
First of all, what is fixed for an order when it comes to production?
The most elementary things that are fixed, are:
- Delivery date
- Quantity to produce
- Lead time (= total time of the constraint buffer + the shipping buffer)
Then, what can we do with this information ?
We can put on a time axis the moment we may expect the order to be at the last constraint operation, i.e. Due date -/- Shipping buffer in days.
We can also say that the material will be issued one constraint buffer time before it is needed
on the constraint. BUT we have in this situation not one but two constraint operations!
What to do with protecting the constraint?
Should we tie the ROPE from the first or from the second constraint operation ?
It must be from the first because otherwise the first constraint operation is considered as non-constraint and
as such not protected. But we still have to do something to protect the second constraint operation. We will also
have to protect the second constraint with a time buffer. This means that we have in this case 2 times a constraint buffer.
What about the length of this second constraint buffer?
We know, that the total factory lead time is equal to the buffer time, so we want to take this second buffer as
short as acceptable (be paranoid and not hysteric). A full buffer size between the two operation is not
necessary because part of the process is already protected by the first constraint buffer. So the solution will be
a figure between 0 (= no protection) and 1 (= full protection). So what about 1/2 a constraint buffer size?
This make the total factory lead time: Constraint buffer + 1/2 Constraint buffer +
Shipping buffer
